A Tui Airways aircraft encountered a serious incident just three days before an unrelated “catastrophic failure” on its approach to Leeds Bradford airport, an official report has found.
The Air Accidents Investigation Branch (AAIB) found that the Boeing 737-8 failed to pressurise after certain engine systems had been “inadvertently” been left off for departure.
The 737, registered G-TAWD, had departed from Manchester airport for Kos at 6.06am on October 17 last year, carrying 187 passengers and six crew.
But the aircraft was forced to return to Manchester after the aircraft failed to pressurise over north Lincolnshire. There were no injuries to passengers or crew.
The AAIB. ‘serious incident’ report concluded: “The aircraft departed with the engine bleed air system off because the switches had been incorrectly left off following maintenance activity and had not been turned on during pre-flight procedures.
“The after-takeoff checklist is designed to trap the latter omission, but the incorrect switch selection went undetected by the crew.”
The AAIB said: “The aircraft failed to pressurise, but the crew did not complete the prescribed drills in response to a cabin altitude warning, which remained illuminated for 43 minutes.”
The crew did not don oxygen masks, the report said.
The report outlined the risk of hypoxia (low levels of oxygen), stating: “As the aircraft did not pressurise, the crew and passengers were exposed to the risk of hypoxia. At cabin altitudes above 10,000ft but below 14,000ft, without the pre-existence of significant medical issues, the likelihood of loss of consciousness is very small.
“However, 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.
“In this range of altitudes it is also difficult to separate the relative contribution of hypoxia versus other performance degraders such as fatigue, distraction or other human performance issues.”
The report noted that if the aircraft had continued to ascend, its passenger oxygen system “would have deployed automatically when the cabin altitude reached 14,000ft”.
At a cabin altitude of 15,800ft, the pressurisation auto fail master caution would have been activated.
The report said: “As progressive exposure to hypoxia increased, the likelihood of the crew taking correct recovery actions would have decreased.”
The AAIB added: “On a previous flight the aircraft’s air conditioning packs had not been operating correctly and a maintenance work order had been generated to investigate the issue.
“Immediately prior to the incident flight, maintenance work had been carried out on the air conditioning packs and noted in the aircraft technical log.”
Three days later, on October 20, the same aircraft veered off the runway while landing amid stormy conditions at Leeds Bradford airport.
A separate inquiry by the AAIB discovered that one of the 737’s nosewheel bearings had “suffered a catastrophic failure” during Storm Babet. The aircraft incurred minor damage in the incident and there were no casualties.
All parties including Tui engaged with the AAIB in its investigations but the operator declined to comment on the report’s findings.