LAX04LA254

While performing a maintenance check flight, the collective down lock engaged, which resulted in an uncontrolled descent and collision with terrain. The pilot and a mechanic were to fly a maintenance flight check of the engine and perform a rotor track and balance. He entered a descent at approximately 1,200 feet above ground level (agl) and prepared to level off at approximately 700 feet agl. When he tried to pull up on the collective, the collective would not move and was observed latched by the collective down lock. He and the mechanic tried to unlatch the collective from the down lock, but they did not have enough time before he had to flare the helicopter for landing. With the collective stuck in flat pitch, they landed hard and with forward speed. The flight crew evacuated the helicopter once it had come to rest. An ensuing post accident fire destroyed the helicopter. A new after market avionics control panel had been installed and the collective down lock, which is secured to the panel, was adjusted prior to the flight. The down lock attaches to the lower section of avionics panel. When the collective is lowered to the lower pitch stop the clearance between the collective and the down lock is about 0.16 inches. The down lock is a flexible plate that is free to vibrate with the helicopter's normal rhythms. In the original factory installation, the clearance between the down lock and the collective is 0.3 inches. This is the second known accident where the collective lock has inadvertently engaged in flight with this particular after market avionics panel installed. The manufacturer of the aftermarket avionics panel provided installation information indicating that maintenance personnel must bend the collective locking plate such that the locking plate will spring away from the collective lock button. Examination of the accident aircraft’s locking plate indicated that the locking plate bend was likely reversed, allowing the locking plate to spring toward the lock button.

On July 1, 2004, at 1500 mountain standard time, a Eurocopter AS-350-B2, N513TS, collided with terrain 8 miles northwest of Scottsdale Airport, Arizona, while performing a maintenance check flight. Westcor Aviation, Inc., was operating the helicopter under the provisions of 14 CFR Part 91. The commercial pilot and a mechanic crewman received minor injuries, and the helicopter was destroyed. Visual meteorological conditions prevailed, and a flight plan had not been filed. The maintenance test flight originated at Scottsdale Airport.

The pilot reported in the Pilot/Operator Accident Report that they were performing a maintenance flight to check the engine and performing a rotor track and balance. He entered a descent at 2,700 feet mean sea level (msl) (approximately 1200 feet above ground level (agl)), and prepared to level off at 2,200 feet msl (approximately 700 feet agl). When he tried to pull up on the collective, the collective would not move. It was observed by the mechanic-crewman that the collective lock had engaged. He and the crewman tried to unlatch the collective from the down lock, but they did not have enough time before he had to flare the helicopter for landing. With the collective stuck in flat pitch, they landed hard and with forward speed. The flight crew evacuated the helicopter once it had come to rest. An ensuing post accident fire destroyed the helicopter.

Examination of the helicopter maintenance logbook revealed that the helicopter's factory avionics console was replaced with a Geneva P132 avionics console on June 7, 2004. On July 1, 2004, the Eurocopter 12-year major inspection was entered as complied with. One of the items entered in the logbook for the major inspection was a check of the collective lock, and the installation of a new collective lock button. The collective lock consists of an aluminum metal button that protrudes from the end of the collective grip and an 8-inch (approximately) sprung steel strap or flexible lock plate is attached to the either the cockpit deck or the avionics console depending on how the helicopter is equipped. The flexible lock plate is aligned in parallel with the vertical travel of the collective end. The flexible plate has a circular hole in the upper end that fits over the protruding collective lock button. The pilot locks the collective down by pushing it slightly past the lower pitch stop and pulling the flexible plate over the collective lock button. The collective is held in place by its natural upward force towards the lower pitch stop position and a groove in the collective button that the flexible lock plate engages. The pilot disengages the lock by pushing down on the collective slightly and the flexible plate springs itself forward away from the collective lock button. When the flexible lock plate is not engaged (i.e., normal flight conditions) it is free to vibrate about its base attachment point with the rhythms of the helicopter. A rubber plug placed in the middle of the flexible plate is meant to dampen out vibrations when in contact with the avionics panel.

Section 3.3, adjustment of the "LOW PITCH" lock plate, in the Eurocopter Maintenance Manual, the collective lock is to be adjusted such that when the collective is at the lower pitch stop, with no further downward force on the collective by the pilot, the collective lock button is set 1 mm short of being aligned to engage the flexible lock plate. On the Geneva avionics panel, two setscrews in the base of the plate adjust the flexible lock plate vertically for alignment.

The clearance between the collective and the flexible plate when the collective is at the lower pitch stop was measured in an exemplar helicopter with a factory avionics panel installed. The measurement was approximately 0.3 inches between the end of the collective lock button and the flexible lock strap. Geneva provided documents that depicted the clearance between the collective lock button and the flexible lock plate with a Geneva avionics panel installed as 0.16 inches.

Additional information

National Transporation Safety Board investigators have identified a second occurrence of an AS350 with the Geneva avionics panel installation where the collective lock engaged in flight, which resulted in an accident. Special Airworthiness Information Bulletin SW-05-20 was issued on December 2, 2004, recommending inspection of the collective lock rigging.