What happened
On November 8, 2021, a Robinson R44 II helicopter, registration PR-HHH, was conducting a local aerial reporting flight near Belo Horizonte, Brazil. Approximately ten minutes after departing from Helicentro Zona Sul, the aircraft experienced a sudden loss of engine power, triggering low RPM alarms.
The pilot attempted to execute emergency procedures, initially attempting to manage the engine performance before transitioning into an autorotation profile to reach a nearby soccer field. During the approach, the pilot's attempt to manage the descent led to a flare executed at an altitude significantly higher than ideal. This resulted in a high-energy, hard touchdown. The impact caused the aircraft to lift off in a nosedown attitude; in an attempt to correct the pitch, the pilot applied aft cyclic, which caused the main rotor blades to strike the tail cone, severing it from the airframe. Despite the substantial damage to the aircraft, the pilot and two passengers escaped without injury.
The investigation
CENIPA investigators examined the engine and components to determine the cause of the power loss. Laboratory analysis of fuel and oil samples showed no signs of contamination. However, a physical examination of the engine revealed significant carbon buildup on the cylinder heads and evidence of pre-ignition and detonation in all six cylinders.
During compression tests, cylinders 2 and 4 exhibited low compression. Investigators found that carbon residue had likely detached from the combustion chamber and lodged between the intake valves and their seats, disrupting the combustion cycle. Furthermore, an inspection of the spark plugs revealed that the electrode gaps exceeded the maximum tolerances specified by the manufacturer. While maintenance records indicated that recent engine inspections had been completed, the physical condition of the spark plugs was inconsistent with those records.
Findings
- The engine power loss was related to inadequate maintenance of the spark plugs, which featured electrode gaps exceeding manufacturer limits.
- Carbon deposits within the combustion chamber had detached and interfered with the sealing of the intake valves.
- The pilot's attempt to manage the engine performance during the emergency led to an improper approach trajectory and an excessively high flare altitude.
- The hard landing and subsequent rotor-to-tail-cone strike caused substantial damage to the aircraft.
- Maintenance records for the spark plug inspections did not accurately reflect the actual condition of the components found during the investigation.