WPR16LA108

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.

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.

L71 is located in the Mojave Desert, near California City California. The airport is situated at an elevation of 2,454 feet above mean sea level (msl). The single paved asphalt runway, designated 6/24, measures 60 by 6,027 feet. The airport was not equipped with an air traffic control tower or any automated weather detection or reporting equipment. Three windsocks, one midfield, and one near each threshold, were located north of the runway. Airplane Examination A few days after the accident, an FAA airworthiness inspector examined the recovered airplane. His examination did not reveal any apparent anomalies or pre-impact deficiencies. About one month later, the FAA inspector and the NTSB investigator examined the airplane in detail, in a hangar of the L71 facility that the pilot was planning to visit for his pre-accident sheet metal repair cost estimate. Some interior and exterior components and panels were removed to enable access to the entire rudder/tailwheel control system. A 'Leatherman' type multi-tool (in its case) was found on the right cockpit floor, forward of the copilot's right rudder pedal. When found, it was not in a position/location that impeded rudder pedal travel, but its original (pre-accident) position/location was not able to be determined. The pilot/owner identified the tool as his. He noted that many cockpit/cabin items were displaced during the accident, and that at least one item had not been located. That item was not recovered by the investigators during their examination. All control cables were found to be properly routed, and no components (pulleys, bellcranks, etc) displayed any unusual wear, corrosion, damage, or other anomalies that could be associated with previous or potential control interference. Because the rudder & vertical stabilizer damage precluded full and free travel of the rudder, the deformed sections were cut away just below the upper rudder hinge, in order to free the rudder for normal travel for the examination. The aft fuselage was hoisted to lift the tailwheel off the ground. The bottom end of the rudder and the tailwheel appeared centered/neutral when the rudder pedals were set to the 'neutral' (left and right pedals 'even' with one another) position; this was confirmed for both the pilot's & copilot's pedal sets. Both sets of cockpit rudder pedals were then exercised to their travel limits, and the system operation, and rudder and tailwheel deflections, were noted. The rudder and tailwheel responded similarly, irrespective of which pedal set (pilot vs copilot) was exercised. The rudder travel was bounded in both directions by the rudder stops, which were properly safetied. No binding, unusual noises, or other anomalies were noted when the pedals were exercised. Cable tensions were measured, and found to be within the proper range. Rudder deflections (from neutral/0º) in the plane parallel to the airplane longitudinal and lateral axes were measured to be as follows: - Left: 13º - Right: 16º The tailwheel was a Scott fully castering model, capable of swiveling 360º in either direction, with a centering detent. All cables, springs, and other components were properly routed, connected, and safetied. Manual manipulation of the tailwheel indicated that it operated normally. Refer to the NTSB public docket for this accident for additional examination details. Cessna Rudder Travel Limit Values Per the Cessna Maintenance Manual (MM), the proper rudder travel limits, measured in the plane parallel to the airplane "waterline" and lateral axes, were 24º either side of the neutral/0º position. Those same values were published in the FAA Type Certificate Data Sheet (TCDS) for the airplane. Previous Airplane Condition and Maintenance The pilot arrived at the examination hangar towards the end of the examination. After the examination was completed, as the investigators were preparing to depart, the pilot mentioned that the reason that he had flown the airplane to L71 was to get a cost estimate to have his rudder "re-skinned." He explained that the re-skinning was intended to correct an improper top-to-bottom curvature (bowing) of the rudder, which was apparently the result of a previous accident. The pilot did not know whether the rudder was deformed during that accident, or as a result of improper repair. He discovered the bowing problem shortly before the subject accident, when he attempted to have the airplane re-rigged, because it flew "left wing-heavy." At that time, he was advised by his mechanic that any effort to re-rig the airplane without first repairing/straightening the rudder would be unsuccessful and futile. The pilot reported, and provided substantiating photographic evidence, that the curvature was such that when the top of the rudder was aligned with the fin, the bottom was deflected about 1.5 inches to the airplane right. Thus, with the bottom of the rudder centered (and the rudder pedals aligned with one another), the rudder curvature would result in the trailing edge of the upper portion of the rudder to be deflected to the airplane left. Because the rudder had been damaged in the subject accident, the curvature profile could not be measured and quantified. Review of the NTSB database indicated that the airplane struck a horse during landing in 1974. The airplane maintenance records contained entries or documentation for six separate maintenance activities/events regarding the rudder or rudder control or tailwheel steering systems, as follows: - October 1973 Rudder and vertical stabilizer "replaced due to hangar damage" - September 1979 Replaced right rudder cable - May 1992 Airplane stripped and repainted; control surfaces removed and reinstalled - September 1994 "Rudder reskinned" - August 1999 Tailwheel "spring mount bracket" replaced, and new (accident) tailwheel installed - April 2014 Annual inspection; "Check & adjust cable tensions" Annual Inspection Guidance and Requirements Aircraft operated in the United States under FAR Part 91 rules are required to be inspected on an annual basis; that inspection is formally designated the "Annual Inspection." The FAA delineates guidance regarding the scope and detail of such inspections; the guidance specifies that the person conducting the inspection must use a checklist while performing the inspection. The guidance states that the checklist "may be of the inspector's own design, one provided by the manufacturer of the equipment being inspected or one obtained from another source," and elaborates that the checklist must include the scope and detail of the items contained Appendix D of FAR Part 43. The manufacturer's published inspection guidance for the accident airplane make and model contained a comprehensive enumeration of items to be checked. Under the subheading "Rudder control system" the final line item was "Rudder system for correct rigging and proper travel." The Annual Inspection affirms that the aircraft meets all applicable airworthiness requirements, which in part signifies that the airplane is in compliance with its type design, much of which is specifically cited in the TCDS. The TCDS for the accident make and model airplane specified the rudder travel ranges. Therefore, satisfactory completion of an Annual Inspection requires that the flight control surface travel ranges are in accordance with the type design and TCDS-specified ranges. May 2016 Annual Inspection Information The airframe maintenance record entry stated that the inspection was completed "per 43 Appendix D," and was followed by "checked cables and tension Inspected Pulleys; Rod Ends, Bellcranks; P/P tubes, Bearings, Hinges checked Flight control travels." NTSB communications with the mechanic who conducted the annual inspection revealed that he was the one who advised the pilot about the rudder bow. The mechanic reported that contrary to the pilot's statements, he (the me