airbus Fast 21

AIRBUS INDUSTRIE AIRBUS TECHNICAL DIGEST NUMBER 21 MAY1997 FAST / NUMBER 212 A330/A340 CARGO BAY CONDENSATION AND SMOKE WARNINGS Solutions available In the last issue of the FAST magazine the carriage of perishables and livestock was discussed. In this article a more specific challenge to the cargo smoke detection system, caused by excessive humidity, is examined. Pneumatic, Fire and Ice Protection Engineers, Engineering and Technical Support, Airbus Industrie, Customer Services Directorate Claire Nurcombe By and Mike Carver TheAirbus Air Cargo market forecast indi-cates that transportation of cargo is the fastest growing area of aviation, with the world’s freighter fleet growing at an annual average of 6.7% until 2015. A large amount of the cargo carried will be moisture and heat carrying, e.g., animals, fruit and veg- etables. This moisture and heat has the potential to be released over the period of time that the cargo remains in the hold. Operations in hot and humid en- vironmental conditions can also lead to occurrences of the same phenomenon. With the opening of the cargo doors there is an influx of hot and humid air. This affects the environmental condi- tions within the hold in the same way as the presence of heat and moisture producing cargo. False smoke alarms may occur in both circumstances due to interference of condensation with the smoke detection system. The condensation formation may be affected by the ventilation and heating options for the cargo hold taken by the operator. There are several options for ventilating and heating the cargo bays. In the forward cargo bay there is a ba- sic option for ventilation, and tempera- ture control and/or ground ventilation can also be installed. In the aft com- partment ventilation is a basic option and in the bulk cargo bay ventilation is fitted on all aircraft. In the bulk cargo bay heating and/or ground ventilation can also be installed. The ventilation systems for the for- ward, aft and bulk cargo compartments all have the same architecture. Two fans are fitted, one to draw air into the compartment and one to draw out air. The expelled air is ducted towards the outflow valve, which ensures that most of the air is not recirculated. Since this is only operative in flight there is an extra option to enable ventilation on the ground. The option for heating the bulk cargo bay consists of an heating element heating the incoming air. There is no true regulation of the system; it is only possible to heat the bulk cargo com- partment, and there is no facility for cooling the compartment. This system differs from the forward cargo bay sys- tem, which allows true temperature control, with heating and cooling of the compartment. Both heating and ventilation should ensure that in-flight spurious smoke warnings due to condensation are pre- vented (since the detectors will be warmed by the heated circulating air and the ventilation will help reduce the amount of water vapour in the air). However, in cases of the carriage of extreme humidity producing cargo, in- flight spurious warnings due to con- densation may still occur. Also, with the cargo hold at a nominal tempera- ture of 20°C, condensation formation is still possible if the cargo doors are opened in very hot and humid condi- tions, where 20°C may be below the dewpoint temperature of the outside air. Condensation forms because the de- tectors are cooler than the air entering the cargo hold, either because of venti- lation in the hold, or because of the cold soak during a long flight. When the hot and humid air enters the cargo bay a disparity occurs between the rel- ative humidity within the hold and the temperature of the detectors. This may lead to the situation where the dew- point temperature of the humid air is above the temperature of the detectors. In these conditions condensation can form on the grid in the measuring chamber of the smoke detector. The condensation causes a change in the current in the measuring chamber, which is the criteria for giving a smoke alarm. These false alarms occur on long range aircraft of all types, this for- mation of condensation being exacer- bated by the length of time a long haul aircraft may be airborne. Over the duration of the flight, if no cargo ventilation is present, the humid- ity level in the cargo bays will increase while the temperature of the smoke de- tectors drops. This provides the perfect conditions for condensation to form. A solution has been developed by Airbus Industrie to prevent spurious alarms due to condensation occurring on the A330 and A340 aircraft. FAST / NUMBER 21 3 CARGO COMPARTMENT A330-300 A340-200 MODIFICATION OPTIONS /A340-300 Forward Ventilation (basic option) Mod 40096 Mod 40186 compartment Temperature control Mod 40097 Mod 40188 Ground ventilation Mod 40220 Mod 40220 Aft Ventilation (basic option) Mod 40098 Mod 40190 compartment Bulk Ventilation cargo Compartment heating Mod 40099 compartment Ground ventilation Mod 40221 Cargo compartment smoke detector hood Forward cargo compt. Aft cargo compt. Smoke Smoke Avionics Smoke FAST / NUMBER 21 SYSTEM OPERATION The lower deck cargo compartment (LDCC) smoke detectors on the A330s and A340s are installed in pairs. Each pair of detectors is supplied with power by a dual redundant power sup- ply (see Figure 1). One detector in the pair is installed on the Smoke Detection Control Unit (SDCU) loop A, the other on loop B. To trigger an alarm a signal from each detector in the pair is needed. However, if one loop is not functioning, a signal from only one detector is able to trigger an alarm. The SDCU tests each loop to check whether it is functioning before it acts on a smoke alarm from a single smoke detector. When a smoke alarm is generated by the SDCU the ventila- tion and heating systems (if installed) will be closed automatically. The detectors used on Airbus aircraft are of the ionisation type that detect both visible and invisible fire aerosols (particle diameter between 0.01m to 10 m m). The ionisation detector utilises the phenomenon that air ions are at- tracted by smoke particles. The elec- trodes set up an electric field and the air between the electrodes is ionised (made electrically conductive) by a weak radioactive source (refer to Figures 2 and 3 for schematic diagrams of the smoke detector operation). These ions move under the influence of the electric field, setting up an ionic current. Smoke particles are too large (up to 1000 times larger than the ions) to be ionised and also attract the ions present between the electrodes. These resulting heavy ions are virtually im- mobile, reducing the ionic current, which as a consequence increases the electrical resistance of the measuring chamber. An imbalance is now present between the measuring chamber and a reference chamber. This imbalance in voltage is amplified and compared to four different threshold levels: l The smoke threshold. The voltage at which the detector recognises that smoke is present in the measuring chamber and gives an alarm signal. l The prefault high threshold. The voltage at which the detector senses a rise above the normal operational volt- age range. l The prefault low threshold. The volt- age at which the detector senses a fall below the normal operational voltage range. l The fault threshold. The voltage at which the detector gives a fault signal. The reference chamber in the detec- tor is present to allow for differential pressure and temperature changes en- suring that the detectors operate with the same sensitivity in flight and on the ground. Battery BUS 28VDC Channel 1 Loop A Channel 1 Loop B Channel 2 Loop B Smoke test LDCC smoke lamps Avionics compartment smoke lamp Avionics compt. smoke detector Lavatory smoke detectors Lavatory smoke detectors Crew rest smoke detection control unit Stairwell smoke detector Avionics compt. smoke detector LDCC smoke detectors 1WH 3WH 5WH 7WH 9WH LDCC smoke detectors 2WH 4WH 6WH 7WH 10WH Power channel 1 Power channel 2 Normal BUS 28VDC SDCU Smoke Detection Control Unit 4 Figure 1 Smoke detection loop schematic for A340 INVESTIGATION The investigations into the spurious smoke alarms due to condensation were mainly concentrated with two op- erators, one operating in the Middle East and one in the Far East. Questionnaires were also sent to other A330/A340 operators susceptible to spurious warnings to discover how widespread the spurious alarms were. Some common factors high-lighted in the replies to the questionnaire allowed Airbus Industrie to suggest some short term solutions to help reduce delays and inconvenience. An effective short term solution was drying the smoke detectors with a hot air source, but this was a maintenance burden and not practical for the operators in the long term. It was also suggested that the cabin should be heated to the maxi- mum temperature (28°C) if no passen- gers were present on the flight, to have the cargo ventilation, if installed, on at all times and to heat the bulk cargo hold, if possible. In January 1995 testing took place on an A340 to define the environment and to determine the effect of localised heaters on the smoke detectors. One of each pair of detectors was instru- mented to measure temperature, hu- midity, sensitivity and smoke indica- tion. The cabin temperature, aircraft skin temperature and the ambient con- ditions on the ground were also recorded for each flight. In total five flights were made, the first between Hong Kong and Osaka and the other four between Singapore and Hong Kong. The last two flights made were with heaters fitted in smoke detectors 1WH and 7WH (the two de- tectors seen as being most susceptible to the formation of condensation, see Figure 4 on the following page). This susceptibility to condensation when the cargo doors are opened was shown by information previously taken during the investigation. This susceptibility is probably due to proximity to the door. The conditions on the ground (tem- perature approximately 25°C, relative humidity 50-100% throughout the test period) did not lead to any false alarms, but enough data was collected from flights 2 and 3 to be able to con- clude that there was a direct, although small, influence of hot and humid con- ditions on the smoke detector sensitiv- ity signal. On flight 2 the sensitivity dropped. The signal moved from -4.6V to -4.9V on 1WH (the detector was not heated on this flight, -4.5V being the normal signal and -6.0V a smoke alarm), while on flight 3 the sensitivity dropped, the signal changing from -4.9V to -5.2V on 3WH (an unheated detector). The lowest sensitivity signal was shown after the cargo doors had been shut. Installing a heater to the smoke detectors did not have any detrimental effect on the smoke detector sensitivity signal. FAST / NUMBER 21 Figure 3 Cargo smoke detector - Description of operation during smoke conditions Figure 2 Simple schematic of cargo smoke detection operation Electrode Ionisation sources IonsFire aerosols During smoke conditions the ion flow in the measuring chamber is impeded with relation to the reference chamber. This creates an imbalance between the two chambers and a smoke alarm is generated. Reference chamber Measuring chamber Ionisation source Reference chamber shell Fire aerosols Ionisation source 5 Airbus currently uses the ionisation type of smoke detectors but is also undertaking a review into the latest technology optical smoke detectors. The Scattered Light Detector is the optical smoke detector which is most suited for the use in cargo holds. The photodiodes used in these detectors are semiconductor devices for detecting and measuring radiant energy (as light) by means of its conversion into an electric current. The photodiodes and LEDs are arranged so that light from the LEDs does not fall on the photodiodes under normal conditions. The optical properties of some types of fire aerosol lead to a scattering of the emitted light, some of which will fall on to the photodiodes. This increase in the amount of light detected by the photodiodes causes a change in the electric current output by the photodiode. FAST / NUMBER 216 EVALUATION Following the results of the flight test- ing, it was decided to proceed with a heated smoke detector design, rather than a change to the grille design or adding a curtain to the cargo bay doors. Heating the smoke detector raises both the temperature of the detector it- self and the air inside the detector. Both of these help to reduce the rela- tive humidity within the measuring chamber. Heating the detector also raises the detector temperature higher than the dewpoint temperature of the ambient ground conditions (or the dewpoint of the cargo). These factors reduce the likelihood of condensation forming. It was decided that the optimum way of heating the detector would be to heat the cell cover inside the protective cover, which would ensure a minimum temperature differential between the reference and the measurement cham- bers. It was decided to regulate the temperature of the smoke detector to 15 degrees over the ambient conditions (to a maximum of 40°C) to optimise the detection ability. Each pair of de- tectors has a dual redundant heater power loop and as before, the SDCU would check and verify smoke alarms from just one detector. An Electromagnetic Inductance filter was also required for the smoke detec- tor. Fluctuations in the 28V electrical bus can occur during switches between Forward cargo compartment 3WH 4WH FWD 1WH 2WH Aft cargo compartment Bulk cargo compartment FWD 5WH 6WH 7WH 8WH 9WH 10WH AIRBUS INDUSTRIE IS CURRENTLY EXAMINING NEW ADVANCES IN OPTICAL SMOKE DETECTOR TECHNOLOGY Figure 4 Position of smoke detectors within the cargo bays power sources (ground power, APU and engines). These fluctuations could cause the heater coil to act as a sole- noid, producing a magnetic effect that could either cause a loss of smoke indi- cation capability or false smoke alarms. The electronic filter prevents such adverse side effects. The evaluation units were tested for six months in operational conditions. At the end of the evaluation period it was judged that the heater coil was successful in preventing spurious smoke alarms. During the six months no spurious warnings had occurred, against what could normally be ex- pected (between three or four spurious smoke warnings per month to three or four per week, depending on the opera- tor and the environmental conditions). FAST / NUMBER 21 7 • Mod 43967 - Wiring • Mod 44177 - Heated smoke detectors Available through the A330/A340 LRIP (Long Range Improvement Programme) • SB 26-3009 (A330) and SB 26-4011 (A340) - Wiring for heater and EMI filter box Issue date: Rev. 2, 30.09.9 • SB 26-3014 (A330) and SB 26-4015 (A340) - Fitting of heated smoke detector Issue date: 04.06.96 Two Service Information Letters have also been issued concerning false smoke alarms. These give advice about the environmental and operational conditions that could give rise to false warnings. • SIL 26-003 (A300) • SIL 26-022 (all aircraft types) Heated smoke detector P/N 4370-264 THE SERVICE BULLETINS AND MODIFICATIONS THAT ARE AVAILABLE ARE SHOWN BELOW: CONCLUSION Retrofitting the modifications on in-service aircraft started at the beginning of 1996. The cargo smoke detectors are an essential component of the fire protection system, but are susceptible to false alarms if the conditions in the hold are hot and humid. Long range aircraft of all types suffer from this phenomenon, but Airbus has solved the occurrence of false alarms by introducing heated smoke detectors. There were two main requirements for a new detector: l The relative humidity within the smoke detector measuring chamber had to be reduced without compromising the detec- tors’ effectiveness. l The dewpoint temperature of the detector had to be raised above the dewpoint temperature within the cargo bay. Both of these requirements could be solved by heating the smoke detector to a nominal temperature above ambient condi- tions. The new detector included a heater coil that was capable of causing electromagnetic interference. A filter was therefore added to the design to protect the detector from the effects of electromagnetic induction. Six months of testing took place to ensure that the heated smoke detectors would enter service without the need for further modification. The main uptake of the modification by operators has been in the Far and Middle East, since many European operators have not experienced problems with the cargo fire detection system. This is due to the less extreme environmental conditions encountered in Europe and as the man-hours required for the wiring modification are fairly substantial it is not seen as eco- nomical to perform this modification. Airbus has successfully solved the occurrence of spurious alarms due to condensation on its long range aircraft. There have been no reported smoke alarms due to condensation from operators who have the heated smoke detectors fitted to their A330 and A340 aircraft. n FAST / NUMBER 218 by Frédérique Rigal A330/A340 Maintenance Systems Engineer Engineering and Technical Support Airbus Industrie Customer Services Directorate The concept of on-board centralised maintenance was developed with the A320. The aim was to provide maintenance teams with diagnosis of faults in plain English, through a single location in the cockpit, with homogeneous access to the maintenance information related to the various electronic systems. As a highly interactive tool, the Centralised Fault Display System (CFDS) has evolved with in-service experience, which has also benefited the A330/A340 Central Maintenance System (CMS) (described in FAST 16, April 1994) in terms of homogeneity of interfaces and definition of layout, reports and messages. CENTRALCENTRAL MAINTENANCEMAINTENANCE SYSTEMSYSTEM OPTIONOPTION PPAACKCKAAGEGE A330/A340 Simplifying maintenance 9 TheCMS in the A330/A340family is based on thesame core principles and basic functions as in the A320 family : l fault monitoring and diagnosis is undertaken by the Built In Test Equipment (BITE) of each system; l a dedicated computer, Centralised Fault Display Interface Unit (CFDIU) on A320 and Central Maintenance Computer (CMC) on A330/A340, con- centrates the messages sent by the BITEs, edits maintenance reports and provides an interface to the operator with the maintenance part of the con- nected systems; l a Post Flight Report is generated after each flight; it lists the ECAM warnings and maintenance status triggered during the last flight, as well as the corre- sponding fault messages produced by the BITEs; l test capabilities and access to addi- tional systems maintenance information are provided through the System Report/Test function. In addition to these basic functions, Airbus Industrie, in cooperation with the A330/A340 operators, has devel- oped a batch of new features to enlarge the capabilities of the Central Maintenance System - The A330/A340 CMS Option Package (Figure 1). This package can be divided into three cate- gories : l features improving the Trouble Shooting process by providing addi- tional information such as flags and ad- visories on the Post Flight Report (PFR) and new means of transmission: information downloading on to a disk, and sending BITE reports following up- link requests from the ground; l the Servicing Report gathers a num- ber of parameters, such as oil/liquid levels, status of filters, pressures, etc., with the aim of reducing the servicing workload; l the Configuration Management Reports allow the airline to know which part numbers, serial numbers and data- bases are fitted on their aircraft; every configuration change is also detected, memorised and transmitted in real t