El Mirage, CA, USA
N42KB
Brock Thorp T-18
The amateur-built airplane's tail wheel separated on landing resulting in a loss of directional control, collision with a post, and nosing over. After touchdown on the private dirt strip, the tail wheel went over a bump, "wobbled," and separated. The airplane departed to the left of the runway, struck the post with its left wing, and nosed over, coming to rest inverted. Post accident examination of the tail wheel fracture point revealed corrosion that resulted in fatigue cracking over approximately 30 percent of the fracture surface.
HISTORY OF FLIGHT On October 19, 2001, at 1545 hours Pacific daylight time, a Brock Thorp T-18 amateur-built, experimental airplane, N42KB, collided with a metallic post and came to rest inverted on the landing rollout at a private dirt strip in El Mirage, California. The pilot, who was the airplane owner and builder, operated the tail wheel equipped airplane under the provisions of 14 CFR Part 91. The commercial pilot sustained fatal injuries and the passenger received minor injuries. The airplane sustained substantial damage. Visual meteorological conditions prevailed for the personal cross-country flight that departed the Corona Municipal Airport, Corona, California, at 1500. The flight was scheduled to terminate at the private dirt strip, and a flight plan had not been filed. The National Transportation Safety Board investigator-in-charge (IIC) interviewed the passenger. She stated the flight from Corona to El Mirage was uneventful. Prior to landing they overflew their hangar to check the wind direction via a windsock. She stated that the winds were calm and they landed towards the west, the designated calm wind runway. After touchdown, the tail wheel went over a bump and "wobbled." The airplane departed to the left of the runway, struck the post with its left wing, and nosed over coming to rest inverted. PERSONNEL INFORMATION The commercial pilot held ratings for single engine land and instrument airplanes, helicopters, and gyroplanes. He was issued a second-class medical certificate in July 2001, with the limitation that he wear corrective lenses for near and distant vision. He was also issued a repairman certificate in 1970, for an experimental rotorcraft model KEB-B8M. He was then issued another repairman certificate in 1982, for the accident aircraft. AIRCRAFT INFORMATION The 2-seat, amateur-built airplane was an all metal, low wing, monoplane. The pilot and passenger seats were positioned side-by-side, with the cockpit enclosed in a Plexiglass canopy. No roll bars or other structural members extended into the canopy. The T-18 tail wheel had full swivel capability. The tail wheel was connected to the steering arm by a fork. The fork arm descended down from the steering arm on the right side of the wheel where it attached to the wheel's axle. The aircraft's maintenance records were not provided to the Safety Board. AIRPORT INFORMATION The private airstrip consisted of a dirt runway, which was oriented east-west, and was approximately 3,000 feet long and 50 feet wide. The airstrip elevation was 2,995 feet above mean sea level. The passenger reported that the dirt strip is "dragged" once a year after the first rain. However, it had not been "dragged" since the preceding year. She and the pilot had been discussing utilizing a water truck to aid in the dragging of the strip because it was late in the year and had not rained yet. She also stated that about 2 weeks prior to the accident, the pilot had used a screen to level out the strip. WRECKAGE AND IMPACT INFORMATION A deputy from the San Bernardino Sheriff's department responded to the accident site, and surveyed the area. He walked the length of the dirt airstrip and observed a gouge in the dirt, running from the point where the airplane left the runway back to the airplane's touchdown point. The gouge was approximately 2 inches wide and 1 inch deep. The tail wheel was detached from the landing gear rod and located 200 feet north of the accident scene. The deputy also stated that at the point where the airplane left the runway there was a metal, 4-feet-tall vertical pipe. The outer portion of the left wing was located approximately 4 feet north of the pipe. The pipe had beige paint transfer marks on it that matched the color of the airplane. The tail landing gear was removed and sent, along with the tail wheel, to the Safety Board Material Laboratory, Washington, DC, for further examination. TESTS AND RESEARCH Examination of the tail wheel and gear revealed the wheel separated from the fork just above the wheel's axle, leaving an upper portion of the fork, which remained attached to the steering arm, and a lower portion, which remained attached to the wheel axle. The upper portion of the fork reveled it was badly scored on the lower surface, and material had been removed from, and even formed over, the edge of the fracture face. The fracture faces, on both the upper and lower portions of the fork, displayed a darker area on the inside edge, adjacent to the wheel. Approximately 30 percent of the lower portion's fracture face was covered by a rust-colored grainy deposit, consistent with surface corrosion. In addition, the darkly discolored fracture region contained what appeared to be two ratchet marks. The dark discoloration and the presence of the ratchet marks are typical of fatigue cracking, with initiation from multiple origins on the inner edge of the fork, adjacent to the wheel. The surface of this portion of the fork, adjacent to the wheel, displayed the pitted surface and brown coloration typical of corrosion (rusting). Examination of the entire fork revealed areas devoid of chrome plating, which once covered the entire landing gear fork. Additional areas were observed where a pitted surface and brown coloration typical of the corrosion process were evident. When viewed from the aft, the periphery of the wheel displayed a smooth taper toward the fork by approximately 1/8 inch. PATHOLOGICAL INFORMATION The San Bernardino County Coroner completed an autopsy. According to the autopsy report, the pilot died as a result of "positional asphyxia with blunt head trauma." According to the coroner's report, the pilot was found suspended in the airplane's restraint system; however, his neck was bent to the left with the left ear touching his left shoulder, and his head resting on the canopy. Photographs of the airplane taken prior to the accident revealed the Plexiglass canopy was situated directly over the pilot and passenger's heads. There was no structural support located above the canopy's slide. The only formed metal above shoulder height was where the windshield and canopy came together, which was forward and above pilot and passenger head level. Photographs taken after the accident showed the airplane resting on its upper nose cowling, left wing, and left seat back. The Federal Aviation Administration Toxicology and Accident Research Laboratory, Oklahoma City, Oklahoma, performed toxicological testing of specimens of the pilot. The results of analysis of the specimens were negative for carbon monoxide, cyanide, ethanol, and drugs.
the corrosion and fatigue failure of the tail wheel fork.
Source: NTSB Aviation Accident Database
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