Accident Details
On Sunday, July 21, 1968, at approximately 1723 hours, a Piper PA-18-180 (registration HB-ORW) departed from Kloten airfield with a pilot and a passenger from the cantonal police for a traffic monitoring operation. About 12 minutes into the flight, over the lower Lake Zurich basin near Wollishofen, the engine began to rattle and shake. Oil sprayed onto the windshield, obscuring forward visibility. The pilot switched off the ignition and prepared for an emergency landing.
Emergency Landing
The pilot initially considered landing on the Allmend Wollishofen but abandoned the plan due to obstacles and risk to people. He chose a clear area of Lake Zurich between Zollikon and Tiefenbrunnen. The skis were lowered, but flaps were not extended. The approach was made at relatively high speed and a steep angle. The aircraft skidded on the water surface and then slowly began to sink. Both occupants exited the aircraft and were rescued by a police boat. The aircraft was later located at a depth of 35 meters and recovered.
Pre-Flight Events
Before departure, after starting the engine, the pilot observed low oil pressure while taxiing. He returned to the hangar, where a fuel attendant checked the oil level and found approximately 2 quarts (about 2 to 2.5 cm on the dipstick), which was about 5 liters low. Oil was added, and subsequent indications for oil pressure, oil temperature, and cylinder head temperature were normal. The pilot stated that he remained cautious and monitored the instruments closely during the flight.
Aircraft Information
The Piper PA-18-180 (HB-ORW) was originally built in 1962 but had been rebuilt twice by M. Dätwyler & Co. after prior accidents. The current engine was a Lycoming O-360-A2D (180 HP) assembled from various used and new parts. The engine had accumulated about 45 hours since the rebuild. The aircraft had a valid airworthiness certificate. The weight at the time of the accident was approximately 865 kg, within the maximum limit of 794 kg (note: the source states 794 kg as maximum gross weight, but the actual weight exceeded this; however, the report does not state this as a cause). The center of gravity was within limits.
Investigation Findings
Examination of the engine revealed that the oil regulating ring (PN 73857) used was not approved for the piston type (PN 72967) installed. According to Lycoming, the correct ring should be PN 69457. However, Lycoming also stated that piston PN 72967 was superseded but still serviceable. The investigation could not determine whether the primary cause was oil starvation or a broken oil control ring. No definitive cause could be established.
Damage
The pilot and passenger were uninjured. The aircraft sustained light damage from recovery and significant water damage. No third-party damage occurred.
Weather
The weather at the time was partly cloudy with 4/8 cumulus clouds, visibility 30 km, wind from the northeast at 10 knots gusting to 15 knots, temperature 18°C, and light turbulence.