What happened
On 20 October 2023, a Cessna 208, registered VH-UMV, was performing a climb to 15,000 ft to transport 16 parachutists. During the ascent, at approximately 500 ft, the aircraft experienced a sudden reduction in power, likely caused by the abnormal activation of the torque and temperature limiter (TTL).
Following an internally developed engine power loss checklist, the pilot reduced power further to prepare for deactivating the TTL. However, the pilot did not immediately lower the aircraft's nose to maintain airspeed. This led to an aerodynamic stall and a stall warning. The aircraft subsequently struck water before coming to rest in a field. The accident resulted in 6 serious injuries and 9 minor injuries, including the pilot, and caused substantial damage to the aircraft.
The investigation
The investigation focused on the functionality of the TTL and the operator's response to engine surging. While the TTL is designed to maintain engine limits, the manufacturer had noted it could restrict power as low as 62% torque. The operator had previously experienced six similar surging events in VH-UMV over five months, which maintenance could not resolve. To avoid activation, the operator had instructed pilots to fly at lower torque and EGT limits than the aircraft flight manual allowed.
Investigators also examined the operator's custom engine power loss checklist. While the checklist required a power reduction to prepare for TTL deactivation, it failed to mandate a minimum altitude for such a maneuver. Furthermore, the investigation found that the aircraft's weight and balance calculations were inaccurate because they omitted the weight of bench seating and failed to account for the specific positioning of the parachutists. Additionally, the software used for weight calculations did not alert the crew to overloaded zones.
Findings
- The operator's engine power loss checklist lacked a requirement for a minimum safe height when reducing power, which increased the risk of a loss of control or ground collision.
- The pilot's decision to further reduce power without first lowering the nose contributed to the stall.
- Inaccurate weight and balance calculations occurred because the weight of bench seating was omitted and the loadmaster did not load passengers according to the calculated center of gravity.
- The software used for weight and balance calculations did not provide warnings for overloaded zones.
- Essential safety information regarding emergency exits and brace positions was not provided to the parachutists prior to departure.