History of Flight
On July 19, 2022, about 2200 Pacific daylight time, a Zenith CH-701, registration 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 14 CFR Part 91 personal flight.
The pilot reported that before takeoff, he filled the fuel tanks to full. After departure, he flew clear of surrounding Class B airspace and proceeded toward the destination. The flight was uneventful until, while approaching the airport, he flew east to join the right downwind leg of the traffic pattern for Runway 34L. When about 6 nautical miles from the airport, he began descent to the pattern altitude of 1,600 ft mean sea level. Immediately thereafter, the engine experienced a total loss of power and the propeller stopped.
The pilot attempted to restart the engine by turning on the auxiliary fuel pump and carburetor heat, but the engine did not respond. He maneuvered toward a vacant road for an off-airport landing. The airplane collided with high-tension powerlines and descended onto the road below.
Aircraft Information
The experimental, amateur-built airplane was certificated in 2011. It was powered by a Jabiru 3300A engine (s/n 33A453), rated at 120 horsepower, 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 for the ignition system and cylinder valvetrain, secured by six 5/18-inch cap screws.
A review of maintenance records showed that the airframe had 70.5 total flight hours, and the engine had 252.1 total flight hours. The most recent maintenance was an annual inspection on June 20, 2022, at an airframe total time of 46.6 hours. The engine was installed when the airplane was built and had 133.4 hours at that time.
Records also indicated that the engine had previously been installed on a different experimental, amateur-built airplane involved in an accident in May 2004 (NTSB accident IAD04LA019). The pilot of that accident reported propeller damage. There was no evidence the flywheel screws were replaced after that event.
Wreckage and Impact Information
The engine cowlings appeared intact and remained installed. The engine remained attached to the airframe, with some impact deformation to the engine mount. Engine accessories remained mounted securely. Engine controls were attached to the carburetor and moved freely. Spark plug electrodes were light gray, consistent with normal operation, except the No. 6 cylinder plug which had carbon deposits.
Manual rotation of the propeller did not produce corresponding valvetrain movement. Disassembly of the engine accessory system revealed that the six screws securing the flywheel to the crankshaft had separated, causing the flywheel to detach from the aft end of the crankshaft.
Tests and Research
The NTSB Material Laboratory examined the 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, consistent with repeated contact between the spinning flywheel assembly and bracket during operation.
The six fractured flywheel hex-head screws were backed out about 0.25 inch from their fully seated positions and were loosely threaded. The investigation could not determine if this position was from the failure or subsequent crash and retrieval. Washers under screw heads were deformed consistent with bending forces, and a crack was found in one washer.
Screw threads showed contact damage from impingement on through-hole walls, with complementary wear in the holes. Fracture surfaces displayed ratchet marks near thread bases, indicating multiple fatigue initiation points. Fatigue crack progression marks were present across most faces until final failure with shear lips, consistent with low-stress bending fatigue. Three screws showed secondary fatigue cracks from threads. One screw had multiple fracture planes with contact damage and fatigue progression radiating from several thread root points. Galling was found between the forward face of the rear plate alternator mount and the aft face of the flywheel.
Communication with Jabiru confirmed that the flywheel hardware hole was manufactured with a smooth bore.