Incident Overview
On July 19, 2009, at 1230 central daylight time, a Cessna 172R, registration N77AF, experienced a loss of throttle control while conducting a practice instrument approach to DuPage Airport (DPA) in West Chicago, Illinois. The instructional flight, conducted under 14 CFR Part 91, was not operating on a flight plan and had originated at 1000. Visual meteorological conditions prevailed. The flight instructor and commercial pilot on board were uninjured.
During the approach, the flight instructor, seated in the left seat, intentionally maintained an increased airspeed to determine if the commercial pilot, who was receiving instruction toward a flight instructor instrument rating, would instruct him to reduce airspeed. The commercial pilot did tell the flight instructor to reduce airspeed, but when the flight instructor retarded the throttle, the airspeed did not decrease. The engine speed remained indicated at 2,200-2,300 rpm and did not respond to additional throttle control movements from full forward to full aft.
The flight instructor informed the DuPage Airport Air Traffic Control Tower of the loss of throttle control and then overflew the airport. He discussed with the commercial pilot a plan to attempt an approach and landing on the longest runway at DPA, runway 2L, which measures 7,571 feet by 100 feet and is concrete. The approach and landing were performed without further incident.
Pilot Information
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. His total flight time was 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 had accumulated a total flight time of 549 hours, all of which were in single-engine airplanes.
Maintenance Findings
An airframe maintenance logbook entry, dated June 30, 2008, by non-operator maintenance personnel, indicated that a new lower firewall was installed and that all other undamaged parts were transferred from the removed firewall to the new one at a tachometer time of 1,403.7 hours. No entries after this date recorded engine removal or engine control removal.
Following the incident, the operator's maintenance personnel at DuPage Airport found the throttle control disconnected at the rod end of the throttle control assembly. Two jamnuts were in place on the threaded portion of the assembly connecting to the rod end. The maintenance personnel replaced the Cablecraft 40-02-50 throttle cable (part number 565-549-021) and the throttle cable end (part number S1103), and inspected the throttle cable jam nuts. The Cessna 172R parts catalog specifies one nut per assembly, but the aircraft's depiction shows two jamnuts. According to a Cessna representative, the depicted second jamnut is actually the rod end itself. Another operator aircraft, a Cessna 172R (N53AF), had two jamnuts installed, but one was later removed during inspections of the operator's remaining airplanes.
Inspection of the cable revealed that all threads along the assembly, including the cable and jamnut, were intact. 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 from the rod end should the nut become loose. Some other Cessna models 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.