XL France A332 near Athens on Dec 26th 2014, all three IRUs failed

Last Update: July 26, 2018 / 15:05:53 GMT/Zulu time

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Incident Facts

Date of incident
Dec 26, 2014

Classification
Incident

Airline
XL Airways France

Flight number
SE-1551

Departure
Saint Denis de la Reunion, Réunion

Destination
Marseille, France

Aircraft Registration
F-GRSQ

Aircraft Type
Airbus A330-200

ICAO Type Designator
A332

A XL Airways France Airbus A330-200, registration F-GRSQ performing flight SE-1551 from Saint Denis de la Reunion (Reunion) to Marseille (France), was enroute at FL380 about 280nm south of Athens (Greece) when the crew reported they had lost all their three Inertial Reference Units (IRUs) and decided to divert to Athens. The aircraft landed safely on Athens' runway 03R (active runways 21) about 50 minutes later.

The occurrence aircraft was able to continue the journey after 3:10 hours on the ground in Athens and reached Marseille with a delay of 3:45 hours.

The French BEA reported in their weekly bulletin that the occurrence was rated an incident, the BEA is investigating the occurrence.

On Jul 26th 2018 the BEA released their final report in French only concluding the probable causes of the incident were (the BEA annotated in the absence of the CVR and signficant discrepancies between technical data collected and testimony by the crew):

The incident was caused by the unavailability of the three Inertial Reference Systems (IRS) leading to degradation of the flight control laws and the degradation of the navigation capabilities of the aircraft. This unavailability occurred in 4 stages:

- at 10:34Z IRS 3 transitioned from NAV to ATT mode, probably as result of a manual action by the crew

- at 13:36Z the disconnection of autopilot (AP) and autothrust systems (AT) probably as result of a misalignment between IRS 1 and IRS2, IRS 2 had been drifting since takeoff, which further resulted in the loss of position and navigation data on the Navigation Display (ND) according to the logic used by the Honeywell Flight Management System (FMS). This became revealed two minutes later when the crew activated BACK-UP NAV mode.

- shortly before 13:44Z the loss of the left hand position data as result of output by the BACK-UP NAV left hand side

- At 13:44Z the loss of alignments of all three IRSs probably as result of uncoordinated and improvised actions by the crew to restore position information from the IRSs.

The loss of position information was an important concern for the crew. The new FMS standards would have prevented the loss of position information, better robustness in maintaining autonomous navigation capabilities is of interest while overflying oceans, desert areas or conflict zones (e.g. Korean flight 007, which was shot down on Aug 31st 1983, departed from its planned track towards the territory of the Sovietunion).

The problem of an IRS drifting, previously identified, has not been resolved due to an incorrect labelling on the mounts. The drift of a single IRS is not a major anomaly though.

The following factors probably contributed to the non-standard actions by the crew on the IRS selectors, without the ability to establish the degree of contribution:

- the ergonomy of the ADIRU control panel not taking into account the irreversibility of changes on the IRS modes
- the presentation of the IRS information of the MCDU Position Monitor in different sequence (1,2,3) than those of the selectors (1,3,2), which is representative of the aircraft architecture
- a logic of presentation of information on the ECAM display which did not make obvious the reasons to the crew of why the autopilot disconnected and the position data were lost
- low exposure of crews to IRS blackouts

The BEA reported the crew performed a complete alignment of all three IRS prior to departure from Saint Denis. The captain was pilot monitoring, the first officer was pilot flying, a second first officer complemented the crew and was in the observer seat during departure.

However, shortly after takeoff the crew received MCDU messages "Check IR 2/ FM position" and found IRS 2 was drifting. Throughout the flight the crew regularly monitored the drift of IRS 2, which amounted between 6 and 15nm. The primary GPS mode was active from takeoff.

About one hour after departure the captain left the cockpit to take his scheduled rest. After the return of the captain the first first officer took his rest.

At 10:34Z the IRS 3 changed from NAV to ATT mode, meaning IRS 3 continued to provide attitude data but no longer provided position and speed data, which was now depending on IRS 1 and IRS 2 only. That loss of redundancy has no reflection on the displays to the crew as it requires no immediate or mid-term crew action, hence in occordance with the manufacturer's philosophy the crew is not notified.

At about 13:00Z the second first first officer relieves the second first officer from the cockpit, takes his seat and assumes the role of pilot flying again. Shortly after the first officer, in analysing the drift of IRS 2, remarked that IRS 3 was in ATT mode with no alarm present on the ADIRU panel or ECAM, however, all IRS selectors are on NAV. The crew thus consults with the IRS fault checklists and performed an alignment procedure in ATT mode. The captain recalled he saw an MCDU message asking for confirmation that the IRS should transition into ATT mode. The crew stated in their testimonies that prior to this moment no IRS selector had been changed. The crew confirms the IRS should transition to ATT mode. In that moment the autopilot and autothrust disconnected, the primary GPS was lost, the position information was removed from the ND. The
first officer assumes manual control of the aircraft, then both captain and first officer activate BACK-UP Navigation on their MCDUs. The captain stated that the FMS could not access the IRS information. Internal data showed each IRS recorded the message "IN FLIGHT ALIGNMENT". The flight law changes to direct law, attitude information is removed from the primary flight display (PFD), position, attitude, flight plan information is no longer available for the remainder of the flight. The crew reverted to using electromechanical instruments for the remainder of the flight without autopilot.

A few minutes later the captain assumes role as pilot flying because it is easier to read the stand by instruments from the left hand seat than from the right hand seat. The first officer advises ATC that they had lost their RVSM capability and subsequently declared emergency. The crew explored diversions to Heraklion and Athens and decides to divert to Athens due to favourable weather conditions. The crew receives radar vectors to join the final for runway 21L, the crew accepts the ATC proposal to land on runway 03R (winds from 140 degrees at low speeds). A short video taken by the second first officer on descent towards Athens shows all three IRS switches in NAV position and the IR fault lights flashing.

Following landing without further incident a mechanics, having been informed about the issues by the flight crew, powers the aircraft off and on, successfully aligns all three IRS, checks autoflight systems to be working and releases the aircraft back to service. The aircraft continues the flight after about 3:10 hours on the ground with no further anomalies. Following arrival in Marseille all three ADIRU units were replaced.

The BEA reported that IRS 2 had been noticed to drift on previous flights. The technical director therefore decided to have IRS 2 replaced on Dec 24th 2014 during a stop over in Marseille. However, due to a mismatch in labelling on the mounts for the IRS units IRS 1 was replaced, the reason for the mislabelling could not be established. There had been no maintenance activity since 2007, XL Airways operated the aircraft since delivery from Airbus in 2002, no records were available for the time 2002-2007. Following the occurrence the labels of the other aircraft were checked with no mislabelling found.

The BEA reported no fault was recorded for IRS 3, it recorded its last position at N12°25.0' E36°45.8' at the border between Ethiopia and Sudan at a ground speed of 477.9 knots, total flight time 5:02 hors until then. All values were consistent with the FDR recordings.

Tests by Honeywell established that the only possible scenario to explain the events on board was, that the selector shifted from NAV to ATT. Had the IRS been rejected and transitioned to ATT as result a fault indication would have occurred.

The loss of a single IRS does not require any pilot attention, the crew thus is not notified.

However, when due to the drift of IRS 2 the information of IRS 1 and IRS 2 was no longer coherent the flight guidance and management system rejected both IRS informations causing the autopilot and autothrust to disconnect.

FDR evidence suggests that the captain received valid position information again after BACKUP-NAV was selected. However, the position information is subsequently lost again, most likely because all three IRS were sent into in flight alignment.
Incident Facts

Date of incident
Dec 26, 2014

Classification
Incident

Airline
XL Airways France

Flight number
SE-1551

Departure
Saint Denis de la Reunion, Réunion

Destination
Marseille, France

Aircraft Registration
F-GRSQ

Aircraft Type
Airbus A330-200

ICAO Type Designator
A332

This article is published under license from Avherald.com. © of text by Avherald.com.
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