Safair B734 near George on Nov 22nd 2020, transponder failure and loss of cabin pressure

Last Update: April 14, 2022 / 20:04:50 GMT/Zulu time

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Incident Facts

Date of incident
Nov 22, 2020



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-400

ICAO Type Designator

A Safair Boeing 737-400, registration ZS-OAF performing flight FA-134 from Cape Town to East London (South Africa) with 158 passengers and 6 crew, was climbing out of Cape Town when at about 24550 feet MSL the transponder stopped responding to Seconadary Surveillance Radar (SSR) interrogations. The crew continued the climb to FL330, when at about 32,900 feet the cabin altitude warning activated with the cabin altitude climbing through 10,000 feet at about 700fpm prompting the crew to don their oxygen masks and work the related memory items. However as the crew was unable to control the cabin pressure they initiated an emergency descent to FL100. During the descent the thrust levers retarded to low IDLE with N1 dropping to approximately 30%, an amber warning for both engines activated at the overhead panel, an amber alert speed mode activated at the eletronic attitude director indicator (EADI). The engine thrust was increased to flight idle again. The cabin altitude changes began to normalize, the crew thus decided to not deploy the passenger oxygen masks. After being level at FL100, about 63nm from George, the crew elected to divert to George rather than to return to Cape Town. Tower in George, not equipped with a primary radar, needed to rely on crew position reports or visual identification of the aircraft in the pattern. Emergency services were put on stand by for the arrival, the aircraft subsequently landed on runway 29 without further incident. A passenger requested medical assistance, cabin crew reported three passengers suffered from nose bleeds and five passengers complained about ear pain.

South Africa's CAA (SACAA) opened an investigation. In a preliminary report the SACAA reported 9 passengers received minor injuries. The air duct at the left hand air conditioning system had failed drastically limiting the efficiency of the left air conditioning system.

The captain (37, ATPL, 7,924 hours total, 3,383 hours on type) was assisted by a first officer (31, ATPL, 5,050 hours total, 2,000 hours on type).

The SACAA reported that the cockpit voice recorder had not been secured during the subsequent ferry flight to Johannesburg (South Africa).

The SACAA reported that the transponder was no longer interrogated by SSR shortly after departure due to some activation malfunction. It began to respond to SSR interrogations again when the aircraft descended through 2500 feet MSL on approach to George.

The SACAA stated the investigation is still ongoing.

On Apr 14th 2022 the SACAA released their final report concluding the probable causes of the occurrence were:

The air/ground proximity sensor becoming defective, which rendered certain cues to remain in ground mode and causing certain warning lights to illuminate in the cockpit which required crew intervention, whilst certain cues functioned normally.

- Loss of cabin pressure, which was attributed to air leaking from the fuselage, including the aft cargo compartment door seal that was worn out. This situation was further aggravated by the left air-conditioning pack outlet duct fracture in an area that was previously repaired, which resulted in reduced air supply to the system.

The SACAA analysed:

2.2.1 Crew

The crew was confronted with some ‘non-normal’ situations as they became aware of multiple alerts. It is possible that a single malfunction could result in multiple flight deck indications. Once the cabin altitude warning activated, the crew dealt with the emergency checklist and initiated an emergency descent as required to FL100.

They also made the decision to divert to their nearest suitable aerodrome, which was FAGG, at which a safe landing was executed.

The injuries/medical treatment the passengers received after the occurrence are viewed as minor. The AIID classified the occurrence as a serious incident and conducted an investigation due to the rapid decompression in conjunction with the necessity to use oxygen masks by the crew.

According to the QAR data, the aircraft had nearly levelled off at its cruising altitude when the descent was initiated, and the crew stopped the descent in FL100. The aircraft was landed without further problems.

2.2.2 The Aircraft

What is concluded from an aircraft structure and aircraft systems is that the cabin altitude warning was triggered by multiple system failures and air leaking from the structure. In the summary below, the specific matters and how they were rectified after the aircraft was positioned from FAGG to its maintenance base at FAOR are highlighted:

1. A problem was encountered prior to departure where the two right doors 1R and 2R proved difficult to close; the assistance of a technician was required to close the doors. After several attempts, it was later confirmed that the doors ‘looked closed’ from the outside. According to the PIC, when he conducted a walkaround of the aircraft after landing at FAGG, he noted that the doors 1R and 2R were skew in their frames, with gaps at the top. With the doors not being in the correctly closed position, cabin air leaked from these door seals during flight, contributing to the cabin rate of climb rate of 600 to 700 ft/min.

These doors were adjusted as per the aircraft maintenance manual (AMM) requirements.

2. After take-off the cockpit crew observed several redundancies:

(i) No transponder, the unit remained in ground mode. It initially could not be determined if this was an aircraft problem or an SSR interrogation system error. It was later determined that this was indeed an aircraft system related occurrence.

(ii) No N1 (low pressure rotor speed) engine indication.

(iii) No LNAV or VNAV.

(iv) The flight management computer (FMC) lost position but came back on later with no LNAV.

(v) The auto throttle thrust stayed in the take-off configuration, and the crew disconnected auto throttle manually and continued with the flight with auto throttle disabled. It was determined that if the air/ground sensing system fails to “ground” mode the auto throttle cannot transition to the Throttle Arm position and it was, therefore, not possible to be armed.

(vi) When the crew retarded the thrust levers as they commenced with the emergency descent, the LOW IDLE amber warning light along with the MASTER CAUTION ENG indication activated, which alerted the crew (see Figure 1) that the engines will be entering the sub-idle range which is associated with an N1 rpm of 32% or less when in-flight. A low idle condition is brought about by the system sensing that flight idle is not achieved and the system is still in ground idle, which is several % N1 lower than flight idle. The crew followed the correct procedure by disconnecting auto throttle and advancing the thrust levers until the light went out. The reason behind the LOW IDLE light is that there was a series of flame outs on these CFM56-3 engines earlier on in their service life, for which an Airworthiness Directive (AD) was issued.

In consultation with the aircraft manufacturer fault isolation maintenance procedure, it was determined that these multiple cockpit indications were as a result of a defective Air/Ground proximity switch, which was located in the right main wheel well. The proximity switch was replaced, and the problem did not present itself again during the confidence/post-maintenance acceptance flight.

3. Aircon Bay Pack Compressor Outlet Duct Fracture

The left Aircon Bay Pack/Machine Compressor Outlet Duct Assembly with Part No. 65-51552-8 was found to have ruptured in the same area where it was previously repaired. This component was removed and was metallurgically examined. The cause of the failure of the ducting was due to an incorrect welding procedure, resulting in material embrittlement as the result of oxygen contamination. The mechanism of failure was initial brittle fracture that continued with a fatigue mechanism.

It could not be determined when this outlet duct had fractured; however, the aircraft had no previous defects that could have been associated with such a failure and it could, therefore, not be ruled out to have occurred during this flight. The left and right packs are completely independent of each other and normally operate in parallel. The cockpit requires only a fraction of the air supply provided by the left pack, with the majority of the left pack supply being routed to the mix manifold. The output of the two packs is combined in the mix manifold. According to technical information, the cabin altitude can be maintained with only one pack operating; this was, however, not the case in this serious incident. The investigator noted that the cockpit crew fitted their oxygen masks, which was most probably related to the reduced air supply from the left pack following the failure of this outlet duct. The subsequent replacement of this component improved the efficiency of the left air-conditioning pack considerably as well as the entire system.

4. Aft Cargo Compartment Door Seal

The door seal was found to be torn in the aft left corner. The seal was replaced.

5. Cabin Altitude Warning

The cabin altitude warning horn sounds (actives) when the cabin altitude
exceeds 10 000ft. The passenger oxygen masks will not deploy until 14 000ft cabin altitude although they can be downed manually at any time by the crew. In this flight, the cabin altitude stabilised at around 12 500ft. Due to the slow nature of the leak, the cockpit crew decision was not to deploy the cabin oxygen masks during the flight.

6. The Outflow Valve

After landing at FAGG, the crew noticed a very high cabin pressure. It was not possible to open the doors, which could have delayed an evacuation if it were required. The outflow valve as illustrated in Figure 19 was in the closed position.

This valve was then slowly opened manually by the crew to ensure the cabin was fully depressurised to allow the doors to be opened; if the air ground sensor was operational, the aircraft would have automatically dumped the excess cabin pressure on touchdown.

2.2.3 Factors that Affect Cabin Pressure

To pressurise the cabin, air is supplied continuously from the engines via the airconditioning packs. The pressure is regulated by controlling the exit of air from the aft outflow valve. Therefore, the cabin pressure is affected by the following factors:

- the inflow or supply of air from the engines to the cabin
- the controlled (regulated) leakage of air from the cabin
- any uncontrolled leakage of air from the cabin.

2.2.4 Environment

The flight was conducted during day time with visual meteorological conditions (VMC) prevailing; fine weather conditions prevailed, which had no bearing on this flight.

2.2.5 Summary

No individual component of the cabin pressurisation system was identified as having caused the loss of cabin pressure. The outflow valve, selector panel and cabin pressure controller have internal built-in test functions that report any faults affecting the control of cabin pressure, whether intermittent or constant, to the controller, which records these faults to non-volatile memory. The non-volatile memory data for this incident was examined and no pressurisation system failures were recorded. The tear-down examination of those components revealed no faults.

Experience has shown that older pressurisation system components could have intermittent faults that do not show during subsequent testing. A combination of components with low or marginal performance could have caused the incident, which include the failure of the left air-conditioning duct. It was identified that some uncontrolled air leakages occurred at several places, which were caused by the cabin doors on the right not being properly secured/latched prior to take-off as well as a worn aft cargo compartment door seal.

The effect of the failure of the Air/Ground proximity sensor resulted in the crew having to deal with a ‘non-normal’ situation. This sensor has a direct effect on twenty-seven (27) of the aircraft systems (see list in Appendix B), which include the pressurisation system. It was noted that not all systems on the list were affected by the failure of the sensor, for example, the crew was able to retract and extend the landing gear normally.
Incident Facts

Date of incident
Nov 22, 2020



Flight number

Aircraft Registration

Aircraft Type
Boeing 737-400

ICAO Type Designator

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