GB2544285A

GB2544285A - Ash detector

Info

Publication number: GB2544285A
Application number: GB1519828.6A
Authority: GB
Inventors: Lad Neetin
Original assignee: GREENBANK TEROTECH Ltd
Current assignee: GREENBANK TEROTECH Ltd
Priority date: 2015-11-10
Filing date: 2015-11-10
Publication date: 2017-05-17
Anticipated expiration: 2035-11-10
Also published as: GB2544285B; GB201519828D0

Abstract

An ash detection system comprises a separation unit 4 operatively connected to an optical flow pipe 8, and an optical detector which comprises a particle detector 6, trigger unit 12 and image capture unit. The system may be used on an aeroplane to detect the presence of volcanic ash in the air through which it is flying. The separation unit 4 may comprise a cyclonic separator such as a dual cyclone system. The particle detector 6 may comprise one or more photodiodes, and the image capture unit may comprise a camera 14 and an optical source such as a laser 11. The optical source and image capture unit may also be synchronised with the trigger unit 12. A method of detecting airborne ash comprises the steps of collecting a sample of air, detecting particles, and then capturing and analysing images of the particles. It may further comprise the step of calculating the velocity. The birefringence and surface morphology of the particles may also be measured.

Description

ASH DETECTOR

This invention relates to an ash detector, and particularly, but not exclusively, to an ash detector for detecting airborne volcanic ash. More particularly, the invention relates to an on board detector for detection of volcanic ash presence in the air in which an aeroplane is flying.

The aviation transportation industry is growing year by year on a global basis. As the number of aircraft increases, airspace becomes crowded Volcanic activity is a random but repeatable natural phenomenon The clouds with ash from volcanic eruption spread with time and therefore can cover large areas. These clouds can impact on the safety of aircraft that are forced to fly through or near the ash bearing clouds.

Volcanic ash can cause damage to various parts of an aircraft such as the fuselage, engines, auxiliary power unit (API)), electrical and avionics equipment, fire warning system, erosion of windshield etc.

According to I ATA (international Air Transport Association) which is a trade association covering major Airlines operators carrying 84% of the total available seat-kilometers air traffic, the volcanic ash influences quite often the aviation business.

There are more than 500 active volcanoes in the world Considering the last 10 years statistics, the global expectation on the average is 12 eruptions per year. The eruptions lasts in some instances up to several weeks and the clouds are persistent.

Volcanic ash is made up predominantly of silicates The melting temperature of the silicates in the volcanic ash is 1100°C. This temperature is below the operating temperature of modern commercial jet engines of about 1400 °C. Thus the melted silica adheres to the engine parts making hazardous situations.

Examples of previous volcanic eruptions that significantly impacted airline operations and resulted in airport closures:

• 1980- Mt. St. Helens, USA * 1982- Galungugung, Indonesia

• 1989 - Mt. Redoubt USA • 1991 - Mt, Pinatubo, Philippines • 1997- Mt. Popocatepetl Mexico • 2010- Mt. Eyjafjallajokull, Iceland • 2011 - Puyehue-Cordøn Gaulle, Chile • 2011 - Mt. Grimsvotn, Iceland • 2013-Mt. Etna, Sicily • 2013- Mt. Sinabung, Indonesia

In 2014 in August September and October increased volcanic activity of Bardarbunga (Iceland), Kilauea, Zhupanovsky (Kamchatka), Stromboiy, Uibinas, Sinabung (Indonesia) was noticed.

While there have been some serious incidents, there has been no aircraft accident, injury or loss of life as a result of a volcanic ash encounter However, some eruptions produced as consequence major negative financial Impact to aviation, related services such as business, tourism, freight transportation and passengers and ancillary domains.

As a result of the serious encounters that occurred during the 1980s the International Airways Volcanic Watch (IAVW) was established by the Internationa! Civil Aviation Organization (ICAO), the establishment of which was made possible by the cooperation of States and a number of international organizations. This volcanic ash watch system included nominated states providing Volcanic Ash Advisory Centers (VAAC). Their role is to monitor and advise of volcanic activity by promulgating volcano advisories and graphics to the meteorology offices, airlines, air navigation service providers and governments The role and responsibilities of the VAACs were introduced into iCAO Annex 3 by Amendment 71, which became applicable on 5 November 1998.

After intense volcanic activity in the 1980s, another notable event was Eyjafjaiiajokuii volcano eruption

Eyjafjaiiajokuii volcano eruption In Iceland In April 2010 led to the closure of most of European airspace for six days and some temporary closures in various parts later.

The financial impact to airlines was estimated at 1. 7 billion Euros in lost revenue.

About 10 million passengers and 100,000flights were affected during the six-day period.

The international aviation community took various measures for coping with volcanic eruptions and the ash clouds. 1. The European Aviation Crisis Coordination Ceil (EACCC) was created in 2010, including cooperation/interface with the airlines. 2. The Volcanic Contamination Exercise was held in April 2011 and provided timely feedback ahead of the second European eruption. 3. Generally, apart from the closure of northern German airspace and airspace in Iceland, operational decisions were delegated to the airlines.

The Association of European Airlines (AEA) made their own proposes and recommendations in a document titled "AEA proposal for Operations in airspace at risk of volcanic ash contamination". The purpose of this document is to outline AEA proposals for commercial operations in European and Transatlantic Airspace, taking account of the significant developments made to date in the enhancement of the relativity of London Volcanic Ash Advisory Center (VAAC) information through the provision of particulate density acceptable for engine ingestion by OEMs and the move to measurement at 10-i? units m3 to 10-is units m3

Although VAACs provide general information relating to volcanic ash presence in particular areas, the coverage is not universal. In addition, VAACS do not provide detailed information relating to ash in the air through which a particular aircraft is flying. in direct cooperation with ICAO and through involvement in the work of the Airworthiness sub-group of the Internationa! Volcanic Ash Task Force (IVATF), ICAO Document 9974 Flight Safety and Volcanic Ash was published in 2012. Major players in civil aviation signed the document: Airports Council International (ACI), Civil Air

Navigation Services Organisation (CANSO), internationa! Air Transport Association (!ATA), Internationa! Coordinating Council of Aerospace industries Associations (iCCAiA). internationa! Federation of Air Line Pilots' Associations (iFALPA) and international Federation of Air Traffic Controllers' Associations (iFATCA) This document provides guidance for states in recommending practices to their operators and reguiatory authorities where volcanic ash contamination may be a hazard for flight operations. This is the reguiatory document in force at the moment of drawing this report.

According to ICAO Document 9974 the national CAAs imposed rules for operation of flights in the presence of volcanic ash. Airlines use the information provided by the VAACs to plan their flights in or near ash contaminated areas. The information provided may be used to mitigate the risk of flying in areas potentialiy affected by volcanic ash in accordance with Safety Risk Analysis procedures approved by the competent nationai aviation authority.

Whilst regulations may change in the future, there is nevertheless a need to be able to ascertain quantitatively, the levels of volcanic ash in air through which aircraft are flying.

Current avoidance guidance recommended by ICAO refers to visibie or discernible ash. There is a need to define these terms in a manner that facilitates their use both at dispatch and in the en route phase of flight.

Known ash detection systems are not able to provide or store information regarding actual concentrations and natures of particulates in the air through which an aircraft is flying and thus how much is Ingested by the aircraft, as typically they focus on giving a long-range view of a very broad situation

There is therefore a need to be able to provided information relevant to individual aircraft. This information is important to enable maintenance crews to make decisions on maintenance routines, and operators to decide whether to fly or not in a volcano hazard situation.

According to a first aspect of the present invention there is provided an ash detection system comprising a separation unit operatively connected to an optical flow pipe, and an optica! detector, which optical detector comprises a particle detector, a trigger unit and an image capture unit.

The invention as ciaimed provides an Ash Ingestion Detection Apparatus (AIDA) which is able to enhance existing detection toois available to air and ground operators for countering the volcanic ash threat to flights

The invention may be used by ground and air operators, but is particularly useful to ground crews responsible for maintenance of aircraft.

An ash detection system according to embodiments of the invention is particularly adapted for the characterisation of volcanic ash present in air through which an aircraft is flying

The system is adapted to be installed in an aircraft and is adapted to run in situ, in real time and at high speeds forming accurate analysis on individual particles.

By means of the present invention, particles present in the air through which an aircraft is flying may be detected and then analysed, and data relating to the particles detected and analysed may be accessible to both air crew and ground maintenance crew.

The invention is therefore of particular importance in analysing relatively low levels of volcanic ash, If low levels of volcanic ash are present in the air through which aircraft are flying, the level may not be high enough to prevent aircraft from flying, but may still have an impact on the maintenance programme required for a particular aircraft. In other words, whilst low levels of volcanic ash may not prevent aircraft from flying through air containing the ash, it may be necessary to amend a standard maintenance programme in order to ensure that parts of the aircraft have not been adversely affected by the presence of such volcanic ash.

Because the system is adapted to be used in an aircraft during flight, particles entering the system are fast moving.

The separation unit is adapted to slow down particles and then to direct particles into the optical flow path. in embodiments of the invention, the separator comprises a cyclonic separator adapted to reduce the velocity of fast moving particles drawn in from the outside air. Typically the velocity of such particles wii! be reduced to approximately <1 00m per second at the point that such particles enter the optical detector

As is well known, a cyclonic separator serves to separate particles by cyclonic and inertial moment of particles from within an airflow

In a cyclonic separator, the centrifugal motion of air causes a pressure drop in the centre of the cyclone. The difference in pressure causes a reverse vortex to be established as the air is pulled in by the pressure gradient. Clean air travels in a tight cyclone upwards whilst dust and dirt present in the incoming air is pushed to the outer edge. This means that particles travel downwards within the cyclone.

In embodiments of the invention, the separation unit comprises a dual cyclone system. A dual cyclone system has been found to be more efficient than a single cyclone. This is because, in a dual cyclone system a first cyclone places a particle into a known position at an inlet to a second cyclone. A second cyclone is then able to emit the particle centrally into the optical flow path. This arrangement eliminates choked flow. A choked flow is undesirable as it may cause the particle position to be randomised due to the aerodynamic properties of the particle which is undesired.

In embodiments of the invention the particle detector comprises a photodiode. In some embodiments of the invention the particle detector comprises a photodetector associated with the photodiode, in embodiments of the invention, the particle detector comprises a plurality of photodiodes and a plurality of photodetectors. Each photodiode being associated with a photodetector. The photodiodes may be placed at an exit of the separation unit at which point particles that have passed through the separation unit exit to enter the optical flow pipe, in some embodiments of the invention the particle detector comprises three photodiodes and three photodetectors. The photodiodes may be equally spaced apart from one another around the optical flow path.

The photodiodes detect particles by collecting light scattered from passing particles which light is then detected by a photodetector. This light may then be analysed in order to detect the presence of particles

In embodiments of the invention in which the particle detector comprises three photodiodes and three detectors, a particle is initially detected by a first diode and associated first detector. in embodiments of the invention, the first diode and first detector wiil detect an increase in intensity of the Sight caused by scattering of a passing particie.

Specifically, as a particle passes through the first photodiode, the first detector will detect a disturbance which causes an increase in intensity of incident light. This causes a peak to be generated in the intensity, which peak is then detected. A second photodiode and associated second detector are adapted to also measure a change in intensity caused by passage of a particle.

The system is adapted to register that a particle has been detected if the responses from the first and second diodes match. A third diode and associated third detector are also adapted to detect a change of intensity caused by the presence of a particle.

The signal detected by the third detector may be used to calculate the velocity of the particle, by calculating the time delay between the signal detected by either the first or second photodiode/detector pair, and the signal detected by the third photodiode/detector pair. Since the distance between either the first or second photodiode, and the third photodiode is known, by calculating the time delay, the velocity of the particle may be computed.

In some embodiments of the invention, each photodiode comprises a pin diode

The ash detection unit comprises a trigger unit for triggering the Image capture unit.

The trigger unit is operatively connected to the particle detector. Once a particle has been detected, the trigger unit triggers the image capture unit.

In embodiments of the invention the image capture unit enables analysis of images of particles by two independent means.

The particles may be analysed by birefringence and also surface features of the particles.

This dual approach enables discrimination between particles of ash and other particulates. This is because other aerosol particles may display birefringence. An example of such a particle displaying birefringence is a pollen particle.

Other known ash particles may have a non-uniform shape such as sand, or surface roughness However, typicaiiy the majority of volcanic ash particles will display all three characteristics.

In addition, there is a particle surface structure used for identification of ash which is caused by the breakage of surface gas bubbles in volcanic ash particles that is only apparent when particles are formed in a high pressure gas producing environment such as a volcanic eruption.

The image capture unit may comprise an optical source. In embodiments of the invention, the optical source comprises a laser. The laser may be a pulsed laser.

The laser acts as a light source and also as an optical shutter to enable dynamic image analysis to be carried out. in embodiments of the invention in which the optical source comprises a laser, the trigger unit acts to trigger generation of a laser pulse required to image particles travelling through the optical flow pipe,

The image capture unit must be capable of imaging particles travelling at high speed, and therefore a shuttering system is required. The ideal maximum velocity movement parameter is 1 %, and in order to accurately capture surface feature data down to 1 pm or less in size, the image must be captured in 36ps, and to specify the bounds of 1 Opm particle the image must be taken within 360ps with particles travelling at maximum speeds of 280 mis. The capture time is increased due to the separator slowing the airflow.

Since this requirement is far beyond the technical capabilities of a mechanical shutter, a pulsed laser is required, which must present a frequency both triggerable and rate controllable Sub-nanosecond lasers are available and with a pulse length of 500ps, a speed of 20m/s is required to achieve a 1 % movement parameter The lower limit to the camera frame length defines the maximum pulse frequency as O.SMHz The image capture unit may encounter 25 particles per second; which requires a minimum frequency of 25Hz. This in turn means that a fine focus is required to obtain detailed images and this requires extremely rapid pulses of about I ns for example, in addition to accurate particle control.

The image capture unit may also comprise a polariser for polarising light emitted by the optica! source. in some embodiments of the invention the polariser comprises two polarisers set at about 90° to one another. The polarisers enable birefingent analysis to be carried out.

The image capture unit may comprise a camera operatively connected to the trigger unit. In some embodiments of the invention the image capture unit comprises a plurality of cameras In embodiments of the invention, the image capture unit comprises two cameras, which cameras may be CCD cameras in such embodiments each camera is operatively connected to the trigger unit.

Due to the relatively large field of view (400 pm diagonal) compared to particle size (ΙΟΙ 00 pm), a full CCD camera and objective iens system provides better particle definition than a binary light sensor This will mean individual particle contribution can be analysed compared to an average array value potentially formed of multiple contributions. For the image this means the small sensor display will be replaced with the larger CCD component.

The cameras and laser may be synchronised with the trigger unit which will provide a calibrated timing pulse to fire the laser and open the camera shutters. Due to the high velocity of the particies, the laser pulse provides a very namow optical shutter inside each camera in order to minimise velocity blur.

The image capture unit is arranged such that incident laser light may be detected in both back Sit and forward IHuminated orientations. This is required as a birefingent observation is a Sight transmission property whilst surface morphology investigation is based on reflected light. In embodiments of the invention a first camera measures hirefingence and a second camera records surface morphology

In embodiments of the invention the image capture unit comprises a magnifier for magnifying light from the light source. The magnifier may comprise one or more magnifying lenses which enable the viewing of the particle image at a required pixel resolution

The ash detector system may further comprise an image analyser.

The image anaiyser may take the form of an algorithm adapted to anaiyse data relating to both birefringence and surface morphology of particles detected in the optical flow pipe.

The image anaiyser then enables the ash detection system to indicate a level of ash particles detected by the system.

In some embodiments of the invention the ash detection system further comprises a display unit. The display unit is adapted to display the results of the analysis carried out on the particles detected.

The display unit may take any convenient form and may for example be a genera! interface unit (GIU). In some embodiments the display unit will be a laptop or tablet for example,

According to a second aspect of the present invention there is provided a method of detecting airborne ash in air through which an aircraft is flying comprising the steps of i. collecting a sample of air: ii. detecting particles in the air iii. capturing images of the particles iv analysing the images.

In embodiments of the invention the step of collecting a sample of air includes the step of reducing the velocity of particles contained in the sample of air.

In embodiments of the invention the step of detecting the particles comprises the step of measuring intensity of light scattered by a particle.

In embodiments of the invention the steps of detecting particles comprises detecting a first peak in intensity; detecting a second peak in intensity determining whether the first and second peaks match one another, to thereby verify detection of a particle.

The method may comprise a further step of detecting a third peak in intensity; and detecting a time delay between either of the first and second peaks of intensity and the third peak of intensity to thereby calculate the velocity of a detected particle in embodiments of the invention the step of analysing the images comprises the steps of analysing birefringence and surface morphology of the particles.

In embodiments of the invention the method comprises the further step of displaying data obtained after analysing the images.

The invention will now be further described by way of example only with reference to the accompanying drawings in which:

Figure i is a schematic representation of an ash detector according to an embodiment of the invention;

Figure 2 is a block diagram showing how particles in an air supply are analysed by the ash detector shown in Figure 1;

Figure 3 is a block diagram showing how a particle passes through the ash detector illustrated in Figure 1;

Figure 4a is an exploded schematic representation of the cyclonic separator forming the ash detector of Figure 1;

Figure 4b is a schematic representation of the cyclonic separator of Figure 2a in an assembled format;

Figure 5 is a schematic representation showing how particles are detected by the particle detector;

Figure 6 is a schematic representation showing how partieies are analysed using the ash detector of Figure 1;

Figure 7 is a schematic representation illustrating a particle that is detected as having ash features by the ash detector of Figure 1; and

Figure 8 is a flow diagram of algorithm used to identify ash particles using the ash detector of Figure 1.

The invention will now be described by way of example only with reference to the accompanying drawings.

Referring first to the figures, an ash detector according to an embodiment of the invention is designated generally by the reference numeral 2.

The ash detector 2 is adapted to be installed in an aircraft, and may be retro fitted into existing aircraft, or installed during manufacture of a new aircraft.

The ash detector 2 according to embodiments of the invention is adapted to deliver an advanced airbourne volcanic ash detection apparatus which involves a special machine vision integration of volcanic ash (known as Tephra) and Image analysis software.

The ash detector 2 is adapted to examine air by interrogating foreign bodies in the fresh air entering an aircraft cabin and then by identifying the type, size and number of particles per unit time to thus infer the presence of low level ash particles in the possible presence of other solids in the air.

After detection and cumulative counts, an early warning can be given to ground crew and airline and maintenance teams so that investigations and actions can be carried out before failure of components in the aircraft.

The ash detector 2 comprises a separation unit in the form of a cyclonic separator 4. The cyclonic separator is adapted to focus and slow down particles captured from outside the aircraft.

The ash detector 2 also comprises a particle detector 6, an optical flow pipe 8, an image capture unit 10 comprising an optical light source 11, trigger unit 12, first camera 14, second camera 16, mirror 18, periscope mirror 20 and poiarisers 22. in this embodiment, the optica! source 12 comprises a pulsed laser.

The parts of the ash detector 2 identified above will now be described in more detail.

In this embodiment the particle detector 6 comprises three laser diodes 6a, 6b and 6c and associated detectors (not shown) which are adapted to detect the presence of particles, as will be described in more detail hereinbeiow.

Once particles have been detected, the particles will flow through an optical flow pipe 8 where an image is captured by the image capture unit 10.

Specifically in this embodiment, the cyclonic separator comprises a dual cyclone system 4 illustrated in Figures 4a and 4b. The dual cyclone system is adapted to direct particles into the flow pipe 8 and to remove fast moving particle laden air from the system. The cyclone technology reduces the fast moving air velocity from outside the aircraft to a more manageable 1 OOm per second at the point of imaging.

The cyclonic separator 4 comprises an inlet 40 which is limited to approximately 6mm in diameter in this embodiment in order to avoid a choked flow of air through the cyclonic system. The inlet 40 is sealingly connected to a conicai section 42, the connecting portion including a sea! 44 The first conicai section 42 is in turn connected to a second cyclone inlet 46 which is sealingiy connected to a second conicai section 48. The second conicai section 48 is in turn connected to an adaptor 50 which then connects to the optical flow pipe 8. Further seals 52, 54 ensure that parts are sealingly connected to one another.

In a cyclonic separator, the centrifugal motion of air causes a pressure drop in the centre of the cyclone. The difference in pressure causes a reverse vortex to be established as air is puiied in by the pressure gradient. Clean air travels in a tight cyclone upwards whilst dust and dirt present in the incoming air is pushed to the outer edges and hence travels within the cyclone downwardly.

Although a single separator could be used, it has been found that the dual cyclonic separator provides better results and is more efficient.

Initial detection of particles contained in incoming air is made by the particle detector 6 In this embodiment the particle detector comprises three laser diodes 6a, 6b, 6c and three associated detectors, which are placed at the junction between the cyclonic separator 2 and the optical flow pipe 8

The three laser diodes are equally spaced around the optical flow path and are focussed on the particle path within the optical flow path. Scattered light from passing particles is collected by the three diodes and analysed for the presence of a particle. A particle is detected if the response from the first two diodes matches, and the third diode is used to verify the particle's presence and to calculate the velocity of the particle

This is shown schematically in Figure 5. As shown in Figure 5, the first laser diode 6a detects a particle passing through the optical flow path 8 by detecting a peak in intensity 500.

The second laser diode and detector pair 6b then also detects a peak in intensity 520

The detection of the second peak 520 is a verification signal that verifies that a particie has been detected.

The third laser diode and associated detector 6c then also detects a peak in the intensity caused by scattering of light by the particle passing through the optical flow path. This signal is used to calculate the velocity of the particle passing through by calculating the time between the second peak 520 and the third peak 540.

Since the distance between the two photodiodes 6b, 6c is known, by recording the time between the two peaks, the velocity at which the particle is travelling may be calculated.

Once the presence of a particle has been verified, the trigger box is activated to trigger the optical light source On detection of a particle, the trigger box triggers the pulsed laser light source 12. The laser 12 can be triggered either as a single shot on demand, or may be set to pulse at a constant rate.

The image capture unit 10 comprises two cameras 14, 18 which in this embodiment each comprise a CCD camera. The image capture unit also comprises mirrors 18, 20 and polarisers 22

The cameras and laser are synchronised with the timing trigger which wil! provide a calibrated timing pulse to fire the iaser and which can open the camera shutters. Due to the high particle velocity, the iaser pulse provides a very narrow optica! shutter inside the camera exposure time in order to minimise velocity blur in order to take an image of a particle it must be in an appropriate place, because the cameras are focussed at a particular pointwithin the optical flow pipe 8. This means that in order gather accurate information on all potential ash particles the following three parameters need to be controlled: location of the particle along the flow pipe axis; iocation of the particle across the pipe width and height; and speed of particle

Detection of the presence of birefringence is an important piece of data to accurately capture The white light image and the birefringence image are analysed in tandem by the detection logic to decide whether it is ash or not.

The detection of the presence of birefringence is required in order to detect the presence of a particle At this stage therefore no particle detail is required from the measurement and this means that the requirements with respect to field of view and depth of field are less stringent than for surface feature extraction as will be explained in more detail hereinbelow.

Measurements relating to surface features of the particles take place on image capture of the particle along the optical flow pipe 8,

In order to identify surface features of the particle and therefore determine whether any particular particle is volcanic ash, it is necessary to measure levels of reflected light in order to build up a 30 representation of the particle surface.

Information gathered from images of the particles will be transferred to the data anaiyser.

The captured images may then be transferred to a data base, and a characterisation algorithm reads and analyses the results. The results may be displayed in any convenient manner, such as on a general user interface (GUI) in order to provide information to aircraft maintenance engineers.

As shown in Figure 2. particles are detected by initially introducing partieies at step 200 The particles are contained in an air supply 210 which enters the system The air supply containing partieies enters the particle separation unit 4 which in this case is in the form of dual cyclonic separator.

Air is then removed from the system via the exhaust 220

The particles then pass through the optica! flow pipe 8 where peaks in intensity are identified by photodiodes/detectors 8a: 6b and 6c Detection of a particle then causes the trigger unit 12 to trigger the image capture unit as described hereinabove

After the image of the particle has been captured, the particles exit at exit point 230

The data captured by the image capture unit 10 is then analysed using an algorithm 240 shown in more detail in Figure 8.

Once the particle has been classified, data may be displayed to a user on a genera! user interface (GUI) 250

Turning to Figure 3 other components in the system are shown.

The ash detection unit 2 is powered by a power supply 300 A signal generator 310 generates a signal. The unit 2 comprises a main printed circuit board (RGB) 320 containing a power supply unit 330, a microcontroller 340 and trigger unit 12,

The main RGB 320 is connected to a trigger box motherboard 350 which in turn is operatively connected to the photodiode/detectors 6a, 6b and 6c.

Figure 3 also shows the optical source in the form of a laser 12 and cameras 14. 16 These components are controlled by both the microcontroller 340 and the trigger unit 12

After data has been analysed using the aigorithm shown In Figure 8, the resuits may be displayed via any display unit, and in Figure 3 the display unit used is a laptop 360.

Turning now to Figure 6, a method of classification of detected partieies is shown in more detail.

An initial ciassification of the particles takes piace in order to determine whether a particular particle is liquid or ice (600), whether If has a spherical shape (620), whether its size is out of the range of the unit 2 (630), whether it is not possible to categorise the particle (840) and whether any vesicles have been identified (650). if the particle is analysed as 600 (liquid or ice) then no further analysis is required to determine it is not ash. If the particle has been categorised as 810 or 620, then birefringence is then measured if birefringence is displayed the particle is not ash. if no birefringence is present, then the surface morphology of the particles must be taken into account. if the surface morphology Is smooth, then the particle is not ash. If however surface vesicles are identified, then the particle is ash. if a particle is identified as 830 (non-categorised) then birefringence is taken into account. Whether or not bifringence is present the surface morphology must again be taken into account. Again, a smooth surface indicates that the particle is not ash, whereas if surface vesicles are identified, the particle is likely to be ash.

Finally, if vesicles are identified (640), no further measurement is required in order to determine that the particle is ash.

Turning now to Figure 7, a typicai display is shown. This shows that for example particle 700 has ash features.

By means of embodiments of the invention therefore an ash detector and a method of identifying the presence of ash particles in air is provided in which either ground crew or air crew may be presented with a visual indication of the level of ash particles in the atmosphere through which an aircraft is flying.

Claims

1. An ash detection system comprising a separation unit operatively connected to an optical flow pipe, and an optica! detector, which optica! detector comprises a particle detector, a trigger unit and an image capture unit.

2. An ash detection system according to claim 1 wherein the separation unit comprises a cyclonic separator, optionally a dual cycione system

3. An ash detection system according to any one of the preceding claims wherein the particle detector comprises a photodiode.

4. An ash detection system according to any one of the preceding ciaims wherein the particle detector comprises a plurality of photodiodes, each of which photodetectors is associated with a photodetector.

5 An ash detection system according to any one of the preceding ciaims wherein the particle detector comprises three laser diodes, and three associated photodetectors.

6. An ash detection system according to any one of the preceding claims wherein the image capture unit comprises an optical source, optionally a laser.

7. An ash detection system according to any one of the preceding claims wherein the image capture unit comprises a polariser for polarising light emitted by the optica! source.

8. An ash detection system according to any one of the preceding claims wherein the Image capture unit comprises a camera operatively connected to the trigger unit.

9. An ash detection system according to claim any one of the preceding claims wherein the image capture unit comprises a plurality of cameras.

10 An ash detection system according to any one of the preceding claims wherein the optica! source and the image capture unit are synchronised with the trigger unit.

11. An ash detection system according to any one of the preceding claims comprising a magnifier for magnifying light from the light source.

12 An ash detection system according to any one of the preceding claims further comprising an image analyser for analysing detected particles

13. An ash detection system according to any one of the preceding claims comprising a display unit for displaying particle data.

14. A method of detecting airborne ash in air through which an aircraft is flying comprising the steps of: I. collecting a sample of air; ii. detecting particles in the air iii. capturing images of the particles iv. analysing the images.

15. A method according to claim 14 wherein the step of collecting a sample of air includes the step of reducing the velocity of particles contained in the sample.

16. A method according to claim 14 or claim 15 wherein the step of detecting the particles comprises the step of measuring intensity of light scattered by a particle.

17 A method according to any one of claims 14 to 16 wherein the step of detecting particles comprises detecting a first peak in intensity; detecting a second peak in intensity; determining whether the first and second peaks match one another, to thereby verify detection of a particle

18. A method according to claim 17 comprising the further step of detecting a third peak in intensity; detecting a time delay between either of the first and second peaks of intensity and the third peak of intensity to thereby calculate the velocity of a detected particle.

19. A method according to any one of claims 14 to 18 comprising the further step of analysing data captured during the image capture stage of the method

20 A method according to claim 19 wherein the step of analysing data comprises the step of measuring birefringence and surface morphology of the particles.

21. A method according to any one of claims 14 to 20 comprising the further step of displaying data obtained after analysing the images.

22. An ash detection system substantially as hereinbefore described with reference with the accompanying drawings,

23. A method substantially as hereinbefore described with reference to the accompanying drawings.