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A320, en route, north of Marseilles France, 2013

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Summary
On 12 September 2013, pressurisation control failed in an A320 after a bleed air fault occurred following dispatch with one of the two pneumatic systems deactivated under MEL provisions. The Investigation found that the cause of the in-flight failure was addressed by an optional SB not yet incorporated. Also, relevant crew response SOPs lacking clarity and a delay in provision of a revised MEL procedure meant that use of the single system had not been optimal and after a necessary progressive descent to FL100 was delayed by inadequate ATC response, and ATC failure to respond to a PAN call required it to be upgraded to MAYDAY.
Event Details
When September 2013
Actual or Potential
Event Type
AGC, AW, HF
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft AIRBUS A-320
Operator Swiss
Domicile Switzerland
Type of Flight Public Transport (Passenger)
Origin Zürich Airport
Intended Destination Valencia
Flight Phase Cruise
ENR
Location En-Route
Origin Zürich Airport
Destination Valencia
General
Tag(s) Event reporting non compliant
Inadequate Aircraft Operator Procedures
Inadequate ATC Procedures
AGC
Tag(s) Phraseology
Flight Crew Oxygen Mask Use
HF
Tag(s) Inappropriate ATC Communication
Inappropriate crew response (technical fault)
Procedural non compliance
EPR
Tag(s) Emergency Descent
MAYDAY declaration
PAN declaration


Outcome
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Technical
Safety Recommendation(s)
Group(s) None Made
Investigation Type
Type Independent

Description

On 12 September 2013, the crew of an Airbus A320 (HB-IJU) being operated by Swiss International Airlines on a scheduled international passenger flight from Zurich to Valencia as SWR2140 and in the cruise at FL370 in day VMC noted loss of cabin pressurisation. Descent was requested, but due to a lack of effective response from ATC, a PAN and then MAYDAY status had to be declared to achieve the descent required. Once at FL100, the APU was started and the intended flight was completed without further event at a reduced cruising level. The flight had been dispatched with one pneumatic system already de-activated.

Investigation

An Investigation was commenced by the Swiss Accident Investigation Board (SAIB) on 3 October after the handling of the Investigation had been delegated to the Board by the French Bureau d'Enquêtes et Analyses (BEA). The SAIB was subsequently replaced by the new Swiss Transportation Safety Investigation Board (STSB). Because of the delay in commencement of the Investigation both the DFDR and the CVR "had already been overwritten and were no longer available". Also, records available from the operator did not include data from the QAR.

It was noted that the flight crew consisted of a Commander with 10,145 total flying hours including 4765 on the incident type and total of 418 in command (all types) accompanied by a First Officer with 1622 hours total flying experience most of it on the incident type.

It was established that prior to departure from Zurich, the No.1 (left hand) pneumatic system had been de-activated and the aircraft released to service in accordance with the MEL. The flight crew had noted that the QRH stated that in the event of a fault subsequently also occurring in the No.2 pneumatic system, the applicable ECAM procedure should not be used and instead a procedure published in an Airbus OEB should be used. They concluded that they did not need to change either fuel load or cruise altitude and that the flight could therefore be undertaken as planned.

Shortly after reaching the planned cruise level of FL370 and during a period of only four minutes on their first Marseille frequency before being instructed to change to a different one, both pilots reported having detected a pressure change in their ears and observed that cabin altitude was rising at approximately 1700 fpm and the engine 2 bleed air valve was indicating closed. The alert AIR ENG 2 BLEED FAULT appeared on the ECAM soon afterwards. The PM (the Captain) referred to the OEB procedure but found that it was not appropriate to the situation and decided to initiate a descent, which was requested (without any reason being given) immediately after checking in with the new frequency - Marseille sector M123. However, this request was ignored and a 'direct to' clearance given instead. After waiting for an exchange with another aircraft, a repeat of the request to descend was replied to with "Swiss 2140 descend level" which again followed by a transmission to another aircraft. As soon as the frequency cleared, the crew transmitted a 'PAN PAN' message requesting descent which received the response to descend FL350 initially. The crew then stated the reason for requesting descent but received only the response "Roger" which prompted the crew to ask the question "Did you copy the PAN Swiss 2140?" to which the controller responded promptly with "Swiss 2140, call you back". Not wanting to delay further decent, the crew tried again with "Did you copy my PAN PAN Miss, Swiss 2140?" but as the controller responded with "Say again your request", a further crew call "Swiss 2140 PAN PAN, PAN PAN, PAN PAN requesting lower” was made. This transmission received another response from the controller of "Roger" and soon afterwards an instruction to squawk 2200. The crew acknowledged this and added "approaching flight level three five zero requesting lower" to which there was no response from ATC.

Two minutes after the initial request for descent had been made, a MAYDAY was declared followed by "request descent” which received a prompt response to squawk 7700 and descend to FL330. This was followed by a 'CAB PR EXCESS CAB ALT' Alert and both pilots donned their oxygen masks and "initiated an emergency descent". The controller asked for the crew’s intentions and upon the need for further descent being confirmed, the air crew were re-cleared to FL250 and after a further exchange, to FL140. The controller then instructed a frequency change to the Marseille ML sector. Once checking in there with a MAYDAY prefix, all further ATC communications proceeded normally and with due assistance. Descent to FL100 was given and the crew advised that "at the present time they would continue the flight towards Valencia but would also consider a diversion to Barcelona". However, having established that fuel reserves would be sufficient to continue to Barcelona at FL100 if necessary, the MAYDAY was cancelled.

On reaching FL100, the Captain called the senior cabin crew member to the flight deck and was advised that “neither the cabin crew nor the passengers had noticed anything of the emergency descent" and that since the seat belt sign had been switched on during the completion of emergency descent actions, it had been decided to discontinue cabin service. Subsequently, with the APU started and the ENG 2 BLEED successfully re-selected and functioning normally after advice from Company Maintenance Control, the aircraft was, after further review of fuel status and en route weather, climbed to FL210 and the flight continued to Valencia as originally intended without further event.

The cause of the No 2 bleed failure en route was found to be bleed air over-temperature at the precooler outlet temperature. The latter was found to be a result of intermittent malfunction of the Precooler air temperature by the regulating Temperature Control valve (TCT) of the Fan Air Valve (FAV). The TCTs of both systems were subsequently replaced.

The Investigation noted that Airbus had accepted as long ago as 2009 that "the overwhelming majority of second bleed losses on the A320 Family were caused by an over-temperature condition". However, in May 2008, they had also issued SB A320-36-1061 which provided for action to improve "the temperature regulation function of the TCT" and it was found that this SB had not, more than 5 years later, been implemented on the subject aircraft. It was also noted that similar TCT failure to control temperature leading to system air bleed 'lockout' had been known to both the UK and French Investigation Agencies since at least 2001 because of their investigation of almost identical Serious Incidents.

In respect of crew guidance on the response to loss of a bleed due to over temperature, the Investigation found that the poor wording of the OEB procedure which led the crew to ignore it when responding to bleed loss amounted to a "lack of clarity". Had the guidance to crew been properly expressed, they would have turned the no 2 pack flow selector to the 'LO' position after engine start and thereafter been directed to monitor the Precooler outlet temperature during the flight. It was concluded that loss of the remaining system would have most probably been avoided. It was noted that although this lack of clarity had been resolved by the revision, five months prior to the investigated event, of the MEL procedure to remove the need for concurrent referral to the OEB, this revision had not been provided to the Operator until 27 November 2013 - more than two months after the occurrence. The Investigation considered that "it is alarming that the revision of the MEL procedure, dated 13 April 2013 was only available to the operator a little more than seven months later".

Overall, the Investigation, noting that "information concerning the procedure that should be used for the present case was scattered over various sources such as the MEL, ECAM, FCOM/OEB and QRH, considered it unsurprising that the flight crew did not immediately apply the AIR DUAL BLEED FAULT procedure published in the QRH" and concluded that "it is clear that this way of dealing with a fault is not very fault-tolerant and not user-friendly".

The Investigation noted that the (allowable) pre-flight decision to continue to permit despatch with the No. 1 pneumatic system inoperative after a fault the previous day had been taken because of the likely need - evidenced by examination of data overnight - to replace the TCTs in the FAVs of both pneumatic systems when none of these components were immediately available.

The poor performance of the Marseille sector M123 controller during communications with the flight, following the onset of the problem until a MAYDAY was declared, was considered to amount to her having "little situational awareness". It was considered that "although the flight crew in the first instance only requested a descent, it is astonishing that the ATCO did not listen attentively, at the latest by the transmission of the PAN PAN message and the reference to pressure problems" and that "the behaviour of the ATCO provided little assistance to the flight crew in this phase". It appeared to the Investigation that “apparently the ATCO only became conscious of the situation when the flight crew gave further emphasis to their request for further descent by using the distress message MAYDAY". It was also noted that having declared a MAYDAY and their intention to descend, the crew did not need ATC clearance, yet had persisted with the requests necessary to obtain such clearances.

The formally stated Causal Factors for the investigated Serious Incident were determined as:

  • The flight commenced with only one functional pneumatic system;
  • A lack of clarity in the procedures meant that the use of this system was not optimised;
  • This pneumatic system had characteristics that led to overheating and the system switching itself off;
  • An optional improvement provided by the manufacturer (Service Bulletin) had not yet been implemented;
  • A revision of the MEL procedure provided by the manufacturer was not available to the operator at that time.

One Contributory Factor was also identified in respect of the occurrence and the history of the Serious Incident:

  • The necessary information and procedures for handling a system fault during the flight were not clearly presented to the flight crew.

The Final Report was approved by the Swiss Transportation Safety Board (STSB) on 10 November 2015 and published on 2 December 2015. No Safety Recommendations were made.

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