What happened
While preparing to arrive at BETTY, the flight crew manually adjusted the hold speed in the flight management computer (FMC). The original VNAV profile had calculated holding between FL 150 and FL 160, but the actual hold level was higher. Because the crew did not re-evaluate the speed for the new altitude, the aircraft entered the holding pattern at a speed 15 kt below the required target.
As the aircraft transitioned into the hold, the speed dropped to 220 kt, which was below both the selected speed of 225 kt and the minimum manoeuvring speed of 223 kt. This reduction in speed, occurring during the turn into the hold, resulted in pre-aerodynamic stall buffeting and the likely activation of the stick shaker. The pilot flying attempted to stop the descent before completing the necessary approach-to-stall recovery procedures, while the pilot monitoring failed to identify or call out the incomplete recovery. These actions led to pilot-induced oscillations and subsequent stick shaker activations.
The investigation
The investigation focused on why the crew did not identify the need to update the hold speed for an altitude above FL 200. It was found that the crew had verified the FMC speed by comparing it to the flaps-up manoeuvre speed, a method that did not account for the specific requirements of higher altitudes. Furthermore, the crew did not utilize the "best speed" function on the FMC hold page, which would have automatically provided a speed appropriate for the aircraft's weight and configuration.
Investigators also found that neither the aircraft manufacturer nor the operator provided procedures requiring a hold speed re-evaluation when the hold level changed during the descent planning stage. Additionally, the crew's attention was diverted by other operational tasks, leading to a failure to notice the speed dropping below the minimum manoeuvring and stall warning thresholds on the primary flight display.
Findings
- The crew did not recognize the necessity of recalculating hold speed for the increased altitude.
- The aircraft speed fell below the minimum manoeuvring speed and the selected speed.
- The pilot flying's attempt to arrest the descent rate interfered with proper stall recovery techniques.
- The pilot monitoring failed to call out the incomplete recovery actions.
- The operator provided insufficient training and guidance regarding stall prevention and recovery at high altitudes or with engines above idle.
- A passenger in seat 63C was not wearing a seatbelt during the activation of the stick shaker.