Synopsis
On 31 March 2022, a Robinson R44 helicopter, registration CS-HFU, operated by Helibravo, departed from Évora Aerodrome (LPEV) on a private flight to Moura, Beja. The sole occupant, the pilot, was alerted by audible and visual low rotor RPM warnings about 10 minutes into the flight at 1,500 feet en route. The pilot immediately lowered the collective to reduce power demand and recover rotor RPM; engine RPM was at 100% so the throttle was not used.
Event Description
After rotor RPM recovered to 100%, the pilot observed engine RPM fluctuations between 100% and 104%. He communicated to the flight information service his intention to return to LPEV. During the return, the oscillations continued, prompting the pilot to prepare for a possible governor failure by controlling engine RPM manually with the throttle. En route to LPEV, the low rotor RPM warning activated again; the pilot reduced collective to recover RPM. However, when attempting to regain power to maintain altitude, rotor RPM dropped again.
Unable to maintain altitude, the pilot decided to perform a precautionary landing and selected a location at the intersection of two rural roads. During the final phase of the controlled descent, when he tried to establish a steady flight, rotor RPM dropped below limits and the warning activated. The pilot performed a "rolled" landing with little horizontal speed, immobilizing the helicopter within a few meters. He then executed normal engine shutdown and egress procedures. Outside, he observed signs of a significant oil leak from the engine cowlings and on the ground.
Injuries and Damage
The pilot left the aircraft unharmed. The engine suffered substantial damage to cylinder #2, specifically the exhaust valve camshaft. The aircraft sustained no damage during the precautionary landing.
Relevant Findings
The pilot held a valid CPL(H) licence and a class 1 medical certificate and was duly authorized for the flight. The Robinson R44 is a two-bladed, single-engine light utility helicopter. The helicopter was used by the operator as a private aircraft by its owners, in commercial initial pilot training, and for private sightseeing and leisure flights. The engine is a Lycoming IO-540 six-cylinder, air-cooled, direct drive, horizontally opposed, fuel-injected engine.
An assessment of the aircraft and components included a detailed analysis of the engine, which found that the exhaust valve control rod had failed due to a stuck valve. According to the report, it is likely that the stuck valve originated from accumulation of deposits from combustion and solidification of lubricating oil between the valve stem and its seat, causing surface erosion, degradation, and mechanical blockage.
Comments
Engines in rotorcraft operate under significantly different regimes compared to fixed-wing aircraft. Rotorcraft rely on maintaining a near-constant main rotor speed (NR) throughout all phases of flight. For the R44, 100% NR corresponds to 2,665 rpm. Power management is achieved through a collective-linked throttle control system. These operating regimes, combined with the cooling system design, expose engine parts such as valves and guides to high thermal stress. Valve guide wear has been identified in numerous cases as a root cause of premature low compression cylinder failures. Lycoming introduced 'hi chrome' exhaust valve guides in 1996 to minimize wear; all engines and cylinder kits have used this material since March 1998, and Lycoming Mandatory Service Bulletin No. 388 requires periodic inspections of valve guide condition.