Brakes – Safety Check
Though performing a thorough preflight inspection is critical to the safety of every flight, there are some items that are especially important to check before settling into the cockpit for engine startup. For example, on the Diamond DA-20, the brake linings (or brake pads) should be thoroughly examined before each flight. Because the DA-20 has a castering nose wheel, all steering on the ground is accomplished through the use of differential braking. This, combined with the fact that our DA-20s are used primarily for flight instruction, means that the brake linings are high-wear items. Checking the condition of these linings is easy, and the pilot can do it without the help of a mechanic.
It is important that you get down on your knees and position your head directly above the brake caliper, as this is the only way to get a good idea of the condition of the linings. The first thing to look for is lining thickness. The manufacturer states that the linings should be at least 0.10″ thick (approximately the thickness of a nickel). This thickness, combined with the proper thickness of the brake disc, makes it impossible to lose hydraulic brake pressure past the caliper O-ring. This minimum thickness also ensures that the rivets that hold the linings to the back plates are not making contact with the brake disc.
One thing to know is that the linings typically wear faster on the bottoms, where it is more difficult to inspect. Therefore, if the tops of the linings measure 0.10″, it is a sure bet that the bottoms of the linings are beyond the wear limit, and that the lower brake rivet is scraping the disc. The linings should be replaced if this is the case. (During an inspection, I use a mirror and a flashlight to inspect the bottoms of the linings, as opposed to looking only at the tops.) Another thing to look for is brake fluid dripping from the brake caliper. The presence of red 5606 hydraulic fluid means that the caliper O-ring is not functioning correctly, or it may have even popped out of its groove. If this happens, your brakes will not work properly.