Wheels and brakes: overlooked, underappreciated.
In the great hierarchy of airplane systems, pilots tend to think little of wheels and brakes. After all, we're dealing with flying machines here, and anything having to do with ground operations usually generates about as much interest as a PBS special on the history of soap. Superfluous systems are rare on airplanes, so the maintenance-savvy owner ought to spend some time learning about airplane wheels and brakes.
Braking systems on light airplanes more resemble those of a motorcycle than an automobile, what with separate circuits for each wheel and the general compactness of the components. By applying foot power to the toe pedals, you are increasing hydraulic pressure in sealed lines running to the wheel-mounted calipers. Action begins at the rudder pedal, which is attached to a brake master cylinder. In most instances, the master cylinder is mounted vertically between the floorboard and the rudder pedal. Location of the mounting holes on the pedal dictate the degree of mechanical leverage your feet have over the master cylinder's small piston; this ratio typically falls between 2:1 and 3:1.
Depressing a brake pedal causes the piston in the master cylinder to move, which bears down on the pressure side of the brakes' hydraulic system. A combination of metal and flexible lines carries this hydraulic fluid to the wheel, terminating at the slave cylinder. In retractable-gear airplanes, there are usually several flexible segments of brake line; some retractable Cessnas use the inside of the tubular gear leg to carry fluid to the wheels. At the slave cylinder, which is really just part of what is called the brake caliper, the fluid causes one or more pistons to squeeze friction pads against the brake disc. (Here we are assuming your airplane has, as do the vast majority, hydraulic disc brakes.)
In that description are the essentials of what make disc brakes work. The disc, which is generally made of steel (plain carbon or stainless) and sometimes chromed to prevent corrosion, is mated firmly to the wheel, spinning with it. The caliper is, of course, bolted to the landing gear and straddles the disc with two small friction pads, built of organic or sintered metallic particles; asbestos is no longer used due to the associated health risks. (Components of the organic pads include brass, iron, carbon, rosin, and manmade fibers.) You'll need to know which type of pad is on your airplane because the break-in procedures vary dramatically. Metallic pads need fast break-in using high taxi speeds and numerous quick stops, with no time between them for cooling. Organic pads need two or three gentle stops to set the rosins, but too much will percolate the adhesive and leave a glossy finish on the pads, a bad thing for friction.
Because the brake pads will in time sacrifice themselves in the pursuit of friction, the whole caliper apparatus needs to accommodate their diminishing thickness. By far the most common wheel and brake combination today is the Cleveland style. In Clevelands, the caliper clutching the pads is located on a pair of rods. While Cessna was making piston airplanes, its McCauley division came up with a wheel and brake system of its own, very similar the Cleveland system. Imagine standing over a main landing gear wheel: The caliper is free to move left and right, but the locating pins are supposed to hold the pads parallel to the face of the disc. This is called a floating caliper. (Some airplanes still employ Goodyear brakes, which maintain the caliper in a fixed position and allow the disc to change its position inside the wheel; this compensates for pad wear.)
Okay, so the system is simple, but that doesn't mean things never go wrong. We spoke with several maintenance shops and compiled a short list of the most common brake troubles. First among them is the mushy pedal. Though there are a number of root causes, this symptom indicates a breach of the hydraulic system. Other maladies: a totally flat pedal (that travels with little resistance right to the floor) is indicative of a large loss of fluid or massive air entrapment. You might also encounter a pedal that starts flat but will pump up, as well as the odd one that will not hold firm even after pumping. The latter can be either a loss of fluid downstream or a problem within the master cylinder. In this case, fluid is being allowed to flow from beneath the piston to the reservoir side of the system. A brake that will pump up and stay firm for a short time can indicate a scoring of the master cylinder bore.
Rubber O-rings provide a seal at both the master cylinder and caliper ends, and a leak in either could be the cause of a system breach. At the wheel end, you will usually see hydraulic fluid, dyed red for easy identification, oozing from the brake components. If the friction pads are severely worn, the wheel piston can be driven far enough out of its bore to cause leakage past the o-rings. Any leaks at the wheel are cause for a stop at the maintenance hangar.
Often a mushy pedal, the result of air bubbles within the hydraulic lines, can be traced to leaking seals at the master cylinder. Not always will these leaks indicate hydraulic fluid; frequently, the seal breach will merely allow air into the system without expelling fluid. First, you need to find out why there's air in the lines. Naturally, you need to ascertain if the seal is bad or merely has some debris between it and the master cylinder bore. Diagnosing a mushy pedal in Pipers can be complicated by the hydraulic parking brake, which exerts pressure to both brake circuits simultaneously. Occasionally, the parking brake cylinder will allow air into both sides of the normal system, rendering both pedals soft.
With the seals cleaned or renewed, you'll need to refill the system with fluid. Typically, there will be a small external reservoir for brake fluid (check your maintenance manual for details), although on many single-engine Cessnas the reservoirs are integral with the master cylinders, ahead of the rudder pedals themselves.
Next, you must bleed the lines of air. Though it's actually a simple matter to bleed the brakes, most shops have the special tools — i.e., a power bleeder — to make the procedure a snap. If you can do this yourself, you will need the help of your mechanic, because bleeding the brakes isn't on the list of owner-performed preventative maintenance. Two schemes are used. One involves injecting brake fluid under pressure at the caliper side and forcing it up through the lines and master cylinder into the reservoir. This system is fast and does a fine job of eliminating trapped air. The other scheme is the top-down method; the pressure generated by the master cylinders is used to pump fluid through the system, where it is bled from the calipers. If you have ever bled the brakes on an automobile, this is probably the system you used.
Pilots often complain of poor braking action even though the pedal feels plenty firm. In Clevelands, this is usually due to either excessive pad wear or poor alignment of the caliper to the disc. Dry or corroded alignment pins can keep the pads from making proper contact with the disc, diminishing brake performance. Not only that, but the pads will wear rapidly if this misalignment is allowed to continue. You can check for this during preflight because the caliper should be free to move slightly.
By far the most common cause of poor braking action concerns corroded brake discs. Commonly made of carbon steel, the discs will suffer surface corrosion rather quickly. Actually using the brakes wipes the corrosion from the faces. Two alternatives to the standard steel disc help slow formation of corrosion — there are stainless-steel and chromed steel discs. Each material has its advantages, with stainless providing good friction properties and chrome somewhat better heat dissipation properties than stainless. We heard from some mechanics that chrome can in time crack and flake off, however. Costs? For a Cessna 172, replacement standard steel discs will set you back about $70 each, chrome discs about $100, and stainless about $180.
As to the wheels themselves, the most common problems involve cracking and corrosion. Unlike automobile wheels, common aircraft parts are two-piece (or more) that split to ease tube and tire replacement. The halves are secured by long bolts that also anchor the brake disc. Loose through-bolts, cited by mechanics as one of the more typical problems, allows the wheel halves to chafe and wear quickly. Many Cessnas were produced with the so-called five-piece McCauley wheel, which suffers a high degree of cracking; retrofit kits for the Cleveland two-piece wheel provide the preferred fix. Most light airplane wheels are constructed of magnesium, although amphibians usually have aluminum rollers thanks to corrosion concerns.
Correct through-bolt torque and yearly greasing of the wheel bearings are said by our source shops to be the best tactics for making the wheels last. Frequent flying helps, too, since regular use keeps all parts limber and precludes accumulation of corrosion. That, and resisting the temptation to make the first turnoff each landing by trouncing the pedals, will help improve both wheel and brake life.
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