What happened
On January 21, 2012, a Robinson R44 helicopter, registration PH-WMW, was conducting a passenger flight near Yerseke, Netherlands. During the flight, the pilot first noticed a visual and acoustic low rotor RPM warning near Stellendam. Shortly thereafter, the cylinder head temperature (CHT) indicator began displaying impossible readings, dropping below zero degrees Celsius.
As the aircraft crossed the Oosterschelde, the low RPM warning recurred. Despite the pilot's attempts to manage the power by adjusting the collective and throttle, and even switching to manual control by disabling the automatic governor, the engine performance remained unstable. The helicopter experienced moderate shaking and occasional yawing. The pilot eventually decided to land at the intended destination in Kruiningen to conclude the flight safely.
The investigation
The investigation focused on the cause of the engine power loss and the erratic instrument readings. Investigators examined the engine components and the fuel used during the flight. The aircraft was operating on automotive gasoline (Mogas) under a Supplemental Type Certificate (STC).
Laboratory analysis of the intake manifold's rubber hose revealed that the internal surface had degraded, with small rubber particles breaking away. The investigation also included a chemical analysis of the fuel, which contained a small amount of ethanol (0.42%) and ETBE.
Findings
- The partial loss of engine power was caused by an overheated engine, which subsequently triggered the CHT indicator failure.
- The overheating was driven by an excessively lean air-fuel mixture, resulting from a leak in the intake manifold's rubber hose.
- The primary failure mechanism was fatigue. The rubber hose had swollen due to the long-term use of Mogas, which increased internal material tension.
- This swelling, combined with engine vibrations and heat, led to the formation of fatigue cracks and the eventual breakdown of the rubber material.
- The damage to the hose was not detectable during standard 50-hour visual inspections because the exterior of the hose appeared undamaged and the interior was not required to be inspected.
Safety action
- The pilot's decision to perform a return flight to the base without a functioning CHT indicator was found to be in violation of the aircraft's flight manual regulations.