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While cruising at Flight Level (FL) 370 on a flight from Perth to Adelaide, the crew of the Airbus A320 noticed that the left engine bleed-air fault warning had illuminated. The aircraft pressurisation and airconditioning systems then automatically shut down, and the cabin pressure altitude began to increase at approximately 700 ft per minute. The crew made an unsuccessful attempt to reselect the left engine bleed air to on, and the aircraft auxiliary power unit (APU) was started.

The pilot in command (PIC) then contacted air traffic control and requested an emergency descent because of the decreasing cabin pressure, gaining a clearance to descend to 10,000 ft. A short time after commencing the descent the PIC informed the cabin crew and passengers that he was conducting an emergency descent.

As some of the cabin crew were beginning to feel the effects of the increased cabin altitude, all donned portable oxygen breathing equipment. They then took their assigned seating positions in the aircraft. At approximately FL200, the pressurisation and airconditioning systems were restored utilising the APU bleed air supply. The crew then levelled the aircraft at FL180 and told the cabin crew and passengers the reason for the descent. They continued to Adelaide where they completed a normal approach and landing.

The aircraft departed from Perth with a minimum equipment list (MEL) MEL 36-11-07 restriction applied following the failure of the right engine high-pressure valve (HPV). Part of the MEL restriction required that the right engine bleed air HPV be locked in the closed position by a locking pin.

The operation of the engine HPV normally supplemented the bleed air supply to the aircraft at low engine speed. At higher engine speeds, such as occur during normal flight, the bleed-air system was supplied with enough air to operate the airconditioning pack, even with the HPV locked closed.

MEL 36-11-07 was titled "Engine Bleed High Pressure Valve (HPV)" and was composed of two parts. Part (a) detailed the actions to be taken if the bleed-air system was considered to be inoperative, and indicated that the bleed-air system was to be isolated and not used. Part (b) detailed the actions to be taken if one HPV was inoperative, "locked closed". However, the intention of the MEL was that the bleed-air system from that engine could still be used except in specified circumstances.

The MEL was part of an operator customised publication, which had been developed from the aircraft manufacturer's master MEL (MMEL). Part (b) of the Operations area of the operator's MEL stated:

"(1) At low engine power (around idle thrust) setting:
(a) Associated bleed is selected OFF.
(b) Cross bleed valve is selected open.
(c) If wing anti-ice is required, one pack is selected OFF".

This differed from the wording of the manufacturer's MMEL, which stated that:

"At low power setting (during descent when thrust setting is in idle position).
Affected ENG BLEED - OFF
If wing anti ice is required

The crew interpreted the operator's MEL to mean that at engine "idle thrust" they were to turn the bleed air from that engine to off. That prevented any supply of bleed air for the pressurisation and airconditioning system coming from that engine. They then opened the bleed air cross-bleed valve and operated both airconditioning packs from the right engine only.

The aircraft then flew with a usable bleed air system isolated. Therefore, when the left engine bleed air system failed, there was a loss of pressurisation and airconditioning. It wasn't until descending below FL200, that pressurisation was able to be restored using the aircraft's APU bleed air source.

A maintenance investigation carried out by the aircraft's operator, found that the left engine bleed-air system was not able to be reselected `on' due to the failure of a temperature control thermostat. The thermostat controlled the temperature of the bleed air from the engine, commanding the position of the fan-air valve. When the signal to control the fan-air valve was lost, the bleed-air system was automatically isolated.

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