August 2005-
Have you ever wondered where those, one-eyed brown gremlins, commonly referred to as rust or oxidation come from? These growing creatures can destroy an aircraft faster than a student pilot on his maiden flight.
Corrosion, to quote Advisory Circular 43-4A, is the “electrochemical deterioration of a metal because of its chemical reaction with the surrounding environment.”
Corrosion will occur more often in moist than dry climates. In the United States corrosion areas are mild to moderate along the West Coast. The Gulf Coast states along with the Southeastern coastal states range from severe to moderate. Northeastern states range from extremely severe to severe. Parts of the Midwest range from mild to negligible. The rest of the United States is negligible in corrosion.
One of the many reasons the boneyards are located in the desert is because that is where the humidity is well below 60 percent, where aircraft are less susceptible to corrosion.
Frequency of inspection required depends on the environment (severe, moderate or mild) of operation.
Corrosion comes in several different flavors of destruction as listed below:
• Uniform Etch Control
• Pitting and Crevice Corrosion
• Intergranular Corrosion
• Exfoliation Corrosion
• Galvanic Corrosion
• Concentration Cell Corrosion has three general types:
Metal ion concentration cells
Oxygen concentration cells
Active-passive cells
• Filiform Corrosion
• Stress-Corrosion Cracking
• Fatigue Corrosion
• Fretting Corrosion (also known as the “Smoking Rivets”)
• Weld Corrosion
• Micro-Organisms
There are other types of corrosion that are not listed above.
Inspecting for corrosion is accomplished through Nondestructive Inspection (NDI), which can be performed by one of the following methods:
• Visual
• Liquid Dye Penetrant
• Magnetic Particle
• Eddy Current
• Ultrasonic
• X-Ray
Trained personnel are required to perform these types of NDI testing. For purposes of this article, we will stick to visual inspection because this one can be performed during the most import part of flying—preflight.
While doing our walkaround looking for strange things like bird nests in the engine compartment(s), field mice in the vent openings, bats hanging from the wing struts or a platypus attach to the belly of the aircraft, we need to keep an eye open for corrosion. This is a good reason to keep the aircraft thoroughly clean and lubricated.
Here are some of the revealing signs we should be looking for:
• Corrosion
• Cracks
• Condition of finishes or painted surfaces:
• Damaged
• Chipped
• Missing
• Lift or bumps under the paint
• Faying
• Blistering of paint
• Dished and popped rivets
• Skin bulge or lifted surfaces
• Discoloration
If one of these types of corrosion is detected, bring to the attention of a trained professional, which is required for removing and treating corrosion.
There are several locations on an aircraft where corrosion can be visually spotted as listed:
• Engine exhaust area(s)
• Engine mount structure
• Engine frontal areas
• Intake vents
• Access panels
• Battery compartments and vents
• Wheels wells and landing gears
• External skin areas
• Window seals
• Cavity drains
• Piano hinges
• Control surface recesses
• Bilges
• Control cables
• Welded areas
• Electronic equipment
Check all areas which do not require removal of fasteners, panels, etc. Preventive maintenance or early detection and treatment reduce cost, out of service time, and possibility of flight related incidents. For further, and more in depth information, I highly recommend reading AC 43-4A. Happy Corrosion Hunting.
Norm Ellis has is an instrument-rated private pilot, flown 24 different aircraft, has been a Mechanical Design Engineer Consultant for the last 16 years. Ellis holds three Multi-STCs certified on 118 aircraft. Send comments or questions to editor@www.piperflyer.com.
For More information about protecting your plane from corrosion:
Corrosion Technologies
800-638-7361
he �ig� �e of, do not remove them from the oil cooler when sending it out for overhaul. The best advice is to leave fittings undisturbed unless absolutely necessary to do otherwise.
WHEN BACKUP IS CALLED FOR
One other extremely common—and entirely avoidable—form of oil cooler injury is caused by not using a backup wrench when dealing with fittings and oil line connections. Each oil cooler manufacturer has its own uniquely-shaped boss for the threaded holes that the fittings screw into. Most of them are hex or square in shape so that it is possible to place a wrench on them. Except for Harrison oil coolers, which have a round boss that requires the use of pliers or a pipe wrench.
In every case, failing to steady the boss on the oil cooler when attempting to connect or disconnect the oil lines or the fittings from the cooler will allow the cooler’s threaded boss to bend from side to side and cause the oil cooler to crack and leak.
AND FINALLY
As with most other things, when it comes to aircraft oil coolers the commonly made mistakes are also the easiest ones to avoid—if you know what to look for and what to look out for. Remember, a fluid ounce of prevention is always better than 11 quarts of cure.
Wayne Thomas is a longtime member of the aircraft maintenance industry and has written a number of articles about issues and procedures concerning aircraft engine maintenance which have appeared in publications such as TBO Advisor and AirMaintenance Update. He soloed in 1973 at age 15 after receiving just 12 hours of dual instruction at El Monte Community Airport’s flight school. Wayne joined Pacific Oil Cooler Service, Inc. of South El Monte, Calif. in 2000.
