SANTA MARIA, CA, USA
N832US
US/LTA 138-S
THE AIRSHIP EXPERIENCE A FRACTURED ROD END BEARING DURING CRUISE FLIGHT. AN ONBOARD MECHANIC WAS ABLE TO MANUALLY MANIPULATE THE CONTROLS UNTIL THE PILOT MADE A PRECAUTIONARY LANDING. EXAMINATION OF THE ROD END BEARING REVEALED FATIGUE CRACKS IN THE THREADS AT THE FRACTURE. EXAMINATION OF THE THREADS DISCLOSED THEY WERE NOT ROLLED AFTER THE MACHINING PROCESS. THE MANUFACTURER'S SPECIFICATIONS DID NOT SPECIFY ROLLING IN THE THREADED AREAS. THE MANUFACTURER INDICATED THE BEARING MAY HAVE BEEN BINDING AS A RESULT OF TEFLON IN THE BEARING AREA ADHERING TO THE METAL SURFACE CONTRIBUTING TO THE STRESSES THAT MAY HAVE FATIGUED THE ROD END BEARING.
On May 25, 1993, at 1015 hours Pacific daylight time, a US-LTA Corporation 138-S Airship, N832US, diverted to Santa Maria, California, after experiencing a flight control malfunction. The airship was being ferried to Chino, California, from the manufacturer's facilities in Eugene, Oregon. The incident flight leg originated in Paso Robles, California, and was to continue non-stop to Chino. The US-LTA Corporation 138-S airship is a recently certified aircraft. The incident airship, serial number 2, was manufactured on March 17, 1993, and had accumulated about 30 hours of flight time. The pilot reported a loss of rudder control while making a normal control input during cruise flight. The mechanic on board the airship opened the floor boards and discovered that a rod end bearing on the rudder input tube was broken at the pilot's stick control end. The mechanic then manually manipulated the rudder control system for the pilot. The airship diverted to Santa Maria Airport and landed without incident. The rod end bearing was sent to the Safety Board's Materials Laboratory for metallurgical examination. The rod end was fractured at the thread termination at the bearing end of the assembly. Details of the examination by the Safety Board's metallurgist indicated a presence of ratchet marks indicative of metal fatigue. The evidence of fatigue was found along the outer portions of the fractured surface of the rod end. The rod end was sectioned along the longitudinal axis of the threaded piece of the rod end. The sectioned piece of the rod end was examined at high magnification. The examination revealed the presence of cracks in the thread roots in three of the four threads adjacent to the fracture. The examination disclosed that the threads had not been rolled during the manufacturing process. The rod end bearing was machined to the manufacturer's specifications and drawings. Review of the specifications and drawings indicated cut threads were defined and that rolling the threads was not specified. Rolled threads are normally specified in AN hardware (hardware manufactured to specifications set up by the Army/Navy that meet or exceed specifications used in aircraft manufacture). The airship manufacturer indicated to FAA that the rod end bearing incorporates Teflon in the bearing and after some time the bearing may bind. The manufacturer explained the binding may be a result of adhesion of the Teflon to the metal in the spherical center of the bearing. According to the airship manufacturer, the binding resulting from the Teflon adhesion could have created the stresses that contributed to the rod end failure. The airship manufacturer also reviewed the rigging of the airship flight controls to determine if rigging may have contributed to the failure of the rod end bearing. As a result of this review, the airship manufacturer issued several "Engineering Change Orders" and a "Change Proposal" to change the airship rigging procedures and adjustment of the control stops to eliminate any errors in rigging and thus prevent a further occurrence. The manufacturer also added a procedure to free the rod end bearing from binding. The procedure simply heated the bearing to between 200 and 250 degrees Fahrenheit to break the adhesion of the Teflon.
INADEQUATE SPECIFICATIONS IN THE DESIGN AND MANUFACTURE OF A RUDDER CONTROL TUBE ROD END BEARING BY THE AIRSHIP MANUFACTURER.
Source: NTSB Aviation Accident Database
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