ANC02LA034

On May 16, 2002, about 1020 Alaska daylight time, a Cessna 170B airplane, N4669C, sustained substantial damage when the right main landing gear separated from the airplane during landing at Bradley Sky-Ranch Airport, North Pole, Alaska. The airplane was being operated as a visual flight rules (VFR) personal flight under Title 14, CFR Part 91, when the accident occurred. The solo private pilot was not injured. The flight originated at the Fairbanks International Airport, Fairbanks, Alaska, about 0945. Visual meteorological conditions prevailed, and no flight plan was filed.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on May 16, the Federal Aviation Administration (FAA) aviation safety inspector who responded to the accident site said the pilot told him that during the landing roll on the gravel airstrip, the right main landing gear separated from the airplane, resulting in damage to the right wing and fuselage. The pilot also told the inspector the main landing gear had recently been rebuilt, and this was the second landing since the work was completed.

An on-site inspection by the FAA aviation safety inspector revealed that the retaining nut on the right main landing gear attaching bolt was missing. The right main landing gear attaching bolt remained attached to the separated gear leg. The AN7-20A bolt was bent, and the threads were damaged. The bolt's retaining nut was found inside the landing gear bulkhead assembly. The nut was longitudinally fractured in the center of one of its hex flats. The nut was identified as an MS21044C stainless steel self-locking nut.

The Cessna illustrated parts catalog indicates the proper replacement nut for the main landing gear attachment is an AN365-720C nut. The AN365-720C is an all steel self-locking nut. The AN365 series steel nuts, with the exception of the AN365-720C size, have been superceded by MS20365 steel nuts, which incorporate some alloys such as copper and aluminum in their makeup, but are considered essentially steel nuts.

The stainless steel nut was installed by a mechanic from Smith Aero Service, Inc., Fairbanks, during the recent rebuild of the airplane's main landing gear. The mechanic told the IIC he went to his supplier, Tamarack Air, Ltd., Fairbanks, and requested the appropriate replacement nut for the main landing gear assembly on the Cessna 170B airplane. He said he was given the MS21044C nut, which he installed on both main landing gear of the accident airplane. He said he relied on the supplier that the stainless steel nut was a suitable replacement.

The NTSB IIC contacted a representative of Tamarack Air, Ltd., and inquired why they provided the mechanic with the MS21044C stainless steel nut instead of the AN365-720C steel nut. The representative said the MS21044C nut was provided by their supplier, Aviall Aircraft Parts, of Anchorage, Alaska, as a suitable replacement for the AN365-720C.

During a telephone conversation with the IIC on July 9, a representative for Aviall Aircraft Parts stated they substituted the MS21044C for the AN365-720C pursuant to a military standard substitution chart which states, in part: "MS21044 nuts can universally replace AN365, MS20365 and NA31021 nuts of like material, plating and thread size." The representative sent a copy of the military standard substitution chart to the IIC. The chart reflected the same information as given by the Aviall representative, but also noted that the AN365 and MS20365 in the 7/16" diameter 20 threads per inch size were excluded from the universal replacement.

The fractured nut and the bolt were sent to the NTSB Materials Laboratory for examination by a specialist. According to a metallurgical report prepared by the specialist, the fracture mechanism for the nut was circumferential overstress tension separation. The report also notes that the all steel MS21044N is the appropriate replacement nut, and that steel and stainless steel are not considered "like materials" as specified in the military specification substitution chart.

Examination of the bolt revealed fractured threads in the region adjacent to the bolt grip. The fractured threads included a portion of the starter thread adjacent to the bolt grip. The remaining threads nearest the end of the bolt were heavily deformed but not fractured. Remnants of the fractured threads were found embedded between areas of the intact threads moving from the head toward the end of the bolt.

Additional information

A "Locknut Selection Chart" provided by the Genuine Aircraft Hardware Company also states that the replacement nut for the AN365-720C is the MS21044N, which is a full height, all steel nut. The chart indicates the rated tensile strength of the AN365, MS21044C, and MS21044N series nuts are all 125 ksi. During the 1930's the Department of Defense (DOD) created a standard (military specification) for aircraft hardware based on the needs of military aircraft. Nuts and bolts were matched so that the corresponding nut was stronger than the bolt, and the bolt would break off in an over-torque situation. The designation (AN) as in AN365, referred to the Army/Navy standard. Subsequently, civil and military aircraft manufacturers adopted the AN standard for aircraft hardware. As an example of the metamorphosis of hardware, the AN365 nut specification was superseded in 1988 by the MS20365 nut specification, but the design specification remained the property of the Department of Defense, and its manufacture and testing subject to DOD standards. In 1994, the DOD sold the AN365/MS20365 specifications to the Automotive Society of Engineers (ASE). Once the specification was sold, there was no requirement for the hardware to meet the old AN standard, and the hardware was no longer inspected by the federal government.