HISTORY OF FLIGHTOn August 18, 2017, about 1400 Alaska daylight time, a tailwheel-equipped Cessna A185F airplane, N92DC, sustained substantial damage from a propeller blade separation at the Aniak Airport (ANI), Aniak, Alaska. The private pilot and three passengers sustained no injury. The airplane was registered to and operated by the pilot, dba Adams Guiding Service, as a visual flight rules business flight under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91. Day visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The flight originated from ANI, about 1400.
The pilot reported the purpose of the flight was to depart from ANI and take the three passengers on a remote guided hunting operation south of Aniak, Alaska. While the airplane was taxiing for takeoff at about 15 mph and about 2,700 rpm, a blade from the metal two blade propeller separated. After the separation, the pilot was able to maintain control of the airplane and shutdown the airplane without further incident as shown below in figure 1. No injuries to personnel on the ground occurred after the propeller blade separation.
Figure 1 – View of the airplane after it was shutdown (courtesy of the pilot).
Prior to the propeller blade separation, the pilot reported there were no known mechanical malfunctions or failures with the airframe, the engine, and the propeller. AIRCRAFT INFORMATIONThe airplane was equipped with a McCauley Propeller Systems controllable pitch (also known as a constant speed) aluminum propeller, model number D2A34C58-0 (serial number 951968), which had amassed 1,575.5 hours since the propeller was overhauled (conducted on April 15, 2008).
A review of the maintenance records revealed that the propeller had undergone a 100-hour inspection on July 13, 2017, at which time it had accumulated 1,544.6 hours since the propeller was overhauled. The propeller had also undergone a 100-hour inspection on March 16, 2017, at which time it had accumulated 1,489.2 hours since the propeller was overhauled.
An examination of the maintenance records revealed no evidence of uncorrected mechanical discrepancies with the propeller. AIRPORT INFORMATIONThe airplane was equipped with a McCauley Propeller Systems controllable pitch (also known as a constant speed) aluminum propeller, model number D2A34C58-0 (serial number 951968), which had amassed 1,575.5 hours since the propeller was overhauled (conducted on April 15, 2008).
A review of the maintenance records revealed that the propeller had undergone a 100-hour inspection on July 13, 2017, at which time it had accumulated 1,544.6 hours since the propeller was overhauled. The propeller had also undergone a 100-hour inspection on March 16, 2017, at which time it had accumulated 1,489.2 hours since the propeller was overhauled.
An examination of the maintenance records revealed no evidence of uncorrected mechanical discrepancies with the propeller. WRECKAGE AND IMPACT INFORMATIONA postflight inspection by the pilot revealed the engine mount sustained substantial damage from excessive vibrations caused by the propeller blade separation as shown below in figure 2.
Figure 2 – View of the fractured engine mount (courtesy of the pilot). ADDITIONAL INFORMATIONBackcountry Flight Operations
Mountain, Canyon, and Backcountry Flying by Amy L. Hoover and Richard K. Williams provides guidance on preflight inspection areas that are unique for airplanes used for backcountry flight operations. This book discusses the preflight inspection of a propeller and states in part:
The propeller should be inspected before every flight, and blade nicks, cuts, or gouges must be properly dressed. TESTS AND RESEARCHThe intact propeller blade (referred to as blade #1), including the separated blade (referred to as blade #2), were recovered and transported to the National Transportation Safety Board Materials Laboratory in Washington, District of Columbia, for an examination. The complete examination report with pictures is in the public docket for this accident.
The #2 blade of the submitted propeller separated into two pieces as shown below in figure 3. The separation was located approximately a third of the blade length from the root of the blade. The crack extended from the leading edge and was roughly v-shaped along the chord of the blade. The blades were disassembled from the hub.
Figure 3 – View of the separated blade (blade #2).
The fracture surface was sectioned from the root half of the blade. There was a distinct thumbnail-shaped pattern emanating from a small nick in the leading edge of the blade.
The size of the nick measured approximately 0.07 inches by 0.08 inches, with a maximum depth of roughly 0.02 inches. A long grain was observed on the fracture surface adjacent to the forward side of the blade. The orientation of the long grain likely contributed to the v-shape of the crack path along the chord length of the blade.
The nick exhibited no signs of a prior field repair. It could not be determined when the fatigue crack was initiated.
The fracture surface was examined using a Zeiss Auriga 40 field emission scanning electron microscope. The fracture surface had transgranular features consistent with fatigue. Crack arrest lines on the fracture surface indicated the fatigue emanated from a corner of deformed material that resulted from the nick in the leading edge.
A portion of the #2 blade was sectioned and sent to Lehigh Testing Laboratories (New Castle, Delaware) for chemical analysis and mechanical property testing. The chemistry of the blade material was inspected using inductively coupled plasma spectroscopy. The chemical composition was consistent with 2025 aluminum alloy per AMS-QQ-A-367A. The material specified per the forging drawing was 2025-T6 aluminum.
Three tensile specimens were sectioned and machined from the blade and tested to failure per ASTM B557-15A. The 0.2% offset yield strength averaged 37 ksi and the ultimate tensile strength averaged 56 ksi, with an average 19% elongation (in 2 inches). All of these mechanical test data exceed the minimums stated in AMS-QQ-A-367A.
The average hardness of the blade material measured per ASTM E10-15A was 119 HBW, which is above the specified 100 Brinell hardness minimum per AMS-QQ-A-367A. ORGANIZATIONAL AND MANAGEMENT INFORMATIONCessna
The Cessna A185F Owner's Manual discusses the preflight procedures and states in part:
Check propeller and spinner for nicks and security.
The Cessna A185F Owner's Manual also discusses care for the propeller and states in part:
Small nicks on the propeller, particularly near the tips and on the leading edges, should be dressed out as soon as possible since these nicks produce stress concentrations, and if ignored, may result in cracks.
McCauley
McCauley has published Service Letter 1995-4C Field Inspection and Repair of Propeller Blades. This document discusses stress riser damage and the associated repair requirements and states in part:
Stress risers can cause failure of a blade if not repaired.
A stress riser is an increase in stress intensity surrounding an area of reduced cross-sectional area. A sharp stress riser is an increased stress intensity attributed to a notch-like displacement of material resulting from a sharp object impact, leaving a very small radius at the bottom of the displaced material.
Sometimes a stress riser in the propeller may be referred to as a nick.
Stress riser damage found during a routine inspection must be field repaired if it is not beyond the field repair limits. If damage is beyond the field repair limits, the blade must be evaluated and/or replaced by an FAA approved Part 145 Propeller Repair Station or international equivalent.
This document states that for damage that is located on the leading or trailing edges of blades must not exceed 0.094 inches in depth. Damage exceeding 0.094 inches in depth is not classified as field repairable and a 14 CFR Part 145 repair station must perform the repair work.
McCauley has published Service Bulletin 137AF Revised Time Between Overhaul. This document states that for the D2A34C58-0 model propeller, the overhaul frequency is 1,500 hours or 60 calendar months, whichever occurs first (the exception is for agricultural aircraft installations, where the overhaul frequency is 1,200 hours or 60 calendar months, whichever occurs first). This document discusses time between overhaul (TBO) requirements and states in part:
TBO specifications are based on normal aircraft with normal and continuous usage. Flight time and calendar limit must not be the only factors considered in determining when a propeller needs to be overhauled. Factors such as operating conditions or environment often demand that a propeller, governor, or accumulator be overhauled prior to TBO. Even though a propeller may be operating normally and have a good external appearance when the TBO flight time or calendar limit is reached, operation beyond the specified TBO limits is not permitted.
The accident hub assembly and propeller blades do not have a published life limit that requires replacement. The hub assembly and propeller blades can be used indefinitely if an overhaul is preformed every 1,500 hours or 60 calendar months (whichever occurs first), and all components remain in a serviceable condition.
It was undetermined why the propeller was not overhauled during the most recent 100-hour inspection (July 13, 2017 – 1,544.6 hours accumulated) or the second most recent 100-hour inspection (March 16, 2017 – 1,489.2 hours accumulated). For both inspections, the same airframe and powerplant mechanic (with inspection authorization), performed the work. Copies of the maintenance records are in the public docket for this accident.
Federal Aviation Administration
The Federal Aviation Administration has published Advisory Circular 20-37E Aircraft Propeller Maintenance. This document discusses propeller design and causes of propeller failure and states in part:
A propeller is one of the most highly stressed components on an aircraft. During normal operation, 10 to 25 tons of centrifugal force pull the blades from the hub while the blades are bending and flexing due to thrust and torque loads and engine, aerodynamic and gyroscopic vibratory loads. A properly maintained propeller is designed to perform normally under these loads, but when propeller components are damaged by corrosion, stone nicks, ground strikes, etc., an additional unintended stress concentration is imposed and the design margin of safety may not be adequate. The result is excessive stress and the propeller may fail.
Additional causes of overstress conditions are exposure to overspeed conditions, other object strikes, unauthorized alterations, engine problems, worn engine vibration dampers, lightning strike, etc. Most mechanical damage takes the form of sharp-edged nicks and scratches created by the displacement of material from the blade surface and corrosion that forms pits and other defects in the blade surface. This small-scale damage tends to concentrate stress in the affected area and eventually, these high-stress areas may develop cracks. As a crack propagates, the stress becomes increasingly concentrated, increasing the crack growth rate. The growing crack may result in blade failure.
This document also defines a nick and a crack on a propeller blade and states:
Nick. A sharp notch-like displacement of metal usually found on leading and trailing edges.
Crack. A physical opening or fissure within the body of a material. May be either internal within the material or at the surface (surface breaking). On a propeller, cracks can be started by cuts, nicks, or corrosion.
Adams Guiding Service
The pilot is the owner of Adams Guiding Service and the airplane was used in conjunction with the business. The airplane was used throughout various remote locations in western Alaska for guided fishing and hunting operations, including usage on unpaved runways and airstrips.