What happened
On 10 July 2015, a Boeing 737-36E operated by Bluebird Cargo was performing an international cargo flight departing from the Airport of Liège (ECLG). During the initial takeoff roll, the first officer noted a slight discrepancy in N1 engine readings. Shortly after lift-off, the crew noticed that the cabin was not pressurizing, accompanied by physical pressure changes in the pilots' ears. Although the pressurization panel settings were verified as correct, the aircraft struggled to maintain a stable cabin altitude.
As the flight progressed through FL050, an auto-fail light illuminated. In an attempt to stabilize the environment, the crew transitioned the pressurization mode selector to standby and adjusted the cabin altitude setting to 6400ft, following the aircraft's placard instructions. While the crew was performing the Quick Reference Handbook (QRH) checklist, the cabin altitude appeared to stabilize briefly near FL100, but subsequent pressure changes occurred. Due to fuel constraints and the inability to reach the planned flight level, the captain decided to limit the climb rate and prepare for a return to Liège.
Upon reaching FL140, the cabin altitude warning horn activated, signaling a significant loss of pressure. The crew immediately donned oxygen masks and initiated an emergency descent to FL100. The flight crew declared an emergency and requested vectors back to the departure airport, eventually performing a normal landing on runway 23L at Liège.
The investigation
The AAIU(Be) examined the operational and engineering data surrounding the event. The investigation focused on the sequence of the gradual loss of cabin pressure and the mechanical or system failures that led to the activation of the warning horn during the climb.
Findings
- The flight experienced a gradual loss of pressurization during the climb phase.
- The crew followed established procedures, including the use of the QRH and transitioning to standby mode, to manage the cabin altitude.