What happened
On 20 October 2010, a Boeing 737-600, registration LN-RRR, was operating a scheduled service from Malmö/Sturup to Stockholm/Arlanda. At the time of the incident, the aircraft was operating under the Minimum Equipment List (MEL), which meant only one of the two available air conditioning systems was functional. This configuration limited the maximum allowable flight altitude to 25,000 feet.
While climbing to flight level 220 (approximately 6,700 meters), the crew received a cabin altitude warning. Following established emergency procedures, the pilots initiated an emergency descent and utilized oxygen masks. The aircraft was successfully stabilized at an altitude of 3,300 meters and proceeded to land normally at Stockholm/Arlanda. There were no injuries reported among the crew or the 69 passengers on board.
The investigation
SHK investigators examined the aircraft's engine air bleed system and the air conditioning units. Physical inspections of the hardware revealed several mechanical and electrical discrepancies. Technicians discovered that the High Stage Valve (HSV) was stuck in a half-open position and the Pressure Regulating Shutoff Valve (PRSOV) was experiencing stiff movement. Both components were subsequently replaced.
Further testing identified that the bleed air regulator was not performing as intended. Additionally, the investigation found that the overhead control panel (bleed module) was providing inaccurate feedback; the switch for the second system indicated it was closed when the valve was actually open. The crew's oxygen equipment and the cabin's oxygen generators were also replaced as a precaution.