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.