Incident Overview
On 27 February 2020, a Robinson R44 II helicopter, registration SP-HPL, was conducting a continuation of power line inspections in the vicinity of Wrocław, Poland. The helicopter was stationed in Jankowice, Oława County. The crew consisted of a pilot and an inspection system operator.
Flight and Malfunction
At approximately 10:20 LMT, the pilot performed a pre-flight inspection in accordance with the Pilot's Operating Handbook, noting no abnormalities or technical defects. The engine was started around 11:10 LMT, and after a successful engine run-up, the helicopter departed at 11:16 LMT for the planned inspection area. After about 19 minutes of flight, while cruising at 500 ft AGL, the pilot noticed a sudden drop in rotor RPM, accompanied by an increase in manifold pressure readings to 24-25 inHg and a sudden onset of vibrations felt in the cabin.
Precautionary Landing
In response to the abnormal indications, the pilot decided to execute a precautionary emergency landing in unprepared terrain. He reduced collective pitch to prevent a further decrease in main rotor RPM. During deceleration and transition to a hover, an audio and visual "LOW RPM" warning activated, indicating the main rotor RPM had dropped to 97%. The landing was completed with a short roll at approximately 11:35 LMT.
Post-Landing Inspection
After landing, the pilot observed white smoke emanating from the rear of the helicopter and immediately shut down the engine. Once the main rotor blades stopped, he conducted a preliminary inspection of the engine compartment, discovering that the lower spark plug of cylinder No. 5 had been dislodged and damaged. The defect was recorded in the aircraft technical log, and the CAMO/AMO organization and the Polish State Commission on Aircraft Accident Investigation (PKBWL) were notified. Upon arrival of AMO maintenance personnel, the side engine cowlings and the valve rocker cover of cylinder No. 5 were removed. Extensive damage to the valve rocker chamber was found, along with metal fragments from the valve head on the lower engine compartment panel.
Engine Disassembly and Repair
The helicopter was transported on a trailer to the AMO facility. After disassembly of cylinder No. 5, the following damage was documented:
- Stripped lower spark plug mounting seat and destruction of the spark plug
- Bent pushrod of the exhaust valve rocker
- Damaged exhaust valve rocker
- Damaged combustion chamber surface of the cylinder
- Damaged intake valve
- Damaged piston
- Damaged exhaust valve guide
- Destroyed exhaust valve
The helicopter was removed from service. The engine was sent to an overhaul facility for verification and repair after consultation. Following verification, the engine was overhauled, reinstalled on the helicopter, and a successful engine run-up was completed before the aircraft was returned to service.
Conclusion
The cause of the incident was the separation of the exhaust valve of cylinder No. 5, its destruction, and the tearing out of the spark plug, which consequently led to a loss of engine tightness. No preventive recommendations were issued by the investigating entity, and the PKBWL did not determine additional safety recommendations.