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Report: Boeing 737-800 mis-configured pressurization system and emergency descent
31 July 2014
Bleed Air Controls and Indicators on the 737-800 overhead panel (AAIU)

Bleed Air Controls and Indicators on the 737-800 overhead panel (AAIU)

The Irish AAIU published the final report of their investigation into a serious incident involving cabin pressurization issues on a Boeing 737-800 over the Irish Sea.

The incident happened on May 20, 2011 when a Ryanair flight took off from Liverpool, UK on a scheduled passenger service to Dublin, Ireland. There were 50 passengers and 6 crew on board. The Captain was the Pilot Flying (PF) and the First Officer was the Pilot Monitoring (PM).
After a few minutes at the flight’s cruising altitude (18,000 ft), the Cabin Altitude Warning Horn sounded in the cockpit. The crew immediately donned their oxygen masks and quickly realised that both air conditioning pack switches were in the OFF position. Both packs were selected to AUTO. The crew subsequently came off oxygen. Shortly after coming off oxygen, the PM became concerned that the pressurization system was not functioning correctly.
A MAYDAY call was broadcast and the aircraft made an emergency descent to 10,000 ft. Passenger emergency masks were deployed throughout the cabin. The aircraft then descended to 6,000 ft and entered a hold to allow the Flight Crew to review and confirm their understanding of events and the status of the aircraft. The aircraft subsequently landed without further incident at Dublin. No injuries were reported.

Probable Cause:

The Air Conditioning Packs were not selected to AUTO following engine start.

Contributory Causes:
1. The “After Start” checks and the checks at 3,000 ft and 10,000 ft did not identify the mis-configuration or that the aircraft was not pressurising.
2. Concern about the effectiveness of the pressurization system after both packs had been selected to AUTO caused an emergency descent to be initiated and the manual deployment of the cabin oxygen masks.

Three safety recommendations were issued to the operator, a.o. regarding communicating with distressed passengers. The Boeing Aircraft Company was advised to consider amending the After Take-Off and 10,000 ft checklists for B 737 operators to require the PM to verbalise the reading on the cabin differential pressure gauge.

More information: