Aviation safety investigations & reports

Door failure and depressurisation involving a Cessna 441, VH-LBY near Broome, Western Australia, on 22 July 2020

Investigation number:
AO-2020-036
Status: Completed
Investigation completed
Phase: Final report: Dissemination Read more information on this investigation phase

Final

Download Final report
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What happened

On 22 July 2020, a Skippers Aviation Cessna Aircraft Company 441 departed on a charter flight from Broome Airport, Western Australia, for Browns Range Airport, with two pilots and six passengers on board. Shortly after reaching the planned cruising level of flight level 270, a rapid depressurisation of the cabin occurred. The pilots and passengers donned their oxygen masks. The pilots performed an emergency descent and landed at Browns Range without further incident. After landing at Browns Range, they noticed the lower rear corner of the emergency exit door outer skin had separated from the inner skin and was the likely source of the depressurisation.

What the ATSB found

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.

As a result of this incident and the subsequent discovery of two further disbonded doors, all for aircraft operating under a supplementary type certificate life extension program, it was concluded that the existing visual inspection procedures for the emergency exit door bondline were inadequate.

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.

What has been done as a result

On 6 August 2020, the Civil Aviation Safety Authority issued airworthiness bulletin 52-004 issue 1: Cessna 441 in Flight Depressurisation due to Emergency Exit door bonded skin failure. The bulletin provided an interim detailed visual inspection procedure for the emergency exit door bondline, based on a scheme provided by the operator’s aeronautical engineer.

On 1 January 2021, the operator’s aeronautical engineer submitted an engineering order with supporting documentation to the Civil Aviation Safety Authority for a major modification to their Cessna 441 aircraft. This included the installation of rivets through the bondline and subjected the door to additional inspection requirements.

On 21 January 2021, Textron Aviation released Conquest Service Letter (CQL-99-02) for the Model 441 Conquest/Conquest II maintenance manual. This introduced inspection A522005 - Emergency exit door bond inspection - with an interval of 2,000 hours or 4 years, whichever occurred first. The purpose of the inspection was to verify the integrity of the emergency exit door bondline utilising the method of ultrasonic bond inspection.

The ATSB identified a safety issue for the inspection procedures in the West Star Aviation maintenance manual supplement for the Cessna 441 aircraft life extension program as inadequate for detecting the progressive disbonding of the emergency exit door skin. West Star Aviation considered the proactive safety action taken by Textron Aviation was sufficient to detect defects with the emergency exit door fitted to Cessna 441 aircraft, as this inspection procedure will apply to aircraft on the life extension program. The ATSB has accepted the response from West Star Aviation as closing the safety issue.

The operator enrolled their Broome base pilots in additional human factors training, to capture the elements of communication, decision-making, leadership and threat and error management. In addition, the chief pilot scheduled a revision session in the understanding of the minimum equipment list and its application, for the next visit to Broome.  

Safety message

After landing at Browns Range and discovering the disbonded door skin, the pilots elected to continue with the return flight with the pressurisation system inoperative, which was consistent with the initial advice they received from their company. However, whenever new information about an abnormal situation becomes available, decisions may need to be re-evaluated, as the initial reasoning may no longer be valid. In addition, for the scenario of a structural failure, the special flight permit process should be followed to manage the potential risks associated with a damaged aircraft.

Download Final report
[Download  PDF: 1.14MB]
 
 
 

The occurrence

Context

Safety analysis

Findings

Safety Issues and actions

Sources and submissions

Safety Issue

Go to AO-2020-036-SI-01 -

Maintenance manual supplement

The inspection procedures in the West Star Aviation maintenance manual supplement for extended life program Cessna 441 aircraft were inadequate to detect the progressive disbonding of the emergency exit door.

Safety issue details
Issue number: AO-2020-036-SI-01
Status: Closed – Adequately addressed
General details
Date: 22 July 2020   Investigation status: Completed  
Time: 0933 WST   Investigation level: Defined - click for an explanation of investigation levels  
Location   (show map): near Broome   Investigation phase: Final report: Dissemination  
State: Western Australia   Occurrence type: Air/pressurisation  
Release date: 30 June 2021   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer Cessna Aircraft Company  
Aircraft model 441  
Aircraft registration VH-LBY  
Serial number 4410023  
Operator Skippers Aviation Pty Ltd  
Type of operation Charter  
Sector Turboprop  
Damage to aircraft Nil  
Departure point Broome, Western Australia  
Destination Browns Range, Western Australia  
Last update 30 June 2021