Fokker B.V. F27 MK 50, VH-FNA, Southern Cross, (ALA), 9 January 2003

When the seat belt sign illuminated, the cabin crew were unaware
that the aircraft was in an emergency descent due to a loss of
cabin pressure and continued cabin duties until the flight crew
advised the reason for the descent. When advised that the aircraft
had levelled out and it was safe to move about the cabin, the cabin
crew continued duties without breathing the supplemental
oxygen.

Oxygen deprivation can be insidious and cabin crew may not be
the best judges of their own oxygen intake following decompression.
Factors that may have contributed to the effects of mild hypoxia
reported by the cabin crew include the continued physical activity
during the initial descent, lack of intake of supplemental oxygen
after the aircraft had levelled out and activity in the cabin
during the remainder of the unpressurised flight.

Cabin crew performance can be critical during emergencies. If
the cabin crew had used oxygen after the descent had been
completed, it would have assisted in recovery from the effects of
hypoxia. That use, in turn, would have provided some assurance that
cabin crew were able to perform their duties appropriately in any
subsequent emergency situation during the remainder of the
flight.

The Fokker B.V. F27 MK 50 was maintaining flight level 250
(FL250), when the flight crew was alerted to a pressurisation
problem by a triple chime, master caution and cabin altitude
annunciation that indicated that the cabin altitude was climbing
above 10,000ft. The normal cabin altitude for flight at FL250 was
8,000 ft. The flight crew donned oxygen masks and initiated the
procedure for an emergency descent. That procedure included
activation of the cabin fasten seat belt sign, broadcasting their
intentions to air traffic control and commencing a descent at
maximum speed with the engines at flight idle.

Cabin crew were alerted to a problem by the illumination of the
fasten seat belt sign and a change in aircraft attitude. They
advised passengers by the public address (PA) system to fasten
their seat belts, then walked through the cabin to check compliance
with the instruction. Shortly after, the flight crew used the
interphone to advise the cabin crew of the loss of cabin pressure.
The cabin crew made another PA to advise passengers of the
situation and secured the galley before sitting in their crew
seats.

The maximum altitude that the cabin attained during the descent,
or how long the cabin was at that altitude, could not be
determined. The pilot reported that the decompression was not rapid
and the descent to a safe altitude was carried out with minimum
delay.

The flight crew advised the cabin crew by interphone when a safe
altitude had been reached. The cabin crew then checked the cabin
safety and security for landing. The remainder of the flight was of
short duration and was continued, with the aircraft unpressurised,
at an altitude of 10,000 ft. None of the passengers or crew
reported any injury or ear distress and supplemental bottled oxygen
was not used during the flight. The cabin crew subsequently
reported symptoms of mild hypoxia including the tingling of hands,
feet and lips.

Emergency descent

The operator's operations manual states that if time permits,
the captain should make a broadcast on the PA about the emergency
descent.

The pilot reported that he alerted the cabin crew to the
emergency descent by interphone instead of the PA, as he considered
that the quality of PA transmissions could be affected by the
wearing of a crew oxygen mask. As the passenger cabin was not
equipped with drop down oxygen masks, he also considered that the
flight crew's priority was to manage the descent to a safe altitude
as quickly as possible.

Oxygen

Oxygen masks stowed beside their seats provided immediate
emergency oxygen for the flight crew. Five portable oxygen bottles
located in the cabin were available for use by the cabin crew and
passengers, if required, when the aircraft had reached a safe
altitude.

Cabin crew use of oxygen

Research conducted by the Civil Aeromedical Institute, Federal
Aviation Administration (FAA), found that physical activity such as
that performed by cabin crew will significantly shorten the time of
useful consciousness during an aircraft decompression. Based on
that research, the FAA's recommended procedure for cabin crew
during a decompression was for them to immediately don the nearest
oxygen mask, sit down or grasp a fixed object, and hold on in order
to brace themselves until given clearance to move about the cabin
by the flight crew.

The operator's flight operations manual loss of cabin pressure
(decompression) procedure, 'Immediate Action for All Cabin Crew',
required cabin crew to secure the bar/meal cart, sit down if a seat
was available, or hold on securely to a rigid structure and, if
near a PA handset, advise passengers to fasten their seat belts.
The procedure also advised cabin crew to use portable
(supplemental) oxygen themselves, if required, once the aircraft
had reached a safe altitude. None of the cabin crew felt the need
to use supplemental oxygen.

Electrical junction box

An electrical junction box on the right main landing gear oleo
contained electrical wiring and connectors for the right main
landing gear weight on wheels microswitch. The microswitch
activates 12 different relays that are linked to avionics systems,
warning and inhibit systems, the pressurisation system and engine
ground controls.

A subsequent inspection by company engineers found that the
junction box had been contaminated with moisture through inadequate
sealing of the box cover following routine maintenance. The
moisture ingestion led to spurious electrical signals being sent to
the aircraft's pressurisation system, resulting in erratic cabin
altitude control.