Final Report: Accident highlighted collective lock problems

 - September 26, 2006, 9:17 AM

Eurocopter AS 350-B2, Scottsdale, Ariz., July 1, 2004–The NTSB determined that the accident was the result of “the inadvertent in-flight engagement of the collective down lock, which resulted in an uncontrolled descent and ground impact. The collective down lock engagement was due to a combination of the reduced clearance between the lock plate and the collective with this avionics panel design, the collective down lock alignment/adjustment and the tendency of the flexible lock plate to vibrate with the natural harmonic rhythms of the helicopter.”

A Westcor Aviation pilot and mechanic were doing a maintenance flight check of the AS 350’s engine and were performing a rotor track and balance when the helicopter crashed eight miles from the Scottsdale Airport.

The pilot began to descend from 1,200 feet agl and planned to level off at about 700 feet. He tried to pull up on the collective but it would not move. He noticed that it was latched by the collective down lock. The pilot, with the help of the mechanic, tried to unlatch the collective, but he didn’t have enough time before he had to flare for landing. With the collective stuck in flat pitch, the helicopter landed hard, with forward speed. The helicopter was destroyed by fire, but the commercial pilot and mechanic sustained only minor injuries.

A new aftermarket Geneva P132 avionics control panel had been installed in the AS 350, and the collective down lock, which is attached to the lower section of the panel, was adjusted before the flight. When the collective is lowered to the lower pitch stop, the clearance between the collective and the down lock is about 0.16 inches. (In the original factory installation, the clearance is 0.3 inches.) The down lock is a flexible plate that is free to vibrate with the helicopter’s normal rhythms.

A previous accident involved the collective lock being inadvertently engaged in flight after the same avionics panel had been installed. Special Airworthiness Information Bulletin SW-05-20 was issued on Dec. 2, 2004, recommending inspection of the collective lock rigging.