What happened
On 30 July 2021, during a scheduled Search and Rescue (SAR) training flight near the Heads of Ayr, a Leonardo AW189, registration G-MCGT, experienced unexpected flight path oscillations in the pitch axis. During the flight, the crew was attempting to replicate an event from a previous flight on 5 July 2021, where similar pitch instability had occurred after selecting the Transition Down (TD) mode of the Automatic Flight Control System (AFCS).
As the crew configured the aircraft for an FMS-directed Sector Search using Autonomous Groundspeed (NGSPD) and selected the TD mode, the helicopter began to oscillate in pitch. The crew noted a sensation of reduced 'g' force and estimated the pitch changes reached approximately 20° nose-up followed by 40° nose-down. To regain control, the pilot deselected the autopilot modes and returned the aircraft to a stable flight path. There were no injuries and no damage to the aircraft.
The investigation
The AAIB investigation focused on the interaction between the AFCS software modes and the flight data recorded during both the July and May occurrences. Analysis of the flight data monitoring (FDM) revealed that the oscillations were triggered by a conflict in the software's logic.
Investigators found that when the TD mode is engaged while the NGSPD mode is active—specifically when the radar altitude is above 150 ft and airspeed exceeds 80 knots—the system attempts to satisfy two competing speed targets simultaneously. The software tries to maintain a specific groundspeed via the NGSPD mode while also attempting to reach a target airspeed via the TD mode. Because pitch is used to control these speeds, the conflicting requirements caused the aircraft to pitch up and down in a continuous cycle.
Additionally, the investigation established that the flight crew received erroneous mode indications on the Primary Flight Display (PFD). Due to the way the software encoded mode status, the simultaneous activation of the TD and NGSPD modes resulted in the display incorrectly showing the Winch Trim (WTR) mode.
Findings
- The primary cause of the pitch oscillations was a software design shortcoming in the Phase 5 version of the AFCS software.
- A priority conflict existed between the groundspeed control algorithms (NGSPD) and the airspeed control algorithms (TD/IAS) when certain altitude and speed thresholds were met.
- The flight crew received misleading mode annunciations on the PFD due to a binary encoding error when multiple modes were active on the same axis.
- The crew's decision to replicate the previous event was driven by a desire to understand whether the issue was caused by manual configuration errors or an automated system fault.