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Airplane Maintenance for the DIYer: Changing a Tire

Airplane Maintenance for the DIYer: Changing a Tire

In this article—the first in a series describing various preventive maintenance actions—A&P Jacqueline Shipe goes through the entire process of removing and reinstalling an aircraft tire. 

All aircraft owners will periodically have to replace a tire. Even planes that don’t get used much eventually require tire replacement due to dry rot and sidewall cracking of the rubber. 

Tire replacement is one of the items the FAA considers preventive maintenance that owners may legally perform on their aircraft. Changing a tire isn’t mechanically complex, but it does require the owner to use some caution. 

Proper aircraft jacking procedures have to be used, and if the plane is jacked outside, first check the weather conditions to ensure that the wind is not going to be too high. Major damage can be done to the airframe structure whenever a plane falls off a jack. 

Once the plane is properly jacked, the removal of the wheel assembly can begin. (For information about aircraft jacks and proper procedures, take a look at Shipe’s “Airplane Maintenance for the DIYer: First Steps” in the May 2016 issue. —Ed.)

If the plane is equipped with wheel pants, the wheel pant has to be removed first. This is usually a pretty straightforward process, just be sure to keep the removed screws and bolts identified as to which receptacle they came out of, because sometimes several different diameters and lengths are installed. 

Removing a main wheel

The next step is to remove the outboard brake backing plate (if the wheel is a main wheel) so that the brake disc will be free to slide off the axle with the wheel assembly. 

There are two or more bolts that connect the backing plates to the caliper that have to be removed. They may or may not have safety wire on them, depending on the design. 

Once the outboard brake is off, the next step is to deflate the tire by removing the valve core. It is important to do this before removing the big axle nut, because if any bolts holding the wheel halves together are loose or damaged, the wheel assembly could blow apart as the axle nut is loosened. 

Most axle nuts have either a clevis pin with a small cotter key or a single large cotter key to keep them from backing off. 

When the axle nut is removed, the wheel assembly will slide off. Some manufacturers employ a spacer that may or may not slide off with the wheel; care needs to be taken to ensure it doesn’t get misplaced.

Removing a nosewheel

Nosewheels usually have a removable axle that slides through the wheel and nose fork. This is generally held in place with a single long bolt and lock nut that secures two cup-shaped retainers. 

Nosewheels are fairly easy to remove unless the steel axle is corroded and frozen in the aluminum fork, in which case the axle has to be driven out. A wooden dowel should be used to drive out the axle because it won’t gouge or mar the aluminum fork.

 

Breaking the tire bead

Once the wheel is removed, the tire bead needs to be broken from the rim of the wheel halves. Get a piece of plywood to lay the wheel on (so it won’t get marred), then forcefully push the tire down all the way around. It will eventually pop down off the rim. This process has to be repeated on the reverse side as well. 

Some folks use flat blade screwdrivers or pry bars to pry the tire away from the wheel, but this can result in major scarring of the relatively soft cast aluminum that the wheels are made of. 

If the bead is really stubborn and just won’t break loose, you may have to enlist the help of a mechanic and a bead-breaking tool made specifically for the task.

Splitting the wheel and removing the tube 

Most wheel assemblies are two-piece, and the wheel halves are split by removing the through bolts and nuts holding them together. After separating the wheel halves, the tube can then be removed from the tire. 

Some mechanics replace the tube with every tire change, and some re-use the old tube as long as it looks good. Tube manufacturers recommend always replacing tubes when replacing a tire because they stretch while in use. 

Once an old tube is removed, it can be barely inflated—just enough to expand it a little—so the entire exterior can be inspected. Tubes with deep wrinkles or that have signs of damage or age, such as dry rot cracks around the valve stem, should always be replaced. 

When ordering a replacement tube, pilots may want to get the type that doesn’t lose air, such as a Leakguard or Airstop inner tube. They do reduce the frequency of having to air up the tires by quite a bit, especially if they are serviced with nitrogen instead of compressed air. 

Reinstalling the tube

The outside of the tube and inner part of the tire need to be coated in talcum powder before installing the tube. The powder keeps the tube from sticking to the sides of the tire and helps prevent chafing. 

The tube should have a balance mark on it. This needs to be aligned with the balance mark on the tire, which is generally a red dot. 

In the absence of a balance mark on the tube, align the valve stem with the red dot. This matches the heaviest part of the tube with the lightest part of the tire and makes it much easier to balance.

 

Reassembling the wheel

Once the tube is installed in the tire, the wheel halves (and the brake disc, if it is a main wheel) can be assembled together. Slightly inflating the tube a tiny amount helps to ensure it won’t be pinched between the wheel halves. 

Place the wheel halves together so the bolt holes align. The bolts can then be slid through and the washers and lock nuts installed. Lock nuts should have enough tension on them so that they cannot be tightened by hand, otherwise they should be replaced. 

The correct torque should be observed when assembling the wheel halves. These are made of cast aluminum and are strong, but over-tightening the nuts and bolts can lead to cracking. 

 

Balancing the tire

A wheel balancer is a fairly expensive tool to buy, and tires aren’t usually replaced on an individual airplane often enough to merit owning one for most folks. Large imbalances in a wheel assembly can be detected by mounting the wheel on the axle and installing the axle nut, but leaving it slightly loose so the wheel rotates freely. 

Once the tire is spun a few times, if the same spot always ends up coming to rest on the bottom, this indicates an imbalance and weights will need to be added to the light side. Using stick-on lead type weights purchased from an aviation parts warehouse or automotive store, add enough weight so that the wheel comes to a stop in random places as it is spun freely on the axle. The balance is more critical on the nosewheel assembly because it will cause a shimmy if there is even a slight imbalance. 

 

Reinstalling the wheel on the aircraft

Once the wheel is balanced, it is ready for installation. The axle should be wiped off and greased, and any corrosion should be removed with an abrasive cleaning pad. 

A general-purpose Scotch-Brite 7447 pad (maroon color) works well and can be purchased anywhere automotive paint products are sold. These pads are abrasive enough to clean off rust, but not so abrasive so as to scratch the metal. Use elbow grease to scrub the axle until it is shiny.  

The nosewheel axle is prone to rusting internally. Any rust should be removed here too, and the internal part of the axle should be either painted with a rusty-metal primer, or coated in LPS 3 or other corrosion-inhibiting compound. 

During the final installation, the axle nut needs to be tightened enough so there is no free play detected as the tire is grasped and pushed inboard on the top while pulling outboard on the bottom, or vice versa. (This checks for side-to-side free play, and there should be none.) 

The wheel should spin somewhat freely, but there needs to be a slight amount of tension on the axle nut. If the nut is too tight, the wheel won’t spin much at all by hand, and the wheel bearings will be more likely to fail from having too much of a pre-load placed on them. 

Once the correct tension is achieved, align the cotter key hole in the axle with the opening in the nut and install the cotter key. Be sure to bend the edges of the cotter key in such a way that they won’t get entangled in the valve stem or rub on the wheel bearing retainer. 

After the wheel is secured in place, the valve stem should be removed and the tube inflated and deflated two or three times to remove any wrinkles. Then the valve stem can be reinstalled and the tire inflated to its proper pressure. (Correct tire pressures are found in the POH.) 

After the outboard brake and/or wheel pant are reinstalled, the tire change will be complete. The aircraft owner will also have the satisfaction of having completed the work himself (or herself)—and will have hopefully have saved a few bucks in the process.

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe attended Kentucky Tech to obtain her A&P license. She has worked as an airline mechanic and on a variety of General Aviation planes, and has logged over 5,000 hours of flight instruction time. Send question or comments to .

RESOURCES >>>>>

Leakguard butyl inner tubes by Aero Classic
Aircraft Spruce & Specialty
Airstop® aircraft inner tubes
Michelin North America Inc.
LPS 3 Premier Rust Inhibitor
ITW Pro Brands
Wheel Bearing Service: Why & How

Wheel Bearing Service: Why & How

A&P Jacqueline Shipe describes how to service wheel bearings in this article, the second in a DIY series for pilots who wish to take on preventive maintenance of their aircraft.

FAR 43 Appendix A lists the preventive maintenance items owners may legally perform on their planes. This list is fairly long—and some of the items are a little involved for a person to perform the first time by themselves, while other tasks on the list are pretty straightforward. 

There are several preventive maintenance tasks pertaining to the landing gear, including tire changes, strut servicing and servicing the wheel bearings. (Last month, Shipe discussed the steps involved in changing an aircraft tire. See the June 2016 issue for more information. —Ed.) 

Bearings: small but mighty

While cleaning and greasing wheel bearings doesn’t seem like too difficult a task, there are some guidelines that need to be followed. The failure of a wheel bearing can cause major damage to the wheel and can even allow the wheel assembly to slide off the axle.

Wheel bearings are relatively small, but are incredibly strong. They have to support the weight of the plane while allowing the wheel to spin freely in all types of temperatures and conditions. In addition, wheel bearings and races on airplane wheel assemblies also have to be capable of withstanding hard landings and both vertical and horizontal loads without failing. 

Types of bearings

The bearings on most airplane wheel assemblies are the tapered roller-type. The outer part of the bearing is larger than the inner part, and the rollers are installed at an angle. 

The bearing itself rides in a metal cup called a race. The race has a “pressed in” fit in the wheel half, and is tapered on the inside to match the bearing. The biggest advantage of tapered bearings is the high load capacity that they can withstand. 

Automotive wheel bearings, on the other hand, usually use spherical rollers (i.e., balls). Ball bearings can withstand prolonged high speeds without building up too much heat, but cannot take high impact loads. 

Tapered bearings will bear up under the not-so-good landings that occur from time to time with an aircraft. In addition, proper servicing of these bearings will keep the wheels spinning freely and will last for a long time. 

Removing the clips

Once a wheel assembly is removed from the axle, the wheel bearings are easily removed by taking out the metal retaining clips that secure the bearings and grease felts. 

There is an indention in the outer part of one end of the clip to allow a screwdriver to be used to pry it out. The clips don’t have a lot of tension on them and can be easily removed. 

Once the clip is off, the bearing, metal rings and grease felts can all be lifted out together. 

Be sure to keep all the rings and clips organized so they can be reinstalled into the same wheel half and in the same place. The metal rings that retain the bearing are sometimes slightly smaller on the outer half than the double rings used on the inner half, and can be easily mixed up. 

Cleaning the parts

A small bucket with 100LL Avgas works well to clean the bearings. Swishing the bearing around and spinning it by hand while it is submerged will clean all of the old grease and gunk out. 

The metal rings and clips should also be cleaned, but the felt material needs to be set aside; it should not be submersed in anything. There is really no way to clean the felt, anyway—as long as it is still in one piece, it’s good to go. Any grease felt that is torn or missing a section needs to be replaced. 

Once all the parts are cleaned, they should be blown out with compressed air (if available) or laid out on paper towels to dry. The parts need to be thoroughly clean and dry before fresh grease is applied. Inspecting the parts

After the bearings, metal rings and clips are clean and dry, the bearing and race should be inspected for pitting or damage. If the race is smooth and has no corrosion, the bearing is generally corrosion-free as well. 

Races that have light surface corrosion can sometimes be smoothed out with a piece of light grit sandpaper (800 to start and 1200 to finish). Deep pits in a race mean replacement is needed. 

Discoloration on the bearing or race, such as a rainbow or gold color, can be a sign that these parts have generated excessive amounts of heat, in which case they should be replaced.

Preventing corrosion

Wheel bearings typically fail for two reasons: corrosion or overheating. 

The greatest threat to airplane wheel bearings is usually corrosion. Almost all bearings and races will eventually require replacement due to water getting past the grease seals and accumulating in the bearing cavity, causing rust and pitting. 

When cleaning a plane, strong degreasers should not be used on wheel assemblies and wheels should never be sprayed with a water hose. The pressurized water will get past the grease seals and ruin the bearings. 

Folks that want their wheels clean can wipe them out with a rag that is lightly moistened with a little Gojo original white cream hand cleaner (the non-pumice kind). Then the wheels can be wiped clean with a dry rag. 

 

Replacing the races

Wheel bearing replacement is easy, but replacement of the races is a little tough to do without the proper tools. 

Because the race has a pressed-in fit in the wheel half, it has to be driven out. This can be accomplished by using either a hammer and punch or a bearing driver tool. 

Occasionally a person encounters a wheel assembly with a race that has broken loose and is spinning in the wheel half itself. In this case, the wheel assembly has to be replaced; there is no permanent way to hold the race in place if the wheel assembly has lost enough metal that the race is no longer fitting tightly. 

The wheel is made of cast aluminum. When reinstalling the steel race, it is very important that it be driven in straight. If it gets cocked—even a little—the much softer aluminum will be gouged and damaged. 

The best tool for the job is a bearing driver, as it allows each blow of the hammer to be applied equally around the circumference of the race. 

Once the race is almost near the bottom of its recess, very light blows should be used to seat it in the wheel half. Many mechanics have driven the race in too far and cracked the fairly thin aluminum ring that retains the race. 

The wheel should always be thoroughly inspected for any sign of cracking on the front and back sides, whether or not a race is replaced.

 

Packing the bearings and reinstalling

Once all of the races are installed and the wheel halves are inspected, the bearings are ready to be packed and installed. A high-quality wheel bearing grease that has good water resistance should be used. 

The grease has to be pushed up through the bearing until it comes out the top between each roller. If it doesn’t squeeze through each opening, the inside of the bearing will have gaps and inadequate lubrication. 

It takes a little while to pack a bearing by hand. There are bearing packers sold in almost any automotive store that make the job a little faster and a little less messy. 

Once the bearing is packed, apply a layer of grease to the entire surface of the race to ensure it is covered as well. 

The bearing can then be reinstalled along with the correct order of retaining rings and grease felts. 

Lastly, reinstall the clip. It is a good idea to make sure the clip is pressed into place all the way around by pushing it outward with a screwdriver. 

After all the clips are in, the wheel bearing service is complete.

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

 

Strut Servicing: The Ins & Outs

Strut Servicing: The Ins & Outs

In the third article in a DIY series for pilots, A&P Jacqueline Shipe goes through the steps an owner can take in order to properly service the struts on their aircraft.

Among the preventive maintenance items listed in FAR 43 Appendix A that pilots may legally perform on an airplane that they own is strut servicing. 

The struts on any airplane serve a critical purpose. They provide the shock absorption necessary to prevent the airframe structure from enduring too much stress from the impact loads incurred on landings. 

Even taxi operations impose stress on an airframe every time the gear hits a bump or uneven surface. 

The strut absorbs the bulk of these loads and prevents them from being transmitted to the airframe. 

Types of struts

There are several different kinds of struts used for shock absorption. Over the years aircraft manufacturers have used different materials to limit the stress from the impact of landing. Some have used rubber biscuits, bungee cords and spring steel. 

The most common type found on most planes (and the only type used on fairly heavy planes from light twins all the way up to airliners) is the hydraulic air/oil cylinder, also referred to in some manuals as oleo struts. The oleo strut is very reliable, can withstand tremendous loads and is fairly simple in its design. 

The oleo strut uses air pressure and hydraulic fluid to create a spring effect. The strut consists of an outer housing called a cylinder and an inner piston that is connected to the nose fork or to the main wheel axle. The piston portion of the strut is the part that actuates up and down. 

There are different styles and configurations, but all struts house hydraulic fluid in the lower section of the strut and compressed air (or nitrogen) in the upper section. As the piston is driven into the cylinder upon landing, the fluid is forced through an opening called an orifice that slows the rate of the flow. 

Some manufacturers make use of a metering pin connected to the piston. The pin is mounted so that it is forced upward through the orifice along with the fluid. It protrudes up through the orifice, is slim in the middle and wider on both ends. 

Its shape is tapered so that as the piston reaches the top portion of its travel, less and less fluid can fit through the opening. This gradually slows the fluid flow and decelerates the piston. Meanwhile the pressure of the compressed air is being steadily increased as the piston travels upward and reduces the volume of space in the upper chamber. 

Eventually the increased pressure of the compressed air overcomes the decreasing fluid pressure and forces the piston to extend. As the fluid flows in the other direction, its flow is impeded at a steady rate by the opposite end of the metering pin, gradually slowing the fluid flow in the opposite direction. This results in dampening out any oscillations and returning the airplane to its normal static height above the ground.

Some models don’t use metering pins but have metering tubes with various sized holes in them that slow the rate of flow as the piston reaches either end of its travel. Some manufacturers don’t use either metering pins or tubes, but instead use restrictor plates with orifices in them to produce the same effect. 

Fluid and air: both are vital 

On any model, the strut has to have the correct amount of fluid and air to work properly. The fluid used for strut servicing is MIL-H-5606 (red) mineral-based hydraulic fluid. 

5606 is sold by the gallon and in quarts. It is nice to keep a supply on hand not only for struts, but also to refill brake and gear reservoirs. Typically it takes around a gallon of hydraulic fluid to service three struts.

Nitrogen is better than compressed air for strut servicing because it is drier and doesn’t vary in pressure as much as air; it is also less corrosive to the inside of the strut housing. 

However, nitrogen is not always readily available. A person needs a regulator and high-pressure hose in addition to a nitrogen bottle, and the cost for all the items can exceed $500. 

If nitrogen is not available, air pressure from a standard air compressor is usually sufficient to air up a nose strut. Nose struts don’t require as much pressure as main struts. 

Typically the main struts on Piper aircraft require at least 200 psi to inflate the strut to its proper level. Most standard air compressors don’t get that high. There are boosters that a person can purchase for around $200 that will increase compressor air to a high enough level to inflate the struts.

It would be best on a twin Piper (or any twin, period) to always use nitrogen, due to the increased weight of a twin engine plane and the more extreme temperature changes.

Servicing a strut

The tools a person needs to service a strut include about three feet of clear flexible tubing with a ¼ inch (inside diameter) opening to fit over the Schrader valve; a valve stem tool; and an empty gallon size container to catch the old fluid. 

To properly service a strut with fluid and air, the airplane needs to be jacked, or at least have the nose raised if only the nose is being serviced. 

With the airplane jacked, remove the valve stem slowly from the filler valve in the top of the strut. It is best to loosen it enough to release the air pressure, and then remove it the rest of the way after the pressure has bled off. A small spray of hydraulic fluid comes out with the air pressure, so it’s a good idea to have a rag handy. 

Once the valve stem has been removed, push the tubing over the open Schrader valve and insert the other end into the empty container. Next, push the strut up to its fully collapsed position. Any old fluid will be shoved out. 

Then remove the container with the old fluid and insert the hose into a can with at least a half-gallon of clean new hydraulic fluid. Next, pull the strut down to its maximum extended position. The suction will pull in the fluid; it will continue to siphon for a few seconds after the strut is fully extended. 

Next, slowly push the strut up to its fully collapsed position. As some of the fluid is pushed back out, air bubbles will come out too. 

Extend the strut again, and repeat the process until all of the fluid comes out as a solid stream on the compression stroke. 

Once all of the air bubbles are removed, the strut will be considerably more difficult to push up to its collapsed position. Once the strut is fully collapsed, the hose should then be removed from the valve and the valve core reinstalled. 

This process is called bleeding the strut, and it’s the only way to get the proper amount of hydraulic fluid into the inner chambers of the cylinder. 

There is no way to simply pump a little fluid in to the strut; the strut must be filled using this bleeding process. If the process isn’t followed, large air pockets in the lower chamber can cause the strut to collapse under a load. 

Once the strut is filled with fluid, it can then be aired up with either nitrogen or compressed air through a strut booster. 

After the airplane is lowered off the jacks, the final adjustments can be made by releasing a little of the pressure by depressing the valve core for a split second at a time. 

Generally main struts should be inflated so that around five inches of the piston is exposed, and nose struts to around four inches. 

The exact range for each model can be found in the service manual. The struts should be inflated so that they are within the proper range even when the airplane is fully loaded. 

 

Troubleshooting struts

Properly serviced struts should have a certain amount of buoyancy about them. 

Struts that are filled with air pressure but are low on hydraulic fluid tend to stick in place. Struts that stay extended for a period of time after a plane has landed and then suddenly collapse are also typically low on fluid. 

Any sort of a knocking noise from the nose strut during taxi operations or upon landing is an indication that it is bottoming out due to it being low on fluid, air, or both.

If a strut is low on fluid, it is usually because the rubber seals have gotten old and hardened. There is generally a rubber wiper ring and a large rubber O-ring with one or more backup rings in the strut housing. These rings harden and become brittle over time, especially in cold weather. 

Granville’s Aircraft Hydraulic and Strut Sealant is an FAA approved product that can be mixed with hydraulic fluid and added to the struts during the servicing process. It doesn’t cause the seals to swell, but it does cause them to soften and become more flexible, much as they were in their original state. 

This additive works well, once enough of it gets into contact with the seals. After it is first added, the strut may still go flat a time or two and need to be re-aired before it finally holds. 

Properly serviced struts help to soften landings and prevent damage to the airframe, and keeping the struts in good shape will pay off big in the long run.

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

RESOURCES >>>>>

Appendix A to Part 43, “Major Alterations, Major Repairs and Preventive Maintenance”
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