What happened
On February 22, 2007, a Boeing 757-200, registration OH-AFJ, operated by Air Finland Ltd, departed Helsinki-Vantaa Airport (EFHK) for Abu Dhabi (OMAA) as a charter flight. During the takeoff roll, the crew received an "R ENG HI STAGE" warning on the Engine Indicating and Crew Alerting System (EICAS). Following the Quick Reference Handbook (QRH) procedures, the pilots shut off the right-side bleed air.
After takeoff, the flight crew consulted with the company's operations department, which advised continuing to the destination as the aircraft remained within operational limits. However, approximately ten minutes into the climb, while cruising at flight level 330 in Estonian airspace, a "CABIN ALT" warning was triggered. The crew donned oxygen masks and transitioned to level flight. In an attempt to restore cabin pressure, the pilots manually re-engaged the right-side bleed air, a decision made outside of standard QRH procedures. While this action initially stabilized the pressure, the crew eventually initiated a descent to flight level 100 and turned back toward Helsinki.
During the descent, at flight level 250, the right-side bleed air was shut off again. The aircraft landed at Helsinki-Vantaa at 15:51 UTC. There were no injuries to the 187 passengers or the 8 crew members, and no damage was sustained by the aircraft.
The investigation
The investigation focused on the mechanical failures within the aircraft's pneumatic and air conditioning systems. Technical inspections following the event revealed that the right-side high-pressure bleed air valve (high stage mod valve) had failed, causing an overpressure condition. This failure necessitated the initial shutdown of the right-side bleed air as per the QRH.
Further examination of the left-side air conditioning system identified a faulty flow control and shut-off valve. This component failure prevented sufficient airflow from entering the cabin, leading to the drop in pressure that triggered the cabin altitude warning. Maintenance records showed that a manufacturer-recommended service letter inspection (757-SL-21-055) had been performed on the aircraft in April 2006.
Findings
- The primary cause of the depressurization was a faulty flow control and shut-off valve in the left-side air conditioning system.
- A contributing factor was the failure of the right-side high-pressure bleed air valve.
- The crew's decision to re-engage the right-side bleed air was an attempt to maintain pressurization, though it was not a prescribed QRH procedure.