WPR11LA238

The pilot was completing an aerial application pass over a field when the engine began to lose power. He initiated a climb, heard a bang, and observed a white puff of smoke issuing from the top of the engine. The engine subsequently lost all power, and the airplane sustained substantial damage during a forced landing into a pasture. Postaccident examination of the engine revealed that it did not exhibit catastrophic internal damage or any preimpact anomalies that would have precluded normal operation. The carburetor was undamaged during the accident and was subsequently installed on an exemplar engine for testing. During the tests, the engine sputtered, backfired, hesitated, and produced puffs of white smoke from the exhaust ports when the throttle was advanced from idle to power. The sputtering stopped when the throttle was advanced slowly. Examination and disassembly of the carburetor revealed excessive wear, particularly to the accelerator pump assembly. The accelerator pump provides increased fuel flow to the engine during rapid throttle advance, and an ineffective pump would cause engine hesitation and stumbling during acceleration. The symptoms reported by the pilot during what would have been rapid engine acceleration after completing a pass was consistent with a worn accelerator pump, and matched the engine characteristics observed during the carburetor test. The carburetor had been installed on another engine since its most recent maintenance event and its total time since overhaul could not be determined. The engine examination also revealed that five teeth from an intermediate gear within the supercharger drivetrain were fractured, with loading opposite of the direction of rotation. Metallurgical tests found that the teeth failed in overload. Given the rotational speeds of the supercharger system, the gear failure was most likely caused by the sudden engine stoppage after the propeller made contact with the ground.

HISTORY OF FLIGHT On May 28, 2011, about 0800 mountain daylight time, a Grumman Aircraft G-164A, N9724, collided with a ditch following a forced landing due to a loss of engine power near Hazelton, Idaho. Crop Jet Aviation LLC., was operating the airplane under the provisions of Title 14 Code of Federal Regulations (CFR) Part 137 as an aerial application flight. The commercial pilot was not injured; the airplane sustained substantial damage during the accident sequence. The local flight departed from Hazelton Municipal Airport at 0740. Visual meteorological conditions prevailed, and no flight plan had been filed for the local flight. The pilot reported that following 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 issuing 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 the airplane’s maintenance logbooks revealed that at the time of the last 100-hour inspection, dated March 30, 2011 (15 flight-hours prior to the accident), the airplane had accrued a total time of 15,789 flight hours. The airplane was powered by a Pratt and Whitney R-1340-S3H1G engine, serial number 42-14186. The engine was a single-row, nine-cylinder, air-cooled radial type, with an integral centrifugal supercharger (blower), driven through a 10:1 ratio gearbox. Maintenance logbooks indicated that the engine was overhauled in November 2005 by Covington Aircraft, Okmulgee, Oklahoma. The records indicated an estimated engine total time of 7,550 hours at overhaul. At the time of the accident, the engine had accrued a total of 526.1 hours since overhaul. The engine was equipped with a Hamilton Standard, three-blade, constant speed propeller; model number 23D40-311, serial number 952601. The propeller was overhauled in April 2005. A maintenance entry dated March 26, 2010, indicated that the propeller was subsequently disassembled, cleaned, and inspected at a time of 1,400 hours since overhaul. TESTS AND RESEARCH Engine The engine and airframe were examined at the accident site by an inspector from the Federal Aviation Administration (FAA). The inspector reported that the crankshaft could only be partially rotated at the propeller before binding. He drained engine oil from the sump, and a gear tooth was present on the chip detector. The engine was removed from the airplane, and transported to Covington Aircraft for further examination in the presence of the NTSB investigator-in-charge (IIC). The forward spark plugs were removed and examined. The electrodes were of the massive electrode type, and were gray in color, with slight ovaling consistent withnormal wear signatures when compared to the Champion Aviation check-a-plug chart AV-27. The rear accessory case was removed, and the lower impeller was inspected. The impeller and shroud appeared free of gouges and scratches. Translucent oil was observed within the shroud, consistent with postaccident engine storage. The floating gear was intact, with serrations noted on the upper faces of three gear teeth. All of the teeth were intact, all other observable gears were wet with oil, and no indications of distress. The impeller moved freely and smoothly when rotated by hand, and became impeded from rotation after about three turns. The impeller gear bearings were intact, rotated smoothly, and appeared undamaged. Four blower intermediate gear teeth were located in the sump and accessory case. The intermediate gear was removed, and five teeth were missing from its small spur gear. The remaining sections of the engine were examined, and no discrepancies or failures were noted which would have precluded normal operation. Carburetor Visual examination of the Bendix/Stromberg gravity-feed carburetor, model NA-Y9E1, serial number 5769298, revealed no indications of damage. The fuel inlet screen was removed, and was free of obstructions and debris. The odor of aviation gasoline was present. The carburetor was subsequently mounted on an exemplar Pratt and Whitney R-1340 engine on a test bench. The engine was started, and run through speed ranges varying between idle, and 2,000 rpm. The engine sputtered, backfired, hesitated, and produced puffs of white smoke from the exhaust ports when the throttle was advanced from idle to power. The sputtering ceased when the throttle was advanced at a slower rate. Maintenance records provided by Aero Engines, Los Angeles, California, indicated that the carburetor was overhauled at their facility in December 2005. At that time it was owned by another operator, and installed on another R-1340 engine. During overhaul the mixture control and economizer linkages, and the accelerator pump assembly, were replaced. The carburetor was also modified for a gravity feed fuel supply. The carburetor was subsequently returned for maintenance in January 2007, while still owned by the same operator, for replacement of the float needle, seat, and pin. The records did not indicate the total time in service between these maintenance events. The carburetor was subsequently examined at the facilities of Aero Engines in the presence of the IIC. The mixture arm and associated economizer-to-butterfly linkage were intact, but loose and worn, exhibiting about 3/16-inch play. The throttle arm was intact, and manual rotation resulted in the actuation of the economizer valve at 30 degrees rather than the manufacturer's recommended angle of 18 degrees. The carburetor case was opened, and both the float and bowl were free of debris. The float valve needle appeared minimally worn, with the needle pin on the float arm exhibiting flat spots. Theaccelerator pump displayed wear, which the Aero Engines representative described as excessive, with compression of the assembly resulting in no pneumatic resistance. The accelerator pump check valve was examined, and appeared to be set to the appropriate tension. A float test was performed, and the fuel level remained at the nominal position, 1/4 inch from the bowl seam for 1 minute, per the manufacturer's specifications. The idle air bleed valves were removed, exhibited coking deposits, but were otherwise unobstructed. Propeller The propeller was examined at the facilities of Precision Propeller Services, Inc., Boise Idaho, in the presence of an NTSB investigator, and an FAA inspector. All three blades exhibited tip twist, and leading edge abrasions and nicks. The blades could be rotated along their longitudinal axis by hand. The blades were removed, and the hub disassembled. All three blades exhibited similar separation of their bushing retention pins at the shaft, and fragmentation of the spider shims. The segmented blade gears and the hub cone gear were all intact. Visual examination of the blade balance plugs within each bore revealed that they remained pressed into the bore, but had become displaced. The cone-shaped plugs were removed, and had become deformed and slanted, exhibiting dull grey and pitted surface features. Intermediate Blower Gears The intermediate blower gears and associated gear teeth were sent to the NTSB Office of Research and Engineering Materials Laboratory Division for examination. Magnified examination of the tooth fractures revealed similar textured, matte gray curved surfaces on all teeth, consistent with overstress fractures. Deformation and smearing patterns were present, indicative of bending loads with the tooth crown bending opposite the direction of gear rotation. No indications of progressive cracking such as fatigue were present. Complete examination reports are contained within the public docket for this accident. ADDITIONAL INFORMATION According to published carburetor documentation, the accelerator pump is utilized to provide an increase in fuel flow to the engine during rapid throttle advance, and that an ineffective pump can cause engine hesitation and stumbling during acceleration.