Introduction
On 27 April 1967, at approximately 1145 hours, a Piper Cherokee-Six (registration HB-OMB) operated by the Motorfluggruppe der Sektion Zürich of the AeCS was engaged in a commercial sightseeing flight from Zurich Airport. The aircraft carried a pilot and five passengers.
Flight and Accident Sequence
The pilot, who held multiple licenses including a private pilot license issued in 1954 and a commercial pilot license with various endorsements, had a total flying experience of 1,115 hours on powered aircraft and 1,122 hours on gliders. His conversion training on the Piper Cherokee-Six amounted to 44 minutes with 6 landings, and he had 2 hours 27 minutes of experience as pilot-in-command on that type.
Before departure, the pilot performed a pre-flight inspection. A helper visually checked the main fuel tanks and reported them full. The pilot checked the left outboard tank and estimated a fuel level of about 4 cm below the filler neck, corresponding to more than three-quarters full. The fuel gauges indicated left outboard, left main, and right main as full, and right outboard as half full. The fuel selector was positioned to the left outboard tank.
Starting the engine at the hangar required several attempts. After taxiing to the holding point for runway 02, the pilot ran the engine at 1,200 RPM for warm-up. During the pre-takeoff check, he again verified the fuel selector and fuel gauges, noting the same indications. He then lined up on runway 02 and received takeoff clearance with an instruction to make a right turn after departure. The pilot set power to 25.0 inches of manifold pressure (not full power) per the checklist. Acceleration was normal, and the aircraft lifted off at an indicated airspeed of just over 70 mph. After liftoff, the pilot reduced RPM to 2,500 and adjusted manifold pressure to 25.0 inches. The engine operated normally.
Shortly after initiating the prescribed right turn, the engine power rapidly decreased. The pilot checked the mixture control, booster pump, and ignition but found no anomalies. He lowered the nose and pumped the throttle, which temporarily restored some power but did not prevent a strong descent. He decided to perform an immediate forced landing, maintaining the current heading. The aircraft cleared a row of trees and touched down normally on a grassy field approximately 20 meters from the field's edge, with landing flaps still in the takeoff position. After the nosewheel contacted the ground, the pilot saw the airport fence ahead. Despite full braking, the aircraft struck and breached the fence frontally between two concrete posts, coming to a stop shortly afterward. All occupants evacuated unaided and uninjured.
Damage
The aircraft sustained substantial damage. The airport fence was damaged to an extent of approximately 800 Swiss francs.
Post-Accident Findings
Examination after the accident revealed that the left and right outboard fuel tanks were empty; the right outboard tank had a fist-sized hole on its underside. The main fuel tanks were full to the point of fuel spilling when the caps were opened, though this may have been influenced by the aircraft's post-accident attitude. The fuel selector was found positioned to the left outboard tank.
A fuel sample from the aircraft tested compliant with aviation gasoline 100/130 specifications. A subsequent engine ground run at the Pilatus Air-Service facility showed normal starting when drawing from the left outboard tank and no difficulties when switching to the main tanks. The fuel injector could not be flow-tested by Transair due to lack of documentation.
Fuel consumption calculations indicated that the aircraft had been fully fueled on 22 April 1967, and three flights totaling 48 minutes were conducted before the accident, consuming approximately 68 liters. The total fuel capacity was 318 liters (two 64-liter outboard tanks and two 95-liter main tanks), with 2 liters per tank unusable. The estimated fuel quantity at the start of the accident flight was about 250 liters.
Probable Cause
The accident was probably caused by the pilot starting the flight with the fuel selector positioned on the nearly empty left outboard tank, which led to an engine failure shortly after takeoff and forced a landing outside the airport.