Background
On 4 June 2006, a regional gliding competition was being held at Hausen am Albis airfield. A morning briefing for tow pilots established a three-step start command procedure: the start coordinator would first call the tug to line up in front of the glider, then order the rope to be pulled out, and finally announce "the rope is taut" as clearance for takeoff. The tug pilot was to decide when to commence the takeoff run.
Sequence of Events
At approximately 14:45 local time, the pilot of the ASW 27 glider (registration HB-3269) prepared for an aerotow from runway 27. The Maule M-7-235 tug (HB-KDM) taxied into position. A ground helper (A) connected the towrope and moved to the right. The glider pilot extended and retracted the airbrakes; with the brakes retracted, the wheel brake was inactive. He then set the flaps to negative for the initial roll.
After signaling readiness, helper B lifted the left glider wing to horizontal, indicating the glider was ready. The start coordinator radioed the tug to pull the rope taut. The tug rolled forward, and when the rope became taut, a jolt caused the unbraked glider to roll forward. The tug pilot then stopped, leaving the rope slack. The glider rolled over the rope with its wheel. The coordinator, unaware of this, radioed "rope is taut," which the tug pilot interpreted as takeoff clearance.
However, the tug pilot noticed a previously landed glider being cleared from the runway and radioed they would wait. The glider pilot heard "we wait" and anticipated a jolt when the tug resumed. Meanwhile, ground helpers, thinking the tug had stopped due to the overrun, moved in front of the glider wings to push it back. The coordinator saw them and radioed "Stop." The tug pilot did not hear the command and, seeing the runway clear, applied full power.
At that moment, the helpers were positioned near the leading edges of the glider's wings. As the glider accelerated, its wings struck both helpers. Helper A rolled backward; helper B fell onto his back, striking his head on the pavement, sustaining serious injuries. Neither pilot noticed the collision and continued the takeoff. Approximately two to three minutes later, the tug pilot heard over the radio that towing was interrupted due to a small accident, and both aircraft landed.
Injuries and Damage
One ground helper (helper B) suffered serious head injuries. The other helper sustained no reported injuries. Both aircraft were undamaged, and no third-party damage occurred.
Probable Cause
The accident is due to the fact that the glider's wings hit two assistants following the application of an inappropriate departure procedure.