Created Wednesday, Jan 3rd 2024 22:55Z, last updated Monday, Nov 4th 2024 16:58Z
A SkyUp Boeing 737-800 on behalf of FlyEgypt, registration UR-SQP performing charter flight FT-8901 from Tallinn (Estonia) to Hurghada (Egypt) with 186 passengers and 7 crew, was enroute at FL360 about 220nm northnortheast of Warsaw (Poland) in Lithuanian Airspace when the crew declared Mayday reporting the loss of cabin pressure. The passenger oxygen masks were released. The aircraft descended to FL140, about 25 minutes later climbed to FL160 and diverted to Warsaw. The crew reported there were no medical issues on board and no assistance was needed for landing, the crew requested to enter a hold. The aircraft landed safely on Warsaw's runway 11 about 80 minutes after leaving FL360.
A replacement Boeing 737-800 registration UR-SQB resumed the flight after about 15 hours and reached Hurghada with a delay of about 16 hours.
The occurrence aircraft was still on the ground in Warsaw about 19 hours after landing.
Polands PKBWL delegated the investigation to Ukraine's NBAAI, who rated the occurrence a serious incident and opened an investigation. The NBAAI reported the aircraft was at FL360 when a depressurization occurred, the passenger oxygen masks were automatically released. The crew performed an emergency descent to FL140 and subsequently landed in Warsaw. There were no injuries.
On Feb 5th 2024 Ukraine's NBAAI released their preliminary report summarizing the sequence of events:
During climb and crossing the FL335, a warning for automatic pressure control system failure was triggered. The crew performed an emergency checklist and switched to manual control of the pressure control system. After reaching FL 360, the pressure control system was recovered in manual mode, however it was not functioning stable. The crew requested Riga FIR dispatcher for the permission to descend to FL 320. To coordinate the descend permission, Riga FIR dispatcher transferred the crew to the frequency of the Vilnius FIR sector. At the same time, the warning for exceeding altitude of 10,000 feet was triggered. The crew performed an emergency checklist, filed a distress signal, and reported on emergency descent to FL140. Due to the loss of pressure, the oxygen masks were automatically activated in cabin. During descend, the crew managed to recover control of the pressure system in manual mode and reduce the altitude in the cockpit to less than 10,000 feet. PIC decided to divert to the alternate aerodrome Warsaw (Poland). After burning off fuel at the holding area, in order to reduce maximum landing mass, the crew made a safe landing at the Warsaw airfield. As a result of the serious incident, no one was injured.
On Nov 4th 2024 Ukraine's NBAAI released their final report in Ukrainian only (Editorial note: to serve the purpose of global prevention of the repeat of causes leading to an occurrence an additional timely release of all occurrence reports in the only world spanning aviation language English would be necessary, an Ukrainian only release does not achieve this purpose as set by ICAO annex 13 and just forces many aviators to waste much more time and effort each in trying to understand the circumstances leading to the occurrence. Aviators operating internationally are required to read/speak English besides their local language, investigators need to be able to read/write/speak English to communicate with their counterparts all around the globe).
The report concludes the probable causes of the serious incident were:
The loss of cabin pressure was caused by the failure of the rear outflow valve, that was caused by water from the aircraft's water system freezing.
The water ingress into the outflow valve was caused by the failure of a coupling of water lines in winter operation probably due to non-compliance with Boeing's Service Advisory "SB 737-38-1057".
Contributing was the crew's untimely execution of the QRH procedures upon occurrence of the cabin altitude warning.
SB 737-38-1057 had been triggered by similiar occurrences in which the 4-way coupling failed due to frozen water.
