Brakes, Parking Brakes and Wheel Chocks

New for X-Plane 12.2

Parking Brake

Unlike on cars with hydraulic brakes and a cable-operated E-brake, most airplanes only have one braking system, that is most commonly hydraulically operated. Most airplanes have individual brakes for the left and right main gear that are actuated with toe brake pedals. Some airplanes, like the PA-28, have a common brake handle that allows to actuate both brakes evenly at once. The common brake handle can be arrested in position to serve as parking brake. The mechanism is essentially the same between PA-28 and C-172, pulling and locking the brake handle applies brake pressure to both wheel brakes, and the handle mechanically locked in place keeps this pressure up. Other airplanes, like the DA-40 or most airliners, employ a different mechanism: A hydraulic valve that traps brake pressure in the gear system, isolating the brake pedals from the gear. In that case, the pilot applies brake pressure with the pedals, and then closes the valve which traps the pressure. The pedals can then be released, but the brake calipers will apply pressure for a long time, if the system is tight and leak-free.

Anti-skid and feathering the brake pedal

Many airplanes have an anti-skid system that prevents the wheels from locking up under excessive brake force. On many airliners this system is so good, that no matter the runway conditions simply pushing the brakes as hard as you can and leaving the work to the anti-skid system will result in the shortest stopping distance. X-Plane works the same and allows you to fully push the toe brake pedals (or pulling the wheel brake handle) even on an icy runway and will feather the brakes for you.

However, if selected in Plane Maker, an airplane can have no anti-skid system installed (or the system could have failed). In this case, you need to carefully apply the wheel brakes to prevent wheel locking.  This is easy with an analogue control axis for the brake, like hardware rudder pedals provide. Users with no hardware pedals can use the “hold brake” commands – they build up brake pressure over time as they are being held down and release it when the key or button for the command is released. With a little practice, quick pushing and releasing of the button can be used to meter brake effort.

Airplane Options in Plane Maker

• “Hydraulic parking brake valve” is the new option to have a valve to trap the pressure downstream from the pedals. Aircraft pre-dating Plane Maker 12.1 will default to not having this valve and using a physically locking handle.
• “Toe brakes can release park brake” is an option that exist on some planes with a parking brake valve that forcefully opens the valve when the toe brakes are pushed. Not all airplanes work that way. Aircraft pre-dating Plane Maker 12.1 will default to having this on, as it matches behavior of X-Plane 11
• “Anti-skid system” equips the airplane with an essentially “always-on” anti-skid system that allows the user to always use full brake pressure and not be afraid of skidding even under slippery conditions. Aircraft pre-dating Plane Maker 12.1 will default to having this on, as it matches the behavior of X-Plane 11.

Wheel chocks

Wheel chocks are another way to secure an airplane in place, as parking brakes tend to lose pressure over time. Placing a chock under the wheel prevents it from moving. It is physically different from a parking brake: in X-Plane, this is simulated by keeping the tire contact patch locked in place, rather than resisting rotation by applying a brake torque. Since the chocks can be placed individually per-wheel, the airplane can be held in place by locking some wheels, even when others need to be able to move freely, such as when the gear geometry changes due to loading or unloading the aircraft while parked.

Commands and DataRefs

• sim/flight_controls/brakes_toggle_regular Applies a moderate braking pressure, depending on the aircrafts weight.
• sim/flight_controls/brakes_toggle_max Applies the maximum braking pressure, regardless of environment. This can lock the wheels on a plane with no anti-skid. When used on a plane with no parking brake valve, equivalent to pulling the brake handle out to maximum and locking it. When used on a plane with parking brake valve, equivalent to holding both brake pedals fully down with continuous foot pressure. Note that over the course of an hour, pressure can seep, and brakes will need to be re-applied. Use the real parking brake valve if you want it!
• sim/flight_controls/brakes_regular If held continuously: applies a moderate braking pressure, depending on the aircrafts weight. Since brake pressure goes away on command release, this can be used to feather the brakes up to a medium pressure.
• sim/flight_controls/brakes_max If held continuously: applies the maximum braking pressure, which can cause skidding in a plane with no anti-skid system. Since brake pressure goes away on command release, this can be used to feather the brakes up to a maximum pressure.
• sim/flight_controls/park_brake_set (new): Applies brake pressure and then locks it in place by closing the parking brake valve, if the plane is so equipped. If not, functionally equivalent to brakes_toggle_max
• sim/flight_controls/park_brake_release (new): Opens the parking brake valve and lets brake pressure escape. On planes with no valve, equivalent to releasing the handle.
• sim/flight_controls/park_brake_toggle (new): toggles between pressurized, valve closed, and unpressurized, valve open on planes with a valve, otherwise functionally equivalent to brakes_toggle_max
• sim/flight_controls/park_brake_valve_close (new): Closes the parking brake valve and maintains whatever pressure existed. Note that when closed without actually applying brake pressure before, it will completely disable the brakes. Does nothing on planes without such a valve.
• sim/flight_controls/park_brake_valve_open (new): Opens the parking brake valve allowing trapped pressure to escape, resulting in brakes being released. Does nothing on planes without such a valve.
• sim/flight_controls/park_brake_valve_toggle (new): Toggles the valve between open and closed on planes that have it.
• sim/flight_controls/install_chocks (new): Chocks all wheels of the airplane.
• sim/flight_controls/remove_chocks (new): Removes wheel chocks from all wheels.
• sim/flight_controls/toggle_chocks (new): Toggles all wheel chocks between installed and removed.

sim/aircraft/gear/acf_has_abs              int         y   boolean   Wheel brakes fitted with an anti-lock system that will automatically prevent skidding
sim/aircraft/gear/acf_park_brake_trap      int         y   boolean   Hydraulic wheelbrake uses a valve to trap pressure for the parking brake
sim/aircraft/gear/acf_park_brake_toe       int         y   boolean   Parking brake (or parking brake valve) can be released by pushing the toe brakes
sim/cockpit2/controls/parking_brake_ratio (this is REPLACED), 
     -Use wheel_brake_ratio dataref below instead.
sim/cockpit2/controls/wheel_brake_ratio    float      y   ratio      The overall brake requested by the master brake cylinder... 0.0 is none, 1.0 is complete.
sim/cockpit2/controls/park_brake_valve     int        y   boolean    Some hydraulic brake systems have a valve that can be closed to trap brake pressure for the parking brake. 0 = Normal (valve open), 1 = Closed (pressure trapped for park brake)
sim/flightmodel2/gear/tire_abs_gain        float[10]  n   ratio      How much ABS is limiting the brake force applied to the caliper (1 means full braking, 0 means brakes fully released)
sim/flightmodel2/gear/is_chocked           int[10]    y   boolean    Is the tire chocked - it's blocked in place and can't be moved regardless of brake setting