On January 5, 2009, about 1115 eastern standard time, a Grumman AA-5B Tiger, N28862, was substantially damaged following an in-flight separation of a propeller blade and a forced landing near Milford, Indiana. The instructional flight was being operated under 14 Code of Federal Regulations Part 91 without a flight plan. Visual meteorological conditions prevailed. The private pilot and flight instructor were not injured. The local flight departed about 1045.
The private pilot stated that the airplane was in cruise flight at 3,500 feet mean sea level (msl) when he heard a "loud bang followed by a violent vibration." He secured the engine and "pulled the nose of the airplane up to stop the rotation of the propeller." He subsequently executed a forced landing to an open field.
A post accident examination revealed that a 20-inch length of one of the blades had separated from the 2-bladed, fixed pitch propeller. The separated section was not recovered. Secondary damage to the engine assembly, engine mounts and firewall was also observed.
Metallurgical examination of the remaining portion of the blade revealed a chordwise fracture approximately 18 inches from the hub centerline (20 inches from the tip). The fracture surface exhibited features consistent with fatigue failure that covered about 70-percent of the area. The remaining surface area appeared consistent with an overload failure. Fatigue striations emanated from an area of corrosion on the cambered side of the blade. Visual inspection using low-powered magnification revealed corrosion pitting distributed over the entire cambered surface of the blade.
Maintenance records revealed the propeller was installed new on the accident airplane in September 1983. It was subsequently reconditioned in May 1987. The most recent annual inspection was completed on October 31, 2008. At the time of the accident, the propeller had accumulated 2,326 hours total time in service, with 8 hours since the annual inspection.
Airworthiness Directive (AD) 69-09-03 R3, effective May 15, 1996, required factory inspection and reconditioning for propellers with 500 hours or more of flight time. Propellers inspected and/or reworked in accordance with that AD were to be identified with the suffix letter "K" after the serial number. The accident propeller included a "K" suffix after the serial number consistent with compliance to the AD. No subsequent AD's had been issued related to the accident propeller.
The manufacturer issued Service Bulletin R-15A on October 4, 1989, recommending inspection and repair procedures to address corrosion issues. Specifically, the service bulletin stated that propeller blades should be inspected for corrosion during annual or 100-hour inspection procedures. Any corrosion should be repaired or the propeller reconditioned depending upon the extent and severity of the corrosion.
Service Bulletin R-17 was issued March 16, 1999, which noted the Federal Aviation Administration's (FAA) prior approval for an increase in the time between propeller overhaul/reconditioning cycles (TBO). The TBO was increased from 1,000 hours total flight time to 2,000 hours for normal and utility operations provided that the specific propeller had not sustained damage that required immediate attention. Aerobatic operations remained at 1,000 hours TBO. However, regulations do not require compliance with TBO limits for non-commercial operations. In these cases, continued airworthiness is based on the condition of the propeller.
FAA Advisory Circular (AC) 20-37E, Aircraft Propeller Maintenance, states that a detailed inspection is completed at each annual inspection or as required by the aircraft maintenance procedures. The detailed inspection should include visual examination of the propeller blades using ten-power (10X) magnification for questionable areas. The visual examination is to look for excessive wear and erosion, damage, nicks, cracks, corrosion, lightning strike, or ground strike.