NTSB Factual Report

On October 24, 2006, at 1313 central daylight time, a Piper PA-24-250, N6090P, owned and piloted by a private pilot, experienced a total loss of engine power during cruise flight about 5,500 feet mean sea level. The pilot executed a forced landing on a bean field near Sherman, Illinois. There was no reported damage to the airplane. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight was not operating on a flight plan. The pilot was uninjured. The flight departed from Abraham Lincoln Capital Airport, Springfield, Illinois, about 1250, en route to Joliet Regional Airport, Joliet, Illinois.

The pilot reported that he was approximately 11 nautical miles north east of SPI at 5,500 feet. He stated that he felt the engine run rough and contacted the departure controller to let them know that he might have an engine problem. A "few seconds later" the pilot declared an emergency and turned back towards SPI. The pilot then reported that the motor stopped producing power and he subsequently landed in a bean field.

The airplane was powered by a Lycoming O-540-A1D5 engine, serial number L-4557-40. The engine was overhauled by the airplane owner and signed off by an airframe and power plant mechanic with inspection authorization. The date of the overhaul completion was December 12, 2001, and the engine accumulated a total time since major overhaul of 155 hours at the time of the accident.

The National Transportation Safety Board (NTSB) Office of Research and Engineering Materials Laboratory examined the engine crankshaft and attached connecting rod components. The #3 connecting rod assembly was separated from the crankshaft. The rod cap bolt was fractured on the numbered side of the assembly and the rod was fractured on the other side. The piston pin area was also fractured. All pieces were heavily damaged and distorted. The #4 connecting rod was also severely damaged but remained attached to the crankshaft. Other rods were also mechanically damaged but to a much lesser extent.

The separated #3 connecting rod assembly showed damage consistent with damage resulting from separation of the mating rod and bolts. Microscopic examination of the mating rod and bolt fractures revealed features consistent with overstress separations. The rod fracture displayed bending deformation and the bolt fracture had tension with bending deformation.

The #3 connecting rod assembly was received with the nut from the unnumbered side separated from the bolt. The bolt had a large radius bend in the grip at the threaded end, and the threads were severely damaged. Close examinations of the threads found them deformed and radially deformed by multiple impacts. Remnants of the nut threads were found between the four full form end threads at the end of the bolt.

Internal examination of the mating nut revealed that the three thread profiles adjacent to the washer face were fractured and smeared. The remaining 5 thread forms in the nut were intact. The directions of the fractures and smearing were toward the washer face consistent with the nut being pulled off the bolt.

The numbered side bolt was fractured through the grip at the split line between the rod and cap. The heavily distorted mating nut was retained on the bolt at the end of the threads. In this position, none of the threads were exposed at the end of the bolt and at least 6 bolt threads were visible between the nut and bolt head. The exposed bolt threads were radially deformed into the valleys between thread profiles.

On rods #1, #2, #5 and #6, approximately 0.29 inch of the bolt was visible and extended past the assembled nuts. This amounted to either 6 or 7 thread crests (including partial threads) at the end of the bolt.

On the #4 rod assembly, a gap was apparent between the rod and cap on the unnumbered side of the rod. The gap measured between 0.055 and 0.058 inch wide at its greatest. No gap was noted on the numbered side of the rod assembly. Also, noticeable radial play could be felt when the rod was manipulated by hand. The nuts and adjacent exposed bolt regions on both sides were heavily impacted and damaged. The damage prevented precise measurement of the length of the exposed threads, but approximately 0.23 inch of bolt threads was visible on the unnumbered side exhibiting the gap. An estimated 0.30 inch of bolt was visible on the numbered side (no gap). About 5 exposed thread crests were visible on the unnumbered side and 6 threads on the numbered side.

The #4 rod cap was removed only after cutting both the nuts (longitudinally) and the bolts (transversely). The internal threads of both bolts showed nonsymmetrical shearing of areas of the threads consistent with the direction and magnitude of distortion of the nuts.

The position of each nut was marked relative to the rod and two sets of torque measurements were recorded for rods #1, #2, #5 and #6 using a 300 inch-pound (in-lbs) maximum, peak recording torque wrench. Breakaway torque was measured during initial disassembly. After the breakaway torque was measured, the torque required to return the nut to its original position was also measured. In all instances the breakaway torque exceeded the 300 in-lbs (25 ft-lbs) rated capacity of the wrench. Most of the return to position torque values also exceeded 300 in-lbs.

The Lycoming Overhaul Manual (Table of Limits, SSP-1776) lists the installation torque for Lycoming connecting rod bolts as 480 in-lbs. Superior Service Letter L03-05 describes installation of connecting rod bolts and lists 480 in-lbs (40 ft-lbs) as the installation torque for torque type bolts.

With the connecting rods disassembled, the faying surfaces of all the rods and caps showed minor amounts of fretting and the bearings displayed minimal wear.

As a test, the bolts on the #2 rod assembly were torqued to 480 in-lbs (40 ft-lbs). The assembly included the rod bearing but not the crankshaft. Approximately 0.30 inch of the bolt extended beyond the nut and seven bolt thread crests were visible, as can be seen in the magnified insert. The breakaway torque was then measured several times using an internal clutch style torque wrench. The breakaway torque for the test nuts was between 360 and 397 in-lbs (30 and 33 ft-lbs).

The crankshaft was marked with raised forging number "72726" on one of the cheeks and vibropeened with "S120451-1" on the circumference of the propeller flange. All readable rods were ink stamped with part numbers "74502". No markings were found on the heavily damaged #3 and #4 rods but the forging numbers and marking were the same as the other rods. All of the rod bolts were marked with Superior Air Parts Inc part numbers "SL75061 E" and "FAA-PMA". When disassembled, all of the rod bearings were also Superior, part number "SL-13521". All bearing shells except one on the cap side of #1 were date stamped "3-01". The exception was date stamped "7-00".

NTSB Probable Cause

The loose connecting rod bolt due to a loss of torque and subsequent failure of the connecting rod during cruise flight.