|Answers to Your Brake Maintenance Questions|
|Written by Jacqueline Shipe|
|Friday, 18 February 2011 15:49|
Airplane brakes are an important part of the airplane and are one of the areas that receives quite a bit of maintenance. Over time, brake linings and discs wear with use and have to be replaced. Master cylinders, wheel cylinders, and hydraulic hoses leak occasionally and have to be fixed.
For obvious reasons, it is important that the brake system on an airplane be in good working condition. While stopping the airplane is the primary job of brakes, they also help the airplane make sharp turns while taxiing. Since the brakes wear with use, maintenance is required periodically.
What parts are involved in the braking system?
The brake system on the average low-wing Piper consists of a firewall mounted reservoir, master cylinders connected to each brake pedal, a parking brake (hand brake) cylinder, wheel cylinders, brake discs and linings with backing plates, and hydraulic hoses and fittings.
This system has some advantages as well as some disadvantages. The reservoir is easy to get to for servicing. It has a reference line for topping off the hydraulic fluid and, unless the lid is stuck, the only tools needed are a flashlight to see the fluid level and a can of fluid. The linings and discs are no harder to change than they are on any other airplane.
The main disadvantage of the system is that it is difficult to bleed all the air out of the lines after performing maintenance that requires opening the lines or cylinders. There are numerous tee fittings and interconnected lines so that as the system is bled, instead of being forced up and out of the reservoir, the air bubble can simply move over into another line.
What maintenance does the master cylinder or hand brake cylinders need?
What can I do to prevent and/or fix leaks?
O-ring leaks can also be caused by worn brake linings or discs that allow the piston to move out further than normal. Wheel cylinders also occasionally develop leaks and need to have the o-ring replaced. The piston gets rough places around the edges over time and has to be sanded with a lightly abrasive paper. The wheel cylinder housing also gets pits and high places and needs to be smoothed out. Sometimes the housing gets so pitted or worn that it leaks even with a new o-ring and has to be replaced. These are pretty expensive items when purchased new. Occasionally salvage yards have the parts but they are often almost as worn as the part that came off.
What precautions should I take when servicing the brake linings?
Brake linings are held on the backing plate with two or more steel rivets. A special brake tool is used to push out the old rivets and flare the end of the new ones. Care has to be taken not to over tighten the new rivets. This can cause either the lining itself or the flared part of the rivet to crack. Under tightening can allow the lining to scoot and chatter on the backing plate.
How do I bleed the brakes?
Different mechanics take different approaches to bleeding the brakes. Some folks have two people, one at each wheel cylinder, pump the fluid up from the bottom until it fills the reservoir. One mechanic I know rigs up a somewhat elaborate system of lines that interconnects the reservoir with each wheel cylinder. He has check valves in the lines going from the reservoir to the cylinders so fluid can go in but not back out. He then fills the system with as much fluid as possible and pumps the toe brakes and hand brakes until they are no longer spongy.
I usually do a combination of different things depending on where I’ve had to open the lines. If a wheel cylinder has just been resealed, I try to have someone pump the brake pedals and hold pressure on both them and the hand brake while I open the bleeder valve on the wheel cylinder and try to repeat the process until I get most of the air out. If a master cylinder, parking brake cylinder, or the lines attaching to them have been opened, I pump the fluid up from the wheel cylinders to refill the reservoir and then I pump up each pedal and crack the lines (then close them quickly) that attach to the master cylinders. I do the same thing with the hand brake. I wrap a rag around it so the fluid won’t make a mess. I can usually get quite a bit of air out this way. (Incidentally, if hydraulic fluid is ever spilled, it’s hard on the carpet or paint, but it’s not corrosive at all to the metal.) A small amount of air in the system will work its way out over time with use, provided the pedals are not too spongy.
Can I use a different fluid than 5606?
The brake fluid used in general aviation airplanes is 5606 red hydraulic oil. Automotive brake fluid causes 5606 to congeal if it comes in contact with it and it also causes the o-rings in the system to swell up. This can result in an extremely hard pedal but absolutely no braking action because the wheel cylinder remains stuck in place. No one should ever attempt to service the reservoir with any other fluid but 5606. Struts and shimmy dampeners also use 5606 fluid.
When should I replace the brake disc and how can I prevent its wear?
Brake disc replacement requires removal of the wheel assembly. The disc manufacturer has minimum thicknesses and wear limits established for linings and discs. The discs occasionally develop cracks in them and have to be replaced prior to these limits. The cracks typically form from the outer edge and progress in toward the center. Sudden cooling of a hot disc causes some cracks. For that reason, I try to avoid taxiing through puddles, especially after landing when the brakes have been used.
Can I install a toe brake if my airplane doesn’t have one?
Some airplanes have toe brakes only on the left side and some have no toe brakes, only a hand brake. I installed toe brakes on the left side of a Cherokee that had only a hand brake and I was surprised at how easy it was to do.
I purchased the pedals and master cylinders along with the hose fittings. Once I started the installation, I found that the airplane was made to have the toe brakes added as an option from the factory, so the aluminum tubing that connects the wheel cylinders was already cut and flared and had a fitting installed where the master cylinder hoses were to connect. The firewall already had holes in it with the bolts installed where the master cylinders mounted. All I had to do was install the new pedals and connect everything. It took longer to bleed the brakes than it did to install them. However, other models might be a little more difficult than this one was.
Aircraft brakes have a very important job. A little maintenance on the brake system will keep everything in top shape so that the airplane stops when needed — a tradeoff any aircraft owner should be happy to make.
From the July 2010 issue of Pipers