Fumes

The co-pilot of the BAe146-300 reported that he noticed both cockpit side windows were open as he entered the flight deck to commence his pre-flight preparations and almost immediately detected the presence of fumes. He stated that these fumes had a distinctive odour that he recognised as being consistent with the contamination of the cabin air supply by lubricating oil from the aircraft's engines. This typically occurs when a faulty bearing seal allows lubricating oil to escape and contaminates one of the sources of cabin air for the aircraft.

The auxiliary power unit (APU) was operating and air conditioning Pack 1 and 2 were supplying air to the flight deck and passenger cabin. As Pack 1 supplies most of the air to the flight deck and because fumes were not apparent in the passenger cabin, the co-pilot immediately suspected that the contamination was associated with Pack 1. He de-selected Pack 1 and noticed an immediate improvement to the quality of the air.

The co-pilot briefed the pilot in command of these observations when he arrived on the flight deck. The crew used engine bleed air to provide cabin air conditioning as the aircraft taxied for take-off and Pack 1 remained de-selected. This was on the basis of the tailwind conditions encountered while taxiing, to reduce the possibility of ingesting fumes from the APU's exhaust into the aircraft cabin and the co-pilot's observations of air quality from Pack 1. The take-off was performed with all air conditioning packs selected off and number 4 engine supplying bleed air for pressurisation control of the aircraft cabin.

The co-pilot was the handling pilot for the sector from Perth to Karratha. After take-off, the crew selected engine bleed air sources from all engines and Pack 1 and 2 were used to provide air conditioning to the passenger cabin and flight deck. No fumes were evident in either the passenger cabin or the flight deck and the flight proceeded normally.

About 10 minutes prior to the top of descent, the co-pilot recalled starting to experience symptoms of a headache. He was able to continue his duties as handling pilot and completed the descent to the Karratha circuit area. The aircraft was on final approach to land, when strong oil-type fumes were again detected on the flight deck. As the aircraft cabin had already stabilised at sea-level atmospheric pressure and the source of fumes appeared to be either the air conditioning packs or one of the engine bleed air supplies, all air conditioning packs and all sources of engine bleed air were selected off. The intensity of the fumes quickly dissipated and the co-pilot completed the landing. Fumes were not detected in the passenger cabin by any of the flight attendants. The co-pilot reported that supplemental oxygen was not used on this occasion due to the critical phase of flight (short final approach to land) and the prompt action taken to isolate the source of fumes.

During the taxi to the passenger terminal, the co-pilot became aware that he was experiencing symptoms of an unusually strong headache, nausea and irritated eyes, nose and throat. His symptoms quickly improved as he conducted the external turnaround duties and he felt capable of performing his duties on the return sector as pilot not flying.

The pilot in command was satisfied that the source of fumes experienced on short final was associated with contamination of the cabin air supply by engine lubricating oil. The return flight was conducted without using APU air and with Pack 1 de-selected to minimise the possible recurrence of fume contamination. The flight was completed without incident.

Subsequent to the incident, the co-pilot recalled that during the outbound sector his bottled drinking water, which had been poured and left standing in his cup, had acquired a rank swampy, slightly metallic taste. The pilot in command was also reported to have made a similar comment about the taste of his drinking water. The cups were resting in the flight deck cup holders adjacent to the air outlet vents. This was noticed prior to the co-pilot reporting symptoms of a headache and fresh cups of water from the bottle tasted normal with no apparent sign of contamination. The ATSB were not able to test the water for contaminants as it had been discarded following the flight.

Following entry of the fume report in the aircraft's defect log, company engineering personnel applied the requirements of Airworthiness Directive AD BAe 146/86, issued 30 March 2001 that required inspection of various components associated with the aircraft's cabin air supply. This inspection revealed no apparent defects or source of contamination to the cabin air.

The operator received subsequent Operating Crew Reports associated with poor cabin air quality on 23 and 26 October 2002. The aircraft commenced a period of heavy maintenance on 28 October 2002. During this maintenance the APU was removed from the aircraft, cleaned, inspected (with nil defects found) and refitted. Air conditioning Pack 1 was also cleaned. The operator received subsequent reports of flight deck odours with respect to this aircraft on 5 and 8 December 2002 and the requirements of the Airworthiness Directive were again applied. On this occasion the inspections revealed slight leakage of engine lubricating oil from a bearing seal on the number 4 engine.

During the take-off roll, the cabin manager of the BAe 146 aircraft became aware of a smoky, burning smell coming from an air vent in the region of her crew seat at the forward (L1) exit door. Initially there was a mild odour. That was followed by the rapid onset of strong fumes for a short period after which the fumes dissipated quickly. The event was of 2-3 minutes duration.

The cabin manager felt overwhelmed by the fumes and was on the verge of passing out when her colleagues became aware of the situation and provided her with portable oxygen. After approximately 10 minutes of using oxygen, the cabin manager felt well enough to attempt a resumption of her duties but was unable to continue due to the effect of the fumes exposure.

The cabin manager, who had ten years of operational experience on the BAe 146, spent the duration of the flight seated at the rear of the aircraft; breathing portable oxygen for most of that time. The cabin manager reported that when she was not using oxygen she felt unwell, she had difficulty in thinking clearly and she found it difficult to coordinate her thoughts with her actions. No other members of the crew or any of the passengers reported being affected by the fumes.

Upon arrival in Kununurra, engineering inspections were performed on the aircraft and further action was deferred in accordance with the Civil Aviation Safety Authority airworthiness directive AD/BAe146/086. That airworthiness directive required certain actions to be performed whenever a cabin air quality problem was identified, which was suspected of being associated with oil contamination of the air supply from the air conditioning packs. Subsequent engineering inspections revealed that the cause of the oil contamination was a worn number one bearing seal in the number 3 engine. The engine was replaced and no further fumes were evident during following flights.

The cabin manager sought medical treatment and tests in Kununurra on the day of the incident and in Perth on the following day. Although she was eventually cleared to return to work, symptoms of anxiety, impaired judgement, and light-headedness remained with her for in excess of one week. Of note was the result of a blood test that revealed she had been exposed to a higher than normal level of carbon monoxide (CO). When inhaled, CO combines with the haemoglobin, the blood's oxygen-carrying molecule, to form carboxyhaemoglobin (COHb). Once in that state, the haemoglobin is unable to carry oxygen. Thus, the blood's ability to carry oxygen to body tissues, including vital organs such as the heart and brain, is inhibited.

CO is the product of incomplete combustion of carbonaceous material. It is found in varying amounts in the smoke and fumes from burning aircraft engine fuels and lubricants. The gas itself is colourless, odourless, and tasteless but is usually mixed with other gases and fumes that can be detected by sight or smell. Individuals that have been exposed to CO should be removed from the exposure and administered 100 percent oxygen through a tight fitting mask until all symptoms have been resolved. If blood testing for measurement of COHb level is required, the samples should be drawn as soon as possible after the exposure, as COHb has a short half-life in the body of 4-5 hours. If an individual is administered 100 percent oxygen, the half-life is reduced to 40 to 80 minutes.

The same aircraft was the subject of a pilot report three days prior to the cabin manager's experience, in which an oil-like smell was evident in the cockpit but not in the cabin. The event was of short duration and occurred just after take-off, when the source of air supply was changed from the Auxiliary Power Unit (APU) to the engines. Inspection by maintenance engineers of the engines, APU, and air-conditioning system revealed no signs of contamination and the defect was cleared.

Evidence from previous incidents of air system contamination on this type of aircraft has indicated that fumes were associated with engine or APU oil contamination of the air conditioning system. The BAe146 is similar to many aircraft in that the supply of cabin air originates in the aircraft's engines. Air is bled from the final stage of the engine's high-pressure compressor just prior to the combustion chamber. The air destined for the cabin then passes through a catalytic converter in order to clean the air of any oil contaminants. Catalytic converters operate at maximum efficiency under highly specific conditions of temperature and contaminant to air ratio. The air is then passed through a heat exchanger and then through one of two air conditioning packs before entering the cabin. During normal operation bleed air from engines one and two is fed to pack one, which in turn supplies conditioned air to the flight deck and cabin. Bleed air from engines three and four is fed to pack two, which normally only supplies air to the cabin. Additionally, bleed air from the APU is used by either pack during the take-off and landing phases or when air conditioning is required on the ground.

The BAe 146 aircraft had departed Brisbane, Qld on a flight to Mackay, Qld with a carried-forward defect that required the number 1 air conditioning pack to be used as the sole source of air for the cabin and flight deck. That situation was permissible under the terms of the aircraft's Minimum Equipment List (MEL), which allowed operation of the aircraft in non-standard configurations. The number 2 pack was not to be used because an intermittent oil leakage in the number 4 engine was a potential source of air contamination to air conditioning pack two.

During normal operation, bleed air from engines one and two was fed to pack one which in turn normally supplied conditioned air to the flight deck and cabin. Bleed air from engines three and four was fed to pack two, which normally supplied air to the cabin only. Additionally, bleed air from the auxiliary power unit (APU) was used by either pack during the take-off and landing phases or when air conditioning was required on the ground.

The flight to Mackay was uneventful. During the approach to land, the APU was selected as the bleed air source for pack one and the configuration remained that way until the aircraft was parked, the engines were shut down and the passengers disembarked.

From the time the aircraft turned off the runway, the crew was aware of a strong oil smell coming from the air-conditioning system. The fumes were detected in the cabin as well as the flight deck. Because it was a short taxi distance and a busy period on the flight deck, the crew did not have time to investigate the origin of the contaminated air. Although the smell was generally described as oil-like, the moderate south-east surface wind may have added to the air contamination by directing engine exhaust fumes into the APU air intake.

The pilot in command vacated the aircraft to get some fresh air and a short time later he suffered headache, itchy eyes, nausea and a bad taste. Company engineers at the Brisbane and Adelaide bases were consulted by telephone and a decision was made to proceed with the scheduled return flight to Brisbane using engine air one and two as the sole source of air to pack one. As the pilot in command was still suffering from the symptoms described above, he checked with the first officer and confirmed that he was unaffected by the fumes incident and requested the first officer to be the handling pilot on the next sector.

The passengers were embarked, the doors were closed and the engine start procedure was commenced. During the turnaround, the air conditioning had been turned off and remained off during the engine start. However, the cabin fan, which distributed air to the cabin through individual louvres above each passenger seat, was on. After starting three of the four engines the pilot in command felt increasingly unwell and the cabin staff also became aware that they were being affected by the fumes. The pilot in command then cancelled the flight and later expressed concern that he had considered attempting a flight while still feeling the effects of the air contamination. He stated that he may have been influenced by his desire to consult his Designated Aviation Medical Examiner in Brisbane as soon as possible. He also noticed that he had made simple errors during the flightdeck preparation and put those errors down to the effect of the fumes on his thought processes. Previous incidents have indicated that operating crews were not aware of their impairment and the subsequent effect on their decision making ability. The seriousness of that aspect was reflected in the decision by the Civil Aviation Safety Authority (CASA) to adopt a United Kingdom Air Accidents Investigation Branch (AAIB) recommendation requiring flight crew to use oxygen masks selected to 100 percent when there is a suspicion of flight deck or cabin air contamination.

A Licensed Aircraft Maintenance Engineer (LAME) was dispatched to Mackay to investigate the source of the fumes. The engineer carried out an inspection in accordance with a CASA airworthiness directive AD/BAe146/086, issued 30 March 2001, and British Aerospace Systems Information Service Bulletin (ISB) 21-150. The ISB required certain actions to be performed whenever a cabin air quality problem was identified, which was suspected of being associated with oil contamination of the air supply from the air conditioning packs. The engineer's inspection of the air conditioning system, engines and APU revealed no signs of oil contamination or oil leaks. The aircraft was ferried to Brisbane where further investigation, including an air test, confirmed that the number 4 engine was producing fumes during the climb and the descent and the APU was continuously producing fumes. Subsequently the number 4 engine and the APU were replaced.

The two cabin staff received medical advice and resumed their flying duties. Medical tests were carried out on the pilot in command but no abnormalities were detected and he resumed flying duties one week after the incident. The co-pilot was unaffected by the fumes.

Particular attention has been paid to this type of problem in Australia since July 1997 due to apparently similar incidents and crew reaction. A number of organisations, including the ATSB, have been conducting investigations into the subject of air quality in BAe146 aircraft. Evidence from previous incidents of air system contamination on this type of aircraft has indicated that the fumes are associated with engine or APU oil contamination of the air conditioning system. As a result, operators have incorporated various modifications to the cabin air system, APU and engines. They have also introduced improved maintenance practises to further address the issue. However, that action has not completely solved the problem. The air supplied to the air conditioning packs is protected from contamination by oil seals in the engines and APU. A technical defect arising in one of these seals can result in oil entering the cabin air conditioning system with the first signal of the defect being an awareness of fumes by the members of the crew. The difficulty of identifying the origin of the contamination is exacerbated by the often intermittent nature of the fume events.

During taxi from the terminal, when the British Aerospace BAe 146-100 was approximately 150 metres from the parking bay, and while making the pre-flight safety public address, a flight attendant began coughing due to a slight irritation in her throat and was unable to finish the presentation. When the second flight attendant went to the front of the cabin to assist, she too developed a cough. Both flight attendants saw what they described as a grey, smoky gas in the airstair region of the left door.

At approximately the same time, the first officer experienced an involuntary cough and stinging eyes and donned an oxygen mask. A short time later a flight attendant opened the flight deck door and advised the flight crew that smoke, or fumes, were filling the forward section of the passenger cabin. The flight crew turned off both air conditioning packs and the Auxillary Power Unit and immediately returned to the parking bay after advising Air Traffic Control and the ground handling company. During the return to the bay, the captain opened the left flight deck window and the flight attendants moved the forward seated passengers to the rear cabin and instructed passengers to cover their nose and mouth.

When the flight crew reported their intention to return to the terminal, the ground handling company requested that passengers remain on board the aircraft at the terminal. The first officer advised the company that this was not possible as there were fumes in the cabin. The aircraft arrived back at the parking bay approximately four minutes after the fumes were first noticed.

Upon arrival at the terminal the captain instructed the flight attendants to open the doors. A flight attendant returned to the front of the aircraft from the rear cabin and opened the forward left door. The second flight attendant opened the rear left door. The flight attendants reported that there were no portable stairs available when the doors were opened. The forward flight attendant called to ground staff in the area and portable stairs were brought to the forward left door. The flight attendants elected not to attempt airstair activation as that was believed to be the source of the smoky gas.

To avoid delay, flight attendants disembarked passengers through the forward door, in the vicinity of the fumes. They reported that, as they were coughing, they could not speak to passengers during the disembarkation.

The company later reported that the urgency of the situation might not have been conveyed to ground services during the initial advice of the aircraft's return to the terminal.

After passengers had disembarked, Aviation Rescue Fire Fighting (ARFF) personnel advised the flight attendants to sit outside the aircraft for ten minutes. The flight attendants later underwent a medical check at the recommendation of the company. Neither the captain nor first officer sought medical attention following the incident. All crewmembers have since returned to duty.

Some passengers later reported that they experienced coughing, watering of the eyes, and respiratory irritation during the event. They also reported that medical attention was not available at the terminal. The ground handling company reported that when passengers reached the terminal, staff did not call a doctor when requested by passengers. They also reported that ARFF personnel had recommended that passengers go outside into the fresh air but few passengers had followed that advice.

Subsequent investigation revealed that neither the operator nor the ground handling company had emergency response procedures in place to cover the situation of an aircraft emergency return to the terminal from a taxi position.

Integral airstair

The aircraft was fitted with an integral stairway (airstair) at the forward left door. However, portable stairs were often used in place of the narrow integral stairs for convenience and ease of boarding.

The operator reported that the forward door airstair had been in use before the flight but had been retracted and replaced by portable stairs prior to boarding passengers.

The airstair was operated from inside the aircraft. It was extended manually, but retracted by hydraulic pressure when the airstair-selector `retract' switch was pressed and held in position. This procedure allowed hydraulic fluid (Skydrol) to pass under pressure through a series of lines and valves to activate airstair retraction. When the airstair was retracted, and the airstair-selector switch was released, the rocker-type switch was designed to spring back to the `off' position. The airstair was then pushed along tracks to a rearward, stowed position out of the doorway.

While the aircraft was on the ground with engines off, an electric stand-by pump was used to top-up and maintain pressure for airstair retraction. After engine start-up, engine driven pumps delivered pressure to the airstair hydraulic system.

An examination of the integral airstair by company engineers revealed that the airstair selector switch had stuck in the `retract' position, and also revealed a failed `o' ring in the retraction jack `banjo' fitting in the airstair actuator. During the examination, other `o' rings had also shown signs of deterioration.

The operator reported that an internal failure caused the switch to be stuck in the `retract' position and also noted that the switch was flush with the surrounding panel. The operator considered it possible that the body of the switch became fouled on the panel and remained in the `retract' position.

The operator also reported that the aircraft maintenance manual stated that the airstair `retract' switch should have had a hinged flap cover. The flap was missing and an engineering order had not been raised for its removal.

Crew emergency procedures and manuals

Flight crew emergency procedures for smoke or fumes required flight deck crew to don oxygen at any time that smoke or fumes were evident. The captain reported that he did not don the oxygen mask when he was alerted to the presence of smoke or fumes as the fumes did not affect him. He also considered it more important to return the aircraft to the parking bay to disembark passengers than to stop and don his oxygen mask.

Flight crew emergency procedures for smoke or fumes also required that the flight deck door remain closed. However, this procedure was not reflected in the flight attendant emergency procedures manual. The flight attendants were unaware of the requirement to keep the flight deck door closed.

The Beech Super King Air aircraft was maintaining flight level (FL)250 on an aerial ambulance flight, when members of the medical crew advised the pilot that they noticed an unusual burning odour in the cabin, similar to that of a bakery. When the smell became stronger, the pilot elected to return the aircraft to the maintenance facility at Jandakot.

A short time later, in addition to the cooking odour, an odour similar to hot plastic and rubber was smelt by the crew. The medical crew elected to don therapeutic oxygen masks and advised the pilot that the smell had become more intense and that they had now gone onto oxygen. The flight nurse observed that the doctor's complexion had changed to ashen gray and he was leaning against the cabin bulkhead with his eyes closed. The nurse then assisted the doctor to don his oxygen mask. The nurse said that the doctor was in a confused mental state. The nurse only recognised the seriousness of the situation "when the doctor's speech became slurred and was running his words together in their sentences". The nurse stated that his own symptoms manifested very quickly and he "felt quite euphoric and light headed". The nurse also estimated that by the time he donned his own oxygen mask, he was rapidly approaching unconsciousness. The pilot donned his oxygen mask and, when he feared he may be sick in his mask, initiated an emergency descent to 10,000 ft. During the descent he depressurised the cabin in an attempt to clear the fumes.

The crew reported that the fumes in the cabin had caused them to experience nausea and confusion soon after they detected the smell. They remained on oxygen for the rest of the flight.

The pilot reported that while inbound he had asked Air Traffic Services (ATS) to repeat instructions on several occasions and this prompted ATS to ask him to confirm that operations were normal. He did not recognise any of the landmarks that he usually used to identify his correct track while on approach to Jandakot. He stated that on that occasion his vision was affected to the extent that he had great difficulty focussing outside the cockpit. The pilot said that following the emergency descent to 10,000 ft, he engaged the autopilot and changed the GPS to the coordinates for Jandakot. The aircraft autopilot then flew the GPS guided track to abeam Jandakot from where the pilot took control and conducted the landing. He could not recall anything about the approach and landing, and later had to ask the flight nurse if he had used any flap, because the flaps were in the retracted position after landing and he did not remember retracting them. After landing, the crew's symptoms did not significantly improve and they were taken to hospital for medical assessment.

Several months after the occurrence, some members of the crew reported to the ATSB that they were still suffering various residual effects including headaches, elevated blood pressure, reduced concentration levels and anxiety. They attributed the symptoms to their exposure to the fumes encountered on the occurrence flight.

A maintenance investigation following the occurrence discovered several air conditioning system defects. A bleed air pressure-reducing valve in the under-floor cabin area was found to be leaking hot bleed air onto an adjacent air conditioning duct. There was also evidence that the insulation was heat affected and had discoloured from the bleed air leak. A "spirap" type loom bundling plastic tie was also found in the vicinity, which had been melted. Samples of those heat-affected items were taken by the ATSB and forwarded to a laboratory for testing, to identify if heat application produced any emissions. The results of that testing and subsequent instrumented flight test showed that the bleed air leak did not reach a sufficiently high enough temperature to be considered a source of hazardous fumes in this incident.

In addition to the faulty reducing valve, two air conditioning refrigerant leaks were detected in the forward air conditioning evaporator refrigerant pipes. Airconditioning compressor lubricating oil was also observed on the evaporator and dripping from a fractured flared fitting onto the surrounding structure. The fitting was located on a pipe that delivered high pressure liquid refrigerant to the expansion valve and evaporator. When the system refrigerant was replenished during the maintenance investigation, it was noted that a considerable quantity of refrigerant was required to refill the system. The initial refrigerant loss was estimated as at least 1.1 pounds or approximately 0.5 kg. As the lubricating oil level could not be determined with any accuracy, maintenance personnel then decided to totally evacuate the system and replenish refrigerant and oil levels from empty. The nature of the leak, troubleshooting and repairs precluded an accurate measurement of the total oil and refrigerant quantities that had escaped and the initial estimate would have been the minimum system loss incurred from the leak. The pipes and fittings to the evaporator were located in a confined space to which proper access for some maintenance activities was very difficult.

The vapour cycle type air conditioning system in the aircraft used a new environment-friendly refrigerant HFC-134a instead of the ozone depleting refrigerant type R12 that had been in service for many years. The design of the vapour cycle systems was such that a significant amount of the oil lubricant for the air conditioning compressor was in solution with the refrigerant. The polyol ester-based lubricating oil had a particular odour when heated, which was similar to the smell detected by the crew.

The Material Safety Data Sheet (MSDS) for HFC-134a stated, "overexposure can cause central nervous system depression with dizziness, confusion, incoordination, drowsiness or unconsciousness. Irregular heartbeat with a strange sensation in the chest, `heart thumping', apprehension, light-headedness, feeling of fainting, dizziness, weakness, sometimes progressing to loss of consciousness and death. Suffocation if air is displaced by the refrigerant vapours".

The MSDS for the polyol ester-based lubricating oil that was used in conjunction with the refrigerant charge stated that inhalation may cause nasal respiratory irritation and dizziness. The operator's engineering manager also stated that the lubricating oil was known to produce valeric acid when heated which was known to cause dizziness and nausea. He stated that prolonged exposure could lead to unconsciousness. The MSDS sheet supported that. In addition to the refrigerant charge of 104 ounces or almost three kg of HFC-134a, the air conditioning system was also charged with 34 ounces or almost one kg of polyol ester oil, of which approximately 26 ounces or almost 75% was held in solution with the refrigerant gas/liquid.

In addition to the refrigerant and oil chemicals, the airframe manufacturer had a maintenance warning that when moisture entered the air conditioning system, it could cause the formation of hydrofluoric acid or hydrochloric acids. The MSDS data for those compounds gave warnings of eye irritations and burns when exposed to the mist or vapours.

The Programme for Alternative Flurocarbon Toxicity (PAFT) found in 1987 that the HFC-134a refrigerant exhibited a very low risk and was considered to be "practically non-toxic". The PAFT tests stated that at very high concentration levels (over 50%), exposure could sensitise the heart to adrenaline and that it could cause irregular heartbeat, even death.

A 1998 University of New South Wales (UNSW) study on the use of HFC-134a in the confined space of motor vehicles concluded that the refrigerant posed a considerable risk to vehicle occupants. It cited two scientific reports in the United States and four scientific papers in peer reviewed journals describing adverse effects expected from human inhalation as all being "scientifically consistent". The study recommended that fresh air vents be kept open at all times when using the air conditioning system and to introduce a pungent odour producing element into the systems to aid in the early detection of refrigerant leaks.

The United Sates Environmental Protection Agency (USEPA) and the US Navy Bureau of Medicine commissioned research to determine safe levels of exposure to HFC-134a. The studies concluded with statements of significantly different safe exposure values. The US Navy-determined levels of acceptable exposure were significantly lower than the values assessed by the USEPA. The US Navy research into the exposure of humans to HFC-134a was conducted in a confined area to simulate the confines of a submarine.

Although there was research data available on HFC-134a and polyol ester-based lubricants, at the time of preparing this report, the ATSB could not find any data on the effects of exposure to HFC-134a at air pressures less than sea level. Similarly, the ATSB could not find data on exposure to a mix or "cocktail" of the two agents HFC-134a and polyol ester lubricants at air pressures at sea level or less than sea level.

The Boeing 767 had just reached cruise altitude at flight level 330 approximately 40 minutes out of Singapore enroute to Perth, when the flight crew noted smoke and electrical fumes on the flight deck. The source of the smoke and fumes could not be readily identified. The pilot in command elected to have the flight crew don oxygen masks, and diverted to Jakarta.

The operator's engineering personnel examined the aircraft and found the right DME (Distance Measuring Equipment) circuit breaker open. Technicians isolated the problem to the right DME interrogator unit. The malfunctioning DME unit was disabled in accordance with the MEL (Minimum Equipment List) guidelines to allow the aircraft to continue to Perth. Following arrival in Perth, the unit was replaced.

Examination of the unit by the manufacturer revealed that the DME unit's A5 modulator had overheated. This failure mode was similar to two other units, which had overheated on a different aircraft in January 2000 (see Occurrence 200000055). The failure mode of those units was such that the A5 modulator had overloaded the positive 86 volt DC power supply and overheated the power transformer. Compliance with service bulletins recommending product improvements to this unit were not mandatory, and the recommended modifications had not been incorporated into this unit, or the previous two units that had sustained failures.