NTSB Factual Report

On August 7, 2012, about 0945 central daylight time, a Cessna 177B airplane, N17201, impacted terrain during a forced landing following a loss of engine power, near Carthage, Texas. The commercial pilot reported that he sustained no injuries. The airplane sustained substantial fuselage damage. The airplane was registered to a private individual and was operated by Hawkeye Helicopters under the provisions of 14 Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules (VFR) conditions prevailed for the flight, which did not operate on a VFR flight plan. The flight departed from the Shreveport Regional Airport, near Shreveport, Louisiana, about 0830.

The pilot indicated in his accident report that he had been flying at 500 feet above ground level in reference to a pipeline that ran into Carthage, Texas, for about 1.5 hours. He stated that the engine lost over 50 percent power without warning or indication. He looked for a place to land and started a climb where he was he was able to gain about 50 feet of altitude. The pilot turned on the electric fuel pump, "pitched prop to full with mixture" and had a descent rate of about 50 feet per minute. The pilot selected an area in an old logging site for the forced landing. The airplane impacted logs and stumps during the roll out.

A Federal Aviation Administration (FAA) inspector examined the wreckage on-scene and confirmed the substantial damage. The engine propeller turned when it was rotated by hand. The accident airplane was relocated and a teardown examination of its engine by the investigator in charge revealed that the no. one cylinder intake valve was not present under its rocker arm tip. This intake valve's keepers were found worn, deformed, and segmented in the bottom of its rocker box. The rocker arm's tip was worn consistent with wear against the valve stem and its keeper. Inboard of the tip, the rocker arm exhibited a wear mark consistent with contact with the valve spring retainer. Removal of the cylinder revealed that the piston exhibited impact marks consistent with valve contact. The cylinder's intake port exhibited a hole consistent with a liberated valve's stem impact contact with the side of the intake port. The intake ports were subsequently disassembled and the liberated valve was found in pieces in the no. three and four intake tubes.

The components associated with the no. one cylinder along with three other intake valves were examined at Engine Components International (ECI), in San Antonio, Texas, and an examination report was produced. The no. one intake valve examination revealed no indications of fatigue in the fractures and the valve face did not exhibit erosion or wear. The no. one intake push rod was not bent and the intake valve and its guide did not exhibit evidence consistent with a sticking valve. According to the examination report, rocker arms are manufactured from relatively low carbon alloy steel. The tip and push rod socket of rocker arms are carburized to provide a hard case about .040 inch effective depth. The surface hardness of the carburized case is approximately Rockwell C-60 for new rocker arms. The remaining intake valve rocker arms were sectioned and those tips' hardness were within overhaul hardness requirements. Those tips exhibited a shallow carburized layer. The valve contact wear surface of the failed rocker arm tip was too badly damaged to accomplish any meaningful metallurgical analysis, and the depth of the carburized layer could not be determined.

A FAA inspector spoke to the inspection-authorized mechanic who performed the engine overhaul and buildup for the accident engine, which was a Lycoming O-360-A1F6D engine with serial number L-21922-36A. The mechanic stated that the cylinder assemblies supplied by ECI were used in the overhaul and that they come preassembled. The last flight log showed the engine had accumulated about 878.5 hours time since overhaul (TSO). The inspector found that a compression check was done on June 6, 2012, with pressures reported as "#1 (79), #2 (72), #3 (72), #4 (79)." An entry indicated that on July 13, 2012, at 779 hours TSO, a rough running engine write-up was corrected by the replacement of the magneto, harness, and ignition switch, which subsequently was operationally checked good and signed off.

The rocker arm with the worn tip was marked with embossed with numbers 5829-2 and 69443-D. That rocker arm was a Lycoming rocker arm with part number 69444, which, according to a work order, was "rebrushed and refaced" by Aircraft Specialties Services. A representative from Aircraft Specialties Services repair station indicated that they use the Lycoming overhaul manual for guidance on rocker arm reconditioning and forwarded an excerpt from that manual. The excerpt from the overhaul manual for Lycoming direct drive aircraft engines, in part, stated:

6-34. VALVE ROCKERS. Damaged, badly worn, pitted

or scored tips and push rod sockets warrant replacement

of the rocker. Check the [inner diameter] of the rocker bushing

at several different positions with a flat plug rejection

gage (64613). This is a double-end gage; be sure to use

the end marked "Rocker Bushing". If the gage enters

the bushing at any point, mark the bushing for replacement.

The above manual excerpt did specify that damaged, badly worn, pitted, or scored rocker arm tips warrant replacement. However, the Lycoming overhaul manual did not list any quantitative limitations/specifications or list a gage to determine the serviceability of rocker arm tips in reference to surface wear or defects, which may not be visually perceptible. Additionally, Lycoming publication Service Table Of Limits And Torque Value Recommendations (SSP1776), specified measurements and tolerances to be used during dimensional inspections of Lycoming parts. This publication also did not list any quantitative limitations/specifications to determine serviceability of rocker arm tips.

Magnetic particle inspection is a non-destructive inspection process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys and must be performed to published standards by qualified personnel. American Society for Testing and Materials (ASTM) publication ASTM E1444/E1444M - 12 Standard Practice for Magnetic Particle Testing, in part, indicated that a specified tool steel ring can be used to verify the performance of the magnetic particle inspection process in reference to subsurface defect detection.

Both the repair station and Lycoming reported that they use magnetic particle inspection during reconditioning of rocker arms.

NTSB Probable Cause

The loss of engine power during a low-level aerial observation flight as the result of the wearing of the rocker arm tip due to a thin carburized case layer, which resulted in a forced landing.