BRAKE SYSTEM

The main landing gear brake system (seeFigure 2-59) utilizes a hydraulically operated, multiple-disk brake on each of the four main landing gear wheels. The nose landing gear wheels do not have brakes. The brakes normally operate from utility hydraulic system pressure, with an alternate supply available through the auxiliary hydraulic system. If electrical power is off, the system with the highest pressure will supply pressure to operate the brakes. Fluid for the normal brake system flows through a brake pressure selector valve to the right- and left-hand brake control valves.

When the fluid leaves the brake control valves, it flows through the dual antiskid valves, brake fuses, and shuttle valves to the brakes. Each brake is controlled by a brake control valve, an antiskid valve, and brake shuttle valve.

The auxiliary system supply flows through the emergency brake pressure selector valve. When the emergency brake system is actuated, fluid is directed to the brake control valves, then through hydraulic fuses and shuttle valves directly to the brakes, bypassing the antiskid valves. Utility or auxiliary system pressure is selected by manually positioning a brake pressure selector switch. Auxiliary system handpump pressure can also be used for brake operation in towing operations when utility or electrically driven auxiliary system pressure is not available. The accumulator in the auxiliary system will provide only one brake application; therefore, the brake pedals should be depressed firmly and held when braking is required. System pressure will not build up when the brake pedals are pumped on and off while the auxiliary system handpump is being operated.

Figure 2-56. Nose Landing Gear Ground Pin

Figure 2-57. Nosegear Uplock Emergency Release Handle

Figure 2-58. Steering Wheel

Figure 2-59. Main Landing Gear Brake System

2.12.1 Brake System Accumulators

Air-charged accumulators are used in both the normal brake and the emergency brake. Hydraulic systems provide a reserve source of hydraulic pressure in the event of certain system failures and absorb pressure surges. The accumulator in the normal brake system is capable of supplying pressure for about two brake applications. The accumulator in the emergency brake system, having one-half the capacity of the normal brake system accumulator, is capable of supplying pressure for about one additional brake application.

2.12.2 Brake Antiskid Provisions

Skidding because of excessive brake application during normal brake operation is controlled by an antiskid system, maintaining maximum braking without excessive wheel skid (refer toparagraph 2.13).

2.12.3 Brake System Controls and Indicators 2.12.3.1 Brake Pedals

Actuation of the brakes is through application of toe pressure on the rudder pedals at either the pilot or copilot station.

The amount of braking force is proportional to the force applied to the brake pedals. The right pedals actuate the right-hand brakes, and the left pedals actuate the left-hand brakes. This arrangement allows directional control of the aircraft through differential braking. When the antiskid system is energized, application of brake pressure before touchdown is prevented by a locked-wheel signal through the touchdown relays.

2.12.3.2 Brake Pressure Selector Switch

A two-position (NORMAL, EMERGENCY) BRAKE SELECT toggle switch located on the hydraulic control panel (seeFigure 2-45) provides selection of either normal or auxiliary hydraulic pressure for applying the brakes. The NORMAL position will supply utility hydraulic pressure to the brakes, and the EMERGENCY position will supply auxiliary hydraulic pressure to the brakes. With the BRAKE SELECT switch in the NORMAL position and the landing gear lever in the UP position, the normal brake selector valve is energized closed by 28-Vdc power from the essential dc bus through the LANDING GEAR CONTROL circuit breaker on the copilot lower circuit breaker panel.

When the landing gear lever is placed to DN, the normal brake selector is deenergized to open. With the BRAKE SELECT switch in the NORMAL position, the emergency brake selector valve is energized closed by 28-Vdc power from the essential dc bus, received through the EMER BRAKE VALVE circuit breaker located on the copilot lower circuit breaker panel. With the BRAKE SELECT switch in the EMERGENCY position, the normal brake selector valve is energized closed by 28-Vdc power from the main dc bus through the ANTI-SKID CONTROL circuit breaker on the copilot lower circuit breaker panel. When the switch is in the EMERGENCY position, the antiskid system is inoperative. Both the normal and emergency brake selector valves are deenergized open.

Note

In case of dc electrical power failure, the deenergized valves admit either utility or auxiliary hydraulic system pressures to the brake system. The shuttle valves are positioned by the system supplying the greater pressure.

Hydraulic fuses are provided in the normal and emergency systems for each of the main landing gear brakes. If a failure is experienced in the hydraulic line between the fuse and the brake, the fuse will allow approximately 10 cubic inches of fluid to bleed out of the system and then close, thereby retaining the remaining hydraulic fluid for operation of the other three brakes.

2.12.3.3 Parking Brake Handle

A parking brake handle (seeFigure 2-60) is located in front of the pilot seat, to the right of the pilot right footrest.

The control handle is mounted on a panel support and is attached to a flexible cable. This cable pulls a pawl into a detent in the brake control lever to lock the pedals in a depressed (brakes on) position.

Note

When a hydraulic pump is not supplying pressure, brake pressure will gradually decrease and eventually cause the parking brake to become ineffective. Wheel chocks shall be used for long-term parking.

The brakes are set for parking by first fully depressing the toe section of the rudder pedals, then pulling firmly on the parking brake control handle while letting off slowly on the brakes. The brakes are released by depressing the toe section of the rudder pedals. When the parking brake is set with power on the aircraft and the ANTI-SKID switch ON, a solenoid in the antiskid valve is deenergized to block the return port of the antiskid valve. This prevents rapid leakage and subsequent release of the pressure used for setting the parking brake. With power on the aircraft, the ANTI-SKID INOPERATIVE light will illuminate when the parking brake is set.

Figure 2-60. Parking Brake Handle 2.12.3.4 Brake Pressure Indicators

Two brake pressure indicators are located on the hydraulic control panel (seeFigure 2-45) at the bottom of the copilot instrument panel. The indicators, which are connected to pressure transmitters in the pressure lines of the brake control system, register the hydraulic pressure available in the brake sections of both the utility and auxiliary hydraulic systems. The indicators are energized by 26-Vac power from the instrument transformers through the BRAKE EMER and NORMAL BRAKE fuses on the pilot lower circuit breaker panel.