Tui flight passengers exposed to 'hypoxia risk' after cabin failed to pressurise

However, the pilot was soon forced to turn back around whilst flying over northern Lincolnshire when it emerged that there was an issue with the switches.

Engineers failed to turn the switches back on following maintenance, and both pilots failed to spot this.

A probe by the Air Accidents Investigation Branch (AAIB) found that those on board “were exposed to a progressive hypoxia risk”, though thankfully no injuries were ever reported.

The aircraft departed “with the engine bleed air system off” because the switches had been “incorrectly left off following maintenance activity” the night before.

As the switches were left off, the aircraft failed to pressurise.

The investigation also found that the pilots did not complete protocols when alerted.

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The crew failed to complete prescribed drills from a Quick Reaction Handbook (QRH) in response to a cabin altitude warning, “which remained illuminated for 43 minutes”.

“The after take-off checklist is designed to trap the latter omission, but the incorrect switch selection went undetected by the crew,” the report states.

Both switches were then turned on and the plane “continued to climb”, after which an additional warning appeared.

This prompted the pilot to seek advice from bosses on the ground, who told them to return to Manchester Airport.

Passengers were told about the situation over the PA system, the report confirmed.

If the plane would have continued to climb to 14,000ft, oxygen masks would have been deployed.

Whilst the possibility of losing consciousness below this height is “very small” for healthy individuals, “cognitive performance and decision-making” could be impacted, the report said.