What happened
On February 27, 2020, an R44 II helicopter, registration SP-HPL, was conducting a planned inspection of power lines near Wrocław, Poland. The crew, consisting of a pilot and an inspection system operator, departed from Jankowice following a standard pre-flight inspection that revealed no technical issues.
Approximately 19 minutes into the flight, while cruising at an altitude of 500 ft AGL, the aircraft experienced a sudden drop in engine RPM accompanied by a significant increase in manifold pressure (reaching 24-25 in Hg) and intense vibrations throughout the cabin. In response to the deteriorating engine performance, the pilot initiated an emergency landing in an unprepared field near Bukowina. During the deceleration and transition to a hover, a "LOW RPM" warning was triggered as the main rotor speed dropped to 97%.
Following the landing, the pilot observed white smoke emanating from the rear of the engine and immediately shut down the powerplant. A preliminary inspection of the engine compartment revealed that the lower spark plug of the fifth cylinder had been dislodged and damaged.
The investigation
Subsequent inspections by maintenance personnel involved the removal of engine side covers and the valve cover for the fifth cylinder. The investigation established that the valve rocker chamber had sustained extensive damage, and metallic fragments from a valve stem were found on the lower engine panel.
Upon removal of the fifth cylinder, investigators identified several critical internal failures, including:
- The destruction of the lower spark plug mounting socket and the plug itself;
- A bent exhaust valve pushrod;
- Damage to the exhaust valve rocker arm and the exhaust valve;
- Damage to the intake valve and the valve guide;
- Significant damage to the piston and the combustion chamber surface.
Findings
The primary cause of the incident was the structural failure and detachment of the exhaust valve in the fifth cylinder, which subsequently tore out the spark plug and resulted in a loss of cylinder sealing.