Accident
On 19 August 2018 at approximately 16:30 local time, a Dynamic WT9 ultralight (registration 68ADW) towing a Centrair 101A Pégase glider (registration F-CGOT) took off from runway 01 (grass) at Colmar Houssen aerodrome. During the tow, the glider moved to a position above the ULM. The ULM pilot radioed "what are you doing?". The air traffic controller observed a height difference of about ten meters between the two aircraft at an altitude of around thirty meters. The ULM then pitched down several tens of degrees, lost altitude rapidly, the tow rope broke, and the ULM struck the ground near the threshold of runway 19. A fire broke out after impact. The glider pilot landed without further incident on the opposite runway direction.
Pilot Information
The ULM pilot, aged 63, held a multi-axis ULM pilot license since 2007, with a glider towing endorsement since 2014. She also held a PPL(A) since 1975 with a glider towing qualification since 2009, and a glider pilot license since 1974 with qualifications for aerotow, self-launch, touring motor glider (TMG), and winch launch. She had over 800 flight hours on airplanes and over 2,000 hours on gliders.
The glider pilot, aged 65, held a Polish glider pilot license since 1996 with 421 total hours (288 as pilot-in-command), a Polish PPL(A) since 1997 with a towing qualification (though not towing for the club), and a multi-axis ULM license since 2006. He had not flown regularly in recent years, resumed in January 2016, was soloed in September 2017, and had about 18 hours between April and August 2018. He recalled that shortly after takeoff he felt very high and that the ULM was descending. He attempted to push the stick forward to adjust altitude without success, then reflexively deployed airbrakes. He looked away to ensure airbrakes were extended, and when he looked back, the ULM had disappeared from his field of view. He then released the tow rope once, turned right, retracted airbrakes, and landed opposite direction on runway 19. He did not remember hearing any radio calls after takeoff clearance. He noted a prior similar experience as a tow pilot years earlier.
Aircraft Information
The Dynamic WT9 ULM was approved for glider towing. Its mass and center of gravity were within limits. It was equipped with optional right-side rearview mirror in addition to standard left mirror. The glider Centrair 101A Pégase was equipped with a central under-fuselage hook (winch type only), forward of the nose wheel.
Examination of Site and Wreckage
The ULM wreckage was found at the end of runway 01. Examination showed the ULM had a zero or slightly nose-up attitude, zero bank, and high vertical speed at impact. A post-impact fire prevented full control cable examination, but no pre-impact fractures or damage were found on observable components. The propeller was rotating when blades struck the ground. The tow rope was broken at the glider end, at the knot inside the fuse holder; the fuses (red 750 daN and yellow 400 daN) were intact and found near the glider at the threshold of runway 19. The ULM's cable-cutting system showed no pre-accident malfunction; the glider's release system was functional.
Tow Rope Fuses
Two fuses were used: a working fuse (red, Tost, 750 daN) and a reserve fuse (yellow, Tost, 400 daN) due to space constraints in the olive. The WT9 manual specifies a 300 daN fuse. The club had progressively increased fuse strength from 300 daN to 400 daN to 750 daN after premature ruptures and ovalized holes, with a weaker reserve fuse because the olive could not hold two 750 daN fuses.
Guidance on High Tow Positions
The "Manuel du pilote vol à voile" (Glider Pilot Manual) states that if the glider rises too high behind the tow plane, it can become dangerous and may cause the entire assembly to enter an uncontrollable dive. It advises: if too high but within tolerance, gently pitch down to return to normal position; if the cable slackens, briefly extend airbrakes to tension it. Beyond the high limit, the glider pilot should pitch down to slacken the cable without deploying airbrakes and then release.