Accident Overview
On May 17, 2021, at approximately 2000 central daylight time, a Flight Design GMBH CLTS, registration N521CT, was involved in an accident near Racine, Wisconsin. The airplane was destroyed, and the student pilot received minor injuries. The flight was being conducted as an instructional solo cross-country flight under Title 14 Code of Federal Regulations Part 91.
Sequence of Events
The student pilot departed Batten International Airport (RAC) at 1741 en route to Burlington Municipal Airport (BUU). The flight to BUU was uneventful until the student pilot reduced engine power in the traffic pattern and encountered severe engine vibration. The pilot returned to RAC and attempted three landings on runway 14, but each attempt was aborted with go-arounds due to excessive airspeed and vibration upon throttle reduction.
During the fourth landing attempt, the airplane touched down with excessive energy and bounced several times on runway 14. The student pilot aborted the landing and applied full engine power to climb, but airspeed decreased. The pilot then headed toward a field to land, but the airplane struck trees, a powerline, and a house. The airplane came to rest inverted. The pilot egressed without further incident.
Postaccident Examination
The airplane was destroyed by impact forces, resulting in numerous fractures through the composite fuselage, damage to the wings, and damage to the empennage.
Examination revealed that the throttle cable leading to the right carburetor was fractured and separated at the carburetor’s throttle control arm. The left carburetor throttle cable and an interconnecting cable between the right and left carburetor arms were intact and secure. The carburetor’s throttle control arms were spring-loaded to the full open throttle position upon cable failure. The left carburetor control cable could be swiveled at its attachment, while the right cable could not. The right throttle cable appeared crushed near the fracture.
Metallurgical examination of the separated cable portion attached to the throttle control arm clamping bolt showed deformation consistent with a previously adjusted clamping location. The clamping bolt head and captive washer bore imprints of the cable wires. The interference between the cable and the clamping bolt’s cross-drilled hole resulted in a tightly focused clamping force, causing deformation and stress risers susceptible to fatigue cracks. Scanning electron microscopy revealed crack progression marks consistent with fatigue fractures.