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

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.

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.

Local safety action

As a result of its investigation, the operator has:

increased the frequency of maintenance inspections of the wiring on the main landing gear, including the junction boxes, and introduced a detailed inspection that includes removal of the junction box cover, inspection of the connections and resealing of the cover, and
amended the 'Loss of Cabin Pressure (Decompression)' section of the flight operations manual to require cabin crew to use portable oxygen for at least 30 seconds to one minute after flight crew advise that an aircraft has reached a safe altitude.