CEN16LA035

While on the downwind leg of the traffic pattern for landing, the private pilot of the experimental amateur-built airplane heard a "clunk" sound from the front of the airplane; however, the propeller continued to rotate and the engine appeared to be operating normally. The pilot continued to the base and final legs of the traffic pattern and attempted to add engine power, but the engine "overreved." The airplane lost altitude as it neared the runway and touched down on the parking apron, then continued into a ditch, where it nosed over and came to rest inverted. A postaccident examination revealed that the propeller gearbox had failed in flight. All of the drive gear and propeller drive teeth were either worn or destroyed, and the gearbox drive gear displayed discoloration and heat signatures consistent with oil starvation. Additionally, there was no usable oil present in the gearbox, and no evidence of an oil leak. Although the airplane owner stated that he had added oil to the gearbox before the flight, it is likely that the flight departed with an insufficient oil supply in the propeller gearbox, which resulted in subsequent oil starvation. When the gearbox failed, the engine continued to operate; however, it ceased to drive the propeller, which resulted in a loss of thrust.

On November 3, 2015 at 1100 eastern standard time, an experimental amateur-built Glimn Kitfox 2-3 airplane, N93TG, impacted terrain and nosed-over during a forced landing at Delta County Airport (ESC), Escanaba, Michigan. The airplane lost propulsive power after the propeller ceased to rotate, but the engine continued to operate while the airplane was in the traffic pattern of ESC. The private pilot received minor injuries and the airplane sustained substantial damage to a wing strut. The airplane was registered to and operated by the airplane owner under 14 Code of Federal Regulations Part 91 as a personal flight that was not operating on a flight plan. Visual meteorological conditions prevailed for the flight that departed from ESC at 1050 and was to remain in the airport traffic pattern. The airplane was flown as part of an aircraft sale. The pilot was an interested buyer. The registered aircraft owner stated that he had added oil to the engine gearbox prior to the accident flight because the oil level was "down." The pilot stated that he saw paperwork that recorded oil being added to the gearbox "on a couple of different pages." He did not recall the oil quantity that was recorded on the paperwork. He did not see any oil added after he arrived to fly the airplane. The pilot stated that while the airplane was established on downwind leg of the airport traffic pattern, he reduced engine power and then heard a "faint klunk" from the front of the airplane. The propeller continued to rotate and the engine sounded "normal." While attempting to add engine power, the engine "raced/overreved." He attempted to add engine power a second time, but the result was the same. He turned the airplane for a left base to runway 36 and made a 180 degree turn with the intent to land on runway 27. He stated that he "ran out of altitude short of the runway." The airplane touched down on the parking apron, rolled out across a grassy strip, the runway 9/27 taxiway, and into a ditch that preceded and paralleled runway 9/27, where the airplane nosed over. The pilot stated that if he had turned earlier toward runway 36, then the airplane would have been able to be landed on runway 36. The pilot said the he did not make the earlier turn because he did not realize he had a problem with the airplane since the engine was still operating and the propeller was still turning. He realized the airplane had a problem when he turned the airplane onto a base leg for runway 36 and needed power, but none was available to maintain flight. The airplane was powered by a Rotex 582LC, serial number 3916418, engine that equipped with a B- model gearbox. The airplane was maintained by the registered owner, who was also an airframe and powerplant mechanic, since he purchased the airplane kit. The engine was examined by a representative from Rotech Flight Safety under the supervision of a National Transportation Safety Board Air Safety Investigator. The examination revealed that no oildrained from the sealed gearbox, which should have contained a level of oil, when the magnetic oil drain plug was removed. The drain plug displayed heavy metal debris deposits consistent with propeller shaft drive gear teeth. There was no gearbox oil residue oil/oil staining present on the engine or gearbox section, consistent with leakage/drainage outside of the gearbox. The gearbox was then disassembled and the reduction section of gearbox was removed. Figure 1: The gearbox drive gear displayed discoloration and heat signatures consistent with no lubrication. All drive gear teeth were worn/destroyed from drive gear. Figure 2: The propeller large gear in the gear set displayed discoloration and heat signatures consistent with no lubrication. All of the propeller large gear teeth were worn/destroyed.