ERA18FA006 · Piper PA 25-235 UNDESIGNAT

A glider was being towed for an introductory flight with the student in the front seat and the glider flight instructor in the rear seat. The glider flight instructor recorded the flight using a digital video camera. At the end of the first video, the tow plane and glider were on the upwind leg of the airport traffic pattern, during initial climb, and the glider was about 200 ft above ground level. The glider's airspeed indicated about 60 knots, and the glider's position relative to the tow plane was consistent with a high-tow position (that is, the glider was positioned slightly above the wake of the tow plane). The video then stopped for unknown reasons. About this time the instructor diverted his attention away from the tow plane. When the instructor looked back toward the tow plane, he did not immediately see it, but then noted that it was below and right of the glider.

Analysis and extrapolation of the tow plane and glider flightpaths revealed that during this time the tow plane turned toward the crosswind leg, stopped climbing, and began to descend while the glider continued to climb. The video then showed that the glider's nose then yawed right, and the yaw string was nearly 90° right. The glider's yellow tow rope release handle moved partially aft, and then a snapping sound was heard followed by the tow rope release handle extending further. The tow plane's elevator was in the "up" position at that time, and the glider then turned left and returned to the airport. The tow plane descended nose down into the backyard of a residence and was consumed by postimpact fire.

Microscopic examination of the tow rope separation revealed features indicative of overstress separation. This evidence and the snapping sound heard on the second video are consistent with the tow rope breaking about 1 second before the glider instructor fully pulled the tow rope release handle. The up-elevator and nose-down tow plane descent was consistent with the tension on the tow rope lifting the empennage of the tow plane. Examination of the tow plane did not reveal evidence of any preimpact mechanical malfunctions that would have precluded normal operation, and there was no evidence that the pilot attempted to cut the tow rope with the tow plane's guillotine.

The tow pilot had a history of hypertension, high cholesterol, and nonocclusive coronary artery disease. The pilot was taking medications to treat these conditions, but they were not considered impairing. According to the tow pilot's autopsy report, his heart was enlarged and he had mild-to-moderate atherosclerosis. Although the pilot's significant coronary artery disease increased his risk for an acute cardiac event, no evidence was found during the autopsy indicating that such an event occurred.

On the basis of the available evidence, the investigation could not determine if the tow plane pilot experienced a medical event during the accident flight or why the airplane stopped climbing at 200 ft rather than continuing to climb to the tow-release altitude.

Verbatim NTSB analysis and probable cause from the NTSB dataset