What happened
On September 19, 2014, an Airbus A319-112, registration OK-NEP, operating a commercial flight from Prague to Yerevan, experienced a severe depressurization event while cruising at FL370 over the Black Sea. At approximately 22:56 UTC, the crew noted a sudden and significant increase in cabin vertical speed, estimated at +10,000 ft/min, without any prior warning. This was immediately followed by an ECAM alert for excessive cabin altitude.
In response to the emergency, the crew donned oxygen masks and initiated an emergency descent. During the descent, air traffic control requested a temporary level-off at FL333 to avoid conflicting traffic, which briefly delayed the descent by approximately 30 seconds. The aircraft eventually reached FL100, where the pressurization system stabilized. The flight diverted to Burgas Airport (LBBG), where the crew requested only technical assistance rather than medical aid. The aircraft landed safely at 23:53 UTC.
In the cabin, flight attendants reported an intense pressure change in the ears and the deployment of oxygen masks. The activation of chemical oxygen generators caused a noticeable odor, which, combined with the sudden descent and the ongoing regional conflict in Ukraine, caused significant stress among the passengers. Most passengers spoke only Russian, complicating communication with the crew.
The investigation
The ÚZPLN investigation focused on the technical cause of the pressure loss. Investigators examined the aircraft's flight data recorder (DFDR), cockpit voice recorder (CVR), and the aircraft condition monitoring system (ACMS). Post-flight inspections of the safety valves (SV) and the outflow valve (OFV) were conducted by technicians.
Upon disassembly of the safety valves, investigators found a small amount of liquid, likely water, within the pneumatic regulation section. The manufacturer confirmed that this liquid could have entered the control part of the valves through the static pressure ports from the non-hermetic rear technical compartment. The investigation also reviewed recent maintenance activities, including a heavy maintenance check performed shortly before the incident.
Findings
- The primary cause of the incident was the unintended opening of one or both safety valves without prior warning.
- This malfunction was most likely caused by the presence of an unspecified liquid and its subsequent freezing within the control mechanism of the safety valves.
- The source of the liquid—whether through external contamination (such as aircraft washing or de-icing) or condensation of atmospheric moisture—could not be definitively determined.
- The crew performed all emergency procedures in accordance with the operator's manual.
- There were no injuries to the crew or the 94 passengers on board.