Amateur-built CASST-SNOSHOO N102MB accident near Reno, Nevada

Probable cause

The movement of the airplane’s modified horizontal stabilizer trim system during an intentional high speed aerobatic maneuver that resulted in exceedence of the design stress limits of the airplane and an in-flight structural failure. Contrubuting to the accident was that the builder’s deviation from the airplane designer's original trim system.

NTSB analysis

During an air race class qualification flight, the pilot was performing a series of maneuvers required to obtain a class license. During a required roll maneuver, as the airplane passed through about 90 to 120 degrees of roll at an estimated speed of 220 to 230 miles per hour, the right wing separated from the airframe, followed almost immediately by the left wing. The amateur built experimental airplane was fitted with an in-flight adjustable pitch trim system, which was a deviation from the original ground-only trim adjustment design. The trim system, designed by the airplane's builder, allowed the pilot to adjust the pitch trim by moving the horizontal stabilizer in flight. The in-flight system was composed of a lever attached to the fuselage structure, and its movement was restricted with friction washers between the fuselage and the lever. Full travel of the lever resulted in one inch of vertical movement of the horizontal stabilizer's leading edge. The manufacturer of the airframe kit stated that the trim system installed on the accident airplane was not of their design. They further stated that any movement of the horizontal stabilizer during an abrupt maneuver could produce an extremely high amount of G-forces and possibly exceed the structural design limits of the airplane. Post accident examination of the wreckage revealed no evidence of pre-existing anomalies with the airframe or flight control system. The wooden wing spar was tested and found to be of proper grade wood. No anomalies were noted with the spar material. The fracture surfaces of both the left and right wing forward wing spars were consistent with a "substantial upward force" and overload. It is unknown if the horizontal trim system was moved in-flight by commanded or uncommanded action. Amateur built airplanes with experimental certificates are not subject to the airworthiness standards in 14 CFR Parts 23 and 47 of the Federal Aviation Regulations, and builders and owners are free to make any modifications to the airplane at will.

Text from the NTSB. Open the NTSB docket for SEA08FA196 for the full record.