Cessna 180 N9370C accident near California City, California

Probable cause

A directional upset on landing due to a thermally induced atmospheric disturbance, which the pilot was unable to successfully counter due to improperly set rudder travel limits.

NTSB analysis

The day before the accident, a satisfactory annual inspection was completed for the tailwheel-equipped airplane, and the owner/pilot conducted an uneventful flight later that day. The following day, he flew the airplane to another airport to obtain a cost estimate for some "cosmetic" maintenance. The flight was uneventful, but as the airplane touched down about 1315 local time on the desert airport runway, it bounced and then touched down again. On the second touchdown, the airplane veered sharply left, and the pilot was unable to regain directional control. The airplane nosed over onto its back when it exited the paved edge of the runway.

Detailed examination of the airplane did not reveal any mechanical reasons for the post-touchdown left swerve. However, the rudder travel stops were found to be set so that the rudder travel limits were 16° right and 13° left, instead of the manufacturer-specified value of 24° in each direction.

After that examination, the pilot reported that the "cosmetic" maintenance he was planning was to correct a top-to-bottom curvature (bow) in the rudder that resulted in an approximate 1.5-inch misalignment between the top and bottom of the rudder. According to the pilot, that bow was either a direct result of a 1974 accident or the consequent improper repair. Because the airplane had been operated uneventfully for 42 years with the deformed rudder, it is unlikely that this deformity caused the left veer. The airplane likely initially veered left due to an encounter with an atmospheric phenomenon common in desert environments on hot afternoons. These thermally induced air disturbances tend to be strong, highly localized, and short term but unpredictable and often undetectable. Thus, the pilot had no means to detect the air disturbance until he encountered it and then the improperly set rudder travel limits significantly reduced his ability to counter the resulting directional upset.

Text from the NTSB. Open the NTSB docket for WPR16LA108 for the full record.