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A388, en-route, northern Afghanistan, 2014
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On 5 January 2014, an Airbus A380-800 (9V-SKE) being operated by Singapore Airlines on a scheduled international passenger flight from London Heathrow to Singapore and in the cruise at FL370 at night in unrecorded flying conditions experienced a progressive loss of cabin pressurisation accompanied by increasing air noise from door 3L. As the cabin altitude approached 10,000 feet, an emergency descent was initiated and a MAYDAY declared. A subsequent diversion was initially planned by turning back to Ashgabat, Turkmenistan but after ATC advised that this airport could not accept an A380, the turn back was continued westwards to Baku, Azerbaijan without further event.
It was noted that the operating flight crew consisted of a 56 year old Captain with 16,815 total flying hours including 4465 on the incident type accompanied by a 37 year old First Officer with 6261 total flying hours including 1903 hours on the incident type. A relief crew was also on board - a 49 year old Captain with 15,041 total flying hours including 1100 on type and a 40 year old First Officer with 2608 total flying hours with 392 on type.
It was established that door 3L was already the subject of an ADD submitted on 23 December 2013 after a "loud noise" coming from it had been reported and a slightly worn seal was found on the door's upper edge. This action allowed a 120 day period for any necessary rectification. After the door had been reported as noisy en route by the Cabin Crew on the previous day’s flight by the aircraft from Singapore to London but inspection by one of the pilots had not found "anything unusual other than the loud noise", maintenance personnel at Heathrow had carried out a full inspection of the door but found "no anomaly other than the slightly-worn seal noted previously" and the existing ADD remained open.
After take-off from Heathrow, the member of cabin crew seated at door 3L heard the noise but did not detect any air leak or vibration. The senior cabin crew was advised and after also not detecting any air leak or vibration, elected to inform the flight crew once the aircraft had reached its cruise altitude of FL370. At this point, the cabin altitude was indicating the normal (for this aircraft type) of 6000 feet. It was noted that "the extent of the noise was such that passengers and cabin crew members in the area around Door 3L could not hear clearly the announcements over the Public Address (PA) system".
Once notified of the situation, one of the flight crew then inspected the door but "did not observe anything unusual other than the noise". Although aware of the PA audibility problem caused by the door noise, the flight crew decided to continue towards the destination whilst monitoring the aircraft pressurisation system, having considered that the issue of a worn seal had already been captured in the ADD, that the pressurisation of the aircraft was normal, that the aircraft had operated normally from Singapore to London, that the noise condition was known and that "maintenance personnel had checked the door and certified the aircraft fit for flight". An ACARS message was sent to the Company requesting that the problem be rectified before the next flight.
Then, about five hours into the flight, an ECAM 'Cabin altitude advisory' was annunciated. It subsequently disappeared but about 30 minutes later the cabin altitude began to climb slowly. Eventually, an ECAM Warning of excess cabin altitude occurred. An emergency descent was initiated as the cabin altitude approached 10,000 feet and a MAYDAY was declared to ATC - Kabul ACC - but received no response until their call was relayed by another aircraft. It was decided to turn back towards Ashgabat, which was the nearest Decompression Alternate (DA) identified on the flight plan and Kabul ATC was informed. Although the applicable Emergency Checklist did not require it, it was decided to manually deploy the passenger oxygen masks as a precaution.
During the diversion, the flight crew reported that Turkmenabat ATC had advised that Ashgabat "is not suitable for A380 aircraft" so after consulting the Company via SATCOM, it was decided instead to continue westwards and divert to Baku in Azerbaijan. This revised plan was successfully accomplished.
After arrival at Baku, the skin of door 3L was found to be "torn and bent backwards at the top left hand corner of the door" with the result that the aircraft was unable to maintain cabin pressurisation. The Door 3 L window shade was also "found collapsed". According to Airbus, this was the first record of such a door skin failure since the start of A380 operations.
The Investigation found that door integrity had been compromised by a fatigue crack which passed through several successive rivet holes in the door Cover Plate (CP). It was found that after a foreign object had been found at the top of the door in 2008 and removed with no related damage found, the aircraft had been released to service. Noise from the door was then reported and the CP was replaced. However, over the next 4 years, reports of noise from the same door continued and eventually, in August 2012, the installed GFRP laminate CP was replaced with an alternative type which had a steel insert embedded in the GFRP laminate to improve its stiffness. The door was subsequently repainted.
It was noted during inspection of the removed door after failure that "some paint and sealant had seeped into the door skin cracks" and that "there were three missing rivet heads at the cracked area with paint and sealant found on two of the missing rivet heads’ countersunk surface". It was therefore concluded that the crack was already present when the door 3L CP was replaced in 2012. It was noted that analysis of the paint showed that it was a top coat paint approved by Airbus.
The Investigation found that the manual deployment of oxygen masks after the emergency descent due to rising cabin altitude had been followed by reports that oxygen supplies from a few PSUs in the passenger cabins and 13 in the Cabin Crew Rest Quarters (CCRC) did not function correctly. Examination of them all found that they were serviceable.
In respect of the noise from door 3L prior to the failure to sustain normal pressurisation, it was noted that it was not known if the flight crew "had appreciated the extent to which the noise had caused discomfort to passengers" and additionally that "the operator did not have guidelines for its flight crews on such noise occurrences" so that decisions on the acceptability of noise would be made on an ad hoc basis. Since in this case, the noise was "apparently loud enough to prevent passengers from hearing clearly the announcements over the PA system", the Investigation considered that in the case of an emergency, there must then be a concern that crew instructions "might not be transmitted quickly and effectively to the passengers through the PA system". It was further considered that "if the operator expects the crew members to use the megaphones to overcome such door noise problem, it has to ensure that the instructions broadcast through the megaphones can be heard clearly over the door noise".
The formally stated Conclusions of the Investigation in respect of the door failure were as follows:
- The Door 3L failure was traced to a crack passing through a number of rivet holes on the door skin. The crack was probably caused by high cycle fatigue under varying amplitude loading due to the fluttering of the Batch 1 CP that was initially installed on the aircraft. Once the crack was initiated, the stiffer Batch 3 CP could not prevent further propagation of the crack.
- In regards to the PSUs in the bunks in the CCRC, some cabin crew members did not seem to be conscious of the fact that oxygen would only flow if the lanyard pin is released though pulling at the oxygen mask.
- There were traces of sealant and paint in the crack surfaces as well as on the countersunk surfaces of two rivet holes, meaning that the crack was present when CP was replaced in August 2012.
Safety Action was taken by both Airbus and Singapore Airlines as a result of the door failure findings.
At this point in the Investigation, one Safety Recommendation has been made as follows:
- that Singapore Airlines assess the effectiveness of the use of megaphones as a means for its crews to give instructions to passengers in an emergency when the noise level in the cabin is such as to prevent passengers from hearing clearly the instructions through the aircraft’s public address system. (R-2015-008)
An Interim Report was completed on 21 September 2015. The Investigation is continuing to look into operational aspects of the event including alternate aerodrome flight planning in respect of the Aerodrome RFFS Category requirement and the Safety Risk Assessment of RFFS requirements by the operator.