HOLBROOK, AZ, USA
N7HB
Beech N35
Moments after lift-off, the propeller separated, & the aircraft (acft) veered off the runway, contacted soft soil, & nosed over. Investigation revealed the propeller hub failed forward of the attachment flange, around the circumference of the hub. The pilot said he had performed a thorough preflight & run-up, & found no discrepancies. Metallurgical exam of the hub revealed crack arrest marks, typical of fatigue, emanating from the 4th thread root on the aft piece of the hub. The crack was estimated to be several inches long before failure occurred. Review of records disclosed that McCauley Service Bulletin 179A, McCauley Service Letter 1989-5, & Airworthiness Directive (AD) 89-26-08 had not been complied with during the previous 5 annual inspections. These referenced propellers which should have incorporated an oil-filled configuration with red dye that would become visible once a hub had begun to crack. The AD cited McCauley '2A36C23/84B-0' Constant Speed Propellers. Both McCauley & the FAA engineering directorate, which wrote the AD, reported the intent was to capture all 2A36C23 hubs regardless of blade models. The hub on the accident acft was a McCauley 2A36C23-P-E, with blade Models S-84B-0 installed (rather than blade Models 84B-0). Unanimous response from mechanics & IA's, who worked on the acft during the previous 5 annuals inspections, was that the AD was not applicable since the propeller was not a '2A36C23/84B-0' as stated in the AD.
On January 9, 1997, at 1405 mountain standard time, a Beech N35, N7HB, collided with the ground following failure and separation of the propeller during the takeoff initial climb at the Holbrook, Arizona, airport. The aircraft sustained substantial damage. The certificated commercial pilot and the two passengers were not injured. The aircraft was owned and operated by the pilot, and was beginning a personal, cross-country flight to Flagstaff, Arizona. Visual meteorological conditions prevailed and a VFR flight plan was filed. The pilot reported that a preflight had been conducted and no evidence had been found of oil discharge on the propeller, spinner or cowling, and further, that the windshield was clean. A complete run-up was conducted after the oil temperature was in the green arc. The propeller was cycled per the checklist. Additionally, the pilot said that at approximately 55 knots, he rotated for takeoff, and at 60-65 knots, a loud bang was heard, which was followed by a secondary bang and a noticeable feeling of physical trauma to the airframe. The secondary bang was simultaneous with a downward pitch and loss of directional control to the right of approximately 15-20 degrees; the cockpit view gave the impression that the nose gear had collapsed. The pilot stated that he then heard the engine over speed and visually confirmed that the throttle was aft. The aircraft continued its 15-20 degree heading right of the runway course, decelerated rapidly in soft dirt, and overturned in a nose-down attitude. An FAA airworthiness inspector from the Scottsdale, Arizona, Flight Standards District Office conducted an on-site examination of the airplane. He reported that the propeller hub, a McCauley Model 2A36C23-PE, had fractured and failed, liberating the propeller blades as the aircraft was taking off. In his report, the inspector indicated that the catastrophic failure of the propeller hub occurred forward of the attachment flange around the entire circumference of the hub, and the propeller had instantaneously separated from the aircraft. Additionally, the inspector reported that while researching Airworthiness Directive (AD) compliance records for the accident aircraft, it was determined that AD 89-26-08 applied to the accident aircraft but had not been complied with. The compliance section of AD 89-26-08 states that [the AD was issued] "to prevent possible cracks in the propeller blade threaded retention area from progressing to blade separation, which can result in loss of aircraft control. . ." According to a representative of McCauley, the purpose of this AD was to force modification of the propeller hub to include a red dye impregnated oil. The idea is that the dyed oil, located in the hub reservoir area, will be driven by centrifugal force into the hub/propeller blade threads where oil would completely travel to the last set of threads and be trapped by an O-ring, remaining inert in the system. Only in the event of a fatigue crack, or other type of component failure would the bright red dye be released from the hub area where it would then become highly visible upon inspection of the propeller area, indicating to the pilot that an unsafe condition exists and that the aircraft is no longer airworthy. The McCauley representative stated that the intent of the AD was to capture all 2A36C23 series hubs, regardless of the propeller blades that are attached, but the applicability section of the AD as written states that the AD applies to McCauley models 2A36C23/84B-O. The "84B-0" designation after the slash refers to a specific blade design. The failed propeller hub is a McCauley model 2A36C23-P-E, which has blade models S-84B-0 attached. The FAA inspector further reported that he presented the propeller model 2A36C23-P-E to several certificated mechanics, IA's and other FAA Airworthiness Inspectors, along with a copy of AD 89-26-08 and asked if the AD was applicable. All of the aviation maintenance professionals polled responded that the AD was not applicable because the AD cites only a specific hub and blade design as being applicable. A complete maintenance record review revealed that the accident aircraft had five annual inspections, completed by two different IA's and a repair station since the effective date of the AD. All three, when questioned why the AD had not been complied with, stated their belief that the AD applied only to the 2A36C23 hubs with the 84B-O blades. TESTS AND RESEARCH The propeller hub was sent to the Safety Board's Materials Laboratory in Washington, D.C. for detailed metallurgical examination. According to the report, visual examination of the fracture in the hub revealed the presence of crack arrest positions, typical of fatigue cracking, that emanated from the fourth thread root. . .[with the] approximate center of the fatigue initiation area on the aft piece of the hub. Measurement of the thread root radius in the various threads generated a value of 0.020 inches throughout most of the radius. This value complies with the manufacturer's specified size of the radius, 0.0187 inches to 0.0206 inches. However, it was noted that the root radius did not completely blend smoothly with the thrust flank of the thread, resulting in an effective radius slightly smaller than 0.020 inches at a position approximately corresponding to the initiation of the fatigue cracking. Additionally, the metallurgist's report also indicated that it was estimated that the length of the crack along the exterior surface of the hub prior to the failure was at least several inches.
fatigue failure of the propeller hub, which resulted in separation of the propeller assembly and reduction of aircraft control. Also causal was: the resultant lack of compliance with Airworthiness Directive (AD) 89-26-08, due to ambiguous and unclear wording of the AD as issued by the FAA. Factors relating to the accident were: non-compliance with McCauley Service Bulletin 179A and McCauley Service Letter 1989-5, and an encounter with soft terrain during the aborted takeoff.
Source: NTSB Aviation Accident Database
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