British Airways B789 near Brussels on Apr 29th 2017, loss of cabin pressure
Last Update: July 12, 2018 / 16:43:55 GMT/Zulu time
A replacement Boeing 787-900 registration G-ZBKC reached Delhi with a delay of 6:40 hours.
The airline confirmed a loss of cabin pressure prompted the release of the oxygen masks and the return to Heathrow.
The occurrence aircraft is still on the ground 31 hours after landing back.
On May 18th 2017 the UK AAIB reported the aircraft suffered a gradual decompression while enroute from London to Delhi and returned to Heathrow. An investigation into the occurrence is ongoing.
On Jul 12th 2018 the AAIB released their bulletin concluding the probable causes of the serious incident were:
Loss of cabin pressure
The inability of the aircraft to maintain normal cabin pressure was found to be have been caused by the right lower recirculation fan becoming detached from the inner duct, which allowed air from the AC unit to leak to atmosphere rather than provide the required cabin pressure on a sector where the left air conditioning system had been disabled before flight. When the aircraft was operated during the incident flight with only the right air conditioning system available, the system did not have sufficient capacity to overcome the effect of the leak. It was concluded that the lower right recirculation fan had not been correctly attached to the inner duct when the fan had been installed on 18 April 2017.
Cabin PRA (prerecorded announcements)
The deployment of the oxygen masks in the cabin was accompanied by the cabin decompression PRA. However, the level of the PRA was reported as not being audible. The apparent lack of an accompanying PRA caused confusion amongst the cabin crew as to the exact nature of the situation and the initial response by the cabin crew was uncoordinated, with some passengers not donning their oxygen masks.
The cause of the low volume of the PRA was subsequently identified as a software issue in the cabin audio system, whereby the volume of the announcement was attenuated following use of the cabin address system from the cockpit.
Digital ‘spikes’ evident in the crew channels, as identified by the NTSB during the readout of the EAFRs fitted to accident aircraft registration JA829J, has been subsequently confirmed as being introduced by the EAFR manufacturers ground replay software, IGS. This issue has now been resolved and the AAIB has communicated this to other accident investigation laboratories.
The NTSB has previously identified deficiencies in the quality of the CVR recordings on the Boeing 787 and issued Safety Recommendation A-14-126 to address these. The AAIB has further identified that ATC communications can be masked when the flight crew are using the oxygen masks, due to the disparity in the recorded dynamic range of the sidetone and oxygen mask signals. A Safety Recommendation has been made to address this.
Testing of the installed performance of CVR systems is currently largely subjective, which has led to variation in their performance. Three Safety Recommendations are made to address this.
The AAIB reported:
Following the return of the aircraft to Heathrow after the incident, an examination took place monitored by the AAIB. It was found that the right-hand inner recirculation duct was disconnected from the lower right-hand recirculation fan (Figure 3). Once the duct and fan unit were removed, there was evidence that the coupling and seal joining the two had been incorrectly aligned when last assembled. The V-band clamp joining the other end of the duct to the heat exchanger was correctly fitted.
Examination of the coupling system used to connect the recirculation fan to the inner duct showed that if the coupling were not installed correctly, there was less visible evidence and a reduced tactile feel compared to that of the V-band clamp used at the opposite end of the duct. The connection of the right-hand recirculation fan is located in a position that requires reaching through an extensive system of ducting and components to install it. Viewing of the connection between the duct and fan is also restricted. Figure 3 shows the relative position of the recirculation fan and the disconnected duct.
This article is published under license from Avherald.com. © of text by Avherald.com.
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