History of Flight
On August 18, 2017, at approximately 1400 Alaska daylight time, a tailwheel-equipped Cessna A185F airplane, registration N92DC, experienced a propeller blade separation while taxiing for takeoff at Aniak Airport (ANI), Aniak, Alaska. The private pilot and three passengers sustained no injuries. The airplane, operated by Adams Guiding Service, was conducting a visual flight rules business flight under 14 CFR Part 91. Day visual meteorological conditions prevailed, and no flight plan was filed. The flight was intended to depart from ANI for a remote guided hunting operation south of Aniak.
While taxiing at about 15 mph with the engine at approximately 2,700 rpm, a blade from the metal two-blade propeller separated. The pilot maintained control and shut down the airplane without further incident. No ground injuries occurred.
Aircraft and Propeller Information
The airplane was equipped with a McCauley Propeller Systems controllable pitch (constant speed) aluminum propeller, model D2A34C58-0 (serial number 951968), which had accumulated 1,575.5 hours since its last overhaul on April 15, 2008. Maintenance records showed that the propeller underwent 100-hour inspections on July 13, 2017 (at 1,544.6 hours) and March 16, 2017 (at 1,489.2 hours). No uncorrected mechanical discrepancies were noted in the records.
Damage and Examination
Postflight inspection revealed the engine mount sustained substantial damage from vibrations caused by the blade separation. The separated blade (blade #2) and the intact blade were sent to the NTSB Materials Laboratory for examination. The #2 blade had separated approximately one-third of its length from the root, with a v-shaped crack extending from the leading edge.
Fractographic analysis showed a fatigue crack emanating from a small nick in the leading edge measuring 0.07 by 0.08 inches, with a maximum depth of 0.02 inches. The nick showed no prior field repair. Fatigue features, including crack arrest lines, indicated the crack originated from deformed material at the nick. Chemical and mechanical testing confirmed the blade material met specifications for 2025-T6 aluminum alloy.
Maintenance Considerations
McCauley Service Bulletin 137AF specifies a 1,500-hour or 60-month overhaul interval for the D2A34C58-0 propeller. At the last two 100-hour inspections, the propeller had exceeded the 1,500-hour TBO (1,544.6 hours at the July inspection). The reason the propeller was not overhauled during these inspections was undetermined. The Cessna A185F Owner's Manual advises inspecting the propeller for nicks and caring for small nicks to prevent cracks.