A SPECIAL US National Transportation Safety Board (NTSB) investigation has exonerated the Robinson R22 and R44 light helicopters of suspected design defects, but it recommends that tougher flight-training requirements remain in force.

The ruling comes after an investigation which started in 1994 after a series of fatal R22/44 accidents involving loss of main rotor control, followed by break-up, while the helicopters were being operated well within their defined operating envelope. At the time, the NTSB expressed concern that "...the stability of the R22 main rotor blades is compromised by an inherent rotor system design deficiency", and recommended that the R22 and R44 be grounded.

The US Federal Aviation Administration rejected the grounding recommendation, but has ordered new pilot and certificated flight-instructor training rules and some flight restrictions.

Investigation of 34 R22/44 accidents between 1981 and 1995 did not identify inherent mechanical failures or material defects. Instead, many accidents most likely stemmed from "a large, abrupt pilot input to a helicopter that is highly responsive to cyclic-control inputs," says the report. As a result, special training is required for student pilots and flight instructors, it adds.

Source: Flight International