NTSB Factual Report

On August 30, 2013, about 2015 eastern daylight time, N389BD, a Stemme GMBH S10-VT motor-glider, sustained substantial damage during a forced landing near Mackinaw City, Michigan. The private pilot/owner sustained serious injuries and the passenger was not injured. Visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. No flight plan was filed for the cross-country flight that originated from the Waukegan Regional Airport (UGN), Chicago/Waukegan, Illinois, about 1530, and destined for Mackinaw County Airport (83D), St. Ignace, Michigan.

The pilot stated that he was on a long, straight in visual approach to land at an altitude about 2,000 feet, when he heard a very loud "bang" in the front of the motor-glider. The bang was followed by a severe vibration. The pilot reduced power, which dampened the vibrations; however, he was unable to maintain altitude. The pilot shut down the engine and made a forced landing to a field. Just prior to landing, the motor-glider struck two dead trees and landed inverted, resulting in substantial damage to the airframe and wings.

An inspector with the Federal Aviation Administration (FAA) performed a postaccident examination of the motor-glider. The inspector stated that the propeller housing, gearbox, forward bulkhead and instrument panel were all pushed aft several inches from impact forces. Both propeller blades were in the stowed position, but the upper left propeller housing mount was bent and the propeller could not be manually rotated. One blade had free folding movement throughout its range with spring tension to return it to the stowed position. The second blade was binding throughout its folding motion and had little to no spring tension. The inspector said enough "usable fuel" was observed in the right fuel tank but the left wing fuel tank was breached and no fuel was observed. The fuel selector valve was in its normally open position. The inspector was unable to gain access to the driveshaft, which ran underneath the cockpit between the propeller and the engine.

The airplane was moved to a repair facility in California that specializes in Stemme motor-gliders for further examination. The examination revealed that the flexible disk (cardanic joint) on the front side of the driveshaft had disconnected. The disk and driveshaft are secured together by three bolts. One of the bolts was partially installed and there was no evidence of Loctite 243 or torque paint. The threads of the bolt were rusted and the head of the bolt was digging into the gearbox housing. The second bolt was sheared off and the third bolt was missing. An FAA inspector who examined the flexible disk (cardanic joint) and driveshaft after it had been removed from the motor-glider said the bolts that secured the flexible disk to the front of the driveshaft appeared to have fallen out and no Loctite 243 was present to indicate the bolts had been properly installed.

The motor-glider was manufactured in 2001. A review of the maintenance logbooks revealed that the last annual inspection was completed on July 26, 2013, at an aircraft total time of 322.7 hours. According to the Stemme S10-VT maintenance manual, the flexible disc (cardanic joint) was to be inspected at each annual inspection. The maintenance manual stated, "Check condition (cracks and embrittlement of rubber) and tight screw connections of cardanic rubber disc joint on freewheel clutch and on forward drive shaft. To inspect rubber discs for cracks and embrittlement, apply a torsional load on the discs and observe rubber surface."

It was also noted that during the last annual inspection that FAA Airworthiness Directive (AD) 2006-19-08, which required a mandatory inspection for deformations and cracks in the exhaust pipes, had not been properly complied with when a weld repair was made to the No.1 exhaust pipe. The AD specifically stated, "If any damage (deformation, cracks, and/or flattening) is found in the inspection...replace the damaged exhaust pipe."

According to Federal Aviation Regulations (FAR) Part 39, ADs are legally enforceable rules that apply to the following products: aircraft, engines, propellers, and appliances. They specify required inspections, conditions and limitations, and any actions to resolve an unsafe condition. Anyone who operates a product that does not meet the requirements of an applicable airworthiness directive is in violation of this section. If a certified aircraft has outstanding airworthiness directives that have not been complied with, the aircraft is not considered airworthy.

Additionally, a review of the maintenance logbooks identified several other maintenance items that were not complied with as per the Stemme S10-VT maintenance manual. In 2010, the propeller was not properly balanced and it had exceeded the required 5 year overhaul inspection.

NTSB Probable Cause

The mechanic’s failure to inspect the tightness of the bolts that connected the flexible disk to the driveshaft, which resulted in the failure of these components.