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 airconditioning 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 airconditioning 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 airconditioning
refrigerant leaks were detected in the forward airconditioning
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 airconditioning 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 airconditioning
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 heart beat 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 airconditioning 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 airconditioning 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 airconditioning 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.
