N72ESGrumman G-164A · 1972

On February 28, 2014, about 0955 Pacific standard time, a Grumman G164-A Ag-Cat , N72ES, was substantially damaged during landing at a private agricultural airstrip in Caruthers, California. The airplane was owned and operated by Trinkle Ag Flying of Kingsburg, California. The pilot was not injured. The aerial application flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no FAA flight plan was filed for the flight.

According to the pilot, the flight was the third one of the day, and he was applying fertilizer. On touchdown to the north, the pilot heard a "huge bang," and the airplane began to roll right wing down. He tried to correct with aileron but was only partially successful. The airplane decelerated, veered to the right after the right wing contacted the ground, and departed the paved surface. As it began descending the embankment, the propeller struck the ground, and the airplane nosed over. The airplane came to rest inverted, partially down the embankment and mostly off the runway. There was no fire or fertilizer spill.

Examination of the airplane revealed that the right main landing gear leg had fractured near the upper bend, just below the fuselage. Preliminary visual examination of the fracture surfaces revealed characteristics similar to a fatigue fracture. The two sections of the fractured gear leg were sent to the NTSB Materials Laboratory in Washington, DC, for detailed examination and analysis.

The pilot held a commercial pilot certificate with an airplane single engine land rating. He reported that he had a total flight experience of approximately 2,434 hours, including about 1,943 hours in the accident airplane make and model. His most recent flight review was completed in September 2012, and his most recent FAA second-class medical certificate was issued in March 2013.

FAA information indicated that the airplane was manufactured in 1972, and was equipped with a Pratt & Whitney R-985 series engine. According to the operator's records, the airplane had a total time in service of approximately 16,615 hours, and the failed landing gear leg had a total time in service of approximately 6,000 hours. The most recent 100-hour inspection was completed in November 2013.

The 0953 automated weather observation at Fresno Yosemite International Airport (FAT), Fresno, California, located about 15 miles north of the accident site, included winds from 130 degrees at 18 knots, visibility 10 miles, few clouds at 3,500 feet, scattered clouds at 15,000 feet, a broken cloud layer at 20,000 feet, temperature 17 degrees C, dew point 13 degrees C, and an altimeter setting of 29.55 inches of mercury.

FAA information indicated that the airplane was manufactured in 1972, and was equipped with a Pratt & Whitney R-985 series engine. According to the operator's records, the airplane had a total time in service of approximately 16,615 hours, and the failed landing gear leg had a total time in service of approximately 6,000 hours. The most recent 100-hour inspection was completed in November 2013.

The airplane came to rest inverted, adjacent to the east side of the runway. The upper wing, fuselage, fin, and rudder sustained substantial damage, primarily crush or bending deformation. The right main landing gear leg was fracture-separated near the fuselage juncture, and the left wheel assembly was fracture-separated from the left main landing gear leg.

FAA records indicated that the airplane serial number was 758. Examination of the wreckage by an FAA inspector revealed that the dataplate installed on the rudder post fairing bore the serial number (s/n) 1710, but another dataplate affixed to the fuselage behind the cockpit bore the correct s/n. In his communication to the NTSB regarding this finding, the FAA inspector noted that when an agricultural operator owns more than one of an airplane type, "parts get mixed up," and that it was not surprising that since the dataplate was affixed to the rudder post fairing, that fairing got installed on the incorrect airplane.

Landing Gear Leg Fracture Evaluation

The examination of the fractured landing gear leg sections by NTSB Materials Laboratory personnel revealed that the fracture face indicated that it was primarily a fatigue crack that progressed to an instantaneous failure. The fracture face was covered by a speckled pattern of rust products, except at the lower forward corner, where a semi-elliptical region was nearly rust free. Visual and magnified examination revealed fracture traces and markings consistent with fracture initiation in the clean region at the lower forward corner of the gear.

Closer optical inspection of the clean area revealed a small, darkly oxidized semi-elliptical region adjacent to the lower gear surface at the forward corner. The oxidized region was then surrounded by a cleaner region that measured about 0.4 inch wide by 0.25 inch high. One crack arrest band was visible in the clean region, and shear lips were evident at the edge of the fracture.

Optical examination of the surface of the spring adjacent to the oxidized region revealed chipped yellow paint with a rough and rusted underlying surface, and removal of more paint uncovered an eroded and globular surface Those features are consistent with localized melting and resolidification, indicative of localized electrical arcing. The damage dimensions and locations indicated a single sustained large arc on the lower surface, and two separate arcs on the upper surface. X-ray energy dispersive spectroscopy of the surface revealed only elements consistent with a plain carbon steel.

The fatigue crack originated at the site of preexisting electrical arc damage to the gear leg, which created a local hard spot. The gear progressively cracked in steps, first to the extent of the oxidized region, then through the first transgranular band, and finally then to the clean region. The presence of paint on the surface of the arc damage indicated that the arc damage occurred prior to that application of paint. The chipped condition of the paint, and the corrosion of the underlying metal, was consistent with the paint not being applied in the recent past.

The airplane manufacturer's instructions for continued airworthiness contained the "CAUTION" that the landing gear legs are "heat treated. NO [emphasis original] welding is permissible." Possible mechanisms for the arc damage included a power line strike, a lightning strike, improper grounding during weld repairs to the airframe, or processing damage during overhaul of the landing gear. The maintenance records were not made available to the investigation, which prevented the determination of the likely mechanism or date of the arc event. The lack of maintenance records also prevented determination of the component's service and inspection history, which precluded the identification of any gear leg inspection interval, type, or process deficiencies.