What happened
On 30 December 2016, a Boeing 737-800, registered CN-ROJ, operated by Royal Air Maroc, was diverted from Paris-Orly to Lyon-Saint Exupéry due to technical malfunctions. During the initial approach to Paris-Orly, the crew attempted to engage the second autopilot, but the flight control computer failed to receive radio-altimeter data, causing the first autopilot to disengage. The crew subsequently requested a diversion to Lyon.
Upon approaching Lyon-Saint Exupéry, the aircraft experienced a sudden deviation from the localizer path. This lateral excursion was caused by erroneous data from the left Air Data Inertial Reference Unit (ADIRU). This malfunction triggered an "IRS FAULT" warning, which led to the disengagement of the remaining autopilot. The crew was forced to fly the second approach manually.
During the final stages of the second approach, the aircraft became unstable. The crew was unable to maintain the glide slope or the localizer path, and the aircraft's speed and configuration were not stabilized. As the aircraft descended, multiple EGPWS warnings, including "SINK RATE," "GLIDE SLOPE," and "TOO LOW TERRAIN," were activated. The aircraft eventually emerged from the cloud layer and performed a side-step to land on runway 35R.
The investigation
The BEA examined the flight data from the Quick Access Recorder (QAR) and analyzed the sequence of events through air traffic control recordings. The investigation focused on the technical failure of the ADIRU and the Flight Control Computer (FCC) logic. Investigators also reviewed the crew's management of the technical malfunctions, the maintenance history of the aircraft, and the communication between the flight crew and Lyon air traffic control.
Findings
- The primary cause of the flight path deviation was erroneous data supplied by the left ADIRU.
- The FCC logic did not monitor the inertial data provided by the ADIRU during this specific flight phase, allowing the undetected error to persist until the fault became critical.
- The crew experienced a progressive loss of confidence in the aircraft systems, leading to high workload and significant stress.
- Crew Resource Management (CRM) deteriorated as the crew struggled to manage the IRS fault and the high workload of a manual approach.
- There was a lack of systematic reporting of intermittent technical malfunctions in the aircraft's technical log book by the operator's crews.
- The ADIRU's internal monitoring logic activated the "IRS FAULT" warning at a late stage in the malfunction process.