What happened
On 8 January 2007, a Blue1 scheduled flight, KF 201, operating an MD-90-30 from Helsinki to Oulu, experienced a serious pressurization failure while cruising at flight level 350. The incident began when the aircraft's cabin pressure altitude started to rise. The flight crew initially noticed an amber FLOW light, indicating insufficient pressurization capability, as the cabin pressure altitude reached 10,000 feet.
In response, the pilots requested a descent to flight level 210. Shortly thereafter, a red CABIN ALT warning light activated, though it subsequently extinguished due to a malfunction in the pressure sensor. As the cabin pressure altitude continued to climb, reaching 14,000 feet, the passenger oxygen masks deployed. The cabin crew and passengers utilized the masks throughout the descent until the aircraft reached flight level 100. The flight crew also used oxygen masks for portions of the descent. The aircraft eventually completed a normal approach and landing in Oulu.
The investigation
The investigation focused on the failure of the air conditioning and pressurization systems. Investigators examined the state of the air conditioning components and the functionality of the temperature control valves. The probe established that the automatic temperature control for the cabin air conditioning system was inoperative, necessitating the use of manual mode for both the cockpit and cabin systems.
Technical analysis revealed faults in the right temperature control valve, the 35°F water separator control valve, and the cabin altitude warning switch. Furthermore, the investigation looked into the effectiveness of the water separators and the crew's adherence to emergency checklists during the depressurization event.