Page 20 - Volume 11 Number 12

Ask the Expert
Parking Brake...Yay or Nay?
by Tom Clements
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
Older position, on the subpanel.
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New position, below the subpanel.
DECEMBER 2017