A recent report by the British Air Accidents Investigation Branch (AAIB) shows that the pilot of the Robinson R44 flying from a private strip in southern Scotland to Manchester, England, found that hills on his track were obscured by an area of low cloud. Recordings from his GPS showed that as he turned to avoid high ground he increased speed and entered a rapid descent.
Soon after, the main rotor blades struck the tailboom, which separated, and the remaining structure impacted the ground at high speed. The pilot held a helicopter private pilot’s license, with 96 hours total, 20 on the R44.
The investigators acknowledged disorientation as a probable cause but also mentioned the possibility that a very low rotor rpm resulted from ensuing flight conditions and caused one of the blades to contact the tailboom. Small delays in applying necessary control inputs can result in a major reduction in rotor rpm to a hazardous degree. They pointed out that Sections 4 and 10 of the R44 Pilot’s Operating Handbook give information on the rotor blade stall hazard caused by low rotor rpm, but do not cover the danger of blade stall caused by rapid and excessive collective pitch movement.
The investigative board has made a formal safety recommendation (2005-021) advising Robinson to consider including a specific warning in the R44 and R22 POH that highlights the possibility of an over-reaction with the collective pitch demand causing a hazardous loss of rotor rpm, as well as guidance on the associated handling of the collective lever.
Since there have been a number of other R22 and R44 fatal accidents resulting from rotor/boom strikes, the investigative board expressed concern about both the adequacy of rotor-to-boom clearance and the maximum time delay that can safely be tolerated in reducing the collective pitch after a sudden power loss.
In 1994 an FAA technical panel convened to study the R22 and R44 and decided (after a comprehensive assessment of the design and behavior of the main rotor system in particular) that the helicopter met or exceeded FAA requirements.
However, it was clear that rotor rpm decay in the event of sudden power loss could be rapid. The AAIB report stated that a delay of as little as two seconds in selecting the collective lever fully down after low rpm warning activation on the R44 could place the rotor rpm in a critical regime.
Finally the report questions the fact that pilots, particularly those with limited experience, are expected to react within what appears to be an unrealistic amount of time. As a result, the agency recommended that the FAA and European Aviation Safety Agency reassess the “corrective action time delay” in reducing the collective control after sudden power loss on a single-engine helicopter (Safety Recommendation 2005-022), with the aim of ensuring, as far as possible, that the minimum reaction time required is realistically within the capability of an average qualified pilot.