Changes to 787 audio systems

UK investigators require Boeing to modify the audio system fitted to the Boeing 787 following the investigation carried out on an incident in which British Airways 787-9 flight from London Heathrow to New Delhi lost cabin pressure on 29 April 2017.

Incident

A British Airways Boeing 787-9 flight BA-143 was on a scheduled flight from London Heathrow to New Delhi, India on 29 April 2017. The aircraft was dispatched in accordance with the Minimum Equipment List (MEL) with the left air conditioning (AC) system disabled. This was because the left No 2 (L2) CAC (Cabin Air Compressor) shaft had failed, which had subsequently damaged the left No 1 (L1) CAC,

An hour into flight at 10:32 the ‘cabin altitude pressure block’ was automatically displayed on the EICAS display. Shortly after reaching FL350 the crew were alerted by EICAS that the cabin altitude was increasing at a rate of about 300 fpm and that both outflow valves were indicating closed.

With no additional Environmental control system (ECS) actions available to control cabin altitude, the flight crew initiated a descent.

Oxygen masks dropped and the crew declared a MAYDAY and made a rapid descent to FL100 with speedbrakes deployed. The Aircraft returned to Heathrow and landed safely.

Cabin crew

The cabin crew in the first-class cabin received interphone call from the co-pilot and the request to stop the cabin service and return to their seats. However, before this message could be passed on to the other cabin crew, the cabin oxygen masks deployed. The deployment of the oxygen masks is accompanied by a PRA that advises of the emergency over the cabin loudspeakers, and contains instructions to don the oxygen masks. However, while the PRA was triggered in the cabin, its volume over the speakers was so low that the content of the message could not be heard above the background cabin noise.
As a result of not hearing the PRA, and in the absence of any other serious indications of depressurisation, the cabin crew were confused as to what was happening. Many thought the oxygen masks had deployed inadvertently, possibly because of turbulence. The cabin crew did not follow their depressurisation actions but returned their cabin service carts to the galleys and took their seats there.

The passengers appeared unaware of the emergency. Some remained asleep and many did not fit their oxygen masks. Of those that did fit the masks, some fitted them incorrectly. As the cabin crews’ awareness of the situation developed, they began shouting instructions to the passengers to fit their oxygen masks.
Due to the uncertainty of the situation, the CSD took a portable oxygen system and went to the cockpit to ascertain what was happening. During this period several of the cabin crew also used portable oxygen systems. Several of them reported that the systems were difficult to extract from their stowage locations and to use due to the “cumbersome” oxygen bottle. They also found it difficult to tell whether the portable oxygen systems were working.
During the return to Heathrow several of the cabin crew and passengers reported feeling unwell, although none required medical attention.

Cabin decompression Pre-Recorded Announcement (PRA)

Manual deployment of the oxygen masks by the crew causes a cabin decompression PRA to be played over the cabin speakers by the passenger address system. This recorded announcement is intended to be heard clearly above the ambient noise in the cabin to alert passengers and cabin crew that they are to don oxygen masks. In this incident the PRA was not clear.

Findings

AAIB investigates found that the right-hand inner recirculation duct was disconnected from the lower right-hand recirculation fan. 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.

Also the aircraft’s AHM system sent Maintenance Alert Message 21‐0209‐C7403 to the operator’s ground-based data system, indicating that a ‘high leakage/low inflow’ of the cabin pressurisation system had been detected. This message is not displayed to the flight crew.

The operator later stated that the AHM system provides just over 1,200 maintenance alerts. From experience, some maintenance alert messages are inadvertently triggered, which has led to refinements to improve the robustness of the system and reduce the level of ‘nuisance’ alerts. The operator had seen alert message 21‐0209‐C740 triggered ‘intermittently’ on other aircraft before and this had caused maintenance staff to question the reliability of this particular alert message.

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.

Recommendations

The aircraft manufacturer has revised the Aircraft Maintenance Manual (AMM) installation procedure for the lower recirculation fans.
● The aircraft manufacturer has made changes to its Fault Isolation Manual for Maintenance Message 21‐34127, the message triggered by Maintenance Alert Message 21-0209-C740 from the AHM. This includes checking for recent maintenance activity on the cabin pressurisation system, including the lower recirculation fans.
● The aircraft manufacturer has made an update to the AHM ‘maintenance alert’ logic for message 21‐0209‐C740. This logic helps to filter out only those instances that are deemed valid and should be presented to the airline.
● The operator of G-ZBKF has revised its process for dealing with AHM ‘maintenance alert’ message 21‐0209‐C740.
● The operator of G-ZBKF is updating the audio system software fitted to its fleet of Boeing 787 to prevent the volume of the cabin decompression PRA from being attenuated.