Amateur-built ZODIAC CH 601 XLB N234S accident near Hubbard, Oregon

Probable cause

A severe propeller vibration due to the pilot's improper installation of the propeller spacer assembly, which necessitated an engine shutdown, an off-airport landing, and a collision with a sprinkler system.

NTSB analysis

While approaching the destination airport in the experimental amateur-built airplane, the pilot felt an engine vibration and elected to shut down the engine and conduct a precautionary landing to a nearby field. During the landing sequence, the airplane's left wing collided with a ground sprinkler system, which resulted in substantial damage.

Postaccident examination of the engine revealed no anomalies. The propeller had been removed from the crankshaft flange by the pilot following the accident. Examination of the flange revealed numerous fractures in the threaded alignment holes, and the alignment screws were worn. Additionally, five of the six "lightning" holes displayed cracking. Examination of the propeller spacer revealed that two bores were missing bushings. All six bolts were fitted with two washers; one affixed to each bolt head and a larger washer between the bolt head and the spacer. The larger washers exhibited rotational deformation consistent with contact with the fixed washers. All six bolts were bent, and it could not be determined if the bolts were safety wired at the time of the accident. The bolt bushings appeared to be "homemade" and of different lengths, with bolt holes drilled off-center. It is likely that the irregular, non-standard bushings prevented the proper installation and torqueing of the propeller bolts, which led to the vibration experienced during the accident flight.

Text from the NTSB. Open the NTSB docket for WPR15LA228 for the full record.