SAS B736 near Umea on Apr 4th 2012, loss of cabin pressure
Last Update: March 29, 2013 / 19:37:15 GMT/Zulu time
The airline commented that the passengers most likely would not have noticed the drop of pressure due to the swift action of the crew. Following the descent the crew explained to the passengers what had happened.
Passengers said they became aware of the diversion upon announcement by the crew contradicting media reports in Sweden, that oxygen masks came down.
Sweden's Statens Haverikommission opened an investigation.
On Apr 25th 2012 the NTSB reported the left bleed system failed while climbing through FL370, the climb was continued with only the right hand bleed system available. While at FL410 the right bleed system failed too, an emergency descent was initiated and the passenger oxygen masks were deployed.
Sweden's Haverikommission (SAIB) released their final report concluding the probable cause of the incident was:
The incident was caused by the remaining cabin pressurisation system not be-ing able to pressurise the cabin at the altitude where the airplane operated.
The Haverikommission reported that an aircraft, that has been dispatched with one air conditioning system inoperative, is limited to FL250. However, if the aircraft departs with both air conditioning systems operative and one air conditioning system fails in flight, no restriction applies.
The aircraft had been dispatched with both air conditioning system working properly. When the aircraft climbed through FL370 a "Bleed Trip Off" message for the left hand air conditioning system activated. The crew continued the climb to FL410 and reset the indication. About a minute later the message illuminated again, the crew did not reset the indication but shut the system down in accordance with the flight crew operating manual. The crew continued the climb to FL410 discussing the possibility to need to conduct an emergency descent should the right hand air conditioning system fail, too. Soon after levelling at FL410 a right hand air conditioning system "Bleed Trip Off" message occurred. The crew declared emergency and descended the aircraft to FL100, the passenger oxygen masks were manually released. During the descent through 14,000 feet the cabin altitude reached 14,000 feet. After levelling at FL100 the crew decided to divert to Umea.
Maintenance identified the left bleed air high stage valve (HSV, at stage 9 of the engine compressor) was half open although it should have been closed. The shaft of the precooler control valve (PCCV) showed signs of extensive wear. Both units were replaced. During examination of the right hand engine maintenance determined that both HSV and PCCV needed replacement as well, in addition the damper for the PCCV was rotating freely and needed to be replaced too. The right hand high stage regulating valve (HSR) needed adjustment.
The Haverikommission issued the safety recommendations to apply the altitude restriction to FL250 like with the minimum equipment list, if one of two operaring air conditioning systems fails in flight.
This article is published under license from Avherald.com. © of text by Avherald.com.
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