What happened
On 2 October 2018, a Grob G115E Tutor, registration G-BYUU, was returning to RAF Wittering, Cambridgeshire, following a successful air test. This flight was conducted as a private operation after a recent engine change had been performed on the aircraft. During the return leg, the pilot observed an uncommanded reduction in engine speed. Although the engine initially regained power, a subsequent drop in speed prompted a PAN call. As power loss continued, the pilot was unable to maintain altitude, leading to a MAYDAY declaration. The pilot prepared for an emergency landing in a field, during which time the engine ceased functioning entirely. The aircraft landed without sustaining any damage, and the pilot escaped with no injuries.
The investigation
Investigators examined the aircraft and the engine following the incident. The inspection revealed that the engine oil sump drain cap was missing. This omission led to a significant oil leak, which ultimately caused the engine to seize. The investigation focused on the maintenance performed prior to the flight, specifically the engine installation process. The maintenance organisation concluded that the most probable cause was that the drain cap had not been properly secured after the new engine was installed. It was noted that the Grob G115E uses a modified, inverted oil system for aerobatic capability, but there is no formal requirement for the oil cap to be wire locked.
Findings
- The engine failure was caused by engine seizure resulting from oil loss.
- The oil loss occurred because the engine oil sump drain cap was not in place.
- The maintenance organisation likely failed to correctly secure the cap during the engine installation process.