What happened
On August 18, 2007, a towing operation at Hoogeveen Airport (EHHO) resulted in a significant accident involving a Diamond HK 36 TTC (registration PH-1224) and a Schleicher ASK-21 glider (PH-952). During the twentieth tow of the day, shortly after the aircraft combination departed the ground, the engine of the towing aircraft began to vibrate violently and run irregularly.
At an altitude of approximately 150 to 200 feet, the tow pilot communicated an engine failure via radio and instructed the glider crew to disconnect. The glider, occupied by an instructor and a student, was at a very low altitude. Upon disconnecting, the instructor attempted a left-hand turn to return to the airfield. During this maneuver, the glider lost airspeed and entered a stall. The aircraft's left wing struck a bicycle shed, and the subsequent rotation caused the tail to strike an advertising sign before the glider ultimately crashed into a parking lot. The accident resulted in one student injury (a spinal fracture) and the total destruction of the glider, along with damage to ground structures and a vehicle.
The investigation
The investigation examined the mechanical state of the PH-1224 and the flight maneuvers performed by both crews. It was noted that the towing aircraft had recently undergone engine maintenance involving a cylinder head replacement. A subsequent test flight by a technician revealed excessively high intake pressure, which was attributed to a stuck waste-gate valve.
Investigators also analyzed the flight path and the surrounding environment. The investigation established that the tow pilot's decision to command an immediate disconnection placed the glider in a precarious position, as the aircraft was already at a low altitude and constrained by nearby buildings. The investigation also looked into the lack of standardized emergency procedures for tow pilots facing engine issues at low altitudes.
Findings
- The primary cause of the accident was an engine malfunction in the PH-1224 occurring at a critical low altitude immediately after takeoff.
- The tow pilot instructed the glider to disconnect before maneuvering the tow combination into a position that would allow the glider a safer landing path.
- The proximity of buildings to the western side of the airfield significantly limited the available maneuvering space for both aircraft.
- The glider entered a stall during a low-altitude turn due to the loss of airspeed following the disconnection.
- The presence of surrounding structures poses a structural safety risk for aircraft with limited climb performance, particularly during towing operations on runway 28.