Accident Overview
On the morning of 20 October 2023, a Cessna 208 aircraft, registered VH-UMV, began takeoff for a planned climb to 15,000 ft to drop 16 parachutists. At approximately 500 ft during the climb, the pilot detected a partial power loss consistent with an abnormal activation of the torque and temperature limiter (TTL). The pilot reduced power to prevent engine surging, but the combination of low power and airspeed caused the aircraft to collide with water before continuing into a field. Six parachutists sustained serious injuries, while the pilot and eight parachutists suffered minor injuries. The aircraft was substantially damaged.
Operator's Prescribed Actions
The TTL system normally reduces fuel flow to maintain the lower of 100% torque or 650°C exhaust gas temperature (EGT). However, the TTL manufacturer indicated it could restrict fuel flow to limit maximum power to about 62% torque. The operator's pilots had reported six engine surging events in VH-UMV over five months, associated with abnormal TTL activations, but maintenance actions could not identify or resolve the cause. To prevent TTL activation, the operator advised pilots to limit torque to 95% and EGT to 640°C, despite the aircraft flight manual supplement defining take-off power as 100% RPM and 100% torque or 650°C EGT. The operator had not declared the TTL and single red line (SRL) systems inoperable, which would have required manual limit monitoring. TTL-induced power reductions were reported as momentary, with power returning after limits fell below thresholds.
The operator created an engine power loss checklist, as no manufacturer checklist existed for partial power loss. The first item was to immediately push the elevator forward if climbing to prevent airspeed decay. The checklist then instructed a significant power reduction if engine RPM was above 60% or surging, in preparation for switching off the TTL. The operator did not specify a minimum height at which such a power reduction was appropriate. The contributing factor noted was that this checklist increased the risk of loss of control or ground collision.
Pilot Actions
During takeoff, the pilot applied full power, reaching 100% torque before reducing slightly to stay under the operator's 95% torque limit. Whether the torque or temperature limit was reached during the initial climb could not be determined, as parameters were not recorded. The pilot detected a power reduction consistent with abnormal TTL activation, which may have limited power to about 62%. According to previous safety reports, such a reduction would require lowering the nose to prevent aerodynamic stall, per the operator's checklist.
However, the pilot initially did not lower the nose; instead, they moved the power lever aft, reducing power. This matched the operator's procedure for TTL deactivation. The as-found switch positions suggested the pilot may have inadvertently selected the SRL switch instead of the TTL, but either action would have deactivated the TTL. Because the nose was not lowered, the aircraft approached a stall, and the stall warning horn sounded. The pilot then lowered the nose, but due to low height, low airspeed, and low power, a collision was unavoidable.
Investigation
The investigation examined the TTL activation, response actions, seating configuration, weight and balance, occupant safety, the number of parachutists, and operational guidance from the Australian Parachute Federation manual approved by the Civil Aviation Safety Authority (CASA).