Accident Summary
On 14 July 1978 at approximately 1829 hours MEZ, a Piper PA-18 B (registration HB-OVZ) operated by Segel und Motorfluggruppe Grenchen was involved in an accident at Grenchen Airport. The aircraft was performing a glider towing mission, its 39th tow flight of the day. After releasing the glider (a K-13) normally, the pilot entered the landing pattern for runway 07. During the final turn, the aircraft continued to bank instead of leveling out, struck the ground with the right wingtip, nosed over, and came to rest in a tail-up position. The pilot was seriously injured and the aircraft was destroyed. Minor third-party property damage occurred.
Pilot Information
The pilot held a valid private pilot license and was authorized for the flight. He had a total of 220 hours and 53 minutes of flight experience, with 170 hours and 57 minutes on the accident aircraft type. In the 90 days prior to the accident, he had logged 166 hours and 55 minutes, all on the accident type. The pilot was born in 1957.
Aircraft Information
The aircraft was a Piper PA-18 B (Super Cub) with registration HB-OVZ. It was owned and operated by Segel und Motorfluggruppe Grenchen, based at Grenchen. The aircraft had a valid airworthiness certificate. Weight and balance were within limits. The flaps were set to position 1. The last 100-hour inspection was on 19 June 1978, and the last 50-hour inspection on 6 July 1978. The elevator trim was approximately neutral (slightly nose-up). No pre-existing mechanical defects were found that could have caused the accident.
Weather Conditions
Wind was from 070° at 6 knots. Visibility was 10 km. Cloud cover: 2/8 altocumulus at 1350 m above ground, 3/8 altocumulus at 3000 m, and 1/8 cumulonimbus at 1350 m.
Accident Sequence
According to witness statements and the pilot's account, after the towing release, the pilot entered the downwind leg and extended flaps to position 1. He noted a crosswind from the Jura but saw no abnormal indication on the windsock. During the turn from base to final for runway 07, the aircraft was at about 50-60 m above ground. The bank angle was approximately 40-45°. The pilot reported that he did not use the rudder, or if he did, unconsciously. He felt the control stick become blocked from the center position. Witnesses observed the aircraft continue to turn instead of leveling out, then enter a spin or spiral. The aircraft struck the ground first with the right wingtip, then slid sideways on the nose and left wing, coming to rest in a nose-down vertical position.
Investigation Findings
The investigation determined that the aircraft's airspeed dropped below the minimum safe speed during the landing turn. The reason for this loss of speed could not be conclusively determined. It was noted that the pilot had performed 39 towing flights that day, and possible fatigue may have affected his attention.
Probable Cause
The accident was due to the aircraft's airspeed dropping below the minimum safe speed during the landing turn.