A TUI flight was forced abort with 187 people on board after they were left at risk of passing out when crew missed a warning light for 43 minutes.
A major error during routine maintenance almost led to a potentially fatal disaster while the aircraft sat in a holding pattern.
The revelation came as part of a report from the Air Accidents Investigation Branch (AAIB), which is called in any time a problem is reported on UK flights.
This particular incident occurred last October, but the findings of the probe into it have only just been revealed.
The report explained: “Both engine bleed air systems had been inadvertently left off for the departure, so the aircraft failed to pressurise.”
These vital safety systems were apparently turned off during maintenance but engineers failed to turn them back on.
They were reactivated by the crew but, during ascent, a “master caution” was activated due to a fault with the Boeing 737’s air conditioning pack.
In agreement with air traffic control, the pilot made the decision to return the plan to Manchester Airport.
But, when it arrived, the crew was informed that the aircraft was overweight and would have to complete a holding pattern until it had burned enough fuel before it could land.
It was during this period that the crew missed an oxygen mask warning light for 43 minutes, which the report said left them and the passengers “exposed to the risk of hypoxia”.
This is when the amount of oxygen stored in your body’s tissue drops below safe levels and can be fatal if prolonged.
The AAIB concluded that, at that altitude, there was little chance of death and a small likelihood of loss of consciousness in the absence of pre-existing medical issues.
But it did say that it could have impacted the “cognitive performance” of those on board, including the pilot.
It added: “In this altitude window, the hypoxic exposure can be sufficient to affect cognitive performance and decision-making to the point where the decline would be observable in cognitive tests.
“In this range of altitudes, there are many variables that affect the severity and impact of hypoxia, including duration of exposure, rate of hypoxia onset (eg rate of climb if no pressurisation), physical workload, fatigue, individual responses and type of task being performed.”
Moreover, the report found that the captain was working on just three hours of sleep after being on-call the previous evening.
It went on: “Though the commander did not believe fatigue was a factor in this event, the analysis of his roster over the eight weeks preceding the event and the rest period immediately before it suggest that fatigue could still have been a contributory factor.
“It should be noted that fatigue, particularly chronic fatigue, can be insidious such that an individual may not recognise the symptoms in themselves.”
Fortunately, the plane landed safely at 8.10am and there were no reports of subsequent medical issues.
