SECTION VI. HYDRAULIC AND PNEUMATIC SYSTEMS
2-41. WHEEL BRAKE SYSTEM.
The main wheels are equipped with multiple-disc hydraulic brakes, actuated by master cylinders attached to the
toe brake sections of the rudder pedals. A reservoir in the nose compartment supplies brake luid to the system.
Braking is permitted from either set of rudder pedals. No emergency brake system is provided. Repeated application
of brakes, with insuficient cooling time between applications, will cause a loss of braking eficiency, and may cause
brake or wheel failure, tire blowout, or destruction of wheel assembly by ire.
2-42. PARKING BRAKE.
Parking brake shall not be set during light.
Dual parking brake valves are installed below the cockpit loor. Both valves can be closed simultaneously by pressing
both brake pedals to build up pressure, and then pulling out the handle placarded PARKING BRAKE , on the left
subpanel. Pulling the handle full out sets the check valves in the system and any pressure being applied by the toe
brakes is maintained. When the brake handle is pushed in the parking brake is released. The parking brake may
be set from either cockpit position.
2-43. PRESSURIZATION SYSTEM.
a. Description. A mixture of bleed air from the engines, and ambient air, is available for pressurization to the
cabin at a rate of approximately 10 to 17 pounds per minute. The low control unit of each engine controls bleed
air from the engine, to make it usable for pressurization, by mixing ambient air with the bleed air depending upon
aircraft altitude and ambient temperature. On takeoff, excessive pressure bumps are prevented by landing gear
safety switch actuated solenoids incorporated in the low control units. These solenoids, through a time delay, stage
the input of ambient airlow by allowing ambient airlow introduction through the left low control unit irst, then 4
seconds later airlow through the right low control unit.
b. Pressure Differential. The pressure vessel is designed for a normal working pressure differential of 6.5 PSI,
which will provide a cabin pressure altitude of 8,000 feet at an aircraft altitude of 29,700 feet, and a cabin altitude of
10,000 feet at an aircraft altitude of 34,000 feet.
c. Pressurization Controller. The pressurization controller, located on the copilot s subpanel (Fig. 2-11), pro-
vides a display of the selected altitude, an altitude selector, and a rate control selector. The cabin and aircraft altitude
display is a mechanically coupled dial. The outer scale, (CABIN ALT ) of the display, indicates the selected cabin
altitude; the inner scale (ACFT ALT ) indicates the corresponding altitude at which the maximum differential pres-
sure would occur. The indicated value on each scale is read per placard ALT FT X 1000 . The rate control selector,
placarded RATE INC ; regulates the rate at which cabin pressure ascends or descends to the selected altitude. The
rate change selected may be from 200 to 2000 feet per minute.
d. Cabin Rate-of-Climb Indicator. An indicator, placarded CABIN CLIMB , is located on the copilot s sub-panel.
It is calibrated in thousands of feet per minute change in cabin altitude.
e. Cabin Altitude Indicator. An indicator, placarded CABIN ALT , is located in the copilot s sub-panel. The
longer needle indicates aircraft altitude in thousands of feet on the outside dial. The shorter needle indicates pres-
sure differential in PSI (pounds per square inch) on the inner dial. Maximum differential is 6.5 ± 0.1 PSI.
f. Outlow Valve. A pneumatically operated outlow valve, located in the aft pressure bulkhead, maintains the
selected cabin altitude and rate-of climb commanded by the cabin rate-of-climb and altitude controller. As the aircraft
climbs, the controller modulates the outlow valve to maintain a selected cabin rate of climb and increases the cabin
differential pressure until the maximum cabin pressure differential is reached. At a cabin altitude of 12,500 feet,
a pressure switch mounted on the back of the overhead control panel completes a circuit to illuminate a red ALT
WARN warning annunciator, to warn of operation requiring oxygen.
g. Safety Valve. Before takeoff, the safety valve is open with equal pressure between the cabin and the outside
air. The safety valve closes upon lift off if the CABIN PRESS DUMP switch is in the PRESS mode. The safety
valve, adjacent to the outlow valve, provides pressure relief in the event of an outlow valve failure. This valve is
also used as a dump valve. The safety valve is opened by vacuum, which is controlled by a solenoid valve operated
by the CABIN PRESS DUMP switch, located adjacent to the controller. It is wired through the landing gear safety