Tailwind B734 near Sofia on Mar 11th 2018, "we have lost the cabin"

Last Update: December 13, 2018 / 16:01:00 GMT/Zulu time

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Incident Facts

Date of incident
Mar 11, 2018

Classification
Incident

Airline
Tailwind Airlines

Flight number
XC-1845

Departure
Antalya, Turkey

Destination
Zurich, Switzerland

Aircraft Registration
TC-TLC

Aircraft Type
Boeing 737-400

ICAO Type Designator
B734

A Tailwind Airlines Boeing 737-400 on behalf of Corendon Air, registration TC-TLC performing flight XC-1845 from Antalya (Turkey) to Zurich (Switzerland) with 89 passengers and 7 crew, was just in the step climb from FL320 to FL340 about 85nm east of Sofia (Bulgaria) when the crew donned their oxygen masks, initiated an emergency descent and radioed "emergency descent". ATC cleared the flight to FL130 immediately. The crew explained they performed an emergency descent "due to we have lost the cabin", the aircraft reached FL130 about 5 minutes later (average rate of descent 4200 fpm). The aircraft diverted to Sofia where the aircraft, cockpit and cabin together, landed safely on runway 27 about 22 minutes after initiating the emergency descent.

Passengers reported the passenger oxygen masks were suddenly released and they felt pressure and pain on their ears.

A replacement Boeing 737-400 registration TC-TLB positioned to Sofia, continued the flight and reached Zurich with a delay of about 6.5 hours.

Bulgaria's AAIU released their final report concluding the probable cause of the serious incident was:

Failure of the heater of the outflow valve seal and possibly icing of the valve during the flight due to its inability to move. The right safety relief valve remains in open position after its activation.

The AAIU summarized the sequence of events:

At about 80 nm east of Sofia Airport the crew of the aircraft decided to climb to FL 340, starting at a vertical speed of about 1000 ft / min. When the FL 337 was reached, a sound alarm for dangerous cabin altitude was triggered, light indicator for cabin altitude was on and the captain saw, that the cabin differential pressure reaches the maximum at the red limiting line. The indicator for a cabin climb speed was at a 3 hrs position, indicating a climb up speed of 4000 ft/min. The cabin pressure altitude indicator pointer was in position between 4 and 5 hrs and the cabin altitude was about 10000 ... 13000 ft. The crew immediately put oxygen masks, selected the "MAN AC" mode on the cabin pressure control selector and, by means of the switch, tried to put the outflow valve in the closed position. The sound alarm for dangerous cabin pressure altitude was switched off. The attempt to close the outflow valve through the selected MAN AC mode switch didn't made the desired effect and the crew decided to perform emergency descending. This was immediately communicated to Sofia Control and the crew performed the checklist "CABIN ALTITUDE WARNING OR RAPID DEPRESSURIZATION AND EMERGENCY DESCENT". Oxygen masks in the passenger cabin were released.

The aircraft descended rapidly to FL 200 when it reaches a speed of 337 KIAS, where the airplane's pressuritation system started to react, although it still remained outside of normal parameters. The aircraft reached FL 130 and was ILS approach for RWY27 at Sofia Airport. The crew decided to land at Sofia Airport without declaring a forced landing. Passengers were warned about the situation that caused forced descend and deviation to Sofia Airport. The approach and landing at Sofia Airport were normal with a landing mass of 51311 kg, at a maximum permitted landing mass of 54884 kg and a residual fuel at landing of 7992 kg. After the landing the cabin crew informed that during the events related to the depressurization in the front and rear kitchens, there was a sudden appearance of a damp mist and it became cold very fast. The damp mist didn't appear in the middle of the cabin. There was no damp mist or temperature change in the cockpit, although there was a large change in the pressure felt by the pilots in their ears. The cabin crew also informed that the passenger cab was under control and there was no panic. There were no injuries to passengers or crew members. After the landing the aircraft was stopped at a stand and the passengers leaved it by normal exits without use of emergency means. There were no damages to the aircraft.

In the AAIU's analysis the AAIU quotes (in part) an e-mail by the operator and analysed:

"In line with our investigation, during the flight the first thing faulted was the seal heater, and after that the outflow valve was ice-covered in half-open position (not fully closed). The jammed outflow valve could not operate in any mode (automatic, backup and manual). Then, due to overloading of the exhaust valve motors, they also failed. Thus the overpressure in the airplane has increased and the safety relief valves were activated automatically. Due to the high discharge pressure, one of the safety valves (right-hand one) fails and didn't return to closed position."

The Commission has accepted this opinion as probable, assuming that the cause of the serious incident was the failure of the heating first and the subsequent ice-coverage of the outflow valve. The weather conditions during this period of the year, when the occurrence was realized, has been related with increased humidity, favouring the realization of the event.

The cause of the jamming of the safety valve may also be related to the quality of its maintenance, but the Commission has no evidences of any breaches in this respect.
Incident Facts

Date of incident
Mar 11, 2018

Classification
Incident

Airline
Tailwind Airlines

Flight number
XC-1845

Departure
Antalya, Turkey

Destination
Zurich, Switzerland

Aircraft Registration
TC-TLC

Aircraft Type
Boeing 737-400

ICAO Type Designator
B734

This article is published under license from Avherald.com. © of text by Avherald.com.
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