Page 12 - Volume 12 Number 6

The Parking Brake
by Dean Benedict, A&P
We all know the “Do’s and Don’ts” of parking brakes.
Don’t set it when you pull into an FBO for any length of time, because they will likely tow your King Air to another spot. I’ve seen the entire nose gear assembly ripped out of a King Air by an over-zealous line guy. I guess he didn’t get the memo to first try a gentle tug and make sure the aircraft will roll.
Don’t rely solely on the parking brake to hold the aircraft. They are notoriously unreliable. I will never forget the morning I pulled into work and found a V-tail Bonanza on the ramp with its tail and rear fuselage sliced neatly into spiraled segments. It looked like a giant slinky. This was years ago at BeechWest in Van Nuys, California. The owner of a Twin Bonanza was getting ready to fly. Line service had pulled his aircraft onto the ramp and chocked it. He hopped in, started the engines, then realized he forgot to pull the chocks. So, he set the parking brake and got out of the aircraft with the engines running. As soon as he kicked the nose chock out of the way, the aircraft started rolling. He tried to stop it by pushing on the nose cone, but he slipped and fell between the prop and fuselage. Miraculously, he was unscathed as his airplane moved beyond him, but the V-tail parked nearby was not so lucky.
The System
Most of us were taught when we first started flying that parking brakes in airplanes are unreliable, if not borderline useless. The parking brake system used by Beechcraft is no exception, although King Airs do have a beefier version than its piston cousins.
Downstream from the master cylinders, there is a valve that traps the hydraulic fluid between the brakes and the park valve. When the brake is set, O-rings in this valve confine the brake fluid on the brake side. This maintains the pressure and keeps the brakes engaged. When these O-rings leak, the brakes begin to release ever so slowly. If the engines are running, the aircraft will creep.
Some pilots are finicky about a leaky parking brake. Others pay it no heed whatsoever because they seldom use it, if at all. If you are fond of your parking brake, pay attention. They can create some serious havoc.
Partial Release and Total Destruction
I got a frantic call from a King Air pilot I did not know. He had flat-spotted a tire on takeoff and had no idea how this happened. Then the brakes failed, and the B200 careened off the taxiway as he tried to make his way
10 • KING AIR MAGAZINE
back to the hangar. He was rattled, the passengers were alarmed, but thankfully no one was hurt.
The brakes, however, were another story. In all my years working on airplanes, I have never seen brakes so thoroughly and completely destroyed. The mechanic who took everything apart had worked for me when I had my shop. He has about as much King Air experience as I do, and he was equally stunned. These brakes didn’t just overheat, they exploded.
The regular pilot of the B200 was unavailable and a temp pilot was hired for the trip. On takeoff he found he was unable to accelerate past 80 knots, so he aborted. While taxiing back, he had no brakes. Unable to slow down for a 30-degree turn in the taxiway, he went into the gravel median. Everyone disembarked at that point. That’s when they noticed the severely blown tire on one side. Line service reported extreme difficulty getting the aircraft onto the tarmac and into its hangar.
At first, all focus was on the side with the blown tire. But the next day, brake fluid was pooled on the hangar floor under both main gear. Initially, the pilot thought the parking brake did not fully release, but later he said he may not have pushed the handle in all the way.
Once they got it up on jacks to address the issue, it was one surprise after another. These were OEM (BFGoodrich®) brakes. Disassembly was difficult at first due to the extreme heat generated during the takeoff roll – those brakes had to have been glowing red. Once they got into it, pieces of the stationary discs dropped on the floor. This was not a good sign.
No Good, Beyond Bad and Just Plain Ugly
The brakes, or what was left of them, were tossed in a box and brought to me for inspection. By this time, I was assisting the owner in assessing the damage and deciding on the best course of action. I was astonished by what I saw.
The right-hand (R/H) outboard froze up and the tire gave out (photo A). The only stationary disc that survived intact was in this brake assembly (photo B). You can see how the lugs fit into receptacles in the caliper housing.
The stationary discs in the other three brake assemblies blew apart. We spent a good hour piecing stationary discs together like giant jigsaw puzzles as shown in photo C. Notice the lugs are mangled or missing. In photo D it shows that even the steel lug receptacles were damaged.
The rotating discs in photo E fared no better, warping under the intense heat; note the plugged-up
JUNE 2018
MAINTENANCE TIP