MIA98LA137

The pilot stated he was on base turn to final for a fly bye, when he experienced a 1 to 1 vertical vibration. He informed the tower that he was going to land, and started a deceleration. The helicopter veered to the right, and the pilot applied left antitorque pedal. The nose started to tuck down, then the helicopter collided with the ground and rolled over on its right side. Examination of the helicopter flight control system revealed a hex nut backed off the cyclic flight control push rod bolt, and the bolt became disconnected from the control yoke teeter block. This resulted in a loss of cyclic control.

On April 20, 1998, at about 1230 eastern daylight time, a Bennett M-500A, N105WB, experimental helicopter, registered to a private owner, operating as a 14 CFR Part 91 personal flight, crashed on landing at the Lakeland-Linder Regional Airport, Lakeland, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter sustained substantial damage. The commercial pilot reported no injuries. The flight originated from the Lakeland-Linder Regional Airport about 1 hour before the accident.

The pilot stated he entered the traffic pattern landing to the south at Chopper Town. He made a base turn and continued to final for a fly by, when he experienced a 1 to 1 vertical vibration. He informed the tower that he was going to land, and started a deceleration at 50 feet. The helicopter started to veer to the right and he applied left antitorque pedal. The nose started to tuck down, estimated at about 50 degrees nose down. The helicopter collided with the ground in a nose down attitude and rolled over on its right side.

Examination of the helicopter revealed that a flight control pushrod became disconnected (hex nut backed off the attaching bolt) from the control yoke teeter block, resulting in a complete loss of cyclic and collective control of the main rotor system. Review of aircraft logbooks revealed no maintenance had been performed on the flight control push rod since assembly of the helicopter 106 hours before the accident.

The manufacturer for the Mini 500 stated the registered owner sent a fax request ordering 20 Teflon DU bushings (part #0562) and 20 Teflon washers (part #0101) that are used in the flight control system. The request was made and filled on March 25, 1998, on invoice #25969. The manufacturer stated in order for the registered owner to remove the bushings and washers that it would be necessary to remove the MS21042L4 nut (part #0470) and the NAS144 bolt (part #0434) to replace the bushings and washers.

The registered owner stated that he ordered the parts that were listed on invoice 25969. He ordered the nuts to comply with Revolution Helicopter Airworthiness Directive (AD) #3101998 dated March 10, 1998. He elected to replace the control yoke (part #0024), bushings (part #0562, 2 each) and the Teflon washers (part #0101, 4 each) The replacement of these parts is accomplished by removing bolts #0365, and the control transfer plate #0023. It does not require the removal of the suspect bolt and nut assembly. Review of the Mini-500 Aircraft Assembly and Maintenance Manual Sub-Assemblies instructions verified the registered owner's comments.