History of Flight
On May 28, 2011, about 0800 mountain daylight time, a Grumman Aircraft G-164A, registration N9724, collided with a ditch following a forced landing due to a loss of engine power near Hazelton, Idaho. The flight was operated by Crop Jet Aviation LLC under 14 CFR Part 137 as an aerial application flight. The commercial pilot was not injured; the airplane sustained substantial damage. The flight departed from Hazelton Municipal Airport at 0740. Visual meteorological conditions prevailed, and no flight plan was filed.
The pilot reported that after an application pass over a field, the engine began to vibrate and lose power. He initiated a climb, heard a bang, and observed a white puff of smoke from the top of the engine. The engine subsequently lost all power, and the pilot attempted a forced landing in a pasture. During the landing roll, the propeller contacted the ground, and the airplane sustained damage to the rudder, lower wing, and forward fuselage.
Aircraft Information
The fixed-gear, tailwheel-equipped biplane, serial number 1244, was manufactured in 1974. A review of maintenance logbooks revealed that at the time of the last 100-hour inspection dated March 30, 2011 (15 flight-hours prior), the airplane had accrued 15,789 total flight hours.
The airplane was powered by a Pratt and Whitney R-1340-S3H1G engine, serial number 42-14186, a single-row, nine-cylinder, air-cooled radial with an integral centrifugal supercharger driven through a 10:1 ratio gearbox. The engine was overhauled in November 2005 by Covington Aircraft. At overhaul, an estimated 7,550 hours were on the engine. At the time of the accident, the engine had 526.1 hours since overhaul.
The engine was equipped with a Hamilton Standard three-blade constant-speed propeller, model 23D40-311, serial number 952601. The propeller was overhauled in April 2005, and disassembled, cleaned, and inspected in March 2010 at 1,400 hours since overhaul.
Engine Examination
The engine and airframe were examined at the accident site by an FAA inspector. The crankshaft could only be partially rotated before binding. Engine oil from the sump contained a gear tooth on the chip detector.
The engine was removed and transported to Covington Aircraft for further examination. Forward spark plugs were removed and appeared gray with slight ovaling, consistent with normal wear. The rear accessory case was removed; the lower impeller and shroud were free of gouges and scratches. Translucent oil was present. The floating gear was intact with serrations on three gear teeth. The impeller moved freely for about three turns then impeded. Four blower intermediate gear teeth were found in the sump and accessory case. The intermediate gear had five teeth missing from its small spur gear. No other discrepancies precluding normal operation were found.
Carburetor Examination
The Bendix/Stromberg gravity-feed carburetor (model NA-Y9E1, serial number 5769298) showed no damage. The fuel inlet screen was clear. Aviation gasoline odor was present. The carburetor was mounted on an exemplar engine; when the throttle was advanced rapidly, the engine sputtered, backfired, hesitated, and produced white smoke. Slower throttle advance eliminated the sputtering.
Maintenance records indicated the carburetor was overhauled in December 2005, with mixture control, economizer linkages, and accelerator pump replaced. It was modified for gravity feed. In January 2007, the float needle, seat, and pin were replaced. The records did not indicate total time between these events.
Examination at Aero Engines showed the mixture arm and economizer-to-butterfly linkage were loose and worn, with about 3/16-inch play. The throttle arm actuated the economizer valve at 30 degrees instead of the recommended 18 degrees. The float bowl was debris-free. The float valve needle showed minimal wear; the accelerator pump displayed what the Aero Engines representative described as excessive wear, with no pneumatic resistance. The accelerator pump check valve had appropriate tension. A float test showed the fuel level remained at nominal position. The idle air bleed valves had coking deposits but were unobstructed.
Propeller Examination
The propeller was examined at Precision Propeller Services. All three blades had tip twist, leading edge abrasions and nicks, and could be rotated longitudinally by hand. Disassembly revealed that all three blades had separation of their bushing retention pins at the shaft and fragmentation of spider shims. The segmented blade gears and hub cone gear were intact. The blade balance plugs within each bore remained pressed in but had become displaced and deformed with dull grey and pitted surfaces.
Intermediate Blower Gears Examination
The intermediate blower gears and teeth were sent to the NTSB Materials Laboratory. Magnified examination showed similar textured, matte gray curved surfaces on all teeth, consistent with overstress fractures. Deformation and smearing patterns indicated bending loads. No indications of progressive cracking such as fatigue were present.