FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin NOVEMBER–DECEMBER 1998 Preventing Fretting Damage Becomes Increasingly Critical as Aircraft Age Typical Fatigue Crack Location Outer or Critical Upper Upper Skin Row of Fasteners Adhesive Stringer Inner or Lower Skin FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin Dedicated to the aviation mechanic whose knowledge, craftsmanship and integrity form the core of air safety. Robert A. Feeler, editorial coordinator November–December 1998 Vol. 46 No. 6 Preventing Fretting Damage Becomes Increasingly Critical as Aircraft Age ............................................................................ 1 Maintenance Alerts................................................................................... 11 News & Tips ............................................................................................. 14 AVIATION MECHANICS BULLETIN Copyright © 1998 FLIGHT SAFETY FOUNDATION INC. 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In keeping with FSF’s independent and nonpartisan mission to disseminate objective safety information, Foundation publications solicit credible contributions that foster thought-provoking discussion of aviation safety issues. If you have an article proposal, a completed manuscript or a technical paper that may be appropriate for Aviation Mechanics Bulletin, please contact the director of publications. Flight Safety Foundation assumes no responsibility for submitted material. The publications staff reserves the right to edit all published submissions. The Foundation buys all rights to published manuscripts. Payment is made to authors upon publication. Contact the Publications Department for more information. Preventing Fretting Damage Becomes Increasingly Critical as Aircraft Age Patrick R. Veillette, Ph.D. Fretting is a combined form of wear, cracks parallel to the major frac- fatigue and corrosion that can lead ture. Fretting was caused by to premature mechanical failure at movement of the bearing shell loads well below structural design within the connecting rod big- limits. It is a time-based failure that end bore. The bearing shell had will require increased attention as not been installed properly. the transport-category aircraft fleet • Fretting corrosion occurred in continues to age. the propeller shaft bearings of a single-engine aircraft operated The following are examples of how in a marine environment. Pro- fretting damage can lead to aircraft peller vibratory stresses appear mishaps:1 to have been sufficient to cause the fretting. (The report did not •A helicopter struck terrain after say whether the damage was a connecting rod broke in its pis- found during routine mainte- ton engine. The primary fracture nance or following an accident surface contained a small area or incident.) of fretting damage. Scanning- electron microscopic examina- • After the engine stopped operat- tion of the fretting area showed ing in flight, the crew of a mili- surface damage and the initia- tary aircraft was unable to restart tion of many small fatigue it because of fretting damage to the electrical system. The crew Whenever a mechanical fastener, made a successful emergency such as a rivet, is used to secure two landing. Investigators found that parts, vibratory stresses can cause the an electrical connector in the fastener to loosen, allowing small engine-starting system had mal- cyclic displacements to occur be- functioned because of fretting tween the two contacting surfaces. action from vibration-induced This is particularly common in the motion between the male pin connections between sheet metal and and the female receptacle. The fuselage frame structural members, incident-investigation board con- and in tail-section connections be- cluded that the vibration-sensitive cause of turbulent airflow. Fretting connector was not suitable for use damage also has occurred between in the aircraft; it was replaced with mating surfaces in oscillating bear- a vibration-resistant connector. ings and flexible couplings. The basic requirements for fretting Small Vibratory are relative motions between two sur- faces in contact; some mechanical Motions Can Cause load applied to the surfaces; and a Fretting load vector sufficient to cause slip between the surfaces. When viewed under magnification, no metal surface appears perfectly Fretting can result in excessive wear, smooth. Rather, surface irregularities surface fatigue, component fracture, appear as peaks and valleys. When loss of clamping pressure and jam- two metal components are placed in ming (by generated debris). Although contact under a load, the peaks (called most reports of fretting damage in- asperities) on one surface will adhere volve metals, composite materials to the asperities on another; simply and ceramic materials also are sus- speaking, the asperities become weld- ceptible to fretting damage.2,3 ed together. When the contacting sur- faces are displaced by some vibratory Critical components such as flight motion, the welded areas rupture. The controls, powerplant controls and tail resulting wear produces debris.8,9 surfaces are especially susceptible to fretting damage because they are ex- Only slight motion is required to posed to the type of vibratory motions cause fretting. Vibratory (back-and- that cause fretting. Also highly sus- forth) motions as limited as 4 x 10-8 ceptible are roller bearings, clamped inch (1 x 10-7 millimeter) have been joints, pivots and a variety of other shown to cause fretting.10,11,12 This aircraft components.4,5,6,7 limited motion between the surfaces 2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • NOVEMBER–DECEMBER 1998 distinguishes fretting from normal increasingly turbulent air flows over wear, which creates debris that typi- the rear fuselage and the empennage. cally is removed from the local area. Pronounced flexing of skin panels typ- Fretting involves such minute relative ically occurs at high angles-of-attack.13 movements that the debris remains in the general area of the damage and Debris resulting from the mechani- forms a “third body” between the sur- cal wear readily oxidizes because of faces. 11 the high temperatures created by fric- tion between the surfaces. Depend- The motions can be produced by ing on the metals involved, the third mechanical sources, such as vibra- body of debris either can act as a lu- tions resonating throughout the ad- bricant and decrease the coefficient joining structure. An example is of friction between the two surfaces, powerplant vibrations that affect or act as an abrasive and exacerbate flight controls. Vibrations also can be the wear damage. caused by aerodynamic sources.13 Determining the source of the motion Debris often is pressed into the sur- sometimes can lead to a preventive faces, causing indentations and fur- measure. Finding and restricting vi- rows. The surface faults create stress brations from mechanical sources, risers that accelerate fatigue.1 (A however, are easier than finding and stress riser, also called a stress rais- restricting vibrations from aerody- er, is a material discontinuity that in- namic sources. duces a local increase in stress.) When fretting occurs between met- A significant aerodynamic source of als of different hardness, the softer airframe vibration is propeller slip- metal will deform the greatest stream (prop wash). Turbulent air- amount. flow within the prop wash strikes the fuselage and empennage, flexing Fretting fatigue cracks are propagat- skin surfaces and creating vibrations ed at very low stresses, well below between the structures. Aft fuselage the fatigue limit. The direction in and empennage are especially sus- which fatigue cracks grow depends ceptible to vibrations caused by prop upon the direction of the contact wash. stresses. The cracks grow perpendic- ular to the maximum principal stress Wing wake has a large influence on in the fretting area. While the overall the airflow over the empennage. Tur- applied loads may be small in the re- bulent flow from boundary-layer sep- gion of contact,