Maintenance Tip: The Parking Brake

Maintenance Tip: The Parking Brake

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 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.

Photo A: Blown Tire

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.

Photo B: The intact stationary disc. Notice the lugs fitting into receptacles in the brake 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.

Photo C: One of the stationary discs pieced back together with what was left after the brakes were removed. Notice the three mangled lugs on the inside; one is missing completely.
Photo D: A close-up of a damaged lug receptacle due to the extreme heat, which dislodged the steel lugs.

The rotating discs in photo E fared no better, warping under the intense heat; note the plugged-up cooling slot. As the heat intensified, the pad material began to break down and melt. Most of the cooling slots in the rotating discs were filled with this gunk.

Check out the heavy gouging in photo F; once the stationary discs broke into pieces, their lugs cut into the caliper housings. This was the worst-case scenario – these brakes had no redeemable cores whatsoever.

And the wheels? All bad. When the stationary discs broke up, the centrifugal force pushed the pieces against the wheel halves and their outer edges dug in (photo G). As for the R/H outboard wheel, the long taxi back at high idle on a mangled tire followed by a plunge into the gravel median took its toll.

Photo E: The cooling slots in the rotating discs were filled with gunk, like the one circled above.

If the brakes were engaged, or partially engaged, the whole time, I was asked why they failed at the taxiway turn. I’m sure the system was overheated so severely that the O-rings melted

Then when the pilot tried to apply the brakes during taxi, the fluid went right through and he got no response.

Replace or Convert?

At first the owner was adamant about sticking with OEM equipment on his King Air. However, after pricing everything out, I concluded it would cost at least $100,000 to buy the BFGoodrich parts outright (i.e., no core credits) plus labor, freight, consumables and taxes. In contrast, a Cleveland conversion kit, with new wheels and everything else, was just about one-third of that amount. It was a no-brainer; he opted for the Cleveland conversion.

Not too long ago I wrote an article for this magazine comparing the OEM and the STC brake systems. In the October 2017 issue, you can read about the BFGoodrich internal disc system versus the Cleveland external disc system. However, I was unable to fit my concerns about the parking brake in that article, so I’m seizing that opportunity here.

Photo F: Extreme gouging on the brake caliper housings, created by the stationary disc lugs once it broke into pieces.
Photo G: Gouging on the wheel halves.

High Idle Taxi Equals High Expenses

I strongly suspect the temp pilot of that B200 was taxiing out at high idle. Why? Because high idle generates enough momentum to get a King Air rolling, even if the parking brake handle isn’t pushed all the way in, or if the brake somehow failed to release all the way.

At low idle, if the parking brake is partially engaged, you can’t get rolling. If you do, it will feel labored, and it won’t feel right. In fact, this is the perfect test to see if your parking brake has failed to fully release – see if you can roll in low idle.

While on the subject, let me point out the many reasons you shouldn’t taxi at high idle. The aircraft is simply going too fast for safe handling on the ground. To compensate, you are forced to ride the brakes and/or go into Beta or reverse to maintain control. Using Beta or reverse is hard on the props and throws ramp debris out in front of the aircraft. Such debris spells damage to your prop blades and potential FOD for your engines.

Accelerated brake wear, unnecessary stress on the prop system, chewing up blades and potential FOD – what is the upside to taxiing in high idle?

Some guys choose to do this, so they can run the air conditioning (A/C) and keep both engines at the same rpm. On 200, 300 and 350 models, with the A/C assembly mounted on the R/H engine, the compressor won’t kick in below 62-65 percent N1 (the exact setting may vary). They don’t want to taxi in low idle and nudge the R/H engine a little higher to get the A/C going, because they don’t like the right side wanting to pull ahead. I understand the reasoning, but if you’re concerned about containing your maintenance costs, consider taxiing in low idle only.

Burn Them In, Part 2

When the Cleveland conversion was completed on that B200, it was understood that I would burn in the new brakes. Since I wasn’t familiar with the airport, I asked the regular pilot to come along with me sitting right seat. And guess what? He fires it up and taxis out in high idle. Whoa Nelly! He was riding those virgin brakes like crazy, which is the last thing you want to do to new brakes before they’re burned in. It was an awkward moment, but I got him to switch to low idle.

As I mentioned in my earlier article on brakes, the proper burning in of brand-new brakes “ensures they have the proper stopping capacity, reduces the possibility of noise or chatter, and makes them wear better.” They wear more evenly and last longer if done properly. When you taxi out to burn in brand-new brakes, use low idle and touch them as little as possible before the burn-in run.

In My Humble Opinion

I never use the brakes when taxiing because: A) I’m never in high idle, and B) I’ve got props to make the turns. Most King Air pilots I know do the same.

The only time I would set a parking brake is when the tower tells me I’m number five in line for takeoff. I say: “Set it, but don’t forget it.” Be on the lookout for creep. It’s common with many aircraft, not just King Airs. Then, when it’s finally your turn to take off, push that parking brake all the way in, roll out and have a great, safe flight in your King Air.

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