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.
The ATSB accepts the response provided by West Star Aviation as the introduction of Textron Aviation Conquest Service Letter (CQL-99-02) supplemental inspection 52-20-03 (specific inspection details for A522005) post-dates the incident and is applicable to all the life extension aircraft.
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.
On 1 January 2021, the Skippers Aviation contracted aeronautical engineer submitted an engineering order (MEO-0400-003 Rev 0: Emergency exit door - bondline modification) with supporting documentation to the Civil Aviation Safety Authority. The purpose was to seek regulatory authority to approve changes to the emergency exit door as a major modification for Skippers Aviation Cessna 441 aircraft.
The engineering order modified the bonded skin assembly of the emergency exit door with the installation of backup rivets through the bondline. After return-to-service, the modified emergency exit door was subject to additional inspection requirements, which included a detailed visual inspection and ultrasonic inspection.
On 6 January 2021, the Civil Aviation Safety Authority provided their design advice response, which approved the modification, but also indicated that a full review of the analysis used for the structural inspection procedures was required. On 2 June 2021, the Civil Aviation Safety Authority reported the proposed structural inspection procedure was still under review and they had provided feedback to an authorised person (CASR Part 21M) regarding the procedure.
On 21 January 2021, Textron Aviation released Conquest Service Letter (CQL-99-02) for the Model 441 Conquest/Conquest II aircraft 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 outer skin bondline utilising the method of ultrasonic bond inspection. It also included an adhesive bonding procedure to support the field repair of the bond joint if a disbond was found within the prescribed limits for repair.
