CSA A319 near Bourgas on Sep 19th 2014, loss of cabin pressure

Last Update: June 17, 2015 / 16:20:43 GMT/Zulu time

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Photo of CSA Czech Airlines OK-NEP, Airbus A319 — CSA Czech Airlines OK-NEP, Airbus A319 (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Sep 19, 2014

Classification
Report

Cause
Loss of cabin pressure

Airline
CSA Czech Airlines

Flight number
OK-930

Departure
Prague, Czech Republic

Destination
Yerevan, Armenia

Aircraft Registration
OK-NEP

Aircraft Type
Airbus A319

ICAO Type Designator
A319

A CSA Czech Airlines Airbus A319-100, registration OK-NEP performing flight OK-930 from Prague (Czech Republic) to Yerevan (Armenia) with 94 passengers and 5 crew, was enroute at FL370 about 40nm eastnortheast of Varna (Bulgaria) over the Black Sea when the crew noticed the cabin pressure was suddenly dropping rapidly and the cabin altitude climbed through 10,000 feet. The crew donned their oxygen masks, declared Mayday and initiated an emergency descent, in response air traffic control instructed the crew to level off at FL330 due to conflicting traffic at FL310. The crew stowed the spoilers already extended for the emergency descend and reduced the rate of descent while turning 30 degrees to the left, about 30 seconds later the crew received clearance for further descent and while descending through FL280 extended the spoilers again descending to FL100 at an average rate of descent of now 4500 fpm. Cabin altitude topped at about 16300 feet, the passenger oxygen masks were released. Sofia Center, concerned that due to the low altitude contact with the aircraft might be lost instructed the crew to contact Ankara Center (Turkey) in case of loss of communication. After levelling off at FL100 the crew advised they would divert to Bourgas (Bulgaria), where the aircraft landed safely about one hour after leaving FL370.

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
- de-icing
- condensation as result of different temperatures at the inside and outside of the pressure bulkhead while flying at high altitude