Investigators believe a damaged retaining pin within the overhead fire-control panel on a TAP Portugal Airbus A321neo led to an uncommanded in-flight engine shutdown, when it released a fire push-button during turbulence.
Portuguese investigation authority GPIAAF says the aircraft (CS-TJL) had been cruising at 37,000ft en route from Milan to Lisbon on 23 October last year.
While the captain was alone in the cockpit – the first officer having left for the lavatory – the jet encountered moderate turbulence. The aircraft subsequently alerted the captain to a shutdown of the right-hand CFM International Leap-1A engine.
After the first officer returned a few seconds later, the crew declared an emergency, commenced a descent to 22,000ft and twice attempted, unsuccessfully, to relight the engine.
The crew diverted to Barcelona where the jet landed safely. None of the 201 occupants was injured.
As the A321neo was parking, the crew noticed that the fire push-button for the right-hand engine had popped from its normal location, and its guard was open.
There had been no fire, and the button did not illuminate. But its release had triggered the automatic shutdown of the engine in flight. This resulted in arming of the extinguishers and an engine-shutdown alert message to the captain.
Airbus’s and TAP’s operating procedures did not require the condition of the overhead fire-control panel to be checked after an in-flight shutdown, says the inquiry.
GPIAAF found that the panel, built in 2009, had originally been installed on an A320.
The inquiry believes the panel was probably dropped at some point, and underwent repair in October 2013 to replace parts and have a distorted base straightened.
TAP received the panel as replacement stock and initially fitted it to one of its A319s before installing it, in 2019, on the A321neo involved in the incident. The panel had logged over 24,500h since the repair.
But GPIAAF indicates that the panel had carried a latent fault – a deformation of less than 6° in the alignment of a fire push-button retention pin – which had gone undetected during repair. The push-button depends on this single pin, just 2mm in diameter, to remain in place.
The inquiry believes the pin’s deformation weakened its ability to retain the fire push-button, and the turbulence experienced by the A321neo was sufficient to cause the button’s release, triggering the engine shutdown.
It points out that the retention system had no redundancy.
Analysis by manufacturer Safran identified 114 fire-control panels with the potential for similar retention-system failure. The inquiry says Airbus and Safran are working to “cleanse” the fleet of affected panels and prevent future installation of units or switches that show signs of damage.
Safran has modified maintenance procedures to include an optical check on the retention pin to ensure alignment within 1°.
Maintenance manuals for Airbus aircraft with line-replaceable fire panels have been updated with a warning not to install panels which have signs of damage, and the airframer has also sought design improvements to increase push-button resilience.
