BEECH A36 — Plainville, MA

What happened

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.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.