What happened
On July 20, 2012, an Airbus A321, registration F-GTAN, operated by Air France, was descending toward Paris Charles de Gaulle. Earlier in the flight, the crew had noted a failure in the SEC1 computer, which required them to avoid using spoilers during the flight.
During the descent, the pilot flying (PF) attempted to increase the rate of descent by moving the thrust levers to IDLE. While this action was intended to accelerate the descent, it inadvertently disconnected the auto-thrust (A/THR) while the autopilot (AP) remained engaged, creating a "mixed" flight control configuration (AP ON A/THR OFF). Because the engines were at idle, the aircraft could not maintain its selected speed of 220 kt.
As the aircraft reached 4,000 ft, the autopilot attempted to maintain altitude by increasing the pitch, which caused the airspeed to decay. Simultaneously, the crew became focused on scanning the external environment for reported conflicting traffic. The airspeed eventually dropped to 177 kt, triggering the Alpha PROT (high-angle-of-attack protection) and disconnecting the autopilot. The aircraft entered a descent of approximately 1,000 ft/min, reaching a minimum altitude of 3,840 ft before the crew applied power and recovered the flight parameters.
The investigation
The BEA examined the crew's management of the auto-thrust system and their coordination during the descent. The investigation focused on why the speed decay went unnoticed and why the crew failed to recognize the activation of the high-angle-of-attack protection. The investigators also reviewed the flight director (FD) commands, which were providing pitch-up instructions that were inconsistent with the need to reduce the angle of attack during a low-speed event.
Findings
- The crew's decision to move the thrust levers to IDLE caused the unintentional disconnection of the auto-thrust.
- Poor crew coordination meant the pilot monitoring (PM) was unaware of the change in automation state.
- The crew's focus on external traffic led to a failure to monitor primary flight parameters, specifically airspeed.
- The pilot flying prioritized maintaining altitude over airspeed, which inadvertently kept the aircraft within the high-angle-of-attack protection mode.
- Training and documentation were found to be insufficient regarding the identification of high-angle-of-attack protection when operating in a mixed automation configuration.
Safety action
- The BEA recommended that EASA and the FAA develop specifications for systems that provide better alerts and protection for low-speed situations across all aircraft phases.
- A recommendation was made to enhance initial and recurrent training regarding low-speed flight and the identification of high-angle-of-attack protection.
- The BEA recommended that EASA and manufacturers reconsider the logic of flight director displays to ensure they provide relevant instructions or disappear when the autopilot disconnects involuntarily.