Aircraft engine-maker Textron Lycoming, Williamsport Pa., is recalling crankshafts used in Robinson helicopters, late-model Cessna 182s, and some Commander 112s. This is on top of an earlier recall that affected only crankshafts in its high-horsepower, six-cylinder engines.
Mandatory Service Bulletin (MSB) 566 affects about 1,200 crankshafts. The MSB comes after Lycoming lost a lawsuit brought by Interstate Forging in Navasota, Tex., alleging that it had improperly called for the addition of vanadium to the crankshaft alloy.
Lycoming countercharged that Interstate had improperly manufactured the cranks and denied that the addition of vanadium was responsible for the crank failures. The jury awarded Interstate $96 million in damages. Despite the growing number of recalled units, Lycoming says it strongly disagrees with the jury's decision and is appealing the case.
In 2002, Lycoming recalled some 400 crankshafts. And by late 2003, after a fifth crash attributed to crank failure, it broadened the recall to include some 1,800 airplanes.
According to a statement by Lycoming, "The level of vanadium was not controlled in the 4340 VAR steel used for the crankshafts. It varied between 0.01% and about 0.1%, with most heats at the high end of the range. There were no subsurface fatigue failures of any crankshafts made with this steel until Interstate began forging the crankshafts."
Lycoming modified the level of vanadium to between 0.07% and 0.11%, "not because it was difficult to 'hot work' or machine the steel, but rather it was determined that crankshafts with higher vanadium were less prone to straightness problems in post-forging heat-treatment." The higher level of vanadium, Lycoming claims, allowed a higher second temper temperature while maintaining the required hardness. This higher second temper temperature also provided better relief of residual stresses, thus reducing straightness issues.
At trial, Lycoming's position was that the cranks were subjected to temperatures in excess of its specification in the furnace used to heat the billets prior to forging (not in the subsequent heattreatment such as normalizing, quench, and temper operations). Dr. John Barsom, formally of U.S. Steel and an expert in metallurgy and fracture mechanics, testified that excessive heat in the forge furnace weakened the steel grain boundaries, creating the potential for the grains to separate in the presence of shear or tensile forces during forging. He also testified that these separations or microcracks were not cured by downstream heattreatment and were large enough to result in fatigue if located in areas of maximum stress.
Dr. Robert Hinton, a metallurgist from Allentown, Pa., testified that he was able to recreate the microcracks by subjecting 4340 VAR steel to temperatures in excess of 2,480°. An analysis of the hammer forger's records showed that, on multiple occasions, the forger reached billet temperatures in excess of 2,480°. Lycoming says it is unclear why this evidence did not resonate with the jury.
According to the FAA report, "The crankshaft has a sufficient design margin between operating stresses and material endurance limit to result in an effectively infinite service life for defect-free material." However, according to Lycoming, the judge excluded this report from evidence.