What happened
On 15 February 2005, a Diamond DA-40 D aircraft, registration SE-LTF, was conducting a training flight near Gråbo, Sweden. The flight, which originated from Göteborg City Airport, was intended to practice takeoffs and landings. After an initial successful practice session, the crew attempted a second simulated emergency landing. During this second maneuver, the instructor applied full throttle, but instead of producing thrust, the engine RPM surged to extreme levels without any corresponding increase in aircraft performance.
Following the loss of thrust, the instructor took control of the aircraft while the student transmitted an emergency radio message. The aircraft subsequently landed in a plowed, partially snow-covered field. While there were no injuries to the two occupants, the impact with the uneven, frozen ground caused substantial damage to the propeller, wings, and landing gear.
The investigation
SHK investigators examined the engine' and FADEC (Full Authority Digital Engine Control) data. The analysis revealed that during the first practice, the engine RPM had exhibited oscillations before stabilizing. However, during the second attempt, the engine RPM increased far beyond normal operating parameters, only stabilizing once the engine's overspeed protection system intervened at approximately 4800 engine RPM.
Physical inspection of the engine assembly focused on the clutch mechanism, which is designed to slip at a specific torque to protect the gearbox. Investigators found that the flywheel exhibited blue discoloration, a sign of extreme overheating. The clutch plate itself showed significant wear, with its friction coating almost entirely depleted. Furthermore, the investigation of the spring stacks revealed that several disc springs had been permanently flattened, indicating that temperatures within the stack had exceeded 130°C.
Findings
- The primary cause of the loss of thrust was a failure of the engine clutch to maintain the required torque.
- The clutch mechanism had been compromised by an improper assembly of the disc springs. Specifically, the springs were arranged in a 3-3-3-2-1 pattern rather than the manufacturer-specified 3-3-3-2-2 pattern.
- This incorrect assembly caused certain springs to reach their maximum compression and flatten, which led to a rapid loss of spring force as the clutch plate wore down.
- The overheating of the flywheel and the depletion of the friction coating were direct results of the clutch slipping excessively during operation.