What happened
On 4 April 2012, a Boeing 737-600, registration LN-RPS, was performing a scheduled commercial flight from Stockholm/Arlanda to Skelle . While climbing through Flight Level 430, the crew received a "Bleed Trip Off" warning for the left engine. Following established procedures, the crew managed the initial failure and continued the climb to Flight Level 410.
Shortly after reaching the target altitude, the right-side bleed system also triggered a "Bleed Trip Off" warning. With both pressurization systems non-operational, the aircraft could no longer maintain the required cabin altitude. The crew declared an emergency and initiated a rapid descent to Flight Level 100. During the descent, the cabin altitude rose to 14,000 feet, triggering an audible warning and prompting the commander to manually deploy oxygen masks in the cabin.
The aircraft eventually diverted to Umeå Airport after finding the initial intended landing site at Sundsvall closed. The landing was completed normally, and the emergency was concluded after the crew confirmed the safety of the passengers.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the mechanical state of the engine bleed air systems. Technicians performing fault isolation discovered that the High Stage Valve (HSV) on the left engine was stuck in a half-open position and showed significant wear. Additionally, the Precooler Control Valve (PCCV) shaft exhibited extensive wear on both engines. On the right engine, the high-pressure bleed air regulator also required adjustment.
Findings
- The primary cause of the incident was the inability of the single remaining pressurization system to maintain cabin pressure at the aircraft's operating altitude.
- Mechanical failure of the High Stage Valve (HSV) on the left engine, which was found in a partially open position.
- Extensive wear was observed on the control valve shafts and links, a level of degradation not previously documented for components with similar operating hours.
- The failure of both systems in rapid succession left the crew with insufficient capacity to maintain a safe cabin altitude at high flight levels.