Graham Warwick/WASHINGTON DC
Certification authorities in Canada and the USA have issued a priority airworthiness directive (AD) requiring Bell 407 operators to install a left pedal stop and limit never-exceed speed (Vne) in a bid to prevent the tail rotor cutting through the tailboom.
This move to make mandatory an earlier Bell technical bulletin follows a third accident involving a tail rotor strike in Brazil in late February. The helicopter autorotated to a safe landing, but two passengers were killed when they were struck by the main rotor as they jumped from the helicopter.
The previous incidents, in the Gulf of Mexico and South Africa, were not fatal. Brazil's Civil Aviation Authority has temporarily grounded all Bell 407s based there.
The tailboom strikes are believed to be caused by excessive tail rotor blade flapping, aggravated by abrupt left pedal input, and are thought to be more likely at higher speeds and altitudes.
Following the first two incidents, an AD was issued last September reducing the 407's Vne by 25kt (46km/h), to 115kt. In December, Bell issued a bulletin recommending the installation of a left pedal stop to limit tail rotor pitch, after which Vne could be increased to 125kt.
The pilot of the Brazilian 407 says that the helicopter, which did not have the pedal stop, was in level flight at 110kt when it experienced an uncommanded left pedal hard over. The new AD limits Vne to 100kt, with the pedal stop.
Bell says a tail rotor redesign has been tested and submitted for approval by Transport Canada, the certificating authority. The fix, which moves the tail rotor outboard from the tailboom and installs bellcrank stops in the pitch control linkage, is being re-examined after the Brazilian incident.
Source: Flight International
