Flight History
On Friday, 26 March 1971, the pilot planned to conduct a non-scheduled commercial flight from Geneva to Saanen, Switzerland, with three passengers aboard the Piper Apache HB-LCA. Before departure, the pilot telephoned Saanen at approximately 12:20 HEC for a weather briefing, obtaining a ceiling of 450 m and visibility of 5 km, which he deemed sufficient. He estimated passenger baggage at 50 kg and, with half-full fuel tanks, inferred that weight and balance were within limits—which was correct. After filing a flight plan, he departed from runway 23 at 14:16 HEC, immediately turning left toward Douvaine, and stabilized at 2,500 ft MSL, as the cloud ceiling was 2,700–3,000 ft. During cruise, he reduced engine speed to 2,400 rpm and manifold pressure to 24 inHg, feeding from auxiliary tanks after climbing on main tanks; electric fuel pumps were engaged during the tank switch.
Pilot Information
The pilot, born in 1947, held a restricted professional pilot license with endorsements for radiotelephony and aerobatics. He was authorized for single-engine aircraft up to 2,500 kg with retractable gear and constant-speed propeller, and for multi-engine aircraft up to 2,500 kg. He had passed the theoretical instrument rating exam and was undergoing IFR flight training. His total flight experience was 216.39 hours with 700 landings, including 24.34 hours on type. In the preceding three months, he had flown 34.32 hours, 7.41 on the Piper Apache. The Federal Air Office records showed no irregularities, and no evidence suggested he was not in good health at the time.
Aircraft Information
The Piper PA-23-160 Apache HB-LCA was an all-metal low-wing twin-engine aircraft, powered by two 160 hp Avco Lycoming O-320-B3B engines with carburetors and Hartzell two-blade constant-speed propellers. Total airframe time was 329 hours with 577 landings; the last periodic check was a 100-hour inspection on 5 February 1971 at 299 hours. Each engine could draw from four wing tanks and had a mechanical fuel pump for normal operation and an electric backup. The system permitted proper feeding even if a tank was empty or a pump failed.
Meteorological Conditions
The weather was characterized by a complex depression over Scandinavia and a high between the Azores and the Bay of Biscay, resulting in a rapid maritime polar air flow from the northwest across Western and Central Europe. In the accident area, cloud cover was 8/8, with 4/8 at 1,000 m MSL and 8/8 at 1,400 m MSL. Geneva Airport reported 2–5/8 stratus between 650 and 900 m MSL and a compact stratocumulus layer around 1,500 m MSL. Visibility was 7–9 km, with a light north wind (5–10 kt). Temperatures and dew points at Geneva: at 13:20, +6°C / 0°C; 13:50, +6°C / 0°C; 14:20, +5°C / 1°C; 14:50, +5°C / 2°C. The 0°C isotherm was at approximately 900 m MSL according to the 11:00 sounding at Payerne. The aviation forecast valid 12–18 HEC indicated intermittent precipitation, snow down to 600–900 m, and open but difficult routes.
Accident Sequence
At 14:23, while over Douvaine, the left engine began vibrating and producing irregular power. The pilot engaged the electric fuel pumps, checked the fuel selector and ignition, then applied left carburetor heat. As power continued to drop, he increased right engine power, which then also showed irregularities. He applied right carburetor heat and turned left, requesting Geneva approach clearance to join the ILS, reporting his position abeam Douvaine at 2,000 ft MSL and citing “reduced visibility and a problem with the left engine.” At 14:32, near Nernier at 1,800 ft MSL, he advised the controller he could not maintain altitude and would attempt to reach La Côte aerodrome. Noticing increasingly black exhaust, he concluded the mixture was too rich and leaned it; when no improvement occurred, he returned the mixture to rich. He contacted La Côte frequency and was informed of traffic. Descending at 100–200 ft/min, he realized he could not make the aerodrome; the left engine was turning about 1,200 rpm and the right about 1,500 rpm. He decided to force-land in a seemingly suitable field between the main road and the lake, bordered by tall trees that the aircraft barely cleared. After clearing the trees, he lowered the landing gear and flaps using the manual pump. The aircraft touched down at about 14:40 on the main gear and nosewheel. He braked lightly and held the elevator full up to relieve the nosewheel. On soft ground with an approximately 9% slope, the nosewheel dug a deepening furrow, causing its collapse after rolling 112 m.
Damage
Occupants were uninjured. The aircraft was seriously damaged. Minor damage to the terrain also occurred from the accident and rescue operations.
Investigation Findings
The investigation found no evidence of technical deficiency in the fuel system or engines. The black exhaust indicated a rich mixture, and fuel samples showed no water contamination, excluding water condensation as a cause. The meteorological conditions at cruise altitude (approximately 2,500 ft MSL) were particularly conducive to carburetor icing, as the aircraft was near the condensation level. All symptoms described—power loss, rough running, black exhaust—were consistent with carburetor icing. The pilot did not apply carburetor heat until after the irregularities began, stating that he had not considered icing possible in VMC during climb and after five minutes of flight, and that heat reduces power by 10–15%. The investigation concluded that carburetor icing caused the engine irregularities. The pilot's late use of carburetor heat and lack of preventive action allowed icing to become severe. The pilot's decisions and maneuvers for the forced landing were appropriate; only the unsuitability of part of the terrain (undetectable from the air) led to the nosewheel collapse. A wheels-up landing would likely have caused less damage to the aircraft.
Probable Cause
The investigation concluded that carburetor icing due to the meteorological conditions encountered and the pilot's delayed use of carburetor heat caused the engine irregularities, leading to the forced landing.