Flight History
On Wednesday, 5 May 1976, at 1106 hours MEZ, the glider pilot departed from Birrfeld Airfield (395 m AMSL) in a K 6 CR glider (registration HB-678) on a 300 km out-and-return flight to L'Auberson and back, using an aeroplane tow. The pilot released the tow cable at 1000 m above the field. Thermal lift was weak during the tow. Believing that lift would be better under the clouds over the Jura despite their low base, the pilot headed directly for the Jura heights after release.
According to the pilot, HB-678 descended at nearly 1 m/s during the flight over the Aare valley towards Staffelegg. He encountered isolated lift zones but no significant altitude gain. After about 20 minutes of flight, he began actively searching for a landing site. West of the village of Kienberg, he spotted a small, gently sloping meadow that he considered suitable for landing. He circled the field at an estimated 200 m above ground, then returned to the nearby hills, hoping to find lift under "nice cumuli." After another 10 minutes of "sweating," he felt forced to land. He approached Kienberg from the south. Shortly before turning west towards the chosen landing field, he observed 0.5 to 1.0 m/s climb on the variometer. The pilot turned east and initiated a shallow right turn. After about 90° of turn, the variometer showed 2.0 to 2.5 m/s sink. He then tightened the turn to quickly reach the intended landing direction, losing control of the glider, which stalled over the right wing. The right wing struck a tree at half height and knocked over a telephone pole. The glider spun about its vertical axis and crashed onto the village street between houses (see annex), coming to rest just short of a garden wall at 1144 hours.
Accident location: Kienberg, Canton of Solothurn, coordinates 639'600/254‘475, elevation 555 m AMSL. Distance from Birrfeld Airfield to Kienberg: approximately 20 km.
Damage
The pilot was seriously injured; the glider was destroyed.
Findings
The pilot, born 1932, held valid glider pilot licence with passenger endorsement, and a national radiotelephony licence. Authorized glider types: all normal gliders and motor gliders. Authorized launch methods: aeroplane and winch tow. Total flight experience: 222 hours 26 minutes, of which 54 hours 54 minutes on the accident type. In the three months prior, the pilot flew 21 hours 31 minutes, including 8 hours 39 minutes on the accident type. The pilot had never conducted an outlanding before. No evidence indicated any health impairment at the time of the accident.
The glider was certified for operation. No pre-existing technical defects were found that could have influenced the accident. Weight and centre of gravity were within allowable limits.
Weather
Gliding weather forecast (excerpt) for 5 May 1976, issued at 0630 hours: General situation: increasing high-pressure influence; warm air advection from west-southwest. Inversions: none. Freezing level: 2500 m. Cloud cover: in the east, especially in the mountains, initially 3-8/8 Ac, base about 2500 m AMSL, and 1-3/8 Ci; then 1-3/8 Cu, base 1800-2100 m AMSL. Thermal lift: moderate to good.
Actual weather at accident site at accident time (from the Central Meteorological Institute): Weather: high-pressure situation, fine. Clouds: 1/8-2/8 Cu at about 1500 m AMSL, above that 3/8 Ci. Visibility: 30 km. Wind: surface: NE-ENE, 5-10 kt; at 1500 m AMSL: W 15 kt. Temperature/dew point: surface: 18°C/10°C; at 1500 m AMSL: 8°C/1°C. QNH: 1019 mbar. Hazards: wind shift between 1500 and 1000 m AMSL. Remarks: small inversions at 1000 and 1500 m AMSL. Thermal lift was generally weak, or at best locally moderate as indicated by individual cumuli.
Barograph Analysis
The aeroplane tow lasted 5 minutes and reached 1000 m above the start point. The subsequent flight until the accident was essentially a more or less flat glide lasting 33 minutes, interrupted only twice by short climbs of about 100 m and 120 m, respectively. Seven minutes before the accident, the glider was at an altitude about 240 m above the crash site. Three minutes before, this altitude was about 150 m; one minute before, 50 m. The average sink rate during the last three minutes was 0.83 m/s.
Regulations
The Swiss Federal Department of Transport and Energy's traffic rules for aircraft of 3 December 1971, Article 60, paragraph 1, states that a flight under visual flight rules must not be conducted over densely built-up areas of cities or other localities or over open-air assemblies of people less than 300 m above the highest obstacle within a radius of 600 m of the aircraft, except for take-off or landing needs or with permission.
Assessment
The gliding weather forecast was too optimistic regarding thermal lift, a fact the pilot apparently recognised too late. Barograph analysis showed that after reaching the Jura slopes, the pilot continued the flight despite precarious lift conditions and insufficient altitude reserve. The decision to land came too late. The attempt to turn into a presumed lift area over the village by turning into the opposite heading at an altitude below 50 m above ground, before initiating the landing turn, was futile and dangerous. During the turn, HB-678 fell below the minimum flying speed and stalled.
Probable Cause
- Poor in-flight decisions
- Exceeding the minimum flying speed at low altitude