NTSB Identification: LAX05LA020.
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Accident occurred Tuesday, November 09, 2004 in Oxnard, CA
Probable Cause Approval Date: 02/28/2006
Aircraft: American Eurocopter AS350-BA, registration: N655TV
Injuries: 2 Minor.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
During a practice autorotation, the collective down lock engaged, which resulted in an uncontrolled descent and collision with terrain. The pilot and an instructor pilot were conducting the first practice autorotation of a planned series, and the pilot entered the autorotation about 500 feet above ground level. After lowering the collective the pilot kept the collective in the down position until he was in the flare. When the pilot tried to pull up on the collective, it would not move and both pilots saw that the collective down lock had engaged. Both pilots attempted to unlatch the collective, but they were unsuccessful. With the collective locked at flat pitch, the helicopter impacted the ground hard in a level attitude with some forward speed and it rolled over. Post accident investigation revealed the helicopter was equipped with an aftermarket avionics control panel. The collective lever lock is installed on this control panel console; it is a spring steel plate with a hole in it to capture the collective locking tab. The lock also has a rubber grommet below the locking hole to dampen any vibration. With the collective in the full down position, the aftermarket console has about a 1/16-inch clearance between the lock plate and the collective lock tab, while the Eurocopter stock console panel has a 1/2-inch clearance. Exemplar aircraft with both the stock and aftermarket consoles were examined. In some of the aircraft, the grommet was not touching the console. This condition would allow the locking lever to vibrate and also decrease the clearance between the locking plate and the locking tab. This accident is the second known accident where the collective lock has inadvertently engaged in-flight with this particular aftermarket avionics panel installed.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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 likely 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 rhythmus of the helicopter. Full narrative available
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