Introduction
On 25 May 2018, at 18:40 local time, a Robinson R44 helicopter (registration HB-ZJW) operated by Héli-Alpes SA was involved in a serious incident at the Staldenhorn mountain landing site (LSVN) in the canton of Bern, Switzerland. The flight was a mountain instruction (MOU) training mission with a student pilot and an instructor. The aircraft sustained minor engine damage, and one crew member received slight injuries.
Sequence of Events
The helicopter departed Sion Airport (LSGS) at 17:50 LT en route to the Sanetsch Pass. After crossing the pass, several mountain landings were performed below 2000 m, including two at the Wallegg landing site. The training proceeded without incident until the landing at Staldenhorn. Shortly after touchdown, with reduced engine power, the crew heard a faint sharp noise. Immediately thereafter, manifold pressure increased, engine rpm decreased, and smoke emerged from the engine compartment.
The engine was shut down, and a visual inspection revealed an oil leak beneath the aircraft. The engine was covered with a protective sheet to collect leaking oil. The helicopter was deemed unserviceable and was heliported the following day to Saanen for detailed examination by qualified mechanics.
Engine Examination
The engine was a Lycoming IO-540-AE1A5 (serial number L-29652-48A), a six-cylinder air-cooled boxer type. It had been fully rebuilt on 16 January 2015. At the time of the incident, the engine had accumulated 804 hours since overhaul (TSO). The last 100-hour inspection was performed at 790 h TSO, and the last intake valve inspection at 694 h TSO. There were no outstanding hold items in the defect log.
Disassembly revealed a fracture of the intake valve of cylinder No. 1. The valve head was split into three pieces and separated from the stem. Examination of engine components indicated that the engine had potentially suffered from detonations, the origin of which could not be determined. These detonations were considered to have induced premature wear of the intake valve on cylinder No. 1. Cylinder No. 2 was inspected and showed no abnormal wear.
Findings
The serious incident was caused by the rupture of the intake valve of cylinder No. 1, which led to a loss of engine power. Premature wear of the intake valve was identified.