What happened
On August 14, 2019, a private flight involving a Mooney M20R (registration HB-DIL) was performing takeoff maneuvers at Lommis airfield (LSZT). During the takeoff roll on the grass runway, the pilot followed a pre-planned decision rule to continue the takeoff if a specific speed was reached by the midpoint of the runway. While the initial acceleration appeared normal, the aircraft failed to gain sufficient altitude or airspeed after lifting off near the end of the runway.
As the aircraft climbed at a low altitude, it struck a traffic sign, which caused the left wing tank to be punctured by a second sign. This resulted in fuel leakage. Unable to maintain flight, the pilot performed an emergency landing approximately 20 meters past the second sign. The aircraft's nose gear collapsed upon impact, bringing the engine to a sudden halt. The two occupants, including a passenger, escaped the aircraft without injury, though the aircraft sustained heavy damage.
The investigation
Investigators examined the engine, the aircraft's maintenance records, and GPS data. The investigation focused on the engine's performance and the pilot's decision-making process regarding the takeoff roll. Analysis of the engine revealed that the Continental IO-550-G engine was suffering from significant mechanical issues. Specifically, the internal baffle of the muffler was destroyed, and an intake valve in cylinder number three was leaking, resulting in a loss of compression.
GPS data and flight logs indicated that the aircraft's acceleration was significantly lower than expected from the start of the roll. The investigation also noted that the pilot's rotation (pitching up) occurred at a speed lower than the recommended takeoff speed, which increased induced drag and prevented the aircraft from climbing out of the ground effect.
Findings
- The primary cause of the accident was the rotation and liftoff of the aircraft at an insufficient airspeed.
- The engine's power output was compromised by a broken muffler baffle and a leaking intake valve.
- The pilot failed to recognize that the aircraft was not achieving the necessary acceleration during the takeoff roll.
- A lack of proper maintenance was evident, as the muffler damage was not detected during the most recent 100-hour inspection.
- The use of the "70/50 rule" for takeoff decisions may have contributed to the failure to abort the takeoff earlier.