Tuesday, December 25, 2012
The Reno Accident
Meticulous probing of wreckage led investigators to some tiny screws
The P-51 first entered production in 1941, and was originally designed and built as a long-range military fighter airplane. The P-51D, which formed the basis of The Galloping Ghost, entered production in April 1944. The stock P-51D's empty weight was 7,635 pounds, and its maximum gross weight was 12,100 pounds. The airplane that became The Galloping Ghost was delivered to the Army Air Forces on December 23, 1944. After World War II, in July of 1946, it was declared surplus and sold. The airplane was raced from 1969 through 1982 with various modifications for racing before being acquired by Leeward through a company in July 1983. Leeward raced it until 1989 and then placed it in storage until 2007. Between 2007 and 2009, the airplane underwent overhaul and further modifications. A modified Rolls-Royce Merlin V-1650-9A engine powered the airplane. Special engine mounts were installed to hold the engine securely at racing speeds. The airplane was outfitted with a racing canopy and hinged to open upward rather than sliding open. The upper fuselage structure was modified to accept the racing canopy. A special system was installed to help cool the engine. Called a "boil-off" cooler, it uses a mixture of water and methanol to absorb heat. The heat makes the mixture boil, and the venting of vapor helps dissipate heat and prevent detonation. The original left wing fuel tank was converted to hold the coolant. The manufacturer's lower air scoop was removed and a custom structure installed.
The outboard section of each wing was replaced with a new wingtip. The wingspan was shortened from just over 37 feet as manufactured to just under 29 feet, making it the shortest of any modified P-51D racing in 2011. The length of each aileron was halved, to about three feet from a stock length of about seven feet. The right aileron trim tab was removed, and the trim for the left aileron was modified to run by an electric motor instead of the manufacturer's manual trim system. The horizontal stabilizer was shortened by about a foot. Modifications to the elevator, including its counterweights, upper rudder counterweight, vertical stabilizer incidence, horizontal stabilizer incidence and rudder were discovered by investigators.
The FAA issued a certificate authorizing the races to take place at the Reno airport. It said that all flights conducted at altitudes less than 1,000 feet AGL must remain north of runway 8/26 and within 1,000 feet horizontally of the depicted course. It also stated that race flight operations may be no closer than 500 feet horizontally from the primary spectator areas. However, an FAA Advisory circular cited by the NTSB stated that aircraft racing at more than 250 miles per hour needed to be 1,000 feet away from spectator areas.
Examination of the elevator trim tabs found that, of the three screws that attach each tab to its respective hinges, two screws in each tab were found intact. All of the intact screws could be rotated easily, even when fully engaged in their locknuts, and the locknuts' insert material was badly deteriorated to the extent that screw-retaining torque could not be maintained. Examination of the remnants of the two fractured screws (one in each tab) and their hinges showed evidence that these screws also had been loose before fracturing. The overload fracture of the right trim tab's center hinge screw showed evidence of directional bending occurring before a shearing action. The left trim tab's inboard hinge screw showed evidence of reverse bending fatigue, and corrosion on the fatigue fracture features indicated that the fatigue cracking had been present for a prolonged time before the screw ultimately failed in overload during the accident flight. The loose locknut connection allowed for larger load oscillations and the initiation and growth of fatigue cracks.
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