ANCHORAGE, AK, USA
N4685F
CESSNA 206
THE THROTTLE CONTROL LEVER ARM USED TO ATTACH THE BALL BEARING TYPE THROTTLE ROD END TO THE CARBURETOR WORE COMPLETELY THROUGH. THE SIDE WALLS OF THE LEVER THAT WERE ADJACENT TO THE BOLT HOLE WERE SCORED AND REPLICATED THE DIMENSIONS OF THE WASHERS THAT WERE USED ON THE BOLT AND NUT SIDE OF THE LEVER. THE BOLT AND WASHERS THAT WERE USED TO CONNECT THE CABLE END TO THE LEVER WERE NOT APPROVED FOR THE APPLICATION. THE PLANE WAS INSPECTED 35 HOURS PRIOR TO THE ACCIDENT.
On July 18, 1994, at 1245 Alaska daylight time, a float equipped Cessna P206A airplane, N4685F, owned and operated by the pilot-in-command, dba Mountain Air of Alaska, collided with terrain approximately 28 miles southeast of Anchorage, Alaska following a loss of engine power while in cruise flight. The commercial certificated pilot and his two passengers, the sole occupants, were not injured, and the airplane sustained substantial damage. The flight, which was being conducted under the on demand rules of 14 CFR Part 135 for the purpose of flight seeing the local area, last departed the Lake Hood float plane base in Anchorage at about 1215. The pilot reported that visual meteorological conditions prevailed in the area of the crash site and that a VFR flight plan was in effect with the Anchorage Flight Service Station (FSS). A post accident inspection of the plane's powerplant assembly by an FAA Airworthiness Inspector revealed that the throttle control lever, Teledyne Continental PN: 632555-23, arm, used to attach the ball bearing type throttle rod end to the carburetor had worn completely through. This allowed the throttle cable attachment bolt to separate from the control lever. The lever is made of brass. The lever and cable attachment components were removed from the airplane and forwarded to the NTSB. An examination of the lever disclosed that the bolt hole was rectangularly elongated to the point that the wear pattern had traveled completely through the lever. The side walls of the lever that were adjacent to the bolt hole were scored. The scoring replicated the dimensions of the washers that were used on both sides of the lever. The parts used to attach the cable rod end to the lever consisted of a predrilled one inch long non AN stove bolt with a 3/4th inch grip, four washers having three different dimensions, and a castellated nut with cotter pin. The correct components are a predrilled 3/4th inch long by 3/16th inch diameter steel AN3-6 bolt with a 1/2 inch grip, three washers - two having similar dimensions, and a castellated nut with cotter pin. The aircraft was last inspected during a 100 hour inspection on July 7, 1994, 35.8 hours prior to the accident. During that inspection, the mixture control lever, also made of brass, was removed and replaced because it was found to have excessive wear at the cable attachment bolt hole. The circumstances surrounding the failure of the throttle control lever were entered into the FAA's (SDR) service difficulty reporting system. In April 1979, Cessna issued Customer Care Service Information Letters SE79-6 and SE79-6A. The letters advised Cessna aircraft owners that the throttle, mixture, and propeller control cable ends should be secured to the engine with a predrilled AN bolt, castellated nut, and a cotter pin. Cessna recommended that, "this improved type attachment should be incorporated on all earlier aircraft at the next 100 hour or annual inspection". The fitting tolerance of the bolt/nut/washer assembly was not specified. Effective January 7, 1987, the FAA issued Airworthiness Directive (AD) 86-2407. The AD made mandatory the recommendations found in Cessna Single-Engine Customer Care Service Information Letter SE79-6.
THE FAILURE OF THE THROTTLE CONTROL LEVER ARM AS A RESULT OF THE IMPROPER 100 HOUR INSPECTION BY COMPANY MAINTENANCE PERSONNEL.
Source: NTSB Aviation Accident Database
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