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Flybe, Loss of Cabin Pressure

December 8, 2016

A Flybe de Havilland Dash 8-400, registration G-FLBB performing flight BE-455 from Manchester,EN to Jersey,CI (UK) with 23 passengers and 4 crew, was enroute at FL250 about 12nm west of Birmingham,EN (UK) about two minutes into cruise flight when the crew donned their oxygen masks and initiated an emergency descent to FL100 due to the loss of cabin pressure, the passenger oxygen masks did not deploy. The crew decided to continue the flight to Jersey at FL100 and landed safely at their destination about 55 minutes after leaving FL250.

The United Kingdom's AAIB released their bulletin concluding the causes of the occurrence were:

Technical cause

The cause of the pressurisation problem was the outflow valve, serial number 00369, which the evidence suggests had a history of being causal or contributory to pressurisation problems in other aircraft. It also appears that the problem did not manifest itself during post installation functional checks as, shown by its fitment to G-KKEV and G-FLBB. In this situation it is sometimes difficult for engineering staff to reject an item which, when subjected to normal checks detailed in the AMM, meets the requirements for release to service.

Effect on the commander and co-pilot

It is probable that the loss of pressurisation was gradual but maintained just within system limits during the climb to FL250. When the aircraft was established in the cruise at FL250 the cabin pressure altitude continued to climb over a period of two minutes until the warning level was reached. Although by no means debilitating in this case, it shows how quickly the flight deck crew appeared to suffer the early signs of hypoxia.

The AAIB analysed:

Aircraft examination

The aircraft was flown back to Manchester, unpressurised, for a system fault diagnosis which found the outflow valve to have been the cause of the depressurisation. The faulty outflow valve was replaced and the aircraft returned to service.

Component history

The operator’s engineering team researched the history of the faulty outflow valve, serial number 00369, and found that it was originally fitted to G-ECOT at build. It was removed from G-ECOT in April 2015 as part of a pressurisation system fault diagnosis, where no specific faulty component could be identified, but the fault was eventually resolved after multiple component replacements. The same outflow valve was fitted to G-KKEV in September 2015. Whilst fitted to G-KKEV, during a climb at FL200, the crew experienced a sudden cabin altitude rate increase with a momentary fault light which appeared to cure itself without intervention. This was a repeat of a similar occurrence three days before and after diagnosis the outflow valve was replaced. It was then fitted to G-FLBB on 7 December 2016 to cure a problem described as ‘pressurisation erratic in descent’. 

This work was carried out the day before the loss of cabin pressure en route to Jersey. The valve has now been removed from service and quarantined and is the subject of a reliability investigation being carried out by the spares provider and the original equipment manufacturer (OEM).

Cockpit crew actions and observations

The co-pilot was quick to react, understand the situation and take appropriate action.

Concurrently the commander was completing a Technical Log entry and at this point the electronic flight bag (EFB) and its mounting fell off the windscreen. The commander saw the co-pilot was ahead of him in donning his oxygen mask and so instructed him to take control and carry out the emergency descent vital action drills, in accordance with the QRH. The commander discarded the Technical Log and moved the EFB out of the way before donning his oxygen mask. Although all of these actions only took a few seconds, both crew describe feeling slightly lightheaded. In the commander’s own analysis, after the event, he realised that he was having difficulty completing the Technical Log, which was a relatively simple task and therefore considered that he was already slightly hypoxic when the pressurisation warning occurred. He also believes that this affected his performance and slowed his initial reactions to the situation.

Initially the descent was on the autopilot but the co-pilot felt the rate of descent was too low and disengaged the autopilot and manually increased the rate of descent from 2,000 fpm to 3,500 fpm. With hindsight the co-pilot felt that he should not have deselected the autopilot during the emergency descent. The EFB falling from the windscreen and the oxygen mask microphone difficulties added to the already heightened workload. With hindsight the commander considered all of this to have influenced the remainder of the flight and the final approach into Jersey which was “not up to the usual standard”. After landing the crew realised the significant effects that hypoxia had had on their performance.

Cabin crew actions

During the event the senior cabin crew (SCC) member and cabin attendant felt the aircraft suddenly adopt a descent profile and saw the seat belt signs illuminate. Although they noticed their “ears popping” they did not associate this with a depressurisation and did not experience symptoms of hypoxia. Initially they were unable to contact the cockpit crew but realised there was a problem and secured the cabin anyway. The cabin crew were unaware of the difficulties the commander was having with his microphone in the early stages of the incident. Communication was eventually established as the aircraft descended through FL150. The cabin crew actions were taken without knowledge of the problem but good crew resource management (CRM) and training meant that cabin and passenger safety was maintained.

Technical Log

The commander had noted the Technical Log entry regarding the pressurisation problem on 7 December 2016. The SCC member was also aware of the problem on this aircraft as she had flown in it over the previous two days. With hindsight the commander felt that had time allowed he would have liked to brief his crew on the potential outcomes in the light of the Technical Log entries. 

 

Source: http://www.avherald.com/h?article=4aa19de4&opt=0

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