NTSB Factual Report

On September 17, 2009, about 0940 Alaska daylight time, a tailwheel-equipped amphibious Grumman G-44A airplane, N86627, sustained substantial damage during a loss of control after landing at the Merrill Field Airport, Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) postmaintenance check flight, under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91, when the accident occurred. The commercial pilot/airplane owner and a maintenance technician were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated at the Merrill Field Airport, about 0915.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on September 17, the pilot noted that the purpose of the flight was to do a postmaintenance check flight after repairs had been completed to the right brake assembly. He said that as he approached the airport, he lowered that airplane’s landing gear, depressed both brake pedals to test that there was adequate hydraulic back-pressure, and then continued the approach. After an uneventful touchdown on Runway 07, a dry paved runway, he applied the brakes to slow the landing roll. He said that while depressing both brake pedals, the right brake pedal unexpectedly "went to the floor" and the airplane swerved to the left. The pilot said that he immediately released pressure on both brake pedals, applied full right rudder, but he could not correct the swerve, and the airplane subsequently ground looped to the left, striking the right wing on the runway. During the ground loop the right wing and right aileron sustained substantial damage. A postaccident examination revealed a broken hydraulic supply line fitting on the airplane’s right brake assembly, with red hydraulic brake fluid sprayed within the right wheel-well area.

During an on-scene interview with the NTSB IIC on September 17, the maintenance technician that did the repairs on the airplane’s brake reported that the hydraulic supply line fitting was not removed during his work. He also said that after his postaccident inspection of the broken hydraulic supply line fitting, he strongly suspected that the broken fitting might be an unapproved or "bogus" aircraft part due to an atypical thread pattern, irrespective of the hydraulic fitting having an "AN" (Air Force-Navy standards) identification stamp.

The pilot/airplane owner said that the hydraulic brake line fittings had not been changed since he had owned the airplane, and a review of the airplane's maintenance logbooks did not disclose any previous work or replacement of the hydraulic fittings.

The broken hydraulic fitting was sent to the NTSB's Materials Laboratory for examination. The hydraulic fitting was inspected on November 18, 2009, at the NTSB's Washington D.C. materials laboratory, under the direction of a senior Safety Board mechanical engineer.

The Safety Board engineer reported that the fracture surface of the 45-degree elbow fitting displayed two distinct surface textures. One was dull and grainy, features typical of an overload event, and one was shiny and smooth. The smooth shiny surface had multiple ratchet marks, features that are consistent with fatigue. He added that an examination of the major fatigue zone revealed numerous zones containing ratchet marks and crack arrest marks, also typical of fatigue.

The Safety Board engineer noted in his report that the broken fitting was marked with the "AN", which signifies it is approved for use in aircraft applications. He said that a nearly identical fitting is available which is intended to be used in industrial applications, and manufactured to meet the standards of the Joint Industrial Council (JIC), but is not intended for use in aviation applications. He wrote, in part: "The two types of fitting are visually indistinguishable except for the 'AN' identification. The most distinguishable difference between the 'AN' and the 'JIC' fitting is in the threads at the flare end."

AN Fittings vs. JIC Fittings

The "JIC" fitting uses a standard series thread with a flat or partially rounded root. The "JIC" fitting is designated as a class 2 component.

The "AN" fitting uses a thread with a controlled root radius, and a tight tolerance thread pattern to achieve an increase in fatigue strength. The "AN" fitting is designated as class 3 component, which has a reported 40 percent increase in fatigue strength over the identical appearing "JIC" fitting.

According to the Safety Board engineer, the thread pitch diameter of the fractured fitting was measured using thread triangles and it was found to be consistent with that of a class 2 or "JIC" fitting, which was not intended for use in an aircraft. He said that without the aid of magnification, the "JIC" fittings and the "AN" fittings look identical.

A complete copy of the NTSB's materials laboratory factual report is included in the public docket for this accident.

On July 22, 2008, the Federal Aviation Administration (FAA) issued an Advisory Circular (AC) 21-29C, change 1, which provides updated information and guidance to the aviation community for detecting suspected unapproved parts (SUP) and reporting them to the FAA. Appendix 1 contains FAA Form 8120-11, Suspected Unapproved Parts Reports, which serves as a standardized means of reporting.

As a result of this accident and the subsequent investigation, the airplane owner/pilot replaced all of the "AN" hydraulic fittings to ensure that no remaining unapproved parts remained on his airplane.

NTSB Probable Cause

A fatigue fracture of an unapproved hydraulic brake line fitting, which resulted in a loss of control during the landing roll. Contributing to the accident was the installation of an unapproved part by an unknown maintenance personnel.