ERA15FA254

The total loss of engine power due to the failure of the alternator drive coupling. Contributing to the accident was the pilot's inability to locate and navigate to a suitable forced landing site due to low cloud ceilings.

The commercial pilot was conducting a cross-country personal flight. About 15 miles from the destination airport, while in instrument meteorological conditions at 3,300 ft mean sea level, the pilot declared an emergency to air traffic control, stating that the airplane was experiencing an "engine problem." The pilot asked the controller about the nearest airport, and the controller provided him with radar vectors to that airport. The pilot then attempted to glide the airplane to the airport, but he reported that he was unable to maintain altitude. The controller then advised that there was a highway right of the airplane's position and about 2.5 miles away, and the pilot responded, "we have no engine we're [in instrument meteorological conditions] I need help." The airplane impacted a house before reaching the highway. It is likely that low cloud ceilings prevented the pilot from locating and navigating to a suitable forced landing site. An examination of the engine revealed that the alternator drive coupling had failed, which resulted in damage to other internal engine components and ultimately resulted in a catastrophic engine failure due to a lack of oil lubrication. The investigation identified two possible conditions that could result in the coupling failure; but was unable to determine which was more likely. First, it is possible for components of the coupling to progressively move out of tolerance due to repeated "slip testing," which is a procedure the engine manufacturer prescribed to be performed anytime the coupling was removed and installed on an alternator shaft. The purpose of the test is to ensure that the coupling's elastomer section, which was designed to slip in the event of an alternator shaft seizure, will not slip under normal conditions. However, it is possible for certain components within the coupling to shift slightly during this test. If they shift far enough, the coupling will not be properly seated when installed on the alternator shaft. This condition is not readily detectable by direct observation. There are currently no published procedures to inspect or measure the coupling for this out-of-tolerance condition. One engine overhaul facility found two new couplings as received from the manufacturer that were out of tolerance. The coupling could also have failed due to one or more of the following: insufficient torque applied to the alternator shaft nut, loosening of the nut to align the cotter pin holes, or failure to lubricate the threads before assembly on the shaft. Either an out-of-tolerance coupling or an improperly installed one can result in insufficient clamping force holding the coupling against the alternator. If there is insufficient clamping force, the coupling can rotate on the shaft and cause unusual wear and the ultimate failure of the coupling, which can lead to catastrophic engine failure. Sets of instructions for the installation of the coupling were available from several sources, including alternator manufacturers, the engine manufacturer, and repair and overhaul facilities. Although the sets of instructions were similar, the steps and details varied among them, and some of the instructions omitted critical guidance. The set of instructions provided by the engine manufacturer was the most complete; however, some steps were generalized and located in separate locations within the maintenance manual. None of the instructions advised that the assembly torque procedure was the designed means to prevent the coupling from rotating on the shaft, not the woodruff key. This may be counterintuitive because of how a woodruff key is generally used, and installers or part suppliers may not realize the importance of each step in the engine maintenance manual. Further, none of the instructions advised that a loose or improperly tightened coupling may lead to a catastrophic engine failure.

A review of maintenance guidance materials and testing in the National Transportation Safety Board (NTSB) Materials Laboratory revealed two conditions that could result in the alternator drive coupling failure identified during examination. First, using improper assembly torque values or procedures when tightening the alternator shaft nut will cause the coupling to progressively loosen over time due to abnormal wear. This may occur by not using a calibrated torque wrench, applying improper torque values, or using inadequate maintenance guidance materials. Six different sets of installation instructions for the coupling were found during the investigation: two from the alternator manufacturer, one from an unidentified repair station, one from the engine overhaul facility, and two (the overhaul maintenance manual and a superseding service bulletin) from the engine manufacturer. Although similar, each set of instructions had different steps and different levels of detail. For example, specific steps in the engine manufacturer's set of instructions were omitted in other instructions. Further, some steps were more clearly prescribed in one set of instructions than in another set. For example, the alternator manufacturer's set of instructions advised to torque the nut to within the range of 300 to 450 inch-pounds and install the cotter pin. Both the engine manufacturer's and the overhaul facility's set prescribed to torque the nut to precisely 300 inch-pounds and then to align the cotter pin holes. If the holes did not align at 300 inch-pounds, the instructions advised to continue applying torque until the holes aligned, up to the maximum value of 450 inch-pounds. The engine manufacturer's instructions also specifically stated not to reverse the nut when attempting to align the holes. Only the engine manufacturer's set of instructions advised that the shaft/nut threads be lubricated with engine oil. That guidance was available peripherally in the preamble to an appendix in the engine maintenance manual that contained tables of torque values. Lubricated threads will result in a much higher "clamping force" holding the coupling against the alternator and resisting rotation on the shaft for a given torque value. None of the sets of instructions noted that the torque procedure and the resulting clamping force are the primary means of preventing the coupling from rotating on the shaft. Although the design of the coupling includes a woodruff key and channels, which are commonly used in other applications to prevent collars from rotating on shafts, that was not the intended purpose for this coupling. In this case, the woodruff key was intended to aid in the coupling installation process by arresting the alternator shaft during the torqueing procedure. The woodruff key was not designed to resist the loads on the coupling during normal engine operation. Figure 2. Exemplar alternator, coupling, and components. The second condition that could result in a coupling failure like that identified during examination is an out-of-tolerance coupling assembly. Specifically, if the gear section of the assembly is not fully seated in the housing and protrudes above the top of the shaft collar, the thrust washer will contact the gear instead of the shaft collar during installation of the coupling (see figure 2). As the nut is tightened against the washer, some of the applied torque will be consumed as the washer presses the gear back into the housing. As a result, less of the applied torque will be converted into the clamping force holding the coupling against the alternator. If that clamping force is insufficient, the coupling will progressively loosen over time due to abnormal wear. This out-of-tolerance condition was observed in the NTSB Materials Laboratory on an exemplar PMA coupling and an exemplar original equipment manufacturer (OEM) coupling that had undergone improper "slip testing," which is a procedure the engine manufacturer prescribed to be performed anytime the coupling was removed and installed on an alternator (slip testing is also performed on all couplings at the manufacturer before delivery). The purpose of the test was to ensure that the coupling's elastomer section, which was designed to slip (allow the gear to rotate independently from the housing) in the event of an alternator shaft seizure, will not slip under normal conditions. The test required that the housing be held fixed and that a specified torque be applied to the gear using a special tool. During this test, if too much torque was applied and the elastomer started to slip, the gear section would slide up and away from the housing a small distance. If this improper testing occurred repeatedly and/or severely, the coupling will become out of tolerance. Currently, there are no published procedures to inspect or measure the coupling for this out of tolerance condition. The engine overhaul facility found two new PMA couplings that were out of tolerance as delivered from the manufacturer; the cause of their condition was not determined. Similar Failures A representative from the engine overhaul facility reported that he had observed several couplings from 2005 to 2015 that exhibited some type of failure or abnormal wear. The damage he observed was consistent with the coupling rotating on the shaft, slipping at the elastomer, or both. Specifically, he observed worn thrust washers, damaged or missing woodruff keys, damage to the key channels on the alternator shaft and the coupling, abnormal wear to the shaft collar section of the housing, separation of the shaft collar section from the housing, and disintegration of the elastomer. He was not aware of any effort to systematically track these failures or estimate the time in service of the damaged couplings; however, he notified the FAA principal maintenance inspector who conducted oversight of the overhaul facility whenever a failure was discovered. He indicated that most of the failures he had observed occurred in engines that were installed in twin-engine airplanes. He noted that the failures were often in the Continental Motors GTSIO series engines, in which the alternator rotated in the reverse direction from the type of engine installed on the accident airplane. He added that the facility only installed PMA couplings during overhaul but that he had observed failures in both the PMA and OEM couplings. A review of records available in the FAA service difficulty reporting system database revealed 10 entries that may be related to an alternator coupling failure; however, the descriptions of the issue and level of detail available in those records varied widely and did not include any photographs. A review of the engine manufacturer's warranty records for the 5 years preceding the accident revealed six claims in which the coupling was described as "failed," "damaged," or as having "come apart." The warranty records did not contain any photographs or sufficient detail to determine if the reported problems were similar to the failure of the accident coupling. Coupling Inspection and Maintenance Procedures According to the engine manufacturer's maintenance manual, the coupling and the face gear that drives it should be examined as part of the alternator inspection during the 500-hour engine inspection. The procedure included examining the coupling for damage or missing material and performing the "Alternator Drive Hub Slippage Inspection" or slip test. The OEM coupling may be reused indefinitely if it satisfactorily passes inspection and slip testing. The PMA coupling manufacturer recommended that it be replaced during major engine overhaul. Alternator Failure Indication System The airplane was equipped with a warning annunciator light labeled "LOW BUS VOLTS" that was designed to illuminate in the event of an alternator failure. Although the alternator may not fail, a coupling failure may affect or stop the rota