Racine, WI, USA
N521CT
FLIGHT DESIGN GMBH CTLS
During a solo cross-country flight, the student pilot returned to the departure airport after the engine began to vibrate when power was reduced. She attempted three landings that were aborted due to excessive airspeed. During the fourth landing attempt, the airplane touched down fast and bounced several times on the runway. The student pilot aborted the landing and added full engine power to climb, but the airspeed decreased. The airplane collided with a tree and a building and was destroyed by impact forces. Postaccident examination of the airplane revealed that one of the two throttle control cables had separated from its carburetor at the control arm. The cable separation from the carburetor resulted in the carburetor’s spring-loaded throttle control moving to full throttle while the second carburetor would have been at a position selected by the cockpit throttle control. When the pilot reduced the cockpit throttle control for landing, the carburetor with the broken cable would have continued operating at maximum throttle regardless of the cockpit throttle control position, which likely resulted in the engine vibration and the excessive airspeed for landing. Metallurgical examination of the separated control cable revealed that a tightly focused clamping force on the carburetor cable attachment bolt resulted in fatigue failure of the cable.
On May 17, 2021, about 2000 central daylight time, a Flight Design GMBH CLTS, N521CT, was involved in an accident near Racine, Wisconsin. The airplane was destroyed. The student pilot received minor injuries. The airplane was operated under Title 14 Code of Federal Regulations Part 91 as an instructional flight. The student pilot stated that she departed from Batten International Airport (RAC), Racine, Wisconsin, at 1741, enroute to Burlington Municipal Airport (BUU), Burlington, Wisconsin, for a solo cross-country flight. The flight to BUU was uneventful until the student pilot reduced engine power in the BUU airport traffic pattern and encountered a severe engine vibration. The student pilot returned to RAC and attempted three landings on runway 14 but initiated go-arounds due to excessive airspeed and the vibration that occurred when the engine throttle was reduced. The student pilot stated that during the fourth landing attempt, the airplane touched down with too much energy and bounced several times on runway 14. The student pilot aborted the landing and attempted a climb by adding full engine power, but the airspeed decreased. The student pilot then headed toward a field to land the airplane, but it hit trees, a powerline, and a house. The airplane came to rest inverted, and the pilot was able to egress from the airplane without further incident. The airplane was destroyed by impact forces that resulted in numerous fractures through the composite fuselage, damage to the wings, and damage to the empennage. Postaccident examination revealed that the throttle cable leading to the right carburetor was fractured and separated at the carburetor’s throttle control arm. The throttle cable leading to the left carburetor and a cable that interconnected the right carburetor throttle control arm to the left carburetor control arm were intact and secure. The carburetor’s throttle control arms were spring loaded to the full open throttle position if a cable failure occurred. The left carburetor control cable could be swiveled at its throttle control arm attachment. The right throttle control cable could not be swiveled on its throttle control arm attachment. The right carburetor throttle cable appeared to be crushed near the fracture, which was at the aft/cockpit side of the throttle control arm. Metallurgical examination of the separated portion of the control cable that remained attached to the throttle control arm clamping bolt revealed deformation to the cable consistent with the clamping location along the cable having been adjusted at some previous time. The act of clamping the control cable to the throttle lever imparts deformation and bending to the cable when it becomes squeezed between the bearing surfaces of the clamping bolt head and captive washer. Close examination of the bearing surfaces on both the clamping bolt head and the captive washer revealed that there were imprints of the cable’s wires depressed into the bearing surfaces. The interference between the control cable and the bearing surfaces of this fixation design resulted in a tightly focused clamping force on the cable where it entered and exited the cross-drilled hole of the clamping bolt resulting in deformation of the cable and its individual wires. This deformation of the wires reduced their diameter and created stress risers that were susceptible to the formation of fatigue cracks. Scanning electron microscopy of the fractured ends of the throttle control cable revealed fracture features exhibiting crack progression marks consistent with fatigue fractures.
The fatigue failure of the carburetor throttle control cable, which resulted in a partial loss of engine control.
Source: NTSB Aviation Accident Database
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