What happened
On October 25, 2006, an ATR 42-500, registration F-GPYF, operated by Airlinair, was arriving at Lyon Saint-Exupéry from Basel-Mulhouse. Upon touchdown, the crew noticed that the 'LO PITCH' light for the right engine failed to illuminate. To prevent asymmetric thrust, the propeller control electronic system automatically reduced the right engine's rotation speed, which subsequently caused the loss of the right engine's AC generator (ACW 2).
During the taxi phase, the crew attempted to diagnose the engine anomaly but were interrupted by communications with ground control regarding a parking stand discrepancy. Following standard airline practice to expedite passenger disembarkation, the commander requested the left engine be placed in feather to shut it down before reaching the final parking position. This action caused the loss of the left engine's AC generator (ACW 1), which in turn deactivated the electric pumps for the green and blue hydraulic circuits.
As the aircraft approached the parking stand, the crew became unaware that the primary hydraulic pumps were no longer powered. While the auxiliary pump maintained enough pressure for steering and emergency braking, the main braking system became ineffective. As the aircraft approached the stand, the pilot attempted to use the brakes, but they failed to stop the aircraft. In an attempt to avoid hitting a ground worker or the runway edge, the pilot steered toward a ground power unit (GPU), resulting in a collision.
The investigation
The investigation focused on the crew's failure to detect the electrical and hydraulic degradation. Investigators examined the Crew Alerting Panel (CAP) and the sequence of alarms. It was noted that the crew had become accustomed to certain 'MASTER CAUTION' alarms during taxiing, such as those resulting from turning off probe heating. The investigation also looked into the maintenance of the propeller pitch mechanism and the accuracy of the aeronautical charts being used by the crew, which contained outdated parking stand numbering.
Findings
- The crew did not verify the status of the CAP alarms during taxiing because they were focused on diagnosing the initial engine pitch anomaly.
- A period of reduced attention during the taxi phase, following the high-workload landing phase, contributed to the oversight.
- The loss of hydraulic pressure in the primary circuits was not immediately apparent because the hydraulic pressure gauges initially showed stable readings.
- The crew's practice of shutting down the left engine before reaching the parking stand created a cascading failure of electrical and hydraulic systems.
- Outdated Jeppesen charts led to confusion regarding the specific parking stand location.