NTSB Factual Report

On May 11, 2004, about 1630 eastern daylight time, a Cessna 172RG, N6562V, registered to and operated by Aviation Atlanta, Inc., exhibited limited elevator control during cruise flight in the vicinity of Chamblee, Georgia. The instructional flight was operated under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The certificated flight instructor and the private pilot-rated student were not injured, and the airplane sustained no damage. The local flight departed Peachtree DeKalb Airport, Chamblee, Georgia, about 1600 on May 11, 2004.

The certified flight instructor reported the airplane did not respond normally to elevator control inputs. He stated full nose-down elevator deflection could not be achieved, and the use of full nose-down elevator trim did not improve the condition. He stated that, when right rudder was applied, an increase in nose-down elevator deflection could be achieved. The flight returned to the airport and landed without further incident.

Maintenance personnel examined the airplane and found the threaded shaft of the nose gear steering bungee assembly was fractured below the sprocket for the rudder trim chain. The fracture separated the upper portion of the shaft and sprocket from the bottom portion of the shaft and bungee assembly, which remained attached to the right rudder weld assembly. Maintenance personnel determined the damaged component physically restricted the movement of the elevator control assembly.

The nose gear steering bungee assembly and fractured shaft were submitted to the National Transportation Safety Board, Materials Research Laboratory, Washington, D.C., for examination. Examination revealed the shaft was fractured at two locations. Examination of the upper fracture through the threaded region of the shaft revealed the fracture surface exhibited smooth crack arrest markings consistent with fatigue; the fatigue crack originated in a thread root. Additional cracks were observed in the two thread roots directly above the fatigue crack. Examination of the lower fracture through the thread relief area revealed the fracture surface exhibited smooth crack arrest markings consistent with fatigue emanating from the forward and aft surfaces of the shaft. The fatigue cracks extended through approximately 95 percent of the shaft radius, and the remaining fracture area exhibited ductile features consistent with overstress.

Hardness measurements of the shaft metal and chemical examination of the lubricating grease revealed no evidence of anomaly and no evidence of improper lubrication. Examination of the profile of the thread relief area of the shaft revealed the region was irregular and asymmetric, and a number of machining grooves were of smaller radii than the 0.050-inch radius specified in Cessna technical drawings. The shaft was absent of identifying markings or a part number. Examination of the maintenance logs for the airplane revealed no record of repair or replacement of the component.

Federal Aviation Administration Airworthiness Directive (AD) 81-14-06 for the Cessna 172RG requires the replacement of part number 2467003-1 rudder trim/nose gear steering bungee with part number 2467003-6 rudder trim/nose gear steering bungee. The AD states the purpose of the replacement is "to ensure the integrity of the rudder trim/nose gear steering bungee," and the AD states a limited elevator movement condition may be encountered, and "Cessna Single Engine Service Letter SE 80-99, Rev. 1, dated June 8, 1981, pertains to this subject." A review of the Cessna Service Letter SE 80-99 revealed the new steering bungee part number 2467003-6 "includes a larger diameter jack screw shaft providing substantially greater unit strength and longer service life."

Both the AD and the Service Letter are applicable to Cessna 172RG airplanes serial numbers 172RG0001 through 172RG0769. The incident airplane was serial number 172RG0775.

NTSB Probable Cause

The fatigue fracture of the nosewheel bungee assembly, which restricted the movement of the elevator control. A factor was the inadequate equipment from an unknown source/manufacturer.