Alvord Desert, OR, USA
N112AJ
Bolkow PHOEBUS
The glider pilot took off utilizing the auto launch (vehicle-tow) method. Photos taken by a witness revealed that the glider overran the towline, which resulted in a slack towline. The glider then pitched up and, after reaching about 100-150 ft above ground level, rolled to the left and impacted terrain in a near vertical attitude. The tow vehicle driver stated that the truck had not yet accelerated to 60-65 mph needed for the release of the tow cable from inside the cockpit; instead, the truck had only accelerated to 55 mph before the spotter told the driver to stop as the glider impacted the terrain. A slack towline can be caused by a change in speed of either the glider or tow vehicle. If not corrected, a slack towline can result in the loss of control in flight. Also, a change in wind gradient can have a noticeable effect on a glider’s speed during ground launches. When a slack towline is encountered, a pilot should communicate with the tow vehicle to change speed, adjust pitch to control airspeed, or release the tow rope and land straight ahead. The investigation could not determine why the slack towline occurred. Nevertheless, the pilot did not follow recommended procedures for responding to a slack towline and subsequently lost control of the glider.
HISTORY OF FLIGHTOn July 17, 2019, about 1225 Pacific daylight time, a Bolkow Phoebus B-1 glider, N112AJ, was destroyed when it was involved in an accident in the Alvord Desert, Oregon. The pilot was fatally injured. The glider was operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. The flight was taking part in an annual soaring club event. The glider was launched using the ground launch, automobile tow method in which the glider is connected to a vehicle by a towline that is also fitted with a small parachute which is used to help find the end of the towline after the pilot disconnects in flight. The tow vehicle’s acceleration causes the glider to gain speed and begin a climb. The crew onboard the tow vehicle consisted of a driver and spotter. Upon arrival to the accident site, local law enforcement personnel interviewed several witnesses. One witness who did not observe the takeoff but witnessed the crash, reported that the weather conditions consisted of sunny, partly cloudy skies with 5 mph wind. He stated that the gliders are launched from the ground using a tow vehicle and about 4,000 ft of towline. He further stated that the gliders typically lift off when the tow vehicle reaches 38 mph (the gliders stall speed) and typically the tow vehicle will achieve 60-65 mph before the pilot releases the towline. The tow vehicle driver indicated that they had only accelerated to 55 mph before the spotter told her to stop as the glider impacted the terrain. The spotter stated that the glider was about 1 mile behind the tow vehicle and connected to the tow line, and the takeoff appeared normal but then without warning the glider crashed. Another witness stated that once airborne, the glider was “nose way up” and he observed the glider rolling on its left side toward the ground. He believed that the left wing and the nose struck the ground at about the same time. Several other witnesses described the takeoff as normal but observed the glider “kitted up” (nosed up) and rotated to the left and impacted the terrain. Review of photos that captured the takeoff through the ground impact revealed that as soon as the glider became airborne, the towline became slack, resulting in the line parachute opening. The glider began to overrun the towline and parachute, while the pitch attitude remained the same. The glider then appeared to pitch up and roll left, at which point the towline caught back up with the glider as it impacted the ground. Figure 1. Slack towline / fully inflated parachute (witness photo) The photos showed that the glider climbed to about 100-150 ft above ground level before rolling to the left; the left wing then dropped, and the glider impacted terrain in a near vertical attitude. Figure 2. Left bank/stall (witness photo) WRECKAGE AND IMPACT INFORMATIONExamination of the glider revealed that all major portions of the glider were accounted for. The flight control system showed no anomalies that would have prevented normal operation. ADDITIONAL INFORMATIONThe FAA-H-8083-13A, “Glider Flying Handbook” chapter 8 “Ground Launch Abnormal and Emergency Procedures” states: Because the launch equipment operator is remote from the glider, it is not uncommon for initial tow to be too fast or too slow. If the towline speed is too great, the glider is not able to climb very high because of excessive airspeed. If the towline speed is too low, the glider may be incapable of lift-off or could stall after becoming airborne. Once airborne, the glider could be incapable of further climb. The pilot should use appropriate signals to direct the launch operator to increase or decrease speed. The pilot must anticipate and be prepared to deal with these situations. In the event these abnormal situations develop, the pilot’s only alternative may be to release the towline and land ahead. Wind gradient (a sudden increase in windspeed with height) can have a noticeable effect on ground launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, indicated airspeed increases that could exceed the maximum ground launch tow speed could occur. The pilot must adjust the airspeed to deal with the effect of the gradient. When encountering a wind gradient, the pilot should push forward on the stick to reduce the indicated airspeed.
Pilot’s failure to properly respond to a slack towline during takeoff, which resulted in a loss of control of the glider.
Source: NTSB Aviation Accident Database
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