NTSB Factual Report

On July 19, 2009, at 1230 central daylight time, a Cessna 172R, N77AF, experienced a loss of throttle control during a practice instrument approach to Dupage Airport (DPA), West Chicago, Illinois. The instructional flight landed without further incident. Visual meteorological conditions prevailed at the time of the incident. The flight instructor and commercial pilot were uninjured. The 14 CFR Part 91 instructional flight was not operating on a flight plan. The local flight originated at 1000.

The flight instructor, who was seated in the left seat, and a commercial pilot who had been receiving instruction towards a flight instructor instrument rating completed several practice approaches at a nearby airport and returned for an approach to DPA. During the approach, the flight instructor intentionally maintained an increased airspeed to determine if the commercial pilot would tell him to reduce airspeed. The commercial pilot told the flight instructor to reduce airspeed, but when the flight instructor retarded the throttle, the airspeed did not decrease, and the engine speed continued to indicate 2,200-2,300 rpm. The engine speed did not change with additional throttle control movements from full forward to full aft. The flight instructor informed the DPA Air Traffic Control Tower that the airplane experienced a loss of throttle control and he then overflew the airport. The flight instructor stated that he discussed with the commercial pilot a plan to attempt an approach and landing on the longest runway at DPA, runway 2L (7,571 feet by 100 feet, concrete). They performed the approach and landing without further incident.

The flight instructor held an airline transport pilot certificate with single-engine land, multiengine land, and instrument airplane ratings. He also held type ratings in Cessna 500, Boeing 737, Boeing 757, Boeing 767, and Beech 1900 airplanes. He held a flight instructor certificate with airplane single-engine, multiengine, and instrument airplane ratings. He accumulated a total flight time of 11,888 hours, of which 2,142 hours were in single-engine airplanes.

The commercial pilot held airplane single-engine landing and instrument airplane ratings. He accumulated a total flight time of 549 hours, all of which were in single-engine airplanes.

An airframe maintenance logbook entry, by non operator maintenance personnel, dated June 30, 2008, shows that a new lower firewall was installed and “all other undamaged parts transferred from removed firewall to new” at a tachometer time of 1,403.7 hours. There were no entries after this date indicating engine removal or engine control removal.

Following the incident, the operator’s maintenance personnel had found the throttle control disconnected at the rod end of the throttle control assembly with two jamnuts in place on the threaded portion of the assembly that connects to the rod end. The operator’s maintenance personnel at DPA replaced the Cablecraft 40-02-50 throttle cable, part number 565-549-021, replaced the throttle cable end, part number S1103, and inspected the throttle cable jam nuts. The Cessna 172R depicts two jamnuts, but the parts catalog specifies one nut per assembly. According to a Cessna representative, the depicted second jamnut is the rod end itself. Another one of the operator’s airplanes, a Cessna 172R, N53AF, had two jamnuts installed but one of these jamnuts was later removed during inspections of the remaining airplanes used by the operator.

Inspection of the cable revealed that all threads along the assembly were in place including the cable and jamnut. The second nut was not retained by the operator after its removal. The throttle control assemblies for the Cessna 172R do not have a key way at the throttle control knob to prevent rotation of the cable and unscrewing of the cable from the rod end should the nut become loose. Some other models of Cessna airplane do have such a key way to prevent rotation of the throttle control knob.

The operator recommended that a safety device and not just a jamnut be installed to prevent the throttle control from disconnecting from the rod end.

NTSB Probable Cause

A partial loss of engine power due to the detachment of the throttle control cable from the throttle assembly during a landing approach. Contributing to the accident was the lack of a redundant means to preclude the detachment of the cable from the throttle assembly.