NTSB Factual Report

On October 4, 1998, approximately 1515 hours Pacific daylight time, a Burson Revolution Mini 500, N418MB, crashed in hilly terrain near Moorpark, California. The amateur built experimental helicopter was destroyed and the private pilot/owner suffered fatal injuries. The flight was conducted under the provisions of 14 CFR Part 91 and originated from a nearby rural area about 1430. Visual meteorological conditions prevailed for the personal flight and no flight plan was filed.

The pilot's daughter reported that she accompanied the pilot to the takeoff/landing site and helped reinstall the main rotor blades, which had been removed to trailer the helicopter. She stated that her father reported that he planned to fly over the park where she would be, and would then fly over the family residence. She left the site at 1417 and asked him to wait about 45 minutes before flying over the park. The daughter further recalled that the pilot reported that he planned to fly for about 1 1/2 to 2 hours. She never saw or heard him fly over the park, and he was not seen over the family house. The pilot's wife reported the pilot as missing and an Alert Notification (ALNOT) was issued at 2040. The Ventura County Sheriff located the wreckage at 0300 on October 5, 1998.

A Federal Aviation Administration (FAA) inspector from the Van Nuys, California, Flight Standards District Office responded to the accident site and interviewed the property owner. The property owner reported that he saw the helicopter in flight, west of the accident site, approximately 1400 to 1430. He stated that the aircraft was flying from east to west about 800 to 1000 feet agl and reported that it "sounded funny, like a go-cart." He reported that the surface winds were calm at the time of the accident. The property owner identified the accident aircraft as the helicopter he had seen earlier.

According to the FAA database, the pilot held a private pilot certificate with a rotorcraft-helicopter rating. The pilot's logbooks indicate he had approximately 100 hours of total flight time, all in helicopters, at the time of the accident. The pilot held a third-class medical dated March 5, 1997.

According to the aircraft logbooks, the experimental helicopter was manufactured by the pilot on April 18, 1998. The engine hour meter indicated that there were 33 engine operating hours on the helicopter at the time of the accident. The records revealed that the pilot made five modifications to the helicopter, which were suggested by the kit manufacturer. A list of those modifications is contained in the party report of Technical Enterprises, attached. A non-factory approved modification was noted on the fuel pump.

The aircraft wreckage was located in an area of low rolling terrain. There was no postimpact fire. The emergency locator transmitter (ELT) was located on the left side of the engine compartment, and the ELT switch was found in the "off" position.

According to the FAA inspectors, at the main impact site, the fuselage was found on its left side. Impact marks were noted about 50 feet upslope from the fuselage. Loose items from the aircraft were scattered in the area between the main wreckage and the site upslope. The fuel tank was ruptured, however, fuel was found in the fuel lines and both carburetors.

Both main rotor blades, tail rotor blades and hub assembly, tail boom, and tail rotor drive shaft were located in a fan-shaped debris field over an area about 1/8 mile east and southeast of the fuselage wreckage location. Main rotor blade "A" was the farthest piece of debris from the fuselage wreckage located approximately 1/8-mile southeast. Within 300 feet east and southeast of the fuselage wreckage were the tail rotor assembly attached to the gearbox and tail boom, a section of the tail rotor driveshaft, and main rotor blade "B." The flight control linkage was separated at several points, however, there was bending in the linkage adjacent to the separations.

The tail rotor drive shaft and tail boom forward of the drive shaft bearing retainer remained attached to the airframe. The tail rotor drive shaft exhibited torsional twisting and displayed a depression between the two bearing retainers, which approximated the leading edge radius of the main rotor. The tail boom was separated from the fuselage approximately 2 feet forward of the tail rotor gearbox. The separation exhibited a diagonal slicing appearance from upper forward to lower aft on the boom. The tail rotor blades remained attached to the tail rotor hub assembly.

Both main rotor blades separated from the rotor hub outboard of the blade root doublers. The leading edge "D" section of each blade displayed downward bending at the point of separation. Main rotor blade "A" displayed damage to the tip of the leading edge and separation of the trailing edge. The leading edge of blade "A" was bent smoothly aft approximately 30 degrees over the entire blade span accompanied by downward bending at the root end. The blade "A" root section was found separated from the hub and the mounting bolts, which attach the blade to the hub, were sheared. Main rotor blade "B" displayed damage to the leading edge and separation of the trailing edge and was bent downward over its entire span. The root section of "B" main rotor blade remained attached to the hub by one of two bolts. The rotor hub remained attached to the mast. The wall of the main rotor drive mast had a dent in one side of the shaft in the vicinity of the main rotor hub attachment. The depth of the dent was approximately 20 percent of the shaft diameter, and the form of the dent resembled the shape of the interior face of the main rotor hub. There was no corresponding dent on the opposite side of the mast.

A representative from Rotax Aircraft Engines assisted in the powerplant examination and a copy of their report is attached. The engine examination did not reveal any evidence of preimpact mechanical malfunction.

NTSB Probable Cause

The improper use of the cyclic flight control by the pilot, which resulted in low g flight load condition, which resulted in rotor disk divergence (mast bumping), and the main rotor blade striking and severing the tail boom.