What happened
On 17 December 2021, a Zenair CH 750, registration G-CIJZ, was conducting a private flight for upper air handling near Lamb Holm Airfield. Approximately 15 minutes after departure, the pilot noticed a hot smell and observed that the engine oil pressure had dropped to zero. As the aircraft flew over hilly terrain, the pilot began a descent toward Kirkwall, considering a precautionary landing in a field.
Shortly after the initial pressure loss, the engine began experiencing intense vibrations and smoke started entering the cockpit. The pilot immediately shut down the engine and initiated an emergency descent from roughly 1,800 ft. While attempting to land in a field west of Wideford Hill, the pilot was unable to see power lines crossing the landing area until the aircraft was at 400 ft due to the low, bright sun. To avoid the wires, the pilot diverted to a nearer grass field. The aircraft struck the soft ground heavily, resulting in no injuries to the pilot, but causing damage to the landing gear and the seizure of the engine.
The investigation
Investigators examined the engine and identified that a brass pipe, which connected the oil pressure transducer to the engine, had failed. This failure caused a significant loss of engine oil. While the standard installation for a Rotax 912ULS engine involves mounting the oil pressure transducer directly onto the crankcase, the transducer on G-CIJZ had been moved to the engine frame.
Unlike a standard remote installation that typically utilizes a flexible hose to absorb movement, this aircraft used a rigid brass pipe. The investigation established that the relocation of the transducer to the engine frame increased the relative movement between the engine and the component during flight.