NTSB Factual Report

On July 2, 2011, at 2040 eastern daylight time, a Wingtip to Wingtip, LLC model Panzl S-330 airplane, N330JK, was substantially damaged when the entire left aileron separated from the airplane during an aerobatic flight over W K Kellogg Airport (BTL), Battle Creek, Michigan. The pilot, who was not injured, was able to land the airplane without further issue. The experimental airplane was registered to and operated by John Klatt Airshows, Inc., under the provisions of 14 Code of Federal Regulations Part 91, without a flight plan. Day visual meteorological conditions prevailed for the local airshow demonstration flight that departed at 2035.

The pilot reported that the left aileron departed the airplane as he was performing an aerobatic aileron-roll maneuver about 50 feet above the ground at 160 miles per hour with a 30-degree nose-up attitude. The pilot was able to recover into level flight and make an uneventful landing. The pilot reported that his aviation mechanic had to retighten the jam nut for the center aileron hinge rod-end after it had been found loose on several occasions before the accident flight. The pilot stated that his preflight inspection before the accident flight did not reveal any mechanical issues with the airplane, including loose jam nuts or any other aileron anomalies.

The left aileron was recovered in three major sections and was sent to the NTSB Materials Laboratory for additional examination. The examination revealed that the center aileron hinge had failed in the shank region of the rod end due to reverse bending fatigue that covered about 90-percent of the fracture face. The remaining 10-percent of the fracture face exhibited overstress signatures. The fatigue zones were located on the upper and lower portions of the fracture face as installed, indicating that the predominant loading was due to the actuation of the aileron. No material issues were identified with the center aileron rod end, which met the specification for a FAFNIR USA part number RE3ML6-2N. The rod end was constructed of AISI 8620 steel that had a specified tensile strength of 90 ksi. The remaining inboard and outboard aileron hinges exhibited fracture features consistent with overstress. The examination did not reveal any material anomalies with the inboard or outboard hinge assemblies.

The center rod end was installed in a 2024-T3 aluminum bracket that was attached to the wing rear spar. The rod end was threaded into the end of the bracket utilizing Loctite on the threads and a jam nut that was seated against the bracket to prevent the rod end from backing out. The accident airplane reportedly had experienced a loose jam nut on several flights before the accident. The rod end failed in the area of the shank adjacent to the aft edge of the bracket beneath the area where the jam nut normally was secured. A Finite Element Analysis (FEA) was performed on the design with a loose jam nut and a tight jam nut. The analysis indicated a maximum tensile stress of about 60 ksi at the upper surface of the shank for the loose jam nut condition and about 47.5 ksi for the tightened jam nut. The fore-aft location of the maximum stress was at the shank-to-bracket interface for the loose jam nut condition and at the shank to jam nut interface for the tight jam nut condition.

Considering a typical airplane operating weight of 1,600 pounds and the design load factor of 10 G, the total aileron load was about 681 pounds due primarily to the bending stress produced by the vertical load at the bearing. The calculated load on the center hinge was about 286 pounds. The maximum bending stress was about 51.0 ksi and 65.7 ksi for the tight jam nut and loose jam nut conditions, respectively. The maximum bending stress occurred at the upper surface of the shank for the aileron deflected full trailing edge up. The total tensile stress at the upper edge of the shank was 52.4 ksi and 67.1 ksi for the tight jam nut and loose jam nut conditions, respectively. The loose jam nut condition yielded a 28-percent increase in the tensile stress versus the tight jam nut condition.

For steel components with a tensile strength less than 160 ksi, the endurance limit is expected to be about 50-percent of the tensile strength for smooth polished specimens. Based on the load estimates, the stress in the rod end was at or above the corresponding endurance limit.

Following the accident, the airplane manufacturer redesigned the aileron bracket assemblies by increasing the diameter of the rod end shank and reducing the moment arm between the bearing and the aileron bracket. The redesign yielded a total stress reduction of about 83-percent in the rod end, which was significantly below the endurance limit for the specified material.

NTSB Probable Cause

The inadequate aileron hinge design, which resulted in the in-flight separation of the left aileron following a fatigue failure of the center hinge rod-end.