16 October 2001
Corrosion of rudder pedal arms and torque tubes
Transport Canada received a service difficulty report describing a fractured rudder pedal arm that had been installed in a Cessna A188. The pedal arm fractured during landing which caused the pilot to lose control of the aircraft, resulting in a ground loop during the landing roll. Fortunately there was no damage or injury as a result of this incident.
The rudder pedal assembly consists of a welded steel torque tube assembly with a stub shaft or spigot welded to the torque tube over which the rudder pedal arm is fitted. The rudder pedal arm is also a welded tube, open at both ends, with an inside diameter equal to or just slightly larger than the outside diameter of the spigot on the torque tube, and is fastened to the torque tube spigot using a single bolt.
The pedal arm tube is open at the top end and consequently contamination of the tube interior from dirt, oil, water and in the case of agricultural aircraft, other chemicals, is possible. In addition to contamination of the rudder pedal arm, any contamination entering the pedal arm tube will also enter the top end of the torque tube spigot, causing corrosion to the spigot and torque tube as well.
The subject aircraft's torque tube exhibited substantial corrosion at the upper end of the spigot and around the rudder pedal arm attachment bolt hole. The rudder pedal arm tube was corroded to just a few thousandths of an inch thick in the area of the fracture (approximately 1 inch above the attachment bolt hole). The other pedal arm, although not fractured or cracked, had severe corrosion on the tube interior and failure was imminent.
The SDR database contains one other report of a corroded rudder pedal arm on a Cessna 188. This type of attachment of the rudder pedal arm to the torque tube is used throughout the Cessna fleet of 100 and 200 Series aircraft, and one report of similar corrosion occurring on a Cessna 180 is contained in the SDR database. The submitter of the Cessna 180 SDR indicated the aircraft had been operated on floats and may have been operated in a coastal area.
Although the problem does not appear to be widespread at this time, Transport Canada recommends that operators of Cessna 188, A188 and T188 aircraft, as well as any other 100 or 200 Series aircraft operating in, or suspected of having been operated in a corrosive environment, consider removing the rudder pedal arms from the rudder pedal torque tubes for inspection and corrosion prevention treatment.
Any defects or further occurrences should be reported by sending a Service Difficulty Report to Transport Canada, Continuing Airworthiness, Ottawa.
For further information, contact a Transport Canada Centre, or Mr. Mark Stephenson, Continuing Airworthiness, Ottawa, telephone (613) 952-4363, facsimile (613) 996-9178 or e-mail firstname.lastname@example.org.
For Director, Aircraft Certification
Chief, Continuing Airworthiness
Tube wall thickness corroded to less than 0.015"
Unbroken pedal arm tube (sectioned).
Tube original wall thickness = 0.058 po
Tube corroded to only a few thousandths of an inch in area of fracture