14 Dec 2018 - TC-TLC 737-400 depressurization and emergency descent of 23 Mar 2018
On 23 March 2018, B737-400 aircraft, operated by TAILWIND AIRLINES, TC-TLC registration marks, was conducting an Antalya-Zurich flight with 89 passengers and 7-member crew on board. At overflying the territory of the Republic of Bulgaria, at an altitude of 33700 ft, the cabin was depressurized. The crew put on oxygen masks and released oxygen masks in the passenger compartment. The crew undertook an emergency descent and performed a forced landing at Sofia Airport at 10:08 UTC. The landing was performed without any problems for the passengers, crew and aircraft. The passengers were transferred to another airplane to continue the flight to the final destination.
The Safety Investigation Commission indicated as a reason for realization of the serious incident: 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.
*** Updated 23 Nov 2020 ***
The Bulgarian MTITC released their Final Report on 13 Dec 2018
Flight preparation and description of the flight
Preparation and description of the flight was made on the basis of explanations given by the captain and crew members after landing at Sofia Airport and flight-related documents. Before the flight all crew briefings, equipment and documents checks were performed. The flight was performed during the day and weather was clear throughout the route. 89 passengers and 7 crew members were on board. The take-off from Antalya Airport was on time at 08:34. The aircraft climbed to FL 320 and kept this FL along the route. A check list for cruise speed flight was completed and it was established that the cabin parameters were stabilized. The flight continued normally as the airplane crossed the Bulgarian border at a reporting point VADEN towards reporting point UTEKA. 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. The flight trajectory of the airplane over the Republic of Bulgaria is shown on Fig. 12 of Enclosure 1. 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.
Damage to Aircraft
During the inspection of the aircraft after realization of air occurrence no damages on the airframe: The external condition of the airplane is visible from the photos shown in Enclosure 1. All oxygen masks in the passenger cabin were activated, Fig. 9 and 10 of Annex 1. A QAR memory card with flight parameters was downloaded. An inspection of the cabin pressure control system performed by CAMO of AO has revealed a failure of the outflow valve gasket heating, failure of the outflow valve, and jamming of the right safety relief valve in open position.
Flight data recorders
B737-400 aircraft, TC-TLC registration marks, took-off from Antalya at 08:34 (UTC). At 08:50 ALT=32 000 ft was reached, and the airplane continues the flight at this level. There were no deviations in system operation and piloting during take-off and climbing. At 09:44:05 in heading of 295° and speed CAS=278 kt A/P switched from “VNAV Path” mode to “VNAV Speed” mode. At the same time A/T switched on N1 mode and started climb from FL=320. At 09:45:00 at ALT=32768 ft and CAS=276 kt A/P changed to “Vertical Speed”, A/T to “MCP Speed”. At 09:45:57 at ALT=33696 ft and CAS=268 kt appearance of “Cabin altitude above 10000 ft” signal was registered. At 09:46:19 at ALT= 34000 ft. A/P switched sequentially through “Altitude acquire” mode for 5 seconds and “Altitude hold” mode for 8 seconds. At 9:46:32, A/P was switched to “Flight level change” и “MCP Speed”. A second later Cabin Altitude Above 10000 ft signal disappeared, and the total duration of the signal was 36 seconds. At 09:46:42 A/P was switched to “Vertical Speed” and the aircraft started to descend. The A/P and A/T modes are listed in the table below during descend to 8500 ft and when they were switched (time, altitude, speed). During descend from FL 340 to ALT=8500ft the maximum reached speed was CAS=337kt, and the maximum vertical speed was Vy= 6000ft/min. between FL300 and FL240. At 09:54:53 at ALT=10996 ft, the "Cabin Altitude Above 10000ft" signal appears again, remaining for 9 minutes and 4 seconds until 10:03:56, when the aircraft reached ALT=5956 ft at a heading of 262° and the flaps were extended to 5 units. At 10:04:25 the landing heading of 270° at ALT=5400ft was taken. At 10:06:50 the aircraft was in landing configuration with extended landing gear and flaps at 30 units at ALT=3000 ft. The aircraft landed at 10:08:34 AM on RWY27 at a speed CAS=140 and g-load of Ny=1.25g. The aircraft mass at touch-down was GW=51311 kg.
When FL 337 was reached, the horn for dangerous cabin altitude was switched on and the "CABIN ALT" light was on. 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 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. Oxygen masks in the passenger cabin were released. The aircraft descended rapidly to FL 200 when it reached a speed of 337 KIAS, where the airplane's pressurization system started to react, although it still remained outside of normal parameters. The aircraft reached FL 130 and was on ILS approach for RWY27 at Sofia Airport. The crew decided to land at Sofia Airport.
It is clear from the above that the serious incident was associated with depressurization of pressurized compartments (increasing of cabin altitude) of the B737-400 aircraft, serial number 25112, TC-TLC registration marks. This depressurization resulted in the necessity of usage of oxygen masks, extreme descent to altitudes, at which the passengers and crew will not experience oxygen starvation and termination of a flight associated with a forced landing at the nearest appropriate airport.
In this case, the increase in cabin altitude above the permissible values was accompanied by audible alert that warned the crew to take immediate measures for elimination of the occurring conditions that pose a safety hazard to the flight, including endangering the lives of passengers and crew. Immediate actions to be taken by the crew are set out in procedures described in the airplane QRH. These procedures in their sequence are described in Para2.6.2. The crew consistently followed these procedures. The crew made an unsuccessful attempt to manually adjust the position of the flap of outflow valve, then activated the oxygen masks and undertook extreme descending to an altitude, which do not endanger the lives of passengers and crew. This process can be traced by the decrypted data of the flight parameters, described in Para2.11. Under this circumstances the crew decided to land at the nearest appropriate airport - Sofia Airport.
The foregoing is related to crew actions to counter the consequences of in-flight situation, which have threatened the safety of flight. As a result of these actions, the flight was terminated by a normal landing at Sofia Airport, with no consequences for the passengers, the crew and the airplane.
Logically, the question arises as to why this situation was threatening the safety of the flight. Its occurrence is related to the disturbance of the difference between the external and the internal pressure maintained in the cabin of the airplane and which is controlled by a pressurization control system. A brief description of this system is made in Para2.6.2 of this report, and in Fig. 11 of Enclosure 1 shows a layout diagram of the components of the system. A study of this system, performed by the AO in order to restore the aircraft airworthiness, reflected in paragraph 2.16 of this report, revealed three failures of components of this system:
Given that in accordance with the requirements of the standards, an aircraft should not be fitted with components with failure rate is greater than 10-3 and that the electric drive of the flap is reserved by duplication, in the heaviest case the frequency of simultaneous occurrence of the said failures should be 10-12 s-1. Such an event in terms of the general theory of reliability is practically unauthentic. If the events themselves occur consecutively, and each of them is preceded and is a prerequisite for the occurrence of the next one, the probability of realization of the event, leading to increasing the cabin altitude above the allowable values can be measured with the probability of occurrence of the first of a group of interrelated events. Paragraph 2.16 quotes Air Operator's opinion, expressed in an e-mail received by the Commission for investigation on March 30, 2018, about the possible cause of the event being realized:
"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.
As a result of the investigation, the Commission concluded:
Based on the analysis performed, the Commission points out that the serious incident resulted based on the circumstances set out in this report and their analysis, the Commission pointed out as a reason for realization of the serious incident: 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.
Taking into account the causes of the serious incident and the deficiencies found during the investigation, the Commission recommends the following safety measures to be implemented:
On the grounds of Art.18, § 5 of Regulation 996/2010, the safety measures instructed shall be recorded in the centralized European System of Safety Measures. Follows: Enclosure 1, which is an integral part of this Report. The Investigation Commission reminds all organizations to which safety measures have been sent, that on the basis of Article 18 of Regulation 996/2010 on Investigation and Prevention of Accidents and Incidents in Civil Aviation and Art19, Para7 of Ordinance No. 13 for investigation of aviation accidents, that are obliged to notify in writing the Directorate AMRAIUD of MTITC for the actions taken on the recommendations made.