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
X BLEED set - OPEN
If wing anti ice is required
ONE PACK - OFF".
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.
