What happened
On December 16, 2016, at approximately 06:19 UTC, a DH8D aircraft departed from Kraków-Balice Airport. During the climb phase, while performing the "AFTER TAKE OFF" procedure, the flight crew identified that the aircraft's pressurization system was not functioning correctly. The crew requested to level off at FL100.
In response to the malfunction, the crew executed the "PRESSURIZATION FAILURE" checklist. Despite these efforts, the pressurization issue persisted. At 06:26 UTC, the crew notified Kraków Approach (EPKK) of their intention to return to the departure airport. The aircraft subsequently performed an ILS approach for runway 25 and landed safely at 06:40 UTC.
The investigation
The investigation focused on determining the source of the pressurization failure. Flight Data Recorder (FDR) analysis showed that the bleed valves were initially in the NORM position and were moved to the MAX position approximately two minutes into level flight at FL100, consistent with the crew's use of the checklist. However, the FDR does not record cabin or pneumatic system pressure parameters.
Technical ground crews performed tests on the pressurization system upon the aircraft's return. These tests were successful, and the aircraft was returned to service following a reset of the cabin pressure controller. The investigation considered three potential failure points: malfunctions in the Pack Valve or Shut-off Valve, failures within the Cabin Pressurization Controller (CPC) or Electronic Control Unit (ECU), or issues with the cargo door seals. The possibility of cargo door seal failure was deemed highly unlikely as no such warning was reported by the captain.
Findings
- The investigation found no recorded "Master Warning" or "Master Caution" alerts on the FDR from takeoff until the return to the airport.
- The most probable cause of the incident was a malfunction of the Cabin Pressurization Controller (CPC).