C2/2007L Passenger aircraft pressurization failure in flight 8 January 2007

A serious incident occurred on a scheduled flight KF-201, a MD-90-30 type passenger aircraft, from Helsinki to Oulu on flight level 350 on eight January 2007. The aircraft pressurization system failed and the cabin oxygen masks deployed. The passengers and the cabin crew used oxygen masks in the cabin and the pilots used their masks in the cockpit. The Accident Investigation Board Finland decided to appoint an investigation commission C2/2007 L for this incident. Accident investigator Tapani Vänttinen was named investigator-in-charge accompanied by investigator Vesa Kokkonen as a member of the commission. The commission requested assistance from Doctor of Philosophy Päivikki Eskelinen-Rönkä to analyse the cockpit voice recording.

The automatic temperature control system of the cabin air conditioning system was faulty but the manual system was in operation and had to be used. The cockpit temperature control was selected in the manual mode. During the flight the water separators of both air conditioning systems probably froze because the prevention of ice accumulation is not so effective in the temperature control manual mode as in the automatic mode. The bleed air from the engines was not enough to pressurize the aircraft when engine power was reduced upon reaching the cruising flight level. The pilots noticed that the amber FLOW-light came on which is indication of insufficient pressurization capability and the pressure altitude in the cabin climbed to 10000 feet. The pilots requested down to flight level 210 (21000 feet) and started to descend. Soon came on the red CABIN ALT light which indicates too high cabin altitude. The CABIN ALT light extinguished after some time due to malfunction in the cabin low pressure warning switch. Cabin oxygen masks deployed when the cabin pressure altitude rose to 14000. Passengers and the cabin crew used oxygen masks down to flight level 100. The cockpit crew used oxygen masks down to flight level 100 except for short periods. The approach and landing to Oulu was normal.

The investigation revealed that there was a fault in the right temperature control valve of the cabin air conditioning system, the 35°F water control valve and in the cabin pressure warning switch. The water separator of the cabin air conditioning system was found to be ice covered still after the ferry flight to Helsinki.

The reason for the serious incident was the fault in the cabin water control valve and temperature control valve and both air conditioning systems being operated in the manual mode which increases the probability of water separator icing. Pilots were not informed about this particular icing problem. The pilots did not read the Cabin alt/Rapid decompression check list logically. The reasons for this may have been the extinguishing of the Cabin Alt light.

The investigation commission issued five safety recommendations to the airline in question. A recommendation was to modify the air conditioning to reduce the probability of water separator icing. The second recommendation was to follow the company flight manual instructions concerning the operation of the air conditioning systems. The third recommendation was to upgrade the pilot’s type and periodic training to cover failures of this nature. The fourth recommendation was to use INOP stickers in the faulty systems. The fifth recommendation was to evaluate cabin personnel’s capacity and ability to move during pressurization failures.

C2/2007L Report (pdf, 0.72 Mt)