Airframe and Powerplant

A little TLC is required on braking systems

Airplane brakes must be powerful enough to prevent airplane movement during pretakeoff magneto checks, complement the nosewheel steering when tight turns are required, work reliably in all weather conditions, be inexpensive to maintain, and be completely dependable. Perhaps surprisingly, modern light-airplane braking systems are up to the task.

Modern light-airplane braking systems In the mid-1950s, Goodyear introduced the disc (or spot) brake. The term "spot brake" comes from the fact that spot brakes rely on a hydraulically actuated caliper — usually referred to as a "wheel cylinder" — and a pair of small, easily replaceable pads to apply friction to the rotating wheel-mounted disc. They were a big step forward because they were light, easy to service and maintain, and powerful.

The Goodyear spot brake was the leading brake system until the mid-1960s. The brake caliper on the Good-year system is bolted to the landing-gear leg. The system automatically adjusts for wear by allowing the wheel-mounted disc to float in the wheel, to slide in and out as necessary for proper alignment with the caliper and pads. The connection between the disc and the wheel depended on teeth around the outer perimeter of the disc that fit loosely into a set of teeth on the inner wheel half.

With time, the teeth on the steel disc caused wear of the teeth on the aluminum wheel. When the teeth were worn beyond a certain point the potential for cocking and jamming increased. Sometimes the wear was so great that a heavy brake application would shear the teeth off the aluminum wheel half.

The discs were prevented from sliding completely out of engagement with the inner wheel half by three or four spring metal clips held in place by spring buttons. Goodyear stopped making replacement wheels years ago. A few companies still sell clips and brake pads, but they are outrageously expensive — a complete set of pads and clips for a Goodyear-brake-equipped Cessna 182 costs more than $1,600.

The industry standard

In the early 1960s, Cleveland Wheels and Brakes, now a division of Parker Hannifin Corp. and renamed Aircraft Wheels and Brakes, introduced a better disc-brake design. Cessna began changing from Goodyear to Cleveland as early as 1963. Cleveland brakes became the standard light-airplane braking system, and they still hold that position today.

Instead of compensating for brake-pad wear with a floating disc-wheel connection, Cleveland bolts the disc solidly to the wheel and permits the anchor bolts of the brake caliper assembly to slide in and out of bushings on the torque plate. The torque plate is secured to the airplane landing gear with the same bolts that secure the axles to the gear legs.

Each brake caliper consists of an aluminum, or magnesium, caliper housing. When pressure is applied to the brake pedal, brake fluid under pressure flows to the back side of the housing-mounted piston or pistons. The piston moves out and the brake pads squeeze on the disc. In addition to the anchor bolts moving in the torque plate bushings, additional pad wear compensation takes place as the piston in the caliper moves farther out in the bore. When new pads are installed, the piston is pushed back into the bore, and the process starts all over again.

A well-maintained set of Cleveland brakes is almost maintenance free as long as the discs and brake pads are replaced when wear limits are reached and the anchor pins and torque plate bushings are kept clean.

Failure to keep an eye on the wear limits will eventually result in a total loss of braking power when the piston seal — a single O-ring — is pushed out of the bore. The published minimum thickness for all Cleveland metallic and organic brake linings is one-tenth of an inch (0.10 inch). A handy no-go gauge is a stack of two U.S. 25-cent coins (quarters).

The wear limits for all Cleveland brake components are available at the Cleveland Web site.

Service and upgrade tips

Goodyear disc-brake systems are still available but manuals for their repair, inspection, and overhaul are pretty scarce. Remember that it's just plain dumb — not to mention illegal according to the federal aviation regulations — to work on components and systems without the proper manuals. If manuals are needed, one source is Essco Aircraft. This company has a tremendous library of manuals, books, and videos. A number of Goodyear manuals are listed. Contact Essco Aircraft online or call 330/644-7724.

Cleveland brake conversion kits are available as brake system upgrades. These are approved for installation under the supplemental type certificate (STC) process. Kits include magnesium wheels — aluminum wheels, which are referred to as "ag wheels" because of aluminum's increased resistance to corrosion, also are available — discs, brake assemblies, torque plates, and installation instructions. Unfortunately, some airplane models are not included and, according to a Cleveland tech support specialist, there are no plans to expand the listing for light airplanes. Fortunately, there are some options.

Cessna issued service letters 65-41 and 66-41 that said that Cleveland wheels and brakes (called Cessna Crafted in the bulletin) can be retrofitted on all earlier models and that interchangeability is covered in the appropriate parts catalogs. According to these letters, a logbook entry referencing the service letter is the only approval required.

Cleveland brake maintenance boils down to two tasks. First, keep the anchor bolts and the bushings in the torque plate clean. Every time the brake caliper is removed, polish the anchor bolts with a little Scotch-Brite and run a bottle-type wire brush through the torque plate bushings. Some technicians spray a little nonstick lubricant on the bolts.

The second critical task is to carefully monitor the torque applied to the back-plate tie bolts. Each brake assembly has two or four bolts called tie bolts that clamp the back plate to the brake caliper assembly. The back plate is the part of the brake assembly that holds a set of brake pads, located opposite the piston side of the brake assembly. Over-torquing these bolts will deform the brake assembly casting and cause the brake piston to bind up within the caliper casting. Since these bolts are used in a shear application, the torque required is very small. The details of this procedure are online.

Conditioning brake linings

A maintenance task often overlooked is the conditioning of new brake pads. This is an easy task, which should be taken care of after replacing the brake pads. If the maintenance facility says it's not necessary, or says it hasn't done it, then the owner should do it.

Most single-engine GA airplanes use organic brake linings. Heavier singles and twins use metallic linings. The conditioning procedure for organic linings is simple — taxi for 1,500 feet with the engine at 1,700 rpm while enough brake pressure is held to keep the airplane speed down to 10 to 15 mph. Then let the brakes cool for 10 to 15 minutes.

Metallic linings are used on heavier airplanes and the procedure is as follows: Do two full stops in rapid succession from 30 to 35 knots; let the brakes cool for 10 to 15 minutes. If the conditioning has been successful, the brakes should prevent wheel roll during a full-power static runup. If the brakes aren't up to the task, go through the conditioning procedure again. An expanded explanation of the proper conditioning procedure is also online.

That misunderstood lock-o-seal

In 1979 Cessna changed the design of the brake master cylinders. The master cylinders are the small fluid reservoir/pump assemblies mounted behind the rudder pedals. These early style master cylinders depended on a poorly understood part called a "lock-o-seal" for successful operation. Remember that as the brake pads wear, the piston in the wheel caliper automatically compensates by traveling farther out of the bore in the caliper housing. To ensure that enough fluid is always available for powerful braking, a method of continually replenishing the fluid that has filled the space behind the piston is required.

Here's how it works. Brake pedal movement causes the master cylinder shaft to move down, contacting the master cylinder piston, causing it to move down and displace fluid. The displaced fluid flows through flexible and rigid tubes to the back of the brake caliper piston, causing it to move and applying brake pressure to the pucks. The master cylinder piston is designed so that it can move up and down slightly, independent of the cylinder shaft, so it lags behind the master cylinder shaft movement. When the brake pedal is released, a strong spring pushes the shaft upward, causing the pedal to return to neutral. Because the piston lags, the drag of the sealing O-ring on the master cylinder piston wall opens a very precise gap between the lock-o-seal and the top of the piston. The lock-o-seal is a washer with a sealing O-ring molded onto the washer inner circumference. Fluid is then free to flow down out of the reservoir past the lock-o-seal and past the piston until the brake pedal is pushed; then the drag of the O-ring causes the piston to lag behind the piston shaft until it contacts the O-ring portion of the lock-o-seal. This seals the opening between the reservoir and the brake cylinder, and brake pressure builds up. There's a very critical dimension of 40-thousandths of an inch (0.040 inch plus or minus 0.005 inch) that must be maintained between the bottom edge of the lock-o-seal washer and the top of the piston — when the piston is pushed down against the adjustment nut — for the system to work properly. The procedure is detailed in all Cessna service manuals.

Cessna braking systems are simple. There's one master cylinder for each wheel. The parking brake locks each master cylinder shaft in position after the brake pedals are pushed. The copilot's brakes are linked to the pilot's master cylinder via mechanical linkages. Bleeding (removing air in the brake system) is a one-step, one-man procedure in Cessnas. Open the bleeder screw in the bottom of each brake caliper, connect a flexible hose to the bleeder screw, and pump hydraulic fluid into the system. A hand-operated oil squirt can, a short length of one-quarter-inch inside-diameter (ID) flexible tubing, and a couple of rags to catch the fluid that flows out of the master cylinder air vent are all that's required. It usually takes about 20 squirts for each side. Close the bleeder screw and test the brakes for a hard pedal. Ninety-nine times out of 100 the brakes will work fine.

Piper brake bleeding

Piper brake systems are much more complex. Not only is there a master cylinder on each of the pilot's pedals, but also there are two additional ones for the copilot's pedals and one for the hand brake. This complexity can create a brake-bleeding nightmare, especially for the technician without a plan. Although the following method takes two people, it works so well that the brakes can be bled in 10 minutes.

Fill the reservoir before each step. Fluid flows from the reservoir to the hand brake to the brake cylinders, and then to the wheel cylinders. The first step is to pull the parking brake handle five or six times before setting the parking brake lock tab. After the parking brake is locked, the helper in the airplane should pump the pilot's and copilot's left (or right) brake pedals a couple of times before holding pressure on both pedals. The person bleeding the brakes then opens the left (or right) brake bleeder at the caliper. The brake-pedal pusher continues to hold pressure on the pedals. The pedals will fall to the floor as the fluid exits the lines. The pedals must be held down until the bleeder screw is shut.

It usually takes more than one run through this process before all the air is bled out of the lines down to the brake cylinder calipers. After removing the pressure from the pedals, release the parking brake and start over. Pump and lock the parking brake, apply pressure to the same pedals that were just bled, and hold the pressure while the bleeder screw is opened. Hold pressure until the bleeder is closed. Repeat the steps until the brake is hard on that side. Repeat the same process on the other wheel. Bleed until the brake is hard. It may be necessary to go through the process again on the first side that was bled, but it's a plan that works.

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