What happened
On 11 September 2011, an Airlinair ATR 72-201, registration F-GVZG, was performing a scheduled passenger flight from Lyon Saint-Exrabéry to Marseille Provence. While executing an ILS approach to runway 13L, the aircraft touched down normally. However, approximately twelve seconds after wheel contact, at a speed of roughly 70 kt, the flight crew experienced a sudden system anomaly.
The captain reported a simultaneous fault in the MFC 1B and 2B modules, noting that the aircraft's steering wheel became unresponsive and that braking performance felt abnormal. In response to the loss of control, the pilot initiated emergency braking. During the landing roll, the aircraft deviated from the runway centerline, oscillating left and right, before coming to a stop two-thirds of the way down the runway, slightly off the pavement. The nose gear was found positioned at a 90-degree angle to the aircraft's longitudinal axis. There were no fatalities or injuries among the 25 passengers and four crew members on board.
The investigation
The BEA examined the aircraft's flight recorders and the MultiFunction Computer (MFC) architecture. The investigation focused on the simultaneous failure of the 1B and 2B modules, which are responsible for processing various aircraft systems and generating crew alerts. The investigators analyzed the synchronization process of the MFC audio module, which generates aural warnings.
Technical examinations of the audio module revealed that when the system attempts to synchronize two modules to generate a single, prioritized warning, a software error can occur. This error can lead to the transmission of erroneous information during the synchronization of microprocessors, resulting in the simultaneous loss of multiple functions, including nose wheel steering, stall warnings, and touchdown protection.
Findings
- The runway excursion was caused by the loss of nose wheel steering control, which resulted in the gear turning perpendicular to the aircraft's axis.
- The loss of steering was triggered by a simultaneous double fault on MFC 1B and 2B modules.
- The root cause of the double fault was a software failure occurring during the synchronization of aural warnings.
- The crew's ability to manage the fault was hindered by the high workload during the landing phase and the fact that the loss of the aural warning meant the fault was often only detected through its physical consequences, such as the loss of steering.
- While the captain reported a sensation of reduced braking power, the investigation could not provide a technical explanation for this specific perception.