Fontana, CA, USA
N523R
DEHAVILLAND TIGER MOTH DH 82A
During cruise flight, the engine lost partial power. The pilot performed a forced landing into a field, and the airplane nosed over. Postaccident examination of the engine revealed that a rocker bracket assembly clamping bolt had failed in fatigue, causing the assembly to partially detach from the cylinder head. The detachment prevented the exhaust valve from actuating. Peening damage sustained to the bolt revealed that it most likely failed during a prior flight, at an undetermined time before the ultimate displacement of the bracket assembly. No airworthiness directives existed for the antique engine, which was overhauled 46 years prior to the accident. However, a “modification news sheet” was issued by the manufacturer 16 years before the overhaul, which detailed a design change intended to preclude the failure of the clamping bolts. The modification was not performed on the failed cylinder; however, the three other cylinders exhibited an undocumented modification to their rocker valve assemblies, which appeared to perform the same function.
HISTORY OF FLIGHT On June 23, 2012, about 1955 Pacific daylight time, a DeHavilland DH 82A (Tiger Moth), N523R, nosed over following a forced landing near Fontana, California. The pilot was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. The airline transport pilot and passenger were not injured. The airplane sustained substantial damage to the aft fuselage, rudder, and upper wings during the accident sequence. The local flight departed Flabob Airport, Riverside/Rubidoux, California, about 1950, with a planned destination of Cable Airport, Upland, California. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that the initial climb after takeoff was uneventful. He leveled the airplane for cruise once they had reached an altitude of 2,000 feet mean sea level (msl), and reduced the engine speed from 2,300 to 2,000 rpm. He then noticed an additional reduction in engine speed, accompanied by a vibration. He applied full engine throttle control, but the engine speed did not increase. With a field in view, he elected to immediately perform a forced landing. The airplane was not at a sufficient altitude for him to turn the airplane into the wind, and during the landing roll, the airplane nosed over. AIRCRAFT INFORMATION Federal Aviation Administration (FAA) records revealed that the two-seat biplane was manufactured in the United Kingdom in 1940, and imported to the United States in 1965, where it was subsequently purchased by the pilot in 1987. The airplane was equipped with a four-cylinder, air-cooled, DeHavilland Gipsy Major Series 1C Engine, serial number 8438. Maintenance records indicated that at the time of the last annual inspection, which occurred on November 1, 2011, the airframe had amassed a total of 3,472 flight hours, with the engine a total of 683 hours since overhaul in 1966. The pilot reported that the airplane had flown for 17 hours since the inspection. TESTS AND RESEARCH Examination The airplane was recovered from the accident site, and subsequently examined by the NTSB investigator-in-charge. Examination of the engine revealed that the forward clamping bolt for the cylinder number four rocker bracket assembly had separated, allowing the assembly to partially detach from the head. As a result, the rocker arm had become displaced from the exhaust valve. The bolt head and shank was loose within the rocker cover, with its separated end located in the cylinder head assembly. Examination of the bolt revealed that it was about 8 millimeters in diameter, and had separated about 4 millimeters past its shank. The threaded bolt tip remained within the nut, with its cotter pin still in place. The bolt sustained peening damage across its entire surface, and examination of the fracture face revealed granular textured features, with a series of semicircular striations emanating from a thread fillet. The bolt head exhibited an indiscernible manufacture marking, which did not correspond to any current Military or National Aerospace Standards. Examination of the remaining cylinders revealed an alternate rocker clamping method had been employed, which utilized three internal-wrenching hexagon drive bolts, secured with interconnecting safety wire. Cylinder Head Modification Rolls Royce Modification News Sheet, Number 1448, issued March 17, 1950, detailed a modification to the rocker bracket assembly designed to obviate failure of the clamping bolts. The modification requires performing a minor alteration to the bracket, and then installing larger, 9-millimeter diameter bolts, along with a one piece locking plate instead of the castellated nuts and cotter pins. Logbook records dating back to 1997 revealed that the engine had undergone maintenance requiring the removal of all four cylinder heads on three occasions. The last removal occurred in January 2009, when new pistons were installed. No record was located indicating the 1448 modification had been performed, and no documentation could be found detailing the installation of the internal-wrenching hexagon drive bolts on cylinders one, two, and three. Examination of FAA records did not reveal the existence of any airworthiness directives for this engine model, and no other instance of this type of failure was located within the FAA service difficulty report database. FAA regulations do not require compliance with service letters, service bulletins, and modification sheets for aircraft operated under part 91.
Partial loss of engine power during cruise flight due to a failed engine cylinder rocker assembly clamping bolt. Contributing to the accident was the failure of unknown maintenance personnel to perform a modification to the assembly that would have prevented the failure.
Source: NTSB Aviation Accident Database
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