It should come as no surprise that more than half of all general aviation aircraft are more than 25 years old, and more than 25% are past their 35th birthdays. Like everything else thats getting older, an aging aircraft can experience a host of problems, including decreases in both performance and structural integrity.
As aircraft age, undetected or uncorrected wear, fatigue, corrosion and creep can decrease an aircrafts ability to sustain the fail-safe loads designed into it. The most graphic example of fatigue and corrosion causing a serious problem in flight comes is the Aloha Air Lines B-737 accident in 1988, and that should be enough to scare any pilot. Clearly prevention, timely detection and control of fatigue, corrosion and wear is important.
Usually failures caused by one of these factors is simply generalized as a fatigue or corrosion failure. However, there is a combined form of wear, fatigue and corrosion called fretting that is particularly troublesome because it can lead to premature mechanical failures at loads well below normal. Fretting can result in either excessive wear, surface fatigue, component fracture, loss of clamping pressure, or jamming due to debris.
Most reported occurrences of fretting concern metallic materials, though newly developed composite and ceramic materials are also susceptible to fretting damage, according to K. Freidrich in the Journal of Material Science and D. Klaffe in Tribology International.
Aluminum sheet metal and rivet joints are very susceptible to this form of damage – a troublesome reality given that many of the structural members in light aircraft are assembled this way. Furthermore, light aircraft are particularly prone to develop fretting because of the vibration levels. These two factors can turn a general aviation aircraft into a prime candidate for fretting.
The basic requirements for fretting are two surfaces that are under load where the contact point is slipping slightly because of vibration. Thats precisely the environment most light planes spend their working hours immersed in. A number of researchers have concluded that flight and powerplant controls, roller bearings, clamped joints, pivots and other aircraft components are especially prone to fretting.
Fretting has led to control jamming or loss of functional control in past accidents. Whenever a mechanical fastener, such as a rivet, is used to secure two parts, vibration causes a loosening of the fastener system, allowing small cyclic displacements to occur between the two contacting faces. This is particularly common in the connections between sheet metal to fuselage structures, or in the tail section due to the beating pulses of the propeller prop wash and other turbulent airflow. Its this latter location that is especially noteworthy because early designers did not take into account the effects of small vibrations and fretting on tail surfaces.
How Fretting Occurs
Fretting can occur because of the characteristics of metal surfaces. When sufficiently magnified, no surface appears perfectly smooth. Rather, it has surface irregularities that look somewhat like peaks and valleys. Metallurgists call the peaks asperities. When two adjoining surfaces are placed under a load, adhesion occurs between the contacting asperities, or in simpler terms, these peaks become welded together. When these welded bodies are then displaced by some kind of vibration, the welded areas rupture and wear fragments are produced, as demonstrated by F.P. Bowden and D. Tabor in their 1964 Oxford University study.
The amount of motion required to produce fretting is surprisingly small. Researchers have discovered that very small vibratory motions, some as small as 0.000001 mm, can cause fretting. Our bodies arent nearly sensitive enough to detect such small motions, but the next time you start your aircraft engine, pay attention to the amount of vibration. It may surprise you.
Fretting creates such small relative movements that the debris remains in the general area of the damage. The debris acts like sand paper and further adds to the wear damage by its abrasive action. The debris is often work hardened, and can be pressed into the surfaces. That causes indentations and furrows, which in turn produce faults at the surface and create stress risers that can lead to accelerated fatigue. When fretting occurs between metals of differing hardnesses, the softer metal will deform the greatest amount.
Fretting cracks can be propagated when stresses are very low. While the overall applied loads may be small in the region of contact, the localized stresses can be much larger due to stress concentrations, thus creating zones that will experience accelerated fatigue crack growth and early failure.
When the fretting zone is exposed to a corrosive environment, failure usually occurs in a shorter time and in fewer cycles than would be the case in a dry or benign environment. The severity of the corrosion attack can vary depending on the metals used, manufacturing care and storage, protective surfaces and other anti-corrosion, films, exposure to corrosive environments, and maintenance programs.
Fretting damage is more severe in aggressive corrosive environments, such as maritime/salt water climates, warm weather locations, and industrial areas. Other factors which accelerate corrosive attack are moisture condensates, high humidity, airborne chemicals, and atmospheric soil and dust. The combined effects of all are common in coastal regions of the southeastern United States, the Mediterranean area, the Middle East, the Philippine Islands and on board naval vessels. Aircraft which are frequently operated in such areas have been found to be more prone to fretting.
The tail and rear fuselage surfaces are especially prone to fretting damage caused by turbulent airflow from the prop wash and the wings wake.
Pulses from the propeller travel downstream, creating turbulence within the propeller slipstream. The prop wash has high velocity and is very turbulent, and thus beats the aircraft surfaces, causing the surfaces to flex.
The wing wake has a large influence on the airflow over the tail. As the angle of attack increases, increasingly turbulent air flows over the rear fuselage and the tail. Skin panel flexure becomes more pronounced as the aircraft proceeds to higher angles of attack.
The next time you fly, take a friend along, preferably a fellow pilot, and put the aircraft into slow flight. While your friend monitors the flight path, turn around and take a look at the tail surfaces. You will be surprised to watch the amount of vibration and skin flexing. Skin and control surfaces flex constantly during the flight, thus creating the very type of motions that are prime for causing fretting damage.
Structural members within the rear fuselage and the tail are very susceptible to fretting damage because of these motions and must be examined carefully during inspections. After its investigation into Piper Malibu accidents in the early 1990s, the NTSB recommended that the FAA should evaluate crown flush rivets where the rivets are subject to vibration from the effects of propeller slipstream impingement on the structure.
Design, Maintenance and Inspection
The most effective step in preventing fretting damage comes during the design state, particularly in specifying materials that are less prone to fretting damage. However, due to aircraft performance factors, aerospace materials are usually chosen for their high strength-to-weight ratio rather than for their corrosion resistance. Strong, heat-treatable aluminum alloys are very susceptible to various forms of corrosion, including fretting.
The susceptibility of aluminum alloys to corrosion can be partially deterred by chemical surface treatments and protection. However, added maintenance attention is required when using such a material, and some cleaning and repair methods can inadvertently increase the probability of fretting.
When parts are replaced, remanufactured or re-machined, the machining processes and heat treatments can develop substantial residual stresses in the surface layer. For example, aggressive grinding of a component creates residual tensile stresses within the surface layers. Tensile stresses within the surface layers allow cracks to grow faster at the surface. Because any cracks are being aided by the pulling apart action of the tensile stresses, the cracks can grow faster – and this directly effects the mechanical properties of the material. It can reduce the endurance limit by 35%, a significant drop in fatigue strength, according to one American Society for Metals study.
Processes that create residual stresses must be scrutinized and used only when necessary. If such processes are unavoidable, then shot peening is a method that can be used to reduce the residual stresses because the residual compressive stress generated in the surface helps to retard the propagation of fatigue cracks. However, heavy shot peening comes at its own price. It might produce surface damage that could outweigh the positive effect of compressive stresses.
All aircraft must be inspected for signs of corrosion and condition of protective coatings during scheduled inspections. These inspections must be conducted on a more frequent basis when the aircraft operates in more corrosive environments, notably under combinations of humidity, warm temperatures, and pollution.
Some locations on the fuselage should receive heightened attention. Some of the fillers used in the structure can absorb moisture. Careful inspection is required any time moisture-absorbing materials such as leather, paper, foam rubber, sound proofing and insulation materials are used. Structures surrounding doors, particularly landing gear doors, landing gear wells, wing skin adjacent to counter-sunk fastener heads, aluminum-faced honeycomb panels, wing-to-body joints, and structures susceptible to vibration and abrasion should receive particular attention.
Ask your mechanic if he has the equipment and skill to use aided visual inspection techniques. Fiber optic probes, magnifying glasses, mechanical probes, gauges, mirrors, etc. can all be used to detect flaws. More advanced non-destructive inspection techniques that have been used to detect fretting fatigue areas include X-ray, magnetic particle, fluorescent penetrant, or ultrasonic inspections.
Methods of Prevention
The best way to prevent fretting is to select materials during the design and maintenance process to preclude many of the commonly recognized corrosion/fatigue phenomena. Materials that show a tendency toward fretting action should not be chosen. Unfortunately, this isnt an option for most older aluminum airplanes.
Fibrous reinforced thermoset composites are now being used on several general aviation aircraft. Regular inspections must ensure than these are in good condition. Fillers and spaces in between these structures can sometimes absorb moisture, and so should be used with caution.
For the majority of the existing fleet, protecting the surfaces from corrosion is the best way to inhibit fretting. Fretting fatigue is less severe in the absence of oxygen and other corrosive agents. Therefore, isolation from corrosive agents may be an acceptable method of preventing more severe forms of fretting. It should be noted that precluding a corrosive environment will not entirely eliminate fretting, however it will significantly lower the rate of fretting.
Protective layers such as chemical coatings, films and paint provide protection from the corrosive environment and physically separate the metal alloy from corrosive agents in the environment, such as moisture and pollution. Protective surfaces are most effective when maintained intact. This becomes difficult when aircraft are operated in environments where foreign object damage can impact and crack the protective surface, thus allowing corrosive agents to seep under and attack the underlying metal. Protective surfaces should be maintained in clean and intact condition.
A corrosive attack can be minimized by reducing the amount of time that the corrosive agent remains in contact with the metal – and that means frequent washing. In addition, make sure paint or other protective layers are kept intact and clean.
Aircraft are washed for many reasons. Washing an aircraft cleans the surface of contaminants which reduces skin friction drag, allowing the aircraft to cruise at faster airspeeds and with lower fuel consumption. Washing can also clean the surface of corrosive agents, namely salts in the environment but also other agents. Washing also improves appearance and prepares the aircraft for maintenance and inspection of aircraft components during servicing.
But cleaning isnt enough. Some cleaning methods have been shown to make fretting worse. To help prevent damage, the airplane must be washed properly.
High velocity heated water cleans the surfaces by rapidly dislodging and dissolving surface dirt, oil, grease and other contaminants. Hot water reduces the viscosity of surface greases and oils, which in turn allows the chemical cleaning solvents to be more effective and improves the overall cleaning action.
Typical high pressure/hot water systems spray 200-degree water at 750 to 2000 psi. Some systems add cleaning solvents to make the pressurized warm water even more effective.
The problem with high pressure spray, however, is that they may force water, chemical solvents and contamination into areas that should remain free of moisture. High pressure water can also force dirt, contamination and moisture into bearings, bushed joints, actuator seals, and electrical connections. In addition, many cleaners have been found to be corrosive or abrasive if not thoroughly rinsed from the aircraft.
Major airline operators have reported corrosion and deterioration in roller bearing elements, TFE lined bearings and bushings, landing gear joints, electrical components and structural elements. Bearings are very susceptible to fretting, caused in part by maintenance procedures. Direct impingement of pressurized water, solvents, or foreign matter can force accumulations of dirt and contaminants into the joints, causing accelerated wear rates, a breakdown of the internal surfaces, and corrosion of the rolling elements. Many of these problems can be traced back to water or foreign material ingress, resulting in corrosion, abrasive wear and reduced service lives.
High pressure water can penetrate the seals on both relubable and non-relubable bearings. Retained water will start corrosion on the bearing surfaces. When water or chemical solvents enter a non-relubable bearing, the contaminant may remain trapped. It is very difficult to purge sealed bearings of this water. Boeing suggests purging the bearing with grease, though the bearing must be rotated to thoroughly coat the bearing members with grease and force the water out.
Boeing also has recommended avoiding direct spraying into joints and roller elements to avoid problems with abrasive wear, fretting and corrosion. The company further recommends that chemical cleaning solvents should carefully be applied and manually scrubbed, followed by a thorough rinsing with generous amounts of free flowing fresh water, preferably warm. Warm water helps dissolve residual dirt and foreign matter.
If you choose to use pressurized washing equipment, make sure you keep the nozzle at least three feet from the surface. Stubborn grease and dirt can be removed by further cleaning applications. Boeing cautions operators not to use the pressure spray nozzle to remove stubborn accumulations.
The landing gear and gear doors are susceptible to fretting damage that begins with contamination by dirt, dust, grime and pollutants that are kicked up from the runway. The problem is compounded by the wheels spraying moisture up into the gear well. This combination of dirt, pollutants, and moisture combines to form a grime layer which adheres to the surfaces and produces a very corrosive environment.
In addition, landing gear components are made from high-strength steels, which typically are sensitive to damage caused by sharp notches. Maintaining a protective surface on the parts is essential to preventing corrosion. One of the methods used to prevent corrosion is a thick semi-permanent film or coating of grease. This layer is applied at initial production and should be reapplied during regular maintenance intervals. Some parts of the gear are inaccessible once manufactured, and therefore rely on the one-time initial application of grease after manufacture. If this protective layer is compromised, it may not be discovered before damage has occurred.
Removing the old layer of grease therefore exposes the components to corrosive environments. A typical preflight inspection of the gear wells usually shows a thick, unsightly grease build-up. This unsightliness tempts technicians and some pilots to remove the build-up. However, often the grease build-ups prevent the joints from water ingress and corrosion. When the build-up is removed, relubrication after washing does not adequately replace the grease.
Excess grease should be removed by a brush, and the joints should be rinsed to remove any chemical solvent overspray, but it is very important that protective layers of grease remain intact. It is interesting to note that few operators cover-up corrosion-prone areas to prevent water ingestion or paint removal. Any pre-surface treatment and protection could be very cost effective in preventing unnecessary damage.
Although designers have the lead role in preventing fretting, in most cases this type of mechanical failure was not considered when older airplanes were designed. Therefore, it is incumbent on the aircraft owner, operator and maintenance technician to be especially wary about this form of fatigue.
Fretting is a form of long-term damage that owners and maintenance technicians must manage together. Finally, owners must examine their cleaning practices, specifically to avoid actions which may remove protective coatings and lead to accelerated fatigue and corrosion.
-by Patrick Veillette
Patrick Veillette is an ATP with more than 11,000 hours. He directs a research program studying human error in high-risk environments.