French investigators have detailed a cascading series of failures which seriously degraded the flight controls of an XL Airways Airbus A330-200 in cruise over the Mediterranean Sea.

Investigation authority BEA states that the aircraft ultimately lost all three inertial reference systems and suffered a degradation of its autonomous navigation capabilities, as well as a reversion to direct flight-control law.

The aircraft – a 12-year old twinjet, F-GRSQ – had departed Reunion for Marseille, on 26 December 2014, with one of its three inertial reference systems demonstrating a significant drift, around 6-15nm.

Although the associated air data inertial reference unit was supposed to have been replaced two days earlier, an identification mix-up meant the wrong unit was swapped. The uncorrected drift played a central role in the subsequent degradation.

BEA did not have access to cockpit-voice recorder information, and it says crew testimonies differ in some details. But it has derived a probable sequence of events.

About 5h into the flight, another of the inertial reference systems, IRS3, switched from ‘navigation’ to ‘attitude’ mode – probably through crew action, says the inquiry. The flight-guidance computer rejected IRS3 and used the information derived from the other two systems, IRS1 and the drifting IRS2.

This change was only noticed by the relief first officer after he took over some two-and-a-half hours later. The crew attempted a procedure to align IRS3.

BEA says, however, that the discrepancy between IRS1 and the drifting IRS2 prompted a disengagement of the autopilot, autothrust and flight directors, and the disappearance of position and flightplan information on the navigation displays.

The inquiry attributes this loss of information to a “consequence” of the previous event and the logic of the Honeywell P3 flight-management system.

While the crew was able to retrieve position and flightplan data by activating a navigation back-up mode, position information on the captain’s side was lost during subsequent troubleshooting efforts.

BEA believes that, as the crew sought to restore the captain’s data, they triggered an in-flight alignment of the three IRSs which, in turn, resulted in a transition of the A330 to direct flight-control law.

No technical fault could be found to explain the triple simultaneous alignment. The inquiry says a “fragile knowledge” of the systems involved combined with the stress of degrading functions and a misunderstanding of the origin of the problem could have contributed to the situation.

Primary flight display information disappeared and the crew had to resort to emergency electromechanical instruments, without the autopilot or autothrust, for the rest of the flight.

As the aircraft approached Greek airspace, the crew declared an emergency and informed air traffic control that the flight had lost reduced vertical-separation minima capability – the height-keeping reliability allowing aircraft to operate at flight levels separated by just 1,000ft.

The crew considered a diversion to Heraklion but opted for Athens, owing to the favourable weather and better air traffic control guidance, and the aircraft was vectored to runway 03R, where it landed without further incident. Following checks the aircraft subsequently flew onwards to Marseille and then Paris.

BEA says that the loss of positional data was a “major concern” for the crew – referencing the accidental drift of a Korean Boeing 747 in August 1983 which led to the jet’s being shot down – and that use of a newer flight-management system standard would have prevented it.

Source: Cirium Dashboard