Good maintenance practices and preventive care on wheels and brakes seem to be one of those neglected tasks. David Schober gives a refresher course on the inspection of wheels and brakes.

As early as 1933, the Department of Commerce recognized that the certification of certain parts should be done independent of the certification of an entire aircraft.

Aircraft wheels are one class of parts that were identified in Aeronautical Bulletin 7F, which was re-codified in 1937 into CAR 15. It is interesting to note that brakes were not considered in Aeronautical Bulletin 7F, yet their design was included in CAR 15. Tailwheels were specifically excluded from CAR 15.

An amendment to CAR 15 in 1942 added further requirements for brakes and specifically called out requirements for transport category aircraft that had not been present up until that time. Finally in 1952, CAR 15 was rescinded and the current Technical Standard Order system in place today was established.

TSO C26 was the original order, and this was revised several times over the years. Finally with TSO C26D in 2004 it was limited to Part 23, 27, and 29 aircraft. In 2004, TSO C135 was developed for wheel and brake assemblies for transport category aircraft. Why spend time on this trivia, you ask? In order to properly maintain an item, we need to know its certification basis so we can have a clear understanding of the requirements for its installation and proper operation.

As we see above, brakes weren’t required for Aero Bulletin 7 aircraft, and specific braking requirements for CAR aircraft weren’t put in place until 1937, and then only for transport category aircraft. FAA’s current TSO system went into place in 1952 and transport category aircraft for which TC application is later than 2004 will require wheels and brakes certified under TSO C135, not TSO C26C or TSO C26D.

Enough with the history lesson, what about my wheels and brakes? Maintenance on wheels and brakes is usually quite straightforward. I guess that’s one of the reasons that good maintenance practices and preventive care on wheels and brakes seem to be one of those neglected tasks or tasks done improperly. "It’s so easy, why get the book out" is a common comment from mechanics when asked about the reference used for maintenance and inspection of the wheels and brakes on a given aircraft. It may be simple, but there are important steps that can be overlooked.

The wheel and brake assembly needs to perform several functions. It needs to support the weight of the aircraft, and transfer all the loads of the airplane during landing, taxi, and takeoff.

The brake assembly needs to convert the kinetic energy of the aircraft’s movement into heat energy from friction as the brake is applied, and the wheel needs to be able to dissipate that heat energy back into the atmosphere or through the bearings to the airframe.

The bearings need to provide low friction to allow the aircraft to roll freely across the ground and allow the heat transfer from the wheel to the axle.

Safety

Whenever removing wheels, a few words of caution. First, be sure the aircraft is properly jacked and won’t fall. This may sound silly, but you would be surprised at the number of insurance claims from holes in wings where the airplane came off the jack and damaged the aircraft.

Second, before removing an axle nut, remove the air from the tire. Third, after reassembling the tire, inflate it in a safety cage. Even a low-pressure tire on a small GA airplane can have fatal consequences if it fails and you are hit by the pieces.

Cleaning

Wheels and brakes by their very nature get dirty. That dirt can hamper performance, and hide flaws that might otherwise cause the wheel or brake to be rejected. Proper cleaning is important to allow a thorough inspection and the replacement of any needed corrosion protection.

In many cases the alloy of the wheel assembly will dictate the cleaning method employed. Be sure that the proper wheel part number is identified and follow the manufacturers instructions for cleaning.

Any time you are using chemicals, be sure that you use the appropriate personal protective equipment. This may include gloves, goggles, apron and respirator.

Inspection

When performing an inspection on a wheel assembly or brake unit, be sure to do it properly. Corrosion is one of the worst problems with these pieces of equipment.

Many of the wheels and brakes used today are magnesium or aluminum alloys. They are in close proximity to the ground and subjected to moisture and abrasion from their operating environment.

Over the years, I’ve seen wheels with holes corroded through, brake units with so much corrosion that the only thing holding the pins in place was the grease and dirt, and disks with pitting so deep that it was a wonder that the pads would last longer that one or two landings.

Each time a wheel is removed, it should be inspected carefully for cracks, deformation, and corrosion. Any surface corrosion on the wheel should be cleaned up and the wheel re-treated according to the manufacturers recommendation.

Keep in mind that the corrosion protection applied to an aluminum wheel is not the same that should be applied to a magnesium wheel. For cracks and deformation, the wheel should be discarded and a suitable replacement installed.

While inspecting the wheels, look at the brake disks and measure the thickness, inspect for cracks around the weld area, and check for flatness of the friction surface.

If the disk is undersized, beyond allowable tolerance for flatness, or pitted, replace it. Inspect the bolts closely. Any signs of corrosion or cracks should be cause for rejection. Finally, look at the nuts. Self-locking nuts have a minimum torque requirement for reuse. If the nuts that hold the wheel halves together don’t meet this minimum torque value, replace them.

Anyone working on Cessna single-engine aircraft with McCauley wheels, keep in mind that Cessna no longer supports these wheels with new parts. When a damaged McCauley wheel is identified, the replacement should be done in accordance with SEB 00-5, which identifies a Cleveland kit for your aircraft. If one wheel or brake is bad, you have to replace the wheel and brake unit on both sides of the aircraft!

Brake inspection should include the disk inspections listed above, pad thickness, and cylinder and back plate assemblies and pressure plate. Signs of weeping at the cylinder can usually be repaired by replacing the O-rings. The cylinder assemblies should also be inspected for cracks where the pins attach and for corrosion, both external and internal. Again, any surface corrosion can be cleaned and the cylinder assembly re-treated. Pads can be replaced provided that the back plates haven’t been damaged from over-torquing. If this is the case, the back plates will also need replacement. Pressure plates need to be inspected for flatness. They can usually be straightened, but may need to be replaced.

As the size and speed of an aircraft increases, so do the stresses on the wheel and brake assemblies. Many corporate-sized aircraft and almost all jets have very specific NDT inspection requirements on their wheels and brakes. If your shop is not equipped for these functions, please send the wheels and brakes to a vendor that is properly equipped and qualified for that inspection.

Maintaining wheels that require NDT inspections is beyond the scope of this article, while many of the techniques and items here are applicable, these wheels and brakes need to be maintained in accordance with the continuous airworthiness program for the aircraft involved.

Corrosion Prevention

Following cleaning and inspection, be sure that all parent metal surfaces are treated with the appropriate coatings. Keep in mind that for many wheels, the simple change of a letter in the designation may change the alloy and the appropriate treatment. Again while using any chemicals or paints, employ the proper personal protective equipment and follow the appropriate environmental regulations while using any paint products.

Reassembly

Following the inspections and replacements or repairs outlined, reassembly of the wheel can begin. If you are dealing with a tube type tire, be sure not to pinch the tube during assembly. If it’s a tubeless tire, don’t forget the O-ring. If replacing the tire, always replace the tube. Install the tube and tire with the balance marks in the appropriate location (red dot on tire with yellow stripe on tube or aligned with the valve stem) and insert the bolts in the proper orientation. Install the washers and nuts as appropriate.

Be sure to read the manufacturers documentation regarding any required thread lubricant and determine the proper torque for the fasteners. Bring all the fasteners down snug before torqueing to the final setting.

Once the wheel is assembled, inflate it in a tire cage and follow the manufacturers recommendation for dwell time before rechecking the pressure. Finally install the assembly back on the aircraft.

Wheel Bearings

Wheel bearings need to be cleaned and inspected each time the wheel is removed. Check for signs of water spots, corrosion or brinelling. Any of these are cause for rejection.

Bearing races should be removed and installed using a punch designed for that purpose.

Use the type of grease specified by the manufacturer. Grease is a very poor heat conductor, so applying too much will reduce the ability of the bearing to transfer the heat from the wheel to the airframe.

Cleanliness is extremely important. The grease seals should also receive your attention. Those felt seals are supposed to have grease on them, not be dry. Part of their job is to keep moisture from the bearing and inside of the wheel. The more moisture, the greater the possibility of bearing and wheel corrosion.

Seaplanes

When working on seaplanes, corrosion is always an issue. An amphibian that spends most of its time in the water will have lots of problems with wheel bearings.

I don’t think anyone has made grease that won’t wash out after just a couple days in the water. I recall changing bearings almost every other week on one airplane years ago.

Cirrus

On the SR20 and SR22, the wheel fairings are tight, the tires are small, and the airplane has a lot of kinetic energy coupled with differential brakes for steering. There have been a couple SBs issued and an AD dealing with break maintenance. On these airplanes it is recommended that the caliper O-rings be replaced every 100 hours. They also have a temperature indicator on each caliper and if the indicator shows an over temp condition, replace the O-rings.

Mechanical Brakes

On all cable-activated mechanical brake installations, inspect the cable assembly closely. Most of these systems have the cables going around pulleys with small diameters and the possibility for wear and fraying is great. Loss of a brake with a tailwheel airplane is no fun on pavement!

Another problem with older aircraft is finding the technical publications that deal with the wheels installed. Several suppliers on the Internet make reprints of many of these old manuals.

Another source is "The Mechanic’s Toolbox" from Sacramento Sky Ranch, which includes data for Goodyear wheels and brakes along with info for Scott and Maule tailwheels.

Brake Conditioning

All new brake pads will require conditioning. If not done properly, the pads will wear much faster than they should, and they will not provide the required friction to stop the airplane under a max breaking event, or may not hold it for the pre-takeoff run up. Follow the manufacturers instructions for the type of pads installed.

Chrome Disks

Chrome-plated disks may look nice when first installed, but keep in mind that the plating is very thin. The brakes function by forcing the pads against the disks and converting the mechanical energy of the rotating wheel into heat energy from friction. You have to ask yourself, how long will that chrome plating remain on the disk, and is it worth the additional cost?

Stainless Disks

Years ago when asbestos linings were still being used, several companies obtained PMA approval for stainless steel brake disks. Asbestos linings are no longer available and stainless steel disks are not compatible with the current organic lining materials in use today.

Stainless has a lower heat coefficient than carbon steel and hence the heat transfer capability of a stainless disk is lower. Simply put, a stainless disk will have less braking capability than the same size carbon disk.

If you are working on an airplane with stainless disks, and are installing organic linings, you have a potential problem. I would recommend contacting the disk manufacturer to determine your course of action.

PMA Brakes: Saving Time and Money With the costs to keep an aircraft flying constantly on the rise, operators are always looking for ways to save time and money. One avenue for lower costs is parts manufacturer approval (PMA) components and systems. Brakes fall into the category of parts that are replaced fairly frequently, and PMA replicas are available from a variety of suppliers. Rapco of Hartland, Wis. makes PMA brake discs, brake linings, vacuum kits, de-ice components, filters, gaskets and other parts for various GA aircraft under 12,500 lbs. Its sister company, Rapco Fleet Support, supplies PMA brakes and fuel pumps for corporate and commuter aircraft. John Wicht, FAA project manager for Rapco, says that PMA parts "are being used just as much if not more now than they ever have been." "With fuel prices going through the roof, owners and operators are looking for any way possible to save some money and keep their airplanes flying," he continues, adding that the sales potential for PMA components is "limitless" with everything that’s happening in today’s market. PMA parts have always been fairly popular because "any time you can offer a customer an aftermarket solution that’s cheaper and better in terms of performance and also quality, you almost have an unfair advantage in the market," Wicht explains. In some cases, OEMs are no longer interested in supporting older aircraft as they age, and don’t stock parts for older airplanes, creating lead times of up to 6 or 8 weeks to get certain parts. Wicht notes that as an aircraft mechanic, when he sees an airplane sitting in the shop "and the owner wants his airplane back to go flying and you can’t get a part, that’s a problem. All it takes is that one time that the OEM didn’t have the part," he continues, for many operators to begin seeking an alternative supplier. According to Wicht, Rapco’s PMA brakes last longer — anywhere from 10 to 15 percent extended life — and are cheaper than their OEM equivalents. In addition, the company uses a worldwide network of distributors to make parts available to an operator in a short timeframe. If by chance one of the distributors does not have a part in stock, then Rapco will drop-ship it to the operator, he explains. Rapco is able to offer more reliable, less expensive products because of modern materials technology — relying on higher-grade steel for PMA brake discs, better additives, and less fillers in the friction material — more efficient manufacturing and quality processes, and greater expertise about particular parts that has developed over time. By using the company’s in-house fabrication inspection system, each PMA part is examined by a Rapco inspector. Another factor that comes into play is lower overhead costs versus the OEMs. Per FAA regulations, Rapco uses modern practices to verify that its PMA brakes are "equal to or better than" OEM equivalents. After a TSO test, the company performs checks on the OEM brakes and their PMA replicas to ensure they meet or exceed the minimum requirements needed for the application. But, Wicht says that after working as a project manager for five years involving brakes and other parts, "I can tell you the FAA will not sign off on equal to. You need to demonstrate that it’s better than." After the TSO testing is complete and accepted by the FAA, Rapco conducts a flight test for the proposed PMA brake components. But again, the company must demonstrate that the components are "equal to or better than what the aircraft was originally type-certificated with," Wicht says. "You need to be very close [to the original part] and have better stopping distances, etc. [with brakes]," he adds. — Andrew D. Parker