Bothell, WA, USA
N8787G
ZENITH CH701
The pilot reported the takeoff and en route portions of the flight were uneventful. While descending to the traffic pattern altitude at the destination airport, the engine lost power and the propeller stopped. The pilot made an emergency landing on a vacant road, colliding with powerlines. A postaccident examination of the engine revealed that the flywheel screws had separated due to fatigue, resulting in an engine failure. Examination of the screws showed multiple fatigue cracks and wear consistent with low-stress bending fatigue. Maintenance records revealed that the engine had been installed in another airplane that was involved in an accident where the propeller was damaged. According to the engine manufacturer, abrupt propeller stoppage can produce a spike of torque along the crankshaft, overloading the flywheel screws and causing failure. There was no evidence that the screws were replaced after that accident.
HISTORY OF FLIGHTOn July 19, 2022, about 2200 Pacific daylight time, a Zenith CH-701, N8787G, sustained substantial damage when it was involved in an accident near Bothell, Washington. The pilot and passenger sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that before takeoff, he topped off the fuel tanks to full. After departure, he flew clear of the surrounding Class B airspace and proceeded to the destination airport. The pilot reported the flight was uneventful. As he approached the airport, he flew east, on a path to adjoin the right downwind leg of the traffic pattern for Runway 34L. When the airplane was about 6 nautical miles from the airport, he began the descent to the traffic pattern altitude of 1,600 ft mean sea level. Immediately thereafter, the engine sustained a total loss of power and the propeller stopped. The pilot further stated that he attempted to restart the engine by turning on the auxiliary fuel pump and the carburetor heat. The engine did not respond, and the pilot maneuvered the airplane toward a vacant road to make an off-airport landing. The airplane collided with high-tension powerlines and descended onto the road below. AIRCRAFT INFORMATIONThe experimental, amateur-built airplane was certificated in 2011. It was powered by a Jabiru 3300A engine (s/n 33A453), rated to produce 120 horsepower, and equipped with a 3-blade Warp Drive composite propeller. The Jabiru 3300 is a direct-drive, six-cylinder, horizontally opposed, air-cooled engine. A flywheel is mounted to the crankshaft to provide rotational energy to drive the ignition system and cylinder valvetrain. The flywheel is mounted to the aft end of the crankshaft using six 5/18” cap screws. A review of the maintenance records revealed that the airframe had accrued 70.5 total flight hours, and the engine had accrued 252.1 total flight hours. The most recent maintenance performed on the airplane was an annual inspection recorded as being performed on June 20th, 2022, at an airframe total time of 46.6 hours. The records indicated the engine was installed when the airplane was built and had a total time of 133.4 hours at that time. A review of maintenance records revealed the engine had been installed on a different airplane before the accident airplane. That airplane was also an experimental, amateur-built airplane that had been involved in an accident in May 2004 (NTSB accident number IAD04LA019). The pilot involved in that accident reported the airplane sustained damage to the propeller during the accident sequence. There was no evidence the flywheel screws were replaced after that event. AIRPORT INFORMATIONThe experimental, amateur-built airplane was certificated in 2011. It was powered by a Jabiru 3300A engine (s/n 33A453), rated to produce 120 horsepower, and equipped with a 3-blade Warp Drive composite propeller. The Jabiru 3300 is a direct-drive, six-cylinder, horizontally opposed, air-cooled engine. A flywheel is mounted to the crankshaft to provide rotational energy to drive the ignition system and cylinder valvetrain. The flywheel is mounted to the aft end of the crankshaft using six 5/18” cap screws. A review of the maintenance records revealed that the airframe had accrued 70.5 total flight hours, and the engine had accrued 252.1 total flight hours. The most recent maintenance performed on the airplane was an annual inspection recorded as being performed on June 20th, 2022, at an airframe total time of 46.6 hours. The records indicated the engine was installed when the airplane was built and had a total time of 133.4 hours at that time. A review of maintenance records revealed the engine had been installed on a different airplane before the accident airplane. That airplane was also an experimental, amateur-built airplane that had been involved in an accident in May 2004 (NTSB accident number IAD04LA019). The pilot involved in that accident reported the airplane sustained damage to the propeller during the accident sequence. There was no evidence the flywheel screws were replaced after that event. WRECKAGE AND IMPACT INFORMATIONThe engine cowlings appeared intact and remained installed over the engine. The engine remained attached to the airframe, though some impact deformation was noted to the engine mount. The engine accessories remained mounted securely to the engine. The engine controls remained attached to the carburetor. Movement of the engine control knobs resulted in the corresponding engine controls moving freely and smoothly. Removal of the spark plugs revealed that the electrodes were light gray in appearance, consistent with normal operation; the exception was the No. 6 cylinder spark plug, which contained carbon deposits. Manual rotation of the propeller did not produce corresponding valvetrain movement. Disassembly of the engine accessory system revealed the six screws securing the flywheel to the crankshaft were separated, resulting in the separation of the flywheel from the aft end of the crankshaft (see figure 1 below). Figure 1. Flywheel and Screws TESTS AND RESEARCHThe National Transportation Safety Board’s Material Laboratory performed an examination of the flywheel, flywheel mounting screws, alternator assembly, and crankshaft timing gear. The timing gear appeared undamaged. Removal of the stator ring and bracket revealed galling wear in a circular pattern near the location of the alternator ring when the two pieces were attached. The wear was consistent with repeated contact and rubbing between the spinning flywheel assembly and bracket during engine operation. The 6 fractured flywheel hex-head screws were backed out of their fully seated positions by about 0.25 inch. The screws were loosely threaded into the holes. According to the Jabiru parts manual, the holes were manufactured with a smooth bore. The investigation could not determine if the screw positions represent their position after the failure or if they shifted during the subsequent crash or retrieval. The washers under the screw heads were deformed in a manner consistent with bending forces and a crack was discovered in one washer. Examination of the screw threads showed contact damage consistent with impingement on the walls of the through-holes, and complementary wear resembling screw threading was visible in the through-holes. Examination of the screw fracture surfaces revealed that several had ratchet marks (indicative of multiple fatigue initiation points) near the base of the threads. Fatigue crack progression marks were present across most of the face until final failure as indicated by a shear lip, consistent with low stress bending fatigue. Additionally, three of the screws displayed secondary fatigue cracks originating from the threads. One screw displayed multiple fracture planes with contact damage on the highest plane and fatigue progression marks radiating from several points at the roots of the threads. Galling wear was found between the forward face of the rear plate alternator mount and the aft face of the flywheel. The flywheel attachment screws remained installed in the flywheel, with contact damage noted on the inside of the flywheel hardware holes. Communication with Jabiru revealed that the flywheel hardware hole was manufactured with a smooth bore. The six flywheel attachment screws were removed from the flywheel, cleaned, and examined using a digital microscope. Four of the screws displayed fatigue crack progression marks across the fracture faces, consistent with low-stress bending fatigue. Additionally, three of the screws displayed secondary fatigue cracks originating from the threads. One screw displayed multiple fracture planes with contact damage on the highest plane. This screw showed fatigue progression marks radiating from several points at the roots of the threads.
A loss of engine power due to the fatigue failure of the engine’s flywheel screws that were likely overloaded when the engine was involved in a previous accident.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
