What happened
On 20 May 2011, a Boeing 737-8AS, registration EI-DLI, was operating a scheduled passenger service from Liverpool to Dublin. While cruising at 18,000 feet over the Irish Sea, the flight crew was alerted by the Cabin Altitude Warning Horn. Upon investigation, the crew discovered that both air conditioning pack switches had been left in the OFF position.
In response, the crew switched the packs to AUTO and initially believed the situation was resolved. However, concerns regarding the continued stability of the pressurization system led the crew to don oxygen masks again. This prompted a MAYDAY call and a manual emergency descent to 10,000 feet. During the descent, the cabin oxygen masks were manually deployed. The aircraft subsequently entered a holding pattern at 6,000 feet to allow the crew to assess the aircraft's status before landing safely at Dublin Airport. There were no injuries among the 50 passengers or 6 crew members.
The investigation
The AAIU investigation focused on why the air conditioning packs were not correctly configured and why subsequent checks failed to identify the error. The investigation examined the cockpit voice recorder (CVR), which revealed that during the after-start and takeoff briefings, the crew verbally confirmed the packs were in the correct position, despite the physical switches being OFF.
The investigators also looked into the crew's management of the emergency. It was noted that the pilot monitoring (PM) deployed the cabin oxygen masks without consulting the pilot flying (PF). Furthermore, the investigation reviewed the operator's procedures for passenger communication and the handling of distressed passengers following the depressurization event.
Findings
- The primary cause of the incident was that the air conditioning packs remained in the OFF position following engine start.
- The crew's initial checks at 3,000 feet and 10,000 feet failed to detect the incorrect configuration.
- Sub-optimal Cockpit Resource Management (CRM) and a flat authority gradient contributed to the incident, specifically regarding the lack of consultation during the emergency descent.
- The crew's concern regarding the continued effectiveness of the pressurization system led to the decision to initiate the emergency descent.