The Czech Air Accidents Investigation Institute (ÚZPLN) reported that a foreign fluid in one or both cabin pressurisation safety valves caused a sudden loss of cabin pressure on an Airbus A319.
The Airbus A319, operated by Czech Airlines, took off from Prague, Czech Republic at 21:21 UTC (23:21 LT), bound for Yerevan, Armenia. On board were 94 passengers and five crew members.
While en route at FL370 at 22:56 (01:56 LT, September 20), approximately half way through the flight, the crew registered a sudden significant cabin pressurization change and CABIN V/S increase without any prior warning. Shortly after that, the ECAM – CAB PRESS EXCESS CAB ALT warning came.
The system display showed an opened Safety Valve. The pilots put on their oxygen masks, declared a MAY DAY and initiated an emergency descent to FL100.
At 22:57 hours, the Sofia Radar ATCo requested the crew to terminate their descent at FL330 due to conflicting traffic at FL310 approaching the A319 from the left. Due to this traffic, the pilot flying changed the heading by 30° to the left and reduced the rate of descent. After about 30 seconds, the flight was cleared to continue the descent.
The subsequent descent was standard at a speed of 335 kt and average vertical speed of – 4,500 ft/min. During the descent, the crew registered cabin altitude of 16,800 ft. FL140 was passed at 23:03 hours UTC. Three minutes later the crew reported reaching FL100. At this level the pressurization system operated normally.
The crew then contacted Sofia Radar, stating that they wanted to divert to Bourgas, Bulgaria. The flight was vectored for an approach and safe landing at Bourgas at 23:53 UTC (02:53 LT).
No further assistance was required and no emergency was declared for the approach and landing phase.
The cause of the serious incident was the spontaneous opening of one SV or both SVs without any prior indication. It is highly probable that this was caused by the presence of not fully specified fluid and its freezing in the SV control section.
Upon the SV opening, the aircraft cabin pressure suddenly and significantly changed. None of the considered options of the fluid getting inside the SV was demonstrably clarified.
- Final report (PDF)