Accident Overview
On 26 April 1979 at 1837 local time, an Aero Commander 680 FL, registration 7T-VBA, made a forced landing in a soft, obstacle-free field near Schneisingen, Switzerland. The aircraft was operated by the Algerian Institut national de cartographie on a private IFR flight from Zurich to Marseille. The crew consisted of a Swiss pilot, an Algerian navigator, and an Algerian mechanic. All three occupants were uninjured. The aircraft suffered fuselage damage and minor third-party land damage.
Flight History
The aircraft had undergone a 100-hour inspection and several repairs by Jet Aviation AG in Zurich, including replacement or sealing of rubber fuel tanks. A test flight on 24 April revealed fluctuating fuel pressure in the right engine when the booster pump was off; the issue was addressed by venting the indicator line.
On the accident flight, the pilot started the engines with pitot heat on and departed Zurich at 1749. After takeoff, the booster pumps were turned off during the after-takeoff check. During climb between FL 70 and FL 80, the right engine fuel pressure began fluctuating. The pilot switched on the right booster pump, stabilizing the pressure.
En route at FL 110, with cruise power set, the pilot turned off the right booster pump. Fuel pressure dropped to zero and the right engine lost power. The pilot reactivated the booster pump, restoring normal operation. The crew contacted Geneva control and decided to return to Zurich, cleared to descend to FL 100. They briefed for an ILS approach to runway 14.
During descent through FL 70, icing conditions were encountered. The pilot activated wing de-icing once. Shortly after, the right engine fuel pressure dropped to zero despite the booster pump being on, and the engine failed. The pilot then switched on the left booster pump as a precaution.
While performing the engine failure procedure, the left engine also showed large fuel pressure fluctuations and power variations. The pilot briefly engaged alternate air but saw no improvement and disengaged it. Between 5000 and 4000 feet, the pilot gained visual contact with the ground. Unable to maintain the assigned altitude of 4000 feet due to insufficient power, he decided at 3500 feet to conduct a forced landing. The aircraft landed with gear retracted and flaps extended.
Findings
Post-accident inspection revealed several fuel system issues. The right auxiliary fuel tank was missing its suction screen. The fuel filters and central tank sump had not been cleaned after maintenance. Laboratory analysis of filter elements showed contamination with textile fibers, wood particles, paint chips, metal flakes, dust, and sand. The left filter contained coarser material, while the right filter had fine, sludge-like debris capable of clogging pores.
Fuel samples from the central and auxiliary tanks were analyzed. The fuel was AVGAS 100 LL but with abnormally high evaporation residue and distillation endpoint. The left outer auxiliary tank sample had a low 75% evaporation point. The central tank sump contained a water-fuel emulsion likely caused by detergent residues, along with sand, slag, fly ash, rust, and unidentified metal compounds. The lower liquid layer was brown, contaminated water with mineral dust and organic particles.
During fuel draining after the landing, the right engine initially had normal flow from the center tank, but after about 15 minutes flow sharply decreased, remaining low when switched to auxiliary tanks. Both engines had worn ignition contacts and aged hoses. A ground test of the left engine on a test stand reproduced the fuel pressure drop and fluctuation seen in flight.