Pilots of a Boeing 737-800 did not notice that the aircraft’s bleed-air system was not active before the jet took off from Manchester, triggering a cabin-altitude warning that remained active for over 40min.
UK investigators found that the bleed-air switches had incorrectly been left off during air-conditioning pack maintenance, and not turned on during pre-flight procedures. Nor was the omission picked up by the crew during the ‘after take-off’ checklist.
The cabin-altitude warning sounded as the TUI Airways aircraft – bound for Kos on 17 October last year – climbed through 13,000ft.
Flight-data recorder information shows the crew reduced the target altitude from 28,000ft to 15,000ft.
Just after the aircraft’s autopilot had captured 15,000ft both bleed-air switches were turned on.
According to the Air Accidents Investigation Branch, the captain recalled that memory items for the ‘cabin altitude warning’ checklist – including the use of oxygen masks – should have been completed. But he felt this would be disproportionate given that the bleed-air system was on and the situation appeared under control.
The target altitude was reset to 28,000ft and the climb recommenced.
But the master caution illuminated, alerting the pilots to a fault in the right-hand air-conditioning pack, and the aircraft was again levelled, this time at 20,000ft.
The right-hand pack was turned off and the left-hand pack transitioned to high-flow. After deciding the pack problem could not be resolved, and realising it would put limitations on the return flight, the crew opted to return to Manchester.
Flight-data recorder information shows the cabin altitude fell below 10,000ft about 15min after the aircraft levelled at 20,000ft. The aircraft started its descent from 20,000ft after a further 15min.
The inquiry says the cabin-altitude warning was active for 43min.
Investigators could not determine the actual cabin altitude during this period, but it points out that the passenger oxygen masks did not deploy – and therefore the cabin altitude must have remained lower than the automatic deployment threshold of 14,000ft.
The passengers and crew would have been exposed to the risk of hypoxia, says the inquiry, although the likelihood of loss of consciousness was “very small”. But it adds that exposure could have been sufficient to affect cognitive performance.
None of the 193 occupants of the aircraft was injured.
Investigators state that the captain’s workload over previous weeks had been above average, and his rest before the early-morning flight had been disturbed, potentially resulting in fatigue. The first officer recalled that the crew had not felt under pressure, but were nevertheless trying to expedite the departure.
As a result, says the inquiry, the crew could have been “more vulnerable” to expectation bias with regard to the position of the bleed-air switches during checks.
Three days after the incident, the same aircraft (G-TAWD) was involved in a runway excursion while landing at Leeds-Bradford airport, after the crew did not use sufficient rudder to maintain direction after a nose-wheel bearing failure.
