The incident, the unavailability of the three IRSs, led to deteriorated flight control laws and to the deteriorated autonomous navigation capability of the aeroplane. This unavailability occurred in four main steps:
- at 10:34, the switchover of IRS 3 from NAV mode to ATT mode, probably the result of a manual action by a member of the crew;
- at 13:36, the disconnection of the AP and A/THR, probably further to a navigational discrepancy between IRS 2, which had been drifting since take-off, and IRS 1, and the loss of position and navigation data on the NDs as a result of the previous event according to the logic specific to Honeywell P3 and earlier FMS standards. This data was found again two minutes later when the crew activated the BACK-UP NAV mode;
- shortly before 13:44, the loss of position data on the left side following the BACK-UP NAV mode being exited on the left (captain’s) side;
- at 13:44, the in-flight alignments of the three IRSs, probably consecutive to improvised actions by the crew possibly without conferring, during attempts to retrieve position data on the captain's ND.
The loss of positions was a major concern for the crew. The new FMS standards would have prevented this loss of data. Greater robustness in maintaining autonomous navigation capabilities is important for flying over the ocean or desert areas, or close to conflict zones(43).
The problem of the previously identified drift of an IRS was not resolved because of the mislabelling on the chassis. However, the drift of a single IRS in flight is not a major abnormal situation.
The following factors were considered to try to understand the reasons for the probable non-standard crew actions on the IRS rotary switches, without it being possible to specify their degree of contribution:
- the ergonomy of the ADIRU control panel not visually recalling the irreversible nature of IRS mode changes;
- the presentation of IRS data on the POSITION MONITOR page of the MCDU in a numerical order (1,2,3) which is different to the layout of the rotary switches (1,3,2), which is representative of the aeroplane’s architecture;
- the logic for presenting ECAM data to the crew preventing them from understanding the reason for the disconnection of the AP and the loss of positions which occurred at 13:36;
- the low exposure of crews to IRS failure situations.
Lastly, the influence of other factors cannot be excluded, specific to the crew (e.g. fatigue or individual knowledge) or the flight environment that the investigation was not able to identify.
Following the incident, Airbus carried out a document review to include the generalisation of the use of the BACK-UP NAV mode to all the cases leading to the loss of positions on the NDs, the most probable case identified still being the double loss of FMSs.