History of Flight
On June 23, 2012, at approximately 1955 Pacific daylight time, a DeHavilland DH 82A (Tiger Moth), registration N523R, nosed over following a forced landing near Fontana, California. The airplane was operated by the pilot under 14 Code of Federal Regulations Part 91. The airline transport pilot and the passenger were not injured. The airplane sustained substantial damage to the aft fuselage, rudder, and upper wings.
The local flight departed Flabob Airport in Riverside/Rubidoux, California, about 1950, with a planned destination of Cable Airport in Upland, California. Visual meteorological conditions prevailed, and no flight plan was filed.
The pilot reported that the initial climb after takeoff was uneventful. After leveling the airplane at 2,000 feet mean sea level for cruise, he reduced engine speed from 2,300 to 2,000 rpm. He then noticed a further reduction in engine speed accompanied by a vibration. Applying full throttle did not increase engine speed. With a field in view, he elected to perform an immediate forced landing. The airplane was not at sufficient altitude to turn into the wind, and during the landing roll, the airplane nosed over.
Aircraft Information
Federal Aviation Administration records show that the two-seat biplane was manufactured in the United Kingdom in 1940 and imported to the United States in 1965. The pilot purchased the airplane in 1987.
The airplane was equipped with a four-cylinder, air-cooled DeHavilland Gipsy Major Series 1C engine, serial number 8438. Maintenance records indicate that at the last annual inspection on November 1, 2011, the airframe had accumulated 3,472 total flight hours, and the engine had 683 hours since overhaul in 1966. The pilot reported that the airplane had flown 17 hours since that inspection.
Examination and Findings
The airplane was recovered from the accident site and 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. Consequently, the rocker arm became displaced from the exhaust valve. The bolt head and shank were loose within the rocker cover, with the separated end located in the cylinder head assembly. The bolt 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. The fracture face exhibited granular textured features with a series of semicircular striations emanating from a thread fillet. The bolt head had an indiscernible manufacture marking that did not correspond to any current Military or National Aerospace Standards.
Examination of the remaining cylinders revealed that an alternate rocker clamping method had been used, employing three internal-wrenching hexagon drive bolts secured with interconnecting safety wire.
Modification and Records
Rolls Royce Modification News Sheet Number 1448, issued March 17, 1950, detailed a modification to the rocker bracket assembly designed to prevent failure of the clamping bolts. The modification required a minor alteration to the bracket and installation of larger, 9-millimeter diameter bolts along with a one-piece locking plate instead of castellated nuts and cotter pins.
Logbook records dating back to 1997 showed that the engine had undergone maintenance requiring removal of all four cylinder heads on three occasions. The last removal occurred in January 2009 when new pistons were installed. No record was found indicating that Modification 1448 had been performed, and no documentation could be found detailing the installation of the hexagon drive bolts on cylinders one, two, and three.
Examination of FAA records did not reveal any airworthiness directives for this engine model, and no other instance of this type of failure was found in 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.