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22nd of July 2020

A Cessna Aircraft Company 441 was scheduled to operate a charter flight with six passengers from Broom Airport in Western Australia to Browns Range Airport. The two-man crew planned the flight to be conducted at a cruise flight level of 270. The estimated flight time was 84 minutes. The aircraft lifted off from the runway at Broome airport at approximately 09.04 lt (local time) and set course for its destination while climbing to FL270.

The disbanded emergency exit door (Source and © ATSB)

28 minutes after departure the aircraft reached its cruise altitude, before the crew could select the required cruise settings a loud noise was heard from the passenger cabin. The Pilot in Command (PIC) initially thought the emergency pressurisation valve had activated (located near the right emergency exit. While checking for the illumination of the warning light for the emergency pressurisation valve the cabin altitude warning light and the corresponding aural warning activated. The cabin altitude was approaching the cruise flight level, indicating a (rapid) decompression of the cabin. Both pilots donned their respective oxygen masks and an emergency descent was initiated. At this time the aircraft was ~108 miles from Broome Airport. The Pilot Monitoring (PM) retrieved the emergency checklist and informed ATC, requesting and were cleared to descent to FL120. 4 minutes later the aircraft levelled off at FL120.

During the descent, the crew confirmed that all passenger were wearing their oxygen mask. After consulting the company the crew was advised to continue to their destination at a lower altitude than planned, with the cabin unpressurised. A subsequent fuel quantity check showed they had sufficient fuel to reach their destination at the new cruise level FL100.

Location of the emergency exit door (Source & © ATSB)

During the remainder of the flight there is regular contact between the pilots of the Cessna and their homebase, with both the Chief Pilot as with the maintenance department, regarding the operation of the aircraft with a (Suspected) defective pressurisation system in accordance with the MEL.

{An approved MEL is a document that allows for the operation of a specific aircraft under specific conditions with a particular item(s) of equipment inoperative at the time of dispatch for the intended flight. Despite the inoperative equipment, the aircraft still complies with its type design standards. This requirement enables the PIC to determine whether a flight may be commenced or continued from any intermediate stop should any instrument, equipment or system become inoperative.}

At about 1046, the aircraft landed at Browns Range Airport without further incident. The passengers disembarked and the PIC applied the MEL item for the pressurisation system. During their walk-around of the aircraft, both pilots noticed the lower aft section of the emergency exit door skin had separated (disbanded)

The aircraft was flown back unpressurised.

Maintenance manual figure for the inspection of the emergency exit door (Source & © ATSB)

An investigation was launched by the ATSB (Australian Transport Safety Bureau) and in their final report (available by clicking here) they recorded the following finding;

"The operator’s aeronautical engineer performed a structural failure investigation, which was reviewed by the ATSB and Textron Aviation (the type certificate holder for the Cessna 441). It was concluded that the emergency exit door progressively disbonded between the inner and outer skin, likely due to the age of the aircraft, facilitated by a combination of corrosion, moisture, and flight cycles. This weakened the structure, which resulted in an accelerated failure of the bondline and rapid depressurisation of the cabin when the aircraft reached flight level 270. In addition, it was found that the decision to apply the minimum equipment list item for the pressurisation system to depart for the return flight precluded an independent assessment of the condition of the aircraft that would have been required under the special flight permit process."

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