Back to home pageTC-TLC 737-400 depressurization and emergency descent

Home > Accident News > TC-TLC

Contents

Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Study notes and technical information Close up photos of internal and external components A compilation of links to major 737 news stories with a downloadable archive Illustrated history and description of all variants of 737 Detailed tech specs of every series of 737 Databases and reports of all the major 737 accidents & incidents General flightdeck views of each generation of 737's Description & news reports of Advanced Blended Winglets Press reports of orders and deliveries A collection of my favourite photographs that I have taken of or from the 737 Details about 737 production methods A compilation of links to other sites with useful 737 content History and Development of the Boeing 737 - MAX A quick concise overview of the pages on this site

spacer

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.

TC-TLC 737-400 depressurization and emergency descent

See more details about the book

All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format.

*** Updated 20 Nov 2018 ***

The 737 Tech Site on Facebook The 737 Tech Site on Twitter The 737 Tech Site on Instagram

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.

Survival Aspects

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.

Analysis

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:

  • - faulty heating of the outflow valve seal;
  • - the outflow valve flap for changing the exhaust air output from the cabin doesn't function (cannot change its position); It was found that this failure was due to the failure of the two electric motors for driving the flap of the valve, one of them on AC and another one on DC;
  • - The right-hand safety relief valve was stuck in open position. The jamming of the safety relief valve in open position connects the interior of the cabin space with the atmosphere and makes it non-hermetic.

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.

Findings

As a result of the investigation, the Commission concluded:

  • 1. В737-400 aircraft, serial number 25112, registration marks TC-TLC, was manufactured in year 1994, manufacturer USA.
  • 2. The aircraft has a Certificate of Airworthiness No 2115, issued on 17.04.2014 by the CAA of the Republic of Turkey.
  • 3. Airworthiness review certificate of the aircraft has been issued on 27.03.2017 and is valid till 30.03.2018.
  • 4. The aircraft has flown 63 976 flight hours and 34 009 cycles since new.
  • 5. A DAILY check of the aircraft was performed before the flight, in which the occurrence was realized. No failures and deficiencies were established during the inspection.
  • 6. The maximum take-off mass of the airplane is 65090 kg. Empty mass of the aircraft is 35 355 kg.
  • 7. At take-off from Antalya Airport according to the load and trim sheet, the take-off mass of the airplane was 56 203 kg and balance was within the permissible limits.
  • 8. The Pressurization Control System of B737-400 aircraft, serial number 25112, is with an analogue controller.
  • 9. When FL 337 was reached, the horn for dangerous cabin altitude was activated and the "CABIN ALT" light was on.
  • 10. The crew performed the sequence of actions provided in the Quick Reference Handbook of B737-400 aircraft for cases when cabin altitude was exceeded.
  • 11. The attempt to close the outflow valve by selection of MAN AC mode switch didn't made the desired effect and the crew decided to perform emergency descending.
  • 12. Oxygen masks in the passenger cabin were released.
  • 13. 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.
  • 14. The crew decided to land at Sofia Airport.
  • 15. The approach and landing at Sofia Airport were normal with a landing mass of 51 311 kg, at a maximum permitted landing mass of 54 884 kg and a residual fuel at landing of 7 992 kg.
  • 16. After the landing the aircraft was stopped at a stand and the passengers leaved it by normal exits without use of emergency means.
  • 17. There is no information that any physiological factors or losses of capacity have influenced the pilots' capacity for work.
  • 18. During the occurrence no dangerous meteorological phenomena have been observed that could affect the flight.
  • 19. An inspection of the cabin pressure control system performed by CAMO of AO has revealed a failure of the outflow valve, failure of the outflow valve gasket heating, and jamming of the right-hand safety relief valve in open position.
  • 20. A functional check of the safety valves was carried out during the last “С“ check in February 2018.

Causes

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.

Safety Recommendations

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:

  • BG.SIA-2018/02/01. Having in mind the potentially high threat related to the consequences of an event similar to the one under investigation, the safety investigation commission recommends that the aviation operator to make a review B737-300/400/500 aircraft maintenance programs operated by this air operator, in the part relating to the service of the cabin pressurization control system and, if necessary, to increase the frequency of the outflow valve gasket heating and safety relief valves checks.
  • BG.SIA-2018/02/02. The outflow valve of the pressurization control system of B737-400 aircraft, serial 25112, TC-TLC registration marks, shall be sent to a service workshop for dismantling and flaw detection in order to identify the reasons for disturbance of its normal operation.
  • BG.SIA-2018/02/03. The right-hand safety relief valve of the pressurization control system of B737-400 aircraft, serial 25112, TC-TLC registration marks, shall be sent to a service workshop for dismantling and flaw detection in order to identify the reason for its jamming in open position.
  • BG.SIA-2018/02/04. The Safety Investigation Commission recommends the Executive Body of ICAO to reconsider the content of Para5.1.2 of Annex 13 to the International Civil Aviation Convention, according to which the investigation of the circumstances of a serious incident when the maximum take-off the mass of the aircraft is over 2250 kg, to be assigned to the State of the Air Operator instead of the State of occurrence, which controls a major part of the information for aircraft and operating personnel.

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.

Footer block

This site has had visitors to date.