GB2534559A - Method and system of controlling aircraft engine emissions - Google Patents

Abstract

A method and system for controlling climate impact of an aircraft engine 10 comprising a location determination system determining a plurality of the engine locations 34 during a current or planned flight, an ozone sensitivity determination system determining ambient air sensitivity to ozone production at the locations, a deployment control system assessing a value of one or more current or future engine operation variable at the locations under a normal operation condition and outputting a revised value or threshold for the variable in order to reduce NOx output by the engine if ozone sensitivity exceeds a predetermined value. Preferably the variable is combustion temperature, fuel flow or composition, throttle setting, etc; the determination system reads ambient ozone, 42, NO2, 44, HO2, 46, OH, 48, H2O, 50 concentration or sunlight (UV) level, 52; a comparator assesses positive and negative climate impact resulting from deployment of a NOx reduction system. Figure 4 illustrates a process 28 calculating the net benefit 30 attributable to a proposed level of deployment 32 of a NOx reduction measure with a flight section.

Description

TITLE OF THE INVENTION

Method and System of Controlling Aircraft Engine Emissions

BACKGROUND OF THE INVENTION

The present invention relates to aircraft engines and more particularly to methods and systems for reducing negative climate impact of aircraft engine emissions.

An important focus of improvements to aircraft engine operation concerns the reduction of greenhouse gas emissions due to fuel consumption. Whilst this is a concern for the whole life-cycle of aircraft engines, there is also a more specific consideration of local air quality (LAO) in the vicinity of airports.

Certain emissions from aircraft are regulated according to international standards which are set by the International Civil Aviation Organization (ICAO) in response to recommendations from ICAO's Committee on Aviation Environmental Protection (CAEP). The scope of the ICAO standards includes emissions taking place during the Landing and Take-Off (LTO) Cycle, which encapsulates aircraft operations below 3000 feet altitude relative to an airport. The standards encompass emissions of oxides of nitrogen (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO), and smoke.

US8311686B discloses a method for assessing engine emissions and/or noise at a collection of ground-based monitoring stations, and presenting the results of that assessment to pilots such that they may determine and manually implement appropriate actions to minimise the emissions/noise at each of the ground stations. The assessment makes use of information such as aircraft location/altitude relative to each ground station, wind-speed/direction, and engine operation parameters influencing the quantity of emissions actually produced.

The stringency of ICAO's nitrogen oxides (NOx) standards has been strengthened several times since their inception in 1986. However ICAO standards remain focussed on emissions during the LTO cycle.

There have been a number of proposals within the prior art for mitigating the negative climate impact of aircraft emissions beyond the LTO cycle, for example during cruise. EP 2 677 139 (ROLLS ROYCE) discloses a method in which regions of ice-super-saturated air are identified such that an aircraft carrying a plurality of fuel compositions can select the best composition to mitigate any negative climate impact associated with vapour trail formation by the aircraft engine emissions. However there are a number of different factors involved in assessing the impact of engine emissions on climate, of which engine vapour trail formation is just one.

NOx emissions from cruising aircraft lead to the creation of ozone and the destruction of methane, both of which are greenhouse gases. It has been determined that the direct ozone-related radiative forcing attributable to aviation's NOx emissions at cruise is a significant consideration and may be similar in magnitude to that arising from aviation-induced CO2 emissions. When the effects of aviation's NOx emissions are averaged over the entire globe, the balance between the resulting ozone-related climate warming and the methane-related climate cooling lies in favour of ozone-related warming. It is generally believed that technologies implemented for reducing emissions of NOx in the LTO cycle also help to reduce emissions of NOx at cruise. However, there has hitherto been little attempt to specifically target cruise NOx emissions.

"Temporal and spatial variability in the aviation NOx-related 03 impact" (Gilmore et al, 2013) analyses ozone production by aircraft engine NOx emissions using computational models and discloses that geographical variation in the efficiency with which ozone is produced from NOx emissions is a significant consideration in addition to variation with altitude. Gilmore et al concludes that aircraft could be rerouted to avoid specific regions in which the sensitivity of ozone production to NOx emissions is relatively high.

The strategy of avoiding regions prone to highly efficient ozone production through the routing of aircraft around, above and/or below such regions has the disadvantage that it increases workload for air traffic control and/or pilots, reduces airspace capacity and increases the duration/distance of a flight, resulting in a fuel-burn penalty. Additionally in the case of climbing so as to fly above regions prone to highly efficient ozone production, additional fuel is burned to provide the increased thrust necessary to perform the climb. If aircraft are scheduled to fly below certain regions, additional fuel may be burned subsequently if the aircraft is to return subsequently to its optimal cruising altitude.

It is an aim of the present invention to provide a system/method for mitigating the negative climate impact of aircraft engine NOx emissions that can be used in addition to, or instead of aircraft re-routing. It may be considered an additional or alternative aim to control aircraft engine NOx emission in a manner which reduces negative climate impact.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of controlling climate impact of an aircraft engine comprising: determining a plurality of locations of the aircraft engine during a current or planned flight; determining the ozone sensitivity of the ambient air at said locations; determining a value of one or more current or future engine operation variable at said locations under a normal engine operation condition; and, outputting a revised value or threshold for said one or more engine operation variable in order to reduce NOx output by the engine in the event that the ozone sensitivity exceeds a predetermined value.

The invention draws from detailed understanding of the mechanisms by which the climate impact of engine emissions can vary and advantageously allows an aircraft to fly through a region of high ozone sensitivity whilst mitigating against ozone production, particularly where re-routing of the aircraft to avoid said region in its entirety would be undesirable. It has been found that the size of regions of high ozone sensitivity may extend across hundreds of miles and avoidance thereof may be constrained by the need for access to large aviation centres or flight paths.

Accordingly the invention is particularly beneficial in allowing a practicable solution to controlling the impact of NOx emissions on atmospheric ozone production.

In the present disclosure, the term "ozone sensitivity" is used to indicate the efficiency with which ozone is produced, typically as a result of emissions of NOx. For example a region of high ozone sensitivity denotes a region in which a significant mass of ozone is produced per unit mass of NOx emitted. The ozone sensitivity determination may be made by sensing or predicting one or more ambient condition and correlating the sensed condition to an indication or rating of the susceptibility of ambient atmospheric gases to production of ozone arising from engine exhaust emissions. The ozone sensitivity determination may additionally take account of the degree of negative climate impact caused by ozone production at each said location.

The determining of a value of one or more engine operation variable may comprise receiving or predicting an engine operation demand, such as for example a thrust demand or an engine operation signal derived therefrom.

The method may comprise correlating said value of one or more engine operation variable to a NOx emission and/or ozone production rating for the determined ozone sensitivity at said location. A revised value or threshold for said one or more engine operation variable may be output if said rating meets or exceeds a predetermined value.

The engine operation variable may comprise a combustion variable, such as for example one or more of combustion temperature, fuel flow, fuel composition, combustor burner selection, air pressure or coolant flow to the combustor. The engine operation variable may comprise a throttle setting for the engine.

In some examples of the invention, the outputting of a revised value or threshold for said one or more engine operation variable comprises outputting a threshold suitable to prevent, or alternatively to constrain the climb-rate within step climb manoeuvres during a flight or portion thereof. In such an example, the higher combustion temperatures and NOx production caused by a step climb within a region of high ozone sensitivity can be avoided or otherwise advantageously constrained. This manoeuvre in particular has been determined to be of significant detrimental impact in contrast to normal cruise engine conditions at substantially constant altitude. The engine operation variable may be set such that a cruise climb is achievable in place of a step climb.

A particular benefit of the invention is that it may avoid the need for bespoke 5 equipment for engine emissions control. The invention may be implemented across a range of engines having different emission controlling equipment or systems. Existing equipment can thus be controlled in an improved manner to further reduce the climate impact of engine emissions. The invention may provide for a common framework which allows deployment across a variety of aircraft 10 and/or NOx reduction techniques.

Additionally or alternatively the engine operation variable may comprise a specific NOx emission control measure. The outputting of a revised value or threshold for one or more such engine operation variable, or other engine operation variables disclosed herein, may comprise a binary (i.e. an on/off) value or else a selection of a revised value from an available range of values.

In some examples, engine core cooling systems may be provided to selectively reduce or suppress NOx emissions in accordance with the invention. Such a system may comprise an engine coolant injection system, for example arranged to inject liquid into the engine upstream of or within the combustor. Any of water, a liquefied gas or another conventional liquid coolant may be used.

In some examples of the invention, the maximum fuel flow rate to the engine may 25 be capped. Determination of any cap or threshold value may take account of prevailing operating conditions and/or the stage of flight. Accordingly a dynamic or variable cap or threshold may be applied according to any aspect of the invention.

The outputting of a revised value or threshold for said one or more engine operation variable may be implemented by an engine controller. Any or any combination of the determining steps of the invention may be performed: substantially in real time during a flight; in advance of a flight; or during a flight but in advance of arrival at one or more of the locations.

Any or any combination of the determining steps of the invention may be performed on board the aircraft or remotely there-from and communicated to the engine prior to a flight or during a flight.

One or more ambient condition sensor may be used to determine the ozone-sensitivity of the ambient air and/or another ambient condition.

The outputting of a revised value or threshold for said one or more engine operation variable may be implemented in one or more specific mode of engine operation or flight phase. The engine operation may or may not be controlled according to the invention during a normal mode of operation, for example such that the invention is used exclusively during a normal mode of operation. Abnormal or emergency operation of the engine require full control of the engine across its entire permissible operational range. The output of the revised value or threshold of the one or more engine operation variable may be inhibited during one or more abnormal modes of operation or flight phases.

The outputting of the revised value or threshold for said one or more engine operation variable may or may not be inhibited during one or more flight phase, for example during an initial climb phase of a flight, e.g. during climb out and/or top of climb. The control of engine operation according to the invention may be enabled during cruise, for example only during cruise or also including initial descent/approach.

The control according to the invention may be enabled or disabled according to one or more predetermined criterion. When enabled, the output of the revised value or threshold for said one or more engine operation variable may be automated in response to the sensing of one or more parameter, e.g. an ambient or engine operation parameter, or a location of the aircraft.

A suitable revised value or threshold for said one or more engine operation variable may be determined according to a computer model, for example including one or more database or look-up table. The model may correlate one or more of an ozone-sensitivity reading or other ambient condition, or desired NOx level/threshold with a value or threshold, such as an engine setting, of the one or more engine operation variable.

According to a second aspect of the invention, there is provided an aircraft engine control system for performing the method of the first aspect.

Ozone sensitivity readings may be taken onboard the aircraft or else remotely there-from and communicated to the control system. Ozone sensitivity readings may be taken for a location in advance of the aircraft arriving at said location, or else substantially in real-time.

An engine controller may output the revised value or threshold for said one or more engine operation variable. The engine controller may determine said revised value or may receive said revised value from one or more further processor either onboard or remote from the aircraft.

According to a third aspect of the invention, there is provided a data carrier comprising machine-readable instructions for the control of one or more processor to perform the method of the first aspect.

In any aspect of the invention an automated deployment decision for a NOx reduction measure or system may be output as a result of the determination process. The deployment decision may comprise the revised value or threshold for the one or more engine operation variable.

Wherever practicable, any of the essential or preferable features defined in relation to any one aspect of the invention may be applied to any further aspect. Accordingly the invention may comprise various alternative configurations of the features defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Practicable embodiments of the invention are described in further detail below by way of example only with reference to the accompanying drawings, of which: Fig. 1 shows a half-longitudinal section through a gas turbine engine according to the present invention; Fig. 2 shows atmospheric reactions relevant to aircraft engine NOx emissions; Fig. 3 shows an example of an aircraft flight for which the invention may be used; Fig. 4 shows a schematic of a determination system for deployment of a NOxreduction measure for a section of a flight according to an example of the invention; and Fig. 5 shows an example of a NOx-reduction measure deployment determination system for a flight, in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Figure 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, and intermediate pressure turbine 18, a low-pressure turbine 19 and a core engine exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 and a bypass exhaust nozzle 23.

The gas turbine engine 10 works in a mechanically conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high-, intermediate-and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting shafts.

Alternative gas turbine engine arrangements may comprise: a two, as opposed to three, shaft arrangement; a gearbox on the low pressure shaft for driving the fan 13; and/or may provide for different bypass ratios. Other configurations known to the skilled person include open rotor designs, such as turboprop engines, or else turbojets, in which the bypass duct is absent such that all air flow passes through the core engine. A further configuration known to the skilled person may comprise an electrical generator which is driven by one or more turbines and which is additional to or alternative to the propulsive fan 13.

The invention concerns the control of the engine operation by an electronic engine controller 24, typically the Engine Control Unit (ECU), which may comprise a pad of a Full Authority Digital Engine Control (FADEC) system. The ECU 24 receives engine demand signals input by the aircraft cockpit controls. The ECU also receives sensor readings from a plurality of engine operation sensors at various locations throughout the engine 10. The engine also receives any or any combination of engine operation demands, external/ambient sensor readings and/or other data pertinent to the engine operation via a signal receiver, e.g. for receiving data from the airframe data bus or from a remote wireless signal source. Whilst the example of Fig. 1 shows a single ECU per engine, it will be appreciated that a one-to-one relationship between the controller and engine is not essential. A common controller could be implemented for a plurality, or all, engines on an aircraft and accordingly the invention applies to one or more controller for use in controlling one or more aircraft engine.

This invention sets out a system for controlling the deployment of additional NOxreduction methods to target specific atmospheric regions in which the sensitivity of ozone levels to aircraft NOx emissions is very high. The ozone sensitivity takes account of factors which influence ozone-production-efficiency. In this way the engine/aircraft efficiency penalty of achieving a certain level of climate-impact reduction is reduced relative to an untargeted deployment of additional NOxreduction methods. This invention balances the disadvantageous reduction in methane destruction against the advantageous reduction in ozone formation so as to identify solutions that yield an overall benefit. This invention may also allow for prioritising any resources required to achieve NOx-reduction to those flights which can use the resources to the greatest benefit.

Whilst it has been found that regions of high ozone sensitivity lie predominantly over the open ocean, it has also been determined that the atmospheric residence time of ozone is of the order of several weeks. Thus the resulting climate warming impact of increased ozone creation is not limited to those geographically remote regions of high ozone sensitivity but is instead spread over a significant proportion of the globe. The targeting of a reduction in ozone creation in such regions is therefore particularly advantageous, despite such regions being in many cases remote from major population centres.

RELEVANT ATMOSPHERIC CHEMISTRY

The following section provides an indication of the atmospheric chemistry influencing control methodologies described herein for reducing the negative climate impact of engine-induced NOx: * At cruise, engine exhaust NOx consists mainly of nitric oxide (NO) and a small proportion of nitrogen dioxide (NO2).

* NO is oxidised to NO2 by ambient hydroperoxy (H02) radicals, a process which gives rise to hydroxyl (OH) radicals.

* The NO2 is then split by ultraviolet light (from the sun) to yield atomic oxygen which can combine with ambient molecular oxygen (02) to yield ozone (03).

* Ozone production is thus enhanced by aircraft NOx emissions.

* OH, created during the oxidation of NO to NO2, is a powerful oxidising agent, and enhances the destruction of ambient methane (CH4) with a corresponding climate cooling effect, since methane is a greenhouse gas.

* The destruction of methane triggers further cooling effects related to a reduction in ozone concentrations and in stratospheric water vapour concentrations.

* Ozone is destroyed by OH and/or H02 and/or photolysis (being broken up by sunlight). NO and NO2 can also consume ozone but are not as significant.

The key reactions are as follows, the inter-relationship between which is shown diagrammatically in Fig. 2: a) NO + H02 -> OH + NO2 b) NO2 + sunlight -> NO + 0 c) 0 + 02 -> 03 d) OH + CH4-> H2O + CH3 In general, if ambient concentrations of NO2 or of 03 are high, then the reaction rates of (a) and (c) are respectively slowed. In cases where reaction (a) is suppressed as a result of re-routing flights around regions of high ozone sensitivity, methane destruction is disadvantageously also suppressed.

NOx REDUCTION OPTIONS The present invention is concerned with a method for determining under what circumstances, and to what extent, a NOx-reduction system or systems should be deployed in order to strike an optimal balance between engine efficiency and a reduction in NOx-related climate impact. The present invention may accommodate a wide variety of NOx reduction techniques and thus may be applicable to various different engine configurations and/or associated NOx-reducing equipment.

NOx is created within the combustor of an engine, through the interaction of oxygen and nitrogen at high temperatures. The mass of NOx produced per unit mass of fuel burned (known as EINOX) is very strongly and non-linearly dependent upon the peak temperatures encountered within the combustor. At higher peak temperatures EINOx rises very quickly. This applies to rich burn and lean-burn combustors. Therefore in various embodiments of the invention, the control scheme implemented aims to limit or constrain 'peak' engine combustion temperatures being achieved, for example by defining a desirable maximum combustion temperature threshold and controlling engine operation and/or associated equipment to ensure that the threshold is not exceeded within an atmospheric region that is sensitive to ozone production.

A number of different techniques for achieving this aim are described below.

In some examples, in regions where the ozone sensitivity lies above a predetermined threshold, the controller could eliminate step-climbs and enforce a slower, continuous cruise-climb manoeuvre, yielding a fuel-burn advantage as well as reduced NOx. This could be achieved by applying a predetermined allowable ascent rate or gradient. Alternatively, in regions where the ozone sensitivity lies above a predetermined threshold, the controller could aim to enforce an altitude threshold that matches the altitude at which the aircraft enters the ozone-sensitive region. Such an altitude threshold may be applied until the aircraft leaves the region. This may lead to a small fuel-burn penalty, but the NOx advantage due to the lack of step climb within the ozone-sensitive region may outweigh the fuel-burn penalty in terms of negative climate impact.

Instead of a binary on/off approach to step climb manoeuvres, a proportional approach could be taken. For example, a policy could be implemented that limits the maximum rate of step-climb, thus reducing the peak combustion temperatures reached during the climb. Although the climb would take longer, the non-linear dependence of EINox upon temperature means that the total NOx emissions during the climb would be reduced. In such examples, the rate of climb could be limited in dependence upon the ozone sensitivity level at the aircraft's location and altitude.

The allowable maximum rate of climb would typically reduce relative to a conventional maximum climb setting as the ambient ozone sensitivity increases.

In one example, a received climb command from the cockpit could be altered such that a modified climb signal is output to the engine. The modified climb signal may be communicated to the other associated aircraft systems, comprising aircraft control surface actuation systems. In other examples, the engine control unit 24 may respond to a climb request to confirm the actual or maximum engine setting being implemented such that the other aircraft systems react accordingly. In another example a maximum or recommended thrust setting could be output as a communication to the aircraft operator for manual implementation. In such respects, the invention accommodates either an executive/automatic control unit or non-executive/informative processing system that outputs control proposals rather than machine-readable control instructions.

In some examples, the peak temperature within the engine combustor(s) could be limited to a level that is defined in dependence upon the ozone sensitivity in the aircraft's current location. A limit on one or more combustion setting may be implemented to achieve this goal, such as fuel flow rate, engine throttle, air compression into the combustor(s), or any combination thereof. The peak engine temperature control could be used in some examples to effectively prevent step climb manoeuvres by limiting the thrust capability of the engine.

The limitations in step-climb rate and/or peak engine core temperatures could be implemented in a number of ways according to different implementations of the invention. In an embodiment which is in many ways preferred, automatic decision-making on-board the aircraft could determine appropriate limits on climb-rate and/or peak engine core temperatures, and then could communicate them to the pilot and/or automatically prevent climb-rate, peak temperatures, etc from exceeding those limits, through the engine operation parameter controls described herein.

Determination of the engine parameter control limits could be achieved by one or more of the following methods: 1. Sensors on the aircraft could observe one or more ambient condition indicative of ozone sensitivity, such as the concentration of one or more compound or species within the ambient air, e.g. one or more of NOx, ozone, H02 etc. A decision-making unit would receive the measured values in conjunction with current aircraft location and altitude to determine the local ozone sensitivity, and would then calculate an upper limit for the relevant engine operation variable. 2. An on-aircraft database or lookup-table describing the ozone sensitivity of any point according to its longitude, latitude and altitude could be interrogated, and the retrieved value corresponding to the aircraft's current location/altitude would then be used to calculate the limit(s) to the relevant engine operation variable(s). This method may potentially avoid the need for bespoke ambient condition sensors on the aircraft itself 3. An on-aircraft database or lookup-table containing predetermined values of engine operation parameter limits could be interrogated to establish the limits appropriate to the aircraft's current location.

Either of the second or third examples above may require population of the database or lookup-table with information received from a ground-based monitoring system in advance of or during the flight. However the current engine operation limitations are implemented dynamically by an on-board controller during the flight, e.g. substantially in real time, in response to the aircraft location.

In another potential implementation of the invention, a ground-based air traffic management (ATM) or flight control centre could monitor flight paths through ozone sensitive regions and issue notifications to receiver systems onboard the aircraft for communication to pilots, for example advising them of the maximum climb rate, or other operational restrictions to be adopted during passage through ozone sensitive regions, e.g. for an imminent step-climb or other manoeuvre.

Thus it will be appreciated that using conventional aircraft data communication systems, e.g. via satellite communication, ozone sensitivity data for a relevant flight portion could be transmitted to an aircraft in flight. Additionally or alternatively, due to the relatively slow-moving nature of the ozone-sensitive regions it is also feasible that such data could be uploaded to an onboard data-store prior to take-off.

In any example of the invention, other ambient conditions may be monitored either on board, or else remotely of, the aircraft and fed into the control scheme in addition to those described above. For example any or any combination of the time of day, time of year, season, or a radiation sensor may be used as a contributing ambient condition, which may impact the local atmospheric sensitivity to NOx emissions. The time of day/year may be used to imply one or more ambient condition, such as the incident radiation by sunlight, e.g. without the need to measure it directly on board the aircraft.

The difference between a step climb and an initial climb phase of a flight would be understood by the skilled person. In situations where a departure airport lay close to or within the region of high ozone sensitivity, it is envisaged that the initial climb requirements would over-ride the NOx reduction scheme. In this, and other scenarios to be described herein, the NOx control scheme would only be implemented in a default or normal mode of engine operation, or at particular flight phases (e.g. cruise), and would be disabled/overridden by the controller to satisfy higher priority engine requirements.

The above examples concern the novel control of engine operation for the purpose of effective NOx reduction using existing engine control parameters. However it is also possible that bespoke equipment for management of the relevant engine operation parameter(s) may be provided, for example to increase/maximise the benefits achievable by way of the present invention. A key feature of such systems is that, in order to reduce weight and/or fuel-burn penalties, the bespoke NOx suppression equipment (as with the engine control parameters described above) would not be used throughout an entire flight. Deployment of such equipment would instead be targeted to specific operating conditions in which their benefit is maximised. A non-limiting list of examples of such equipment is below: * Water iniection into the combustor or compressor. An onboard water reserve may be selectively injected to reduce combustion temperatures within the engine.

* Injection of liquid air into the compressor. An onboard liquid air reserve may be selectively injected to reduce combustion temperatures within the engine. An example of such a system is provided in GB2479001.

* Use of liquefied natural gas (LNG) as a fuel. LNG offers a number of potential advantages as an aviation fuel, for example lower price and lower CO2 emissions than conventional kerosene jet fuel. However, due to LNG's poor volumetric energy density, typically LNG tanks will be sized for a specific mission length, with kerosene used to supplement LNG where necessary. Accordingly the present invention may be used for prioritisation of LNG usage in place of kerosene for parts of the flight where the environmental benefits of LNG are greatest. Burning LNG rather than kerosene enables the same thrust production but with slightly lower combustion temperatures, and hence can enable lower NOx emissions.

* Use of butanol as a fuel or as a fuel component. It has been found that significant reductions in thrust-specific NOx emissions can be achieved by substituting butanol in place of some or all of the Jet-A within the fuel-composition supplied to a small gas turbine engine. However, butanol has a 25% lower specific energy than kerosene, and thus may not be suitable for use throughout the entirety of a flight. Prioritisation of its use (e.g. by the present invention) to those sections of a flight where the NOx-reduction yields maximum benefits would thus be advantageous.

* Variation of pilot vs. main-burner fuel flow in a lean-burn combustor. There is a large change in NOx emissions from a lean-burn combustor at the transition between pilot-only and main-burner operation, and at the transition between half-burners and all-burners operation. Thus independent control of the pilot and/or main burners fuel-flows can be managed so as to minimise emissions at a particular engine operating point. However, there is a variation of combustion efficiency (and hence engine fuel efficiency) across a staging point. Therefore the independent control of fuel-flows to the different burners for NOx emission reduction would benefit from prioritisation by way of the present invention to those situations in which the net climate benefit is the greatest.

NOx CONTROL CONSIDERATIONS Factors influencing determination of a suitable level of NOx reduction according to one or more control technique described herein are provided below.

a. Rate of NOx creation In the absence of other transient changes in engine operation or atmospheric conditions, application of a NOx-reduction technique is likely to yield more value if it is carried out when an engine is creating NOx at a high rate, rather than a low rate. Accordingly a deployment decision for the NOx-reduction technique may be targeted for use within time periods for which engine core temperatures exceed a threshold value, such as top-of-climb and/or during step-climb manoeuvres.

b. Rate of Ozone Creation from NOx With reference to reactions (a), (b) and (c) set out above, we can see that the creation of ozone from engine exhaust NOx is enhanced by the following: i. High ambient H02 concentrations: promotes reaction (a) ii. Low ambient NO2 concentration: promotes reaction (a) iii. Low ambient OH concentration: promotes reaction (a) iv. High levels of incoming ultra-violet light (sunlight): promotes reaction (b) v. Low ambient ozone concentration: promotes reaction (c) However, methane destruction is enhanced by reaction (a). Hence the prioritisation of anti-NOx measures to sensed conditions iv and/or v may help to limit significant ozone production, but without adversely affecting methane destruction. In view of this a deployment decision may be based upon ambient ozone concentration below a low threshold value and/or incident sunlight/UV light above a threshold value. Instead of sensing incident light, a sunlight reading may be approximated according to a clock and/or daylight hours log. A binary light reading may be sufficient in some examples (e.g. having a zero or very low threshold) or else a radiation intensity reading may be used for example by use of on-board radiation sensing or reference to seasonal charts to approximate light intensity for a given time and date. The light intensity could be an instantaneous value, or an average value over the daylight hours, or the average value over 24 hours, or a value representative of the peak intensity likely to be experienced during the daylight hours. The light intensity reading could refer to the intensity within a broad spectrum of wavelengths, or to the intensity within one or more specific wavelength band relevant for reaction (b) listed above.

c. Rate of Ozone Destruction The lifetime of ozone created by engine emissions may be a factor in the control scheme so as to infer a more-detailed model/consideration of climate warming impact. Potential ozone destruction mechanisms may therefore be an input into the deployment control decision. If ozone destruction is rapid then there is less requirement to suppress NOx formation in order to suppress ozone formation. In contrast, in regions where ozone destruction is likely to be relatively slow, then suppressing the formation of ozone may carry greater benefits.

Ozone destruction may be hindered by the following vi. low levels of OH vii. Low levels of H02 viii. Low levels of sunlight ix. Low levels of NO and NO2 These circumstances are broadly the opposite of the factors i-v identified above for enhancing the rate of ozone creation. However, while the ozone creation processes are relatively swift, and can therefore be influenced by actions based on geographically localised measurements of species concentrations, ozone destruction lifetime is of the order of several weeks, meaning that the precise rate of destruction is influenced by conditions over a much broader geographical area. When making an assessment of ozone destruction rates it is therefore proposed to access information from sources beyond the aircraft itself (e.g. satellite measurements, readings taken by other aircraft, forecasts etc). Optionally, the rate of ozone destruction could be omitted from decision making, and the focus placed on minimising ozone creation.

d. Effectiveness of Ozone as a Greenhouse Gas Optionally, the decision to suppress or not to suppress ozone formation could also take into account the effectiveness of the ozone as a greenhouse gas, i.e. the level of disadvantage incurred due to ozone formed as a result of NOx emissions in the aircraft's current location. Such an assessment would take into account factors such as the average ambient temperature in the "region of influence", and the average underlying surface temperature in the "region of influence". The term "region of influence" is used to mean the geographical volume throughout which any ozone, formed as a result of NOx emissions at the aircraft's current location, can be expected to spread during its lifetime. Optionally, the calculation could be weighted according to distance from the point of NOx emissions, or according to distance from a presumed trajectory (originating at or near the point of NOx emissions) dictated by prevailing wind patterns.

OPTIONS FOR DEPLOYING THE DECISION-MAKING LOOP

An aim of the present invention is to determine, for each point in a flight, the extent to which a NOx-reduction method should be deployed. The level of deployment could be binary ("on / off'). Alternatively the level of deployment could encompass a number of discrete deployment levels (for example: "off, low, medium, high"). Alternatively the level of deployment could encompass a sliding scale, for example allowing selection of any value lying within a continuous range from a predetermined lower limit, such as zero, up to a predetermined upper limit.

There are several options for implementing an overall decision-making process. In each case the decision making process makes use of one or more input variable from the following list: ambient ozone concentration, ambient H02 concentration, ambient OH concentration, ambient H2O concentration, ambient NO concentration, ambient NO2 concentration, latitude, longitude, altitude, time of year, engine operating point. It is beneficial if the control scheme proposed by the invention is only activated when required and/or when not superseded by a more important engine/thrust demand. The control scheme may be disabled entirely, for example by not processing the relevant atmospheric and aircraft location data, or else by inhibiting the control of engine operation parameters by the NOx reduction systems described herein. In different examples of the invention, the decision to deploy the NOx control scheme may be the same as, or different from, the decision to implement the control output by the controller once activated.

Implementation options for the "decision-to-deploy" process are as follows: 1. Use a predetermined script which specifies for each point (or section) of a flight whether and to what extent NOx-reduction should be deployed. The predetermined script is calculated pre-flight, specifically for the planned route and altitude profile, taking account of conditions expected to be encountered during the specific flight. The predetermined script would likely take the form of an ordered list of points that the aircraft is intended to pass during the flight, each point being associated with a choice of NOxreduction method and the level of deployment thereof.

2. Use a pre-populated lookup-table or database or mathematical relationship which specifies the extent to which NOx-reduction is required, the extent and choice of NOx-reduction method being specified in dependence upon physical position (latitude, longitude, altitude), time of year, and/or engine operating conditions.

3. Use a predetermined policy which identifies conditions under which NOxreduction measures should not be deployed. In other conditions, a decision-making process (described below) would be allowed to operate and to control the level of deployment of NOx-reduction. The operation of the control decision making process may be restricted, for example to any or any combination of: a. one or more latitude band and/or longitude band b. one or more range of altitude c. a period after initial engine start or warm-up (i.e. inhibited during engine warm-up) d. periods when the engine core temperature is above a predetermined threshold The decision-making process could be invoked under one of the following 15 circumstances: 1. Upon making a transition from operating conditions under which invocation of the decision-making process is forbidden, according to a predetermined policy as described above, to conditions under which invocation of the decision-making process is permitted, according to a predetermined policy of the type described above. If there is no such policy, then the decision making loop would first be invoked upon engine start-up.

2. Immediately after a previous invocation of the decision-making process has completed, or 3. After a delay of a predetermined duration (for example 1 minute or 10 minutes) following the previous invocation of the decision-making process has completed, or 4. When the aircraft's position has changed by a predetermined distance (for example 10 kilometres or 50 kilometres) since the previous invocation of the decision-making process has completed, or 5. In response to a material change in observed ambient conditions (for example ambient species concentrations) since the previous invocation of the decision-making process has completed, or 6. In response to a material change in engine operating point (for example throttle setting, or for example measured temperatures inside the core of the engine) since the previous invocation of the decision-making process has completed, or 7. When the core temperature in the engine exceeds a predetermined threshold.

Each engine could have its own decision-making process, or there could be a single decision making process for all engines on board a single aircraft.

A flight could be viewed as being divided up into a number of sections, where the boundary between two adjacent sections corresponds to one of the changes given in the list immediately above.

MEASUREMENT OF AMBIENT CONCENTRATIONS

The decision making process is informed by assessments of the ambient concentrations of various species such as ozone, H02, OH, H2O, NO and/or NO2 at the aircraft's current position. There are several options for making such assessments: 1. Use a pre-populated lookup-table or database which details the expected ambient concentrations of the various species in dependence upon location (latitude, longitude, altitude) and the time of year. Values contained with the pre-populated lookup-table or database would be determined by measurements taken prior to the flight, and would thus not be real-time measurements. Although such pre-determined data may reduce accuracy compared to real-time monitoring, both the spatial resolution requirements of the invention, and the rate at which measurements for many of the atmospheric species of interest change significantly, may be relatively low, thereby making such an embodiment viable.

2. Use remote sensing capability (satellites, other aircraft in the region) to take measurements of ambient species concentrations, and then communicate the measured values to the aircraft so as to provide quasi real time information. This approach would have the advantage that the aircraft would have access to up-to-date information, specific to its location, but would not need to carry any sensors on board.

3. Use on-board sensors to provide real-time measurements of ambient species concentrations.

The decision-making process is also influenced by the aircraft's current position (latitude, longitude, altitude) which is measured by systems already installed on board the aircraft. The atmospheric sensing options above may be enhanced by communications between aircraft and a ground-based monitoring station, such that up-to-date readings from aircraft having the relevant sensors can be amassed at a ground monitoring station and relevant atmospheric data communicated to aircraft for a remainder or later portion of a flight.

CALCULATION OF EFFICIENCY/FUEL-BURN PENALTY It is assumed in the present invention that the NOx-reduction method to be implemented incurs some form of fuel-burn penalty, either when deployed, or due to increased weight associated with the carriage of the necessary materials (such as coolant) required for its use. In the examples of fuel composition switching for NOx reduction, a less efficient fuel blend/composition may be required to be burned.

Figure 3 illustrates a flight from Airport A to Airport B, along a route which passes through a region 26 characterised by a significantly higher climate change impact per unit mass of aircraft NOx emissions. In cases where the chosen NOx-reduction measure (e.g. water-injection, liquid-air-intercooling) involves the consumption of a resource which has an associated weight penalty, the deployment of the NOx-reduction measure within the region 26 will involve the deployment of a resource which has incurred a fuel-burn penalty for a substantial distance from Airport A. However for the return journey from Airport B to Airport A along the same route, the deployment of a similar NOx-reduction measure could be achieved at lower efficiency penalty cost, since the resource has to be carried less far before it is deployed.

This efficiency penalty calculation may take into account any or any combination 30 of: o The fuel consumption penalty associated with carrying to the point of deployment the required resources, optionally taking account of any headwinds or tailwinds that may be encountered en-route to the point of deployment.

o The fuel consumption penalty or benefit associated with operation of the NOx-reduction measure, taking into account the proposed level of deployment of the NOx-reduction measure and the duration of the period of deployment at the proposed level o Any additional fuel required to carry the above-described resources or fuel to the point of deployment From a practical point of view an airline would be concerned with the cost of implementation of the NOx reduction measure and so any efficiency penalty may be represented as a cost. Such a cost calculation may also include the purchase cost of any resources (e.g. water, liquid air, more expensive fuel type) required to implement the proposed level of deployment of the chosen NOx-reduction measure for the duration of the section.

Accordingly in any example of the invention, the potential engine efficiency/fuel burn penalty or cost may be determined and used as a deployment control factor (either as a determination prior to a flight or else in-flight).

CALCULATIONS

Assessing the Net Benefit in a Section of a Flight In this context, a "section" is a part of a flight in which relevant parameters can be considered relatively unchanging. Boundaries between sections might correspond to any or any combination of the events listed 1-7 above for invoking the NOx reduction decision-making process.

Figure 4 shows an example process 28 for calculating the net benefit 30 attributable to a proposed level of deployment 32 of a NOx-reduction measure within a particular section of a single flight. The result is calculated in dependence upon the following inputs: the aircraft's location (including position and altitude, which may be section-specific) 34, the current date (time of year) 36, the duration of the section 38, and the section-specific operating point 40 of the engine or engines. The term 'operating point' is used to encompass a range of engine-specific parameters whose values are monitored. Of most relevance to the present invention is a temperature measurement indicative of the peak temperature encountered within the engine's combustion chamber. Additionally, or alternatively, such a temperature can be inferred from readings such as ambient temperature and pressure, the engine's overall pressure ratio and the engine's fuel-flow rate. In Figure 4, parameter inputs to the process 28 are understood to be available to any, any combination, or all of the models or sub-processes within the process 28.

The process 28 has access to a number of computational models or the relevant physical (e.g. chemical and/or engineering) models affecting the negative climate impact of engine NOx emissions. The computational models may comprise any or any combination of computational algorithms, databases and/or look-up tables, for which the relevant values be obtained from any of: o measurements taken by sensors on board the aircraft (whether on the current flight or on previous flights) o measurements taken by sensors on other aircraft o near-real-time satellite measurements o a pre-populated database based on time-averaged data and/or forecasts Each model could be flight specific (e.g. flight-path specific) or more general (e.g. regional, hemispherical or global), such that a proportion of the data/knowledge therein would be used on a single flight. The models in this example comprise: a model 42 of the spatial and temporal variation of ambient concentrations of ozone; a model 44 of the spatial and temporal variation of ambient concentrations of NO2; a model 46 of the spatial and temporal variation of ambient concentrations of H02; a model 48 of the spatial and temporal variation of ambient concentrations of OH; a model 50 of the spatial and temporal variation of ambient concentrations of H2O (averaged over some appropriate timescale); a model 52 from which can be established details of the typical strength of incoming sunlight (averaged over for example a 24-hour cycle, or over the daylight hours, or over some other period within the daylight hours), or of the peak strength of incoming sunlight within for example a 24-hour period, based on the latitude, season and optionally any specific conditions such as long-lasting weather patterns (e.g. Monsoon, El Nino).

A process 54 receives values obtained from one or more of models 42-52 and determines the mass of ozone production per unit mass of NOx emitted. Process 56 receives values obtained from one or more of models 42-52 and determines the mass of methane destruction per unit mass of NOx emitted. Processes 54 and 56 may be performed in parallel. Process 56 could be omitted in other embodiments.

Process 58 receives the proposed level of deployment input 32 and determines the corresponding reduction in the rate of NOx production likely to be achieved.

The output of 58 feeds into calculation process 60, which determines the total mass of NOx production which will be avoided within the section of flight being considered, in dependence upon the output of 58 and the duration of the section 38.

62 is a model from which can be determined a value expressing the negative climate impact or penalty arising from the production of a unit mass of ozone in or near the aircraft's current location 34, taking account of the time of year 36.

64 is a model from which can be determined a value expressing the positive climate impact associated with the destruction of a unit mass of methane. Due to the potentially long lifetime of methane in the atmosphere, this model need not be location specific.

Process 66 determines the net benefit associated with the reduction in ozone production made possible by the avoided mass of NOx production determined by process 60. The process 66 will take account of the value determined by the process 54 concerning the mass of ozone produced per unit mass of NOx emitted. From these will be calculated the total mass of ozone avoided due to the reduction in NOx emissions during the section of the flight. This will then be combined with the value determined from the model 62 to produce a total, specific to the current section of a flight, for the ozone-related "benefit" of the proposed level of deployment 32.

Optional process 68 determines the net cost associated with the loss of methane destruction arising from the reduced NOx production determined by the process 60. The process 68 would take account of the value determined by the process 56 concerning the mass of methane destroyed per unit mass of NOx emitted. From these will be calculated the total mass of methane retained (i.e. not destroyed) due to the reduction in NOx emissions during the section of the flight. This will then be combined with the value determined from the model 64 to produce a total, specific to the current section of a flight, for the methane-related climate impact of the proposed level of deployment 32.

Model 70 determines the operational aircraft/engine(s) efficiency penalty of implementing the proposed level of deployment 32 for the duration of the section 38, potentially taking into account the distance and/or altitude of the point of deployment from the flight's departure point, and the altitude of the point of deployment.

72 is a process which determines the net benefit of the proposed level of deployment 32, taking into account the ozone related benefit (from 66), the implementation efficiency/cost penalty (from 70), and optionally the methane related climate impact value (from 68). The net benefit 30 is determined by subtracting the penalty output of model 70 (and optionally process 68) from the benefit output by 66. Additionally or alternatively the net benefit 30 could be expressed as a benefit-to-penalty ratio or another combination of the benefit and total penalty calculations.

Whilst a specific determination process for the net benefit 30 is described above, it will be appreciated that a more complex or more streamlined process may be accommodated provided the consideration of the climate benefits and penalties are juxtaposed in order to achieve a representative value output indicative of the attractiveness of NOx reduction for relevant sections of the flight. The complexity of the process will be determined predominantly by the number and type of parameters accommodated in the calculation and the nature of accommodation (i.e. by measurement or approximation).

SINGLE FLIGHT DEPLOYMENT ASSESSMENT

Figure 5 shows an example process 74 for calculating the flight-specific net benefit 76 attributable to a proposed pattern 78 of deployment levels of a NOx-reduction measure during different sections of a single flight.

The pattern 78 specifies for each section of the flight: * The proposed level of deployment of the NOx-reduction measure * Other section-specific parameters necessary for the evaluation include: o The duration of the section (corresponding to 38 from Fig. 4) o The aircraft location (corresponding to 34 from Fig. 4) o The engine operating point (corresponding to 40 from Fig. 4) Each section is evaluated in turn using the process 28 described above, taking account of the time of year 36 (which is not section-specific and therefore does not form part of 28). Net benefits are accumulated/summed to determine the net benefit for the flight as a whole (i.e. the flight-specific net benefit 76). Positive and/or negative net benefit values may be accommodated.

Whilst this process is described at the level of a single flight, it is to be understood that the assessment for an aircraft could be extended to a number of consecutive planned flights if desired. The time delay between flights may be accommodated as well as refuelling and/or supply of NOx-reduction resources whilst on the ground as necessary.

DISCOVERING THE OPTIMAL PATTERN OF DEPLOYMENT LEVELS

The next step is to use the net benefit or "fitness function/value" calculated in the previous section to identify the most advantageous combination of section-specific deployment levels. The aim is to identify what level of deployment of the NOxreduction method should be employed during each of the sections to optimise usage for the flight as a whole.

This can be achieved using an optimisation algorithm which uses 78 as a parameter vector to define its search space, and the value 76 to guide its search. The search space will be constrained by limits on resource availability (e.g. the maximum quantity of coolant that can be stored or maximum duration of permitted NOx reduction measure) or by limits on additional fuel burn required to implement the proposed pattern of deployment levels, taking account of the duration of each section to which proposed levels of deployment relate. The optimisation algorithm would generate possible candidate solutions comprising proposed NOx reduction levels over all flight sections. Candidate solutions could be scored and penalised if they did not meet the constraint. Alternatively the optimisation algorithm could be configured to generate only feasible (i.e. constraint-satisfying) candidate solutions.

In the event that the search space is prohibitively large, e.g. having a significant number of flight sections, one or more policy may be applied to limit the search space and thereby the available candidate options. One such policy may rule out or permit the deployment of cruise NOx-reduction measures in certain altitude bands or latitude bands or certain geographical areas. An additional/alternative policy may permit NOx reduction only when a specific engine temperature reading exceeds a predetermined threshold. The number of sections for which proposed levels of deployment of the NOx-reduction method must be found can thus be greatly reduced, thereby simplifying the search process.

The search process may be extended to allow the system to compare the net benefits, within a particular section of flight, of a range of NOx-reduction measures, and to select the method or combination of methods and their respective extents of deployment which maximises the net benefit within that individual section.

The search process may further be extended to assess the net benefit over an entire flight of a sequence of proposed patterns of deployment of multiple methods of NOx-reduction, and to select the pattern of deployment corresponding to the greatest net benefit achievable over the flight as a whole.

PROCESS IMPLEMENTATION OPTIONS

An individual aircraft might make use of an optimisation scheme such as that described above to calculate the best distribution of its available NOx-reduction resource across an entire flight. This would provide potentially the best outcome, balancing ozone-related benefits against methane-related disadvantages and implementation costs. Alternatively the NOx-reduction method could be activated only when the expected net benefit:penalty value exceeds a certain threshold, e.g. a positive magnitude value.

In another example, the NOx-reduction method could simply be activated to an extent determined by an altitude-specific model (or lookup-table or database) so as to approximate activation to a greater extent within regions of high ozone sensitivity, and to a lesser extent (or not at all) in regions of low ozone sensitivity.

In any example, the NOx reduction method could be inhibited from operation in a particular flight phase or mode of engine/aircraft operation for which safety considerations require the full range of engine control/thrust to be available to the pilot. Banking manoeuvres, climb out, emergency scenarios (including an engine failure or fault) may all be excluded from the scope of NOx reduction measures as necessary.

FLEET-WIDE RESOURCE OPTIMISATION

Since some flights will present a more cost-effective opportunity for NOx-reduction than others, the present invention may allow a system for deciding how best to allocate limited or costly NOx control resources between a collection of flights and/or aircraft. This could be achieved by a second layer of optimisation which proposes a distribution of resources between flights, then for each flight within the collection uses an inner optimisation loop (or policy) as described above to find the best distribution within each flight of its allocated amount of resource, and thus to assess the net benefit to that flight of its proposed allocation.

Alternatively resources could be allocated to flights pro-rata according to the distance they expect to travel (or the amount of fuel expected to be burned) in a high-ozone sensitivity region. Alternatively resources could be allocated to flights according to a predetermined lookup table in dependence upon the city-pair the flight is serving, for example taking account of the direction of travel as discussed above in relation to efficiency penalty determination.

For a given level of marginal deployment cost of a low-NOx method, this invention enables a greater climate benefit to be obtained due to targeting the deployment to those situations (and ambient conditions) in which the climate benefit will be greatest. Unlike the prior art, this invention can balance the disadvantageous reduction in methane destruction against the advantageous reduction in ozone formation so as to identify solutions that yield a net benefit without the need to circumnavigate ozone-sensitive areas. The present invention may also help to mitigate an emerging risk associated with rapid growth in demand for aviation in parts of the world where ozone sensitivity to NOx emissions is high.

Claims (23)

1. CLAIMS: 1. A system for controlling climate impact of an aircraft engine comprising: a location determination system for determining a plurality of locations of the aircraft engine during a current or planned flight; an ozone sensitivity determination system for determining the susceptibility of ambient air to ozone production at said plurality of locations; and a deployment control system for assessing a value of one or more current or future engine operation variable at said locations under a normal engine operation condition and outputting a revised value or threshold for said one or more engine operation variable in order to reduce NOx output by the engine in the event that the ozone sensitivity exceeds a predetermined value.
2. 2. The system of claim 1, wherein the one or more engine operation variable comprises an engine combustion variable or a variable affecting engine combustion temperature.
3. 3. The system of claim 1 or 2, wherein the one or more engine operation variable comprises one or more of combustion temperature, fuel flow, fuel composition, combustor burner selection, air pressure to the combustor, coolant flow to the combustor and a throttle setting for the engine.
4. 4. The system of any of claims 1 to 3, further comprising a NOx reduction system arranged to control operation of the aircraft engine in response to the 25 output of the deployment control system.
5. 5. The system of claim 4, wherein the NOx reduction system is arranged to reduce or limit engine combustion temperature.
6. 6. The system of any preceding claim, wherein the deployment control system is arranged to receive or predict an engine operation demand.
7. 7. The system of any preceding claim, wherein the deployment control system is arranged to correlate said value of one or more engine operation variable to a NOx emission and/or ozone production rating for the determined ozone sensitivity at said location by reference to a computational model.
8. 8. The system of any preceding claim, wherein the deployment control system is arranged to output a revised value or threshold for said one or more engine operation variable so as to prevent step climb manoeuvres during a flight or portion thereof.
9. 9. The system of any preceding claim, wherein the ozone sensitivity determination system comprises one or more ambient condition sensor onboard the aircraft.
10. 10. The system of any preceding claim, wherein the ozone sensitivity determination system generates readings for any or any combination of: ambient ozone concentration, ambient H02 concentration, ambient OH concentration, ambient H2O concentration, ambient NO concentration, ambient NO2 concentration and/or ambient sunlight level.
11. 11. The system of any preceding claim, wherein the ozone sensitivity determination system communicates ozone sensitivity data to a communication system on board the aircraft in advance of a flight or during a flight.
12. 12. The system of any preceding claim wherein the deployment control system comprises a comparator for assessing positive and negative climate impact 25 resulting from deployment of a NOx reduction system onboard the aircraft.
13. 13. The system of claim 12, wherein the comparator assesses any or any combination of: atmospheric ozone production due to engine emissions; atmospheric methane depletion due to engine emissions; engine efficiency alteration by a NOx reduction system on board the aircraft; and/or change in aircraft weight associated with the carriage of resources to reduce NOx output by the engine.
14. 14. The system of any preceding claim wherein the deployment control system predicts the impact of engine NOx emissions on ambient ozone levels and/or ambient methane levels for a plurality of sections of a flight, wherein a boundary between adjacent sections is defined by a material change in engine operation and/or ambient condition.
15. 15. The system of claim 14, wherein the impact of engine NOx emissions for each section is compared in order to output revised values or thresholds of said one or more engine operation variable for different sections so as to prioritise NOx reduction for one section over another section.
16. 16. The system of claim 14 or 15, wherein the impact of engine NOx emissions for each section is summed for a flight so as to allow comparison between a plurality of flights performed by one or more aircraft.
17. 17. The system of any preceding claim, wherein the deployment control system invokes a search algorithm to determine one or more revised value or threshold of the engine operation variable which yields an optimal net climate impact for the flight or a plurality of sections thereof.
18. 18. The system of any preceding claim, wherein the location determination system outputs the geographical location and altitude of the aircraft.
19. 19. The system of any preceding claim, wherein the ozone sensitivity determination system outputs the extent of an atmospheric region in which ozone sensitivity is greater than or equal to a threshold sensitivity level.
20. 20. An aircraft engine NOx reduction deployment controller comprising machine-readable instructions for the control of one or more processor to: receive a determination of a plurality of locations of the aircraft engine during a current or planned flight; receive or process ozone sensitivity data for ambient air in the vicinity of said plurality of locations; assess the ozone impact for a value of one or more current or future engine operation variable at said locations under a normal engine operation condition based upon said ozone sensitivity data; and output a revised value or threshold for said one or more engine operation 5 variable in order to reduce NOx output by the engine in the event that the ozone sensitivity and/or ozone impact exceeds a predetermined threshold value.
21. 21. A data carrier comprising machine readable instructions for the control of an aircraft engine NOx reduction deployment controller according to claim 20. 10
22. 22. A method of controlling climate impact of an aircraft engine comprising: determining a plurality of locations of the aircraft engine during a current or planned flight; determining the ozone sensitivity of the ambient air at said locations; determining a value of one or more current or future engine operation variable at said locations under a normal engine operation condition; and, outputting a revised value or threshold for said one or more engine operation variable in order to reduce NOx output by the engine in the event that the ozone sensitivity exceeds a predetermined value.
23. 23. A system or method of aircraft engine control substantially as hereinbefore described with reference to the accompanying drawings.