A TUI flight was aborted midair after the plane did not pressurise due to switches being left off by mistake, an investigation has found.

Although the Kos-bound flight returned to Manchester with no injuries, 187 passengers and six crew members ‘were exposed to a progressive hypoxia risk’ from a lack of oxygen.

The Boeing 737-8K5 had only flown as far as northern Lincolnshire when the pilots noticed a cabin altitude warning went off on October 17, 2023.

It had departed ‘with the engine bleed air system off’ because switches regulating air pressure had been ‘incorrectly left off following maintenance activity’ on the air conditioning system the night before.

Engineers failed to switch them back on during pre-flight checks, with one telling the Air Accidents Investigation Branch they thought the switches had been returned to the ‘on’ position’.

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

When the aircraft failed to pressurise, the cabin altitude warning ‘ich remained illuminated for 43 minutes’ because the crew did not complete prescribed drills.

A checklist in the Quick Reaction Handbook includes the ‘immediate use of oxygen masks’.

But the pilot continued the climb after turning both switches to on, until a further warning was activated.

At this point, the pilot turned the plane back to Manchester Airport after discussing the matter with TUI’s maintenance control.

Had the plane continued to ascend, it would have passed 14,000ft, at which point the oxygen masks would have automatically dropped.

Below this altitude, ‘the likelihood of loss of consciousness is very small’ without significant pre-existing medical issues.

However, within this range, hypoxia can still affect decision-making abilities.

The report suggests the pilots thought it would have been ‘disproportionate’ to deploy oxygen masks after turning on the switches.

But ‘donning the oxygen masks as an immediate action would have given both pilots immediate protection from any hypoxia risk and allowed them to clarify the situation with the highest risk removed’, investigators said.

Metro has contacted TUI for comment.

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