What happened
On January 13, 2017, an ATR72 departed from Cologne Bonn Airport (EDDK) and began a climb toward flight level FL160, heading for the DIK waypoint. While the aircraft was passing through FL140, the crew received a "Master Warning" alert indicating an excess cabin altitude (EXCES CAB ALT).
Upon identifying the loss of cabin pressure, the crew interrupted the climb and descended the aircraft to FL100. The crew performed the necessary procedures as outlined in the Quick Reference Handbook (QRH). Because the pressurization issue could not be resolved during flight, the crew decided to return to the departure airport, EDDK.
The investigation
The investigation focused on the mechanical state of the aircraft's pressurization system and the actions taken by the flight crew during the climb. The investigation established that the loss of pressure was not due to a mechanical failure of the aircraft's hardware, but rather a configuration error.
Findings
- The primary cause of the incident was a crew error.
- The crew left the overboard valve (OVBD Valve), which regulates the pressure difference between the aircraft interior and the external environment, in the "FULL OPEN" position.
- The failure to properly set this valve prevented the aircraft from maintaining the required cabin altitude during the climb.