What happened
During a flight, the aircraft experienced a degradation in cabin pressurisation. This occurred because a defect involving the door seal seat was incorrectly categorized as a non-safety of flight defect (NSOFD). Under existing regulations, minor defects that do not impact flight safety can be deferred for repair, but the damage to the door seal seat was actually an airworthiness issue that required immediate attention.
The investigation
The investigation focused on how the defect was identified and approved for deferral. It was found that the flight crew and a licensed aircraft maintenance engineer applied the deferral process based on the mistaken belief that the damage was limited to internal cosmetic trim. While cosmetic issues are eligible for deferral, the damage to the seal seat was not.
Furthermore, the investigation revealed a second, separate defect involving the door seal seat that was not understood by the incoming flight crew. This second issue was not identified until after the incident had occurred. The investigation noted that the combined impact of both damaged sections of the seal seat likely contributed to the failure of the pressurisation system. The lack of clear communication and the failure to use visual aids, such as photos or video, during the maintenance consultation prevented the engineer from recognizing the true nature of the damage.
Findings
- The primary cause of the error was the misidentification of the cabin door seal seat as simple door trim, which led to the incorrect application of the company's deferral process.
- A second piece of the broken seal seat was misidentified as being part of the previously deferred item.
- The cumulative effect of both damaged sections of the seal seat resulted in the degradation of the aircraft's pressurisation capabilities.
Safety message
To prevent similar occurrences, maintenance engineers must accurately identify defects and their impact on aircraft systems before approving any deferral. Effective shift handovers and defect communications rely on the ability of all parties to clearly exchange information. When ambiguity exists, aircrew and maintenance personnel should use supplementary tools, such as photography, to ensure the defect is accurately communicated and assessed.