Singapore A388 near Baku on Jan 6th 2014, loss of cabin pressure
Last Update: April 24, 2017 / 22:24:57 GMT/Zulu time
- 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.
- 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.
- As regards the PSUs in the bunks in the CCRC, some cabin crew members did not seem to be aware that a conscious effort is needed to pull on the mask, when lying down, in order to release the lanyard pin to allow oxygen to flow to the mask.
- The amount of water required to be make available at an aerodrome of Category X can be significantly different from that required of a Category X-1 aerodrome. The smaller amount of water available at the Category X-1 aerodrome could compromise the desired RFFS protection level. ICAO guidance material does not seem to offer a methodology for airline operators to evaluate the extent of the compromise in fire protection level when an aerodrome of a lower category than the aeroplane category is used.
The SAAIB reported that loud noise had been observed from door 3L already on the previous day during the flight from Singapore to London but did not notice anything unusual except for the noise. Maintenance personnel had checked the door already on Dec 23rd 2013 due to noises, no anomaly except for a slightly worn seal was found, the defect was put into the log for deferred defects to be monitored (ADD log). This log permitted the defect to be deferred for 120 days.
After arrival in London the door was checked again by maintenance, who did not find any anomaly except for the already noted slightly worn seal, the defect remained on the ADD log.
After departure from London the noise was again heard, a cabin crew member checked the door but did not detect any air leak or vibration. The aircraft continued the climb to FL370 while cabin crew supervisor rechecked the door again without detecting any leak or vibration. Flight crew checked the cabin pressure after levelling off at FL370, the cabin pressure was stable at 6000 feet. The flight crew was aware of the ADD log entry and decided that it was safe to continue the flight.
About 5 hours into the flight the flight crew was alerted to increasing cabin altitude by an ECAM warning and disappeared again. Flight crew continued to closely monitor cabin pressure and noticed the cabin altitude began to climb again about 30 minutes later, which resulted in a cabin excess altitude warning. The flight crew therefore initiated an emergency descent and declared Mayday with Kabul (Afghanistan) Center, however, did not receive a reply. Another aircraft in the vicinity heard the Mayday Call and relayed it to Kabul Center.
The flight crew decided to turn around into the direction of Ashgabat (Turkmenistan) and informed Kabul about their decision to divert to Ashgabat. Turkmenistan's ATC subsequently informed the crew that Ashgabat was not suitable for A380s. The crew consulted with dispatch and decided to divert to Baku (Azerbaijan), which was the next nearest airport, where the aircraft landed without further incident.
The SAAIB described the damage to the door: "The skin of Door 3L was found torn and bent outwards at the top left hand corner of the door. As a result, the aircraft was unable to maintain pressurisation. The Door 3L window shade was found collapsed." The manufactrer indicated that there had been no other similiar case of a door skin failure.
The damaged door was sent to the manufacturer for laboratory analysis, a crack was found at the door coverplate (CP), which was identified to be a fatigue crask as result of high cycle fatigue failure under varying amplitude loading. The crack likely originated at rivet 4 and propagated away. 3 rivet heads were missing.
The SAAIB analysed:
Traces of sealant and paint were found on the crack surfaces as well as on the countersunk surfaces of two of the rivet holes with missing rivet heads. This means the crack was already present in August 2012 during the CP replacement and subsequent painting work. The rivet heads could also have been missing by then.
When the CP is in place, the lower edge of the CP would almost cover the row of rivets in question and a fillet of sealant is applied to seal the lower edge of the CP. The sealant application job does not include an inspection of the row of rivets. Anyway, it is doubtful if maintenance personnel focusing on the sealant application job would notice any such missing rivet heads, especially if the cavity left by the missing rivet heads had been filled with sealant.
When the CP was removed during maintenance work, one might get to inspect the condition of the row of rivets. However, if the cavity left by the rivet heads had been filled by sealant before, it might not have been possible for maintenance personnel to notice the missing rivet heads. After the CP replacement in August 2012, there was no further work done in that area and thus no opportunity to inspect the condition of the row of rivets in the area.
The SAAIB analysed that the level of fire protection by an ICAO category 9 (category X-1/X minus 1) and category 10 (category X, required by aircraft) aerodrome could make a difference. One or two more minutes of fire protection might result in a different, successful or unsuccessful outcome of an evacuation. There had been risk assessments by ICAO which permitted a category 10 aircraft to use a category 9 aerodrome, which provides about 70-80% of water supplies needed. However, the SAAIB asked: "Put another way, if a flight crew needs to choose between landing at a Category X aerodrome and a Category X-1 aerodrome, would the crew know what the difference in fire protection levels offered by these aerodromes?"
The SAAIB analysed that ICAO regulations state the aerodrome category provided "the level of protection provided shall be not less than one category below the determined category" is ambiguous, in particular on first read by an unprepared person who might conclude from this sentence, that the provided category should not be less than one category below the determined category resulting in two categories below. The wording "may be one lower than the determined category" is clear however.
This article is published under license from Avherald.com. © of text by Avherald.com.
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