NTSB Factual Report

On August 5, 2011, approximately 1430 central daylight time, N7689W, a Piper PA-28-180, sustained substantial damage after a section of propeller blade separated in flight and the pilot made a forced landing on a highway near Millsap, Texas. The private pilot and his passenger were not injured. The airplane was registered to the pilot and two other owners and operated by the pilot. No flight plan was filed for the flight that departed Mineral Wells Airport (MWL), about 1415, and destined for Grand Prairie Municipal Airport (GPM), Grand Prairie, Texas. Visual meteorological conditions prevailed for the flight conducted under 14 Code of Federal Regulations Part 91.

The pilot reported that he was just about to level off at 3,400 feet mean sea level (msl) when the airplane made a "severe jerk" and began to vibrate violently. The pilot then observed smoke coming through the firewall and faceplates on his instrument panel began to fall off. He decided to turn off the engine by pulling the mixture control. Once the engine stopped, the pilot could see that a large section of a propeller blade was missing. He then trimmed the airplane for best glide speed and landed on a road. During the landing roll-out, the airplane's right wing struck a road sign.

According to the pilot, an 18-inch long section of propeller blade was missing and the engine had broken loose from several of its motor mounts. Secondary damage to the engine and components was also reported.

Metallurgical examination of the remaining blade section revealed striations on the fracture surface that were consistent with crack initiation and proporgation by high cycle fatigue. At least three crack arrest marks were observed consistent with the stopping and starting of the cyclic stresses.

Examination of maintenance records revealed that the Sensenich propeller (part number 76EM8-0-60, S/N 13700K) had been overhauled on July 30, 2004, and reinstalled on the Lycoming O-360-A3A engine. The last 100 hour inspection was completed on October 1, 2010 at a total time of 165.73 hours since overhaul.

The Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 69-09-03 R3 on May 15, 1996, that applied to Sensenich 76EM8 propellers mounted to Lycoming O-360-A3A engines. The AD was issued to prevent propeller blade tip fatigue failure, which could result in loss of control of the aircraft. The AD 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. The AD also required the following two instructions:

a) Commencing with the next flight after the effective date of this AD, do not operate the engine in continuous operation between 2150 and 2350 RPM.

b) Within the next 25 hours of service (TIS) after the effective date of this AD, mark engine tachometer with a red arc from 2150 RPM to 2350 RPM.

The pilot stated that he was aware of the AD and that he had complied with the RPM restriction.

NTSB Probable Cause

A sudden loss of engine thrust due to the separation of a propeller blade in cruise flight due to high cycle fatigue.