The pilot and an engineer had been performing flight maintenance checks that had been intermittently interrupted by heavy rain showers. Whilst performing a rotor blade check in the hover during a period of clear weather, the No 1 engine OIL HOT caption illuminated. After landing, a second engineer opened the engine cowl to conduct an inspection of the engine. Following this inspection, the helicopter took off again for a further maintenance check flight. During the flight, a small but unusual vibration was felt by the occupants. The pilot elected to return to land at the airfield, but during his approach a sudden very heavy vibration started. The pilot performed a run-on landing, with no further issues. Subsequently, the engine cowl was found to have contacted the main rotor blades due to it not having been correctly latched shut during the earlier inspection.
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