What happened
On 11 August 2010, a Boeing 737-73V, registration G-EZKG, was operating a scheduled passenger flight from London Luton Airport to Lisbon International Airport. During the initial climb, the crew noticed a discrepancy in duct pressures, which led to the illumination of a left BLEED TRIP OFF caution message.
While performing the associated checklist, the co-pilot incorrectly selected the bleed switch to the off position instead of the pack switch. This action, combined with the existing system issues, caused the loss of cabin pressure. Shortly after the crew completed the checklist and decided to continue the climb to FL3LE390, a second failure occurred: the right BLEED TRIP OFF message illuminated. This caused the cabin altitude to rise at a rate of approximately 3,000 ft/min, triggering the cabin altitude warning horn. The crew initiated an emergency descent and eventually returned to London Luton Airport at FL080. There were no injuries to the 144 passengers or 5 crew members on board.
The investigation
The investigation examined the sequence of cockpit actions and the mechanical state of the bleed air system. It was established that the commander was distracted from monitoring the co-pilot's actions by air traffic control transmissions and a call from the cabin crew. The investigation also looked into the engineering history of the aircraft, noting that the bleed air regulator had been replaced prior to this flight due to previous faults.
Findings
- The primary cause of the initial pressure loss was the co-pilot's error in selecting the bleed switch to the off position during the checklist.
- The right engine bleed system was unable to meet the demand of two air conditioning packs, leading to the second trip.
- The commander's monitoring of the checklist was compromised by simultaneous communications from ATC and the cabin crew.
- The decision to continue the climb to FL390 despite the degraded pressurization system was influenced by the commander's previous simulator training experience.
- During the rapid descent, six passenger oxygen hoses detached from their chemical generators.
Safety action
- The operator inspected all Boeing 737 aircraft in its fleet and applied additional fastenings to oxygen hoses to prevent detachment.
- The airline's training management was notified regarding the commander's decision-making and the impact of previous training on flight's operational choices.
- The commander underwent retraining focused on managing threats and errors, as well as verifying competence through a supervised flight duty.