Do you use your King Air’s parking brake regularly? I surely do, but I have observed that quite a few King Air pilots do not. Why?
I believe that the reluctance to use the parking brake comes from our first training flights, such as in a Cessna 172. “Don’t use that. It’s unreliable and doesn’t work very well,” was what we usually heard from our instructor. And you know what? He or she was probably correct! These primitive systems in which the parking brake knob merely pulled up on a spring clip designed to hold the pilot’s brake master cylinder rods under the rudder pedals in the depressed position were indeed iffy at best.
I think it is unfortunate, however, that too many King Air pilots have not evolved in their understanding and trust of parking brakes. Take my word for it: They’re quite good!
Remember how I have preached often about the benefit of “Judicious Suspicion?” That’s the opposite of complacency and the feeling that “It won’t happen to me.” Instead, it’s the knowledge that “Today’s the day that there will be traffic nearby when I look over my shoulder before turning.” Or, “Today’s the day that the oil pressure will not be in the green arc.” And, “Today’s the day that I will enter the wrong waypoint into the GPS’s flight plan.”
Well, Judicious Suspicion leads us to never trust a parking brake completely. It is certainly wise to stay alert for unexpected airplane movement and to not spend much time without at least one crewmember looking out the window. Yet in most cases the parking brake, when properly used, will prevent undesired ground motion even under the most severe conditions. Let me explain.
Yes, there are variations in the parking brake system – as I will discuss later in this article – between different models and different years of King Air production but they all share some commonality. The knob on the subpanel, in all King Airs, is connected to two (left and right) fluid check valves. When the knob is in the Off position, the valves act as if they don’t exist. They allow the hydraulic brake fluid to freely flow in either direction: From the master cylinders in the cockpit to the wheel caliper to apply the brakes, and from the calipers back into the master cylinders to release the braking force. But when the knob is pulled into the On position, we create the two check valves that permit fluid to only go to, not from, the calipers. To be clear, pulling the knob does not create any braking force. No, that must come from pressure in the lines to the calipers. It doesn’t matter whether you pull the parking brake knob gently or with great force; all you are accomplishing is the creation of the check valves.
Creation of the one-way device – check valve – does not create a stopping force. The force comes from the pressure at the caliper. The more pressure, the greater the stopping and holding force. What creates the pressure is the force applied to the top of the rudder pedals (and from there to the pistons in the master cylinders). If you want and expect the parking brake to work properly – to hold the airplane still even while doing a high-power runup – you need to really push HARD on the brake pedals! Saying, “Oh, the parking brake won’t hold during the Overspeed Governor check; I need to use the brakes myself then,” is almost always wrong and shows a lack of system understanding. If you can hold the airplane with brake pressure, then so can the parking brake, by trapping that same pressure. A leaking check valve is the only valid reason for the parking brake not being able to hold just as tightly as yourself … and leaking brake check valves are rather rare to find.
So, don’t be afraid or reluctant to really stand on the top of those rudder pedals while setting the brake! Doing it two or three times; pumping the brakes is fine, too, and usually helps by forcing even more fluid to the caliper.
Does the order of parking brake control usage matter? Should I press the pedals and then pull the knob, or pull the knob and then press the pedals? It doesn’t matter one whit! Realize that you are creating a one-way valve, not creating a two-way stop. If the brakes are already being applied by you, just pull the knob to trap the pressure. If the knob is already pulled, just pump the brakes until they get good and stiff as fluid is sent downstream of the check valve.
Not all King Airs have a Rudder Boost system, but many do. When checking the proper operation of this system during the runup, it is very important to have the parking brake set strongly enough that it alone will hold the airplane. If you are not using the parking brake, then you have to apply so much rudder pedal force that you tend to mask the motion and feel of the Rudder Boost system.
I think that most King Air simulators do a poor job of replicating the operation of the Rudder Boost system during its test in the real airplane. Almost always, the sims activate too soon, before sufficient difference in engine power is created by advancing only one power lever. Keep in mind that Rudder Boost is triggered by sensing differential power, not total power. The 350 uses torque to measure differential; the other models use raw P3 bleed air pressure instead. (Torque is the better way to go!) Especially in those P3-measuring airplanes with four-blade props – and, hence, higher low idle speeds – often it takes nearly full power on the “good” side before enough differential P3 pressure is created to activate the Rudder boost “kick.”
Just recently I observed a case in which the parking brake was not set strongly enough during the Rudder Boost test. This allowed the airplane to creep forward as the left engine’s power was increased. With more power on the left, the nose wheel deflected to the right as the plane slipped forward and turned away from the powered-up engine. Which rudder pedal went forward? The right one, of course. The pilot said, “There is the Rudder Boost activation,” without realizing that it was backwards! “Good foot; good engine.” The left, not right, pedal should have gone forward … as it did when we released the brake, taxied forward to center the nose wheel, then applied the parking brake HARD and repeated the test.
I wrote earlier that there were differences in the King Air braking system depending upon model and year. If you have learned and know your exact particulars, great. But if you are unsure, or are flying a variety of King Airs, then a dangerous “gotcha” can be discovered accidentally. You see, in many King Airs the copilot’s rudder pedals cannot operate the parking brake! In these models, “Shuttle Valves” separate the pilot and copilot sides and the check valve exits only in the pilot’s lines. If the copilot is flying the leg, taxis up to the parking area, and asks the captain to pull the parking brake knob for him since he cannot reach it, the brake is not set! Only if the copilot releases his pressure, the captain takes over and applies the pressure, can the parking brake be utilized. As stated, this is not true for all King Airs, but it is safer to assume it is the situation than to think the brake is set when actually it may not be.
Finally, how about releasing the parking brake after it has been set? Please – after the chocks are in place – take the time to sit in the pilot’s seat, apply force on the top of the rudder pedals, then push the knob in to release the brake. If the knob is pushed in when pedal force is not present, three “bad” things can happen. First, because the trapped high pressure on the downstream side of the check valve is not balanced by high pressure on the upstream side, it is physically more difficult for the cable to push against that pressure to release the check valve. Second, balancing the pressures prevents a sudden surge of high pressure liquid – a shock wave – that will travel upstream to the master cylinders and increase their propensity to develop leaks. Third, it is safer. By holding the brakes, the airplane won’t roll even if the chocks are not yet in place.
Trust the parking brake totally? Never! Use it often? Always!
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