A schematic diagram of a typical power boost
brakes. As a general rule, this applies to all patrol (VP)
brake system is shown in figure 14-4. The normal
system consists of a reservoir, two power boost master
(VA) aircraft. Because of the weight and size of the
cylinders, two shuttle valves, and the brake assembly
aircraft, large wheels and brakes are required. Larger
in each main landing wheel. A compressed air bottle
brakes mean greater fluid displacement and higher
with a gauge and release valve is installed for
pressures. For this reason, independent type master
emergency operation of the brakes.
cylinder systems are not practical on heavy aircraft. A
typical power brake control valve system is shown in
In this system (fig. 14-4), main hydraulic system
pressure is routed from the pressure manifold to the
power boost master cylinders. When the brake pedals
In this system, a line is tapped off from the main
are depressed, fluid for actuating the brakes is routed
hydraulic system pressure line. The first unit in this
from the power boost master cylinders through shuttle
line is a check valve, which prevents loss of brake
valves to the brakes.
system pressure in case of main system failure.
When the brake pedals are released, the main
The next unit is the accumulator, the main purpose
system pressure port in the master cylinder is closed
of which is to store a reserve supply of fluid under
off, and fluid is forced out the return port, through the
pressure. When the brakes are applied and pressure
return line to the brake reservoir. The brake reservoir is
drops in the accumulator, more fluid enters from the
connected to the main hydraulic system reservoir to
main system and is trapped by the check valve. The
assure an adequate supply of fluid to operate the
accumulator also acts as a surge chamber for excessive
loads imposed upon the brake hydraulic system.
When the emergency air system is used, air
Following the accumulator are the pilot's and
pressure, directed through a separate set of lines, acts
copilot's brake valves. The purpose of a brake valve is
on the shuttle valves, blocking off the hydraulic lines
to regulate and control the volume and pressure of the
and actuating the brakes.
fluid that actuates the brake.
POWER BRAKE CONTROL VALVE
sometimes referred to as orifice check valves, are
A power brake control valve system is used on
aircraft requiring a large volume of fluid to operate the
1. Brake reservoir
6. Shuttle valve
2. Power boost master
7. Air vent
8. Main system pressure
3. Emergency brake control
4. Air release valve
9. Emergency air bottle
5. Wheel brake
10. Emergency air gauge
Figure 14-5.--Typical power brake control valve system.
Figure 14-4.--Power boost brake system.