The Oracle @ Delphi #2 2/15/06 - Hydraulic Wounds (Brake Failure)
It is prophetic that most of the hydraulic fluid used in general aviation is red (MIL-H-5606). You know it's one of those things that you never pay attention to until it fails you then you scratch your head and say "what happened?"
I remember shortly after I got my airplane I was having a problem with my short field landings. For some reason I needed to shift my hips to get my feet high enough on the pedals and depressed far enough to stop the plane. After burning holes in two very expensive tires, I finally went back to my mechanic and asked for a diagnosis. After asking me a few questions and looking at the brake pads and such he prescribed bleeding the brakes. It turns out that the fluid had degraded and the brakes were getting spongy. As a low time pilot, I didn't recognize that the brakes were spongy; I only knew that I needed more pressure to get the effect that I desired. It turns out that at the end I was only really breaking with one wheel.
I have once again found hydraulic fluid seeping from the veins of an aircraft. The mechanic (A+P) that I'm working with got a report about a Piper Warrior that he maintains for a customer was having problems with its breaks. Knowing that I am a bit on the eager side my instructor turned me loose on the problem. So, with driver in hand I jumped into the pilot seat and step on the brakes. Boy they were soft. In fact they went right to the floor. I pulled off the cowl and checked the reservoir. It was low (slightly over the half-full mark). I asked a bunch of questions as to the last time the brakes were serviced, but I didn't get any useful information. With flashlight in hand I got into the favorite position of the mechanic. Sitting in the pilot's seat with my head where my feet belong, I began looking for a big puddle of hydraulic fluid.
I could not find anything that would account for about half of the reservoir's volume anywhere. The carpets appeared new and unstained. There wasn't an oily residue on any of the hoses; in fact I couldn't readily see anything wrong. My instructor said that there was likely a leak in one of the master cylinders and that he did have a kit to rebuild them. He also noted that the cables for the aileron were rubbing against the hydraulic feed hose when the yoke was full forward. This rubbing would act like a saw cutting through the hydraulic hose. I don't know how he can see all these problems that I always seem to miss, but I digress.
Let me take a moment and give you a small primer on how the Warrior's breaking system works. There is a small reservoir (about a pint) attached to the fire wall. The reservoir has a small vent on the top. The reservoir then leads to the intake of the parking brake. Coming out the parking brake through a right 270 degree turn to a T which one side feeds a right angle to the pilot's right master intake, the other side of the T goes to the Left master intake. Then the output of each master cylinder goes through their respective hoses to the matching slave cylinders, which are attached to the brake pads.
Being that we were unsure about the hose we decided to pull it first and see if the cable had breached it. There was a small amount of wear on it but no hole. We cleaned the hose, the T, and the right master brake elbow. We then reinstalled them, hoping that would solve the problem.
We all know to expect that problems that go away by themselves come back by themselves and usually, at the worst possible time. In hind sight, there was no chance that the problem was fixed. After fiddling around with it some more and replacing the feed hose, I noticed a small crack in the right angle fitting where the T screws into it. When we looked at this fitting when it was out we didn't see any problem, however, when we pressurized the intake to the master brake cylinder by actuating the hand brake the pressure in the line caused just the smallest amount hydraulic fluid to leak out. And of course because air is less viscous than hydraulic fluid when the brakes where pressed the air would be sucked into the master cylinder causing the brakes to be spongy. This area of the brake is somewhat special in that it acts in both a pressurized manner with the parking brake and as a feed to the master cylinders for the left and right brake. It turns out that the pressure from the parking brake and the over tightening the T in an attempt to make the feed hose not rub on the control cables causing the right angle fitting to crack.
I won't bore you with the rest of the details of the repair, however, I will caution all of those pilots out there. If you step on those brakes (while you are on the ground) and they don't behave like they should, cancel your flight. This is not the time to go and practice your landings. No, not even your soft field landings!