Preliminary and Flight Sequence
On Monday, 13 July 1987, the pilot planned a VFR flight from Basel-Mulhouse to Bex with two passengers. The pilot had landed at Bex for the first time on 6 July 1987 with the same aircraft and used previous flight preparation data, updated as needed. A prior landing permission (PPR) was obtained by phone on 12 July 1987.
The aircraft departed at 0742 hours and the VFR flight to Bex proceeded normally. The pilot reported positions over the Bex frequency (122.15 MHz): Chillon, Novembre, and downwind, but received no response.
During downwind, the pilot noted the landing T indicated runway 33 and the windsock hung vertically (no wind). The pilot described the approach: landing gear extended, flaps set to position 1 on downwind, position 2 on base, and position 3 at the end of base, with an airspeed of 85 knots. On short final, after clearing the last obstacle with a normal glide angle, the descent rate increased abnormally when power was reduced. The pilot responded by adding power and pulling for flare but felt no response from the elevator control; the aircraft drifted right, turned, and left the runway.
The aircraft struck the grass runway hard approximately 80 meters before the threshold of runway 33, causing the right main landing gear leg to collapse. It came to rest in a cereal field about 130 meters past the threshold after a 150-degree turn.
A witness observed no wind four minutes before the accident. Just before impact, at a height of about 6-8 meters, the aircraft was level but the wind was from behind, creating a tailwind.
Established Facts
- The pilot held a valid license and was qualified for the flight.
- No health issues during the flight were indicated.
- The aircraft was certified for VFR/IFR operations.
- The pilot reported no technical malfunctions related to the accident.
- Aircraft weight and center of gravity were within limits.
- Landing distances per flight manual (from 15 m height): calm wind – 760 m ground roll, 420 m; with 10 knots tailwind – 1000 m ground roll, 600 m.
- Available landing distance on runway 33 was 600 m.
- Minimum control speed (VMCA) was 66 knots indicated.
- Wind was calm when the aircraft was on downwind for runway 33.
- A tailwind arose while HB-LKP was on final approach.
- The pilot did not notice the tailwind during final.
- After clearing the last obstacle, the pilot reduced power.
- The pilot could not arrest the high descent rate with elevator input.
- On ground contact, the right landing gear leg collapsed.
- At impact, the right engine was producing low power, as indicated by ground marks and propeller deformation.
Analysis
The accident resulted from an uncontrolled descent due to insufficient speed following a rapid wind shift. The pilot was familiar with Bex airfield, having landed there without difficulty one week earlier with the same aircraft. He had good experience and recurrent training on type.
Wind was calm during downwind but shifted suddenly to a tailwind on final. This phenomenon is not uncommon at Bex at that time of day. The pilot did not notice the change, nor was he warned by radio as airport personnel had no time to alert him.
Expecting a calm-wind approach, the pilot focused on the aiming point, aware of the short runway. The already low airspeed (aircraft in landing configuration 6-8 m above ground) quickly dropped below the minimum.
Likely, the pilot's concentration on the short runway and obstacles led to insufficient attention to airspeed, allowing it to drop unnoticed. The aircraft was too low for a timely correction.
The geographical situation of Bex airfield subjects it to sudden changes in wind strength and direction, which can surprise an unwary pilot. Although familiar with the field, the HB-LKP pilot was not warned of this possibility.
Combined with the short runway, this phenomenon makes the airfield unsuitable for landings of aircraft like HB-LKP, whose performance and operational margins are marginal under such conditions. Simultaneously monitoring flight parameters and local approach conditions imposes excessive demands.
It would be desirable that visiting pilots, even with aircraft less marginal for local conditions, be warned of the possibility of sudden wind shifts.
Cause
The accident was due to:
- Loss of speed at low height.
- Marginal operational reserves of the aircraft for landing on a short field.
Contributing factors:
- Sudden appearance of tailwind while the aircraft was on final approach.
- Insufficient monitoring of airspeed by the pilot, who was focused on the final approach to a short runway.