CSA A319 near Bourgas on Sep 19th 2014, loss of cabin pressure
Last Update: June 17, 2015 / 16:20:43 GMT/Zulu time
Czech Republic's Institute for Air Accident Investigation (UZPLN) released their final report in Czech concluding the probable cause of the serious incident was:
The opening of one or both cabin pressure safety valves without prior indication.
This was likely due to the presence of some undetermined liquid and its freezing in the controls of the safety valve. The opening of the valve resulted in a sudden and significant drop of cabin pressure.
The investigation could not clearly explain how it was possible for liquid to be present at the safety valve.
The investigation did not release any safety recommendations.
The UZPLN reported that immediately after landing the outflow valves and the safety valves were examined, the inspectors could not find any anomaly at/in those valves and the surroundings. The aircraft was released to ferry to Prague for further examination on Sep 21st 2014.
After arrival in Prague both outflow valves and safety valves were replaced and sent to the manufacturers for further examination. In addition, all passenger oxygen generators were replaced.
Examination of the valves at the manufacturers did not find any anomaly with either of the valves, both outflow and safety valves operated normally and to specifications.
The safety valves were disassembled and traces of liquid, most likely water, were found in the pneumatic control of the safety valves. However, the seals were not damaged, there were no cracks in the housing and the hermetic protection of the valve control had not been breached. The manufacturer argued that the only way then to get water ingress the interior of the valve control would be through the static pressure ports of the valves, that were mounted inside the cabin bulkhead, part of the cabin heating and air conditioning (sensing cabin pressure) as well as the outside of the bulkhead in the non-pressurized aft technical compartment sensing outside ambient pressure.
The UZPLN analysed that the crew acted in accordance with operating procedures including the interruption of the emergency descent for about 30 seconds due to conflicting traffic. Overall, it took 8 minutes to descend from FL370 to FL100 (average rate of descent 3375 fpm).
The UZPLN analysed that the aircraft was enroute at 37,011 feet, the left hand cabin pressure controller CPC1 was active, the outflow valve was 7% open and the cabin altitude variation was 0 feet/min. Then one or both safety valves opened, evidenced by the flight data recording recording a sudden decrease in cabin pressure and the master warning illuminating. At the same time a cabin altitude warning activated indicating the cabin altitude had climbed through 9550 feet. About one minute later the crew received indication that the cabin safety valve had not been closed over a period of one minute, the outflow valve indicated normally at 7% open. After levelling off at FL100 the cabin pressurization system resumed normal operation.
The UZPLN listed that following factors could contribute to the contamination of the safety valve control with water:
- maintenance work in the aft technical compartment
- washing of the outside of the aircraft
- condensation as result of different temperatures at the inside and outside of the pressure bulkhead while flying at high altitude
This article is published under license from Avherald.com. © of text by Avherald.com.
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