APPENDIX: Hydraulic/Brake System
The hydraulic system provides hydraulic pressure to operate the Flaps, Speed Brakes, Landing Gear and Power brakes.
The hydraulic reservoir located aft of the cabin pressure bulkhead. The reservoir contains two markings in the reservoir sight glass: FULL and REFILL. FULL = 1.1 gallons. REFILL = .8 gallons or less. Service if liquid drops to or below the REFILL line. In the aircraft the HYD LEVEL LO annunciator will illuminate when the fluid in the reservoir drops to .6 gal or less.
The reservoir is pressurized with hot bleed air from the engines, at 15 psi. This air serves for several purposes. Pressurized air is needed to purge any air bubbles in the system and prevents foaming of the liquid. Also hot air warms up hydraulic in cold weather which helps keep the liquid its lubricating consistency. Air is filtered and goes through a check valve. This check valve prevents hydraulic fluid from going into the bleed air system and causing smoke. A hydraulic pressure relief valve relieves pressure as fluid level increases and a Vacuum relief valve prevents negative pressure from building as fluid volume decreases.
Hydraulic fuel supply from the reservoir is controlled by a SHUTOFF valve that can be electrically controlled from the flight deck. The ENG FIRE PUSH switch light also closes the hydraulic and fuel valves, trips the generator and arms both extinguishing bottles.
One engine driven pump at each engine is used to pressurize the system. The pumps are driven by the accessory gearbox in each engine. Pump output is varied from 0 – 3.9 gpm by each pump to automatically adjust to demands. One pump can meet all the needs of the system. The pump itself cannot be stopped unless the engine is shut down.
Pump output is monitored by a LH/RH PMP PRESS LO sensor. The annunciator illuminates when the pressure drops below 750psi.
There are a total of 3 filters. One at each pump and one on the reservoir return line. These remove contaminants. They can be bypassed if they become clogged at 50psi. There is no indication on the cockpit of this.
Another check valve after the filter prevents reverse flow into one pump from another pump and enables us two have two different pump pressure sensors because if not, the output of a good pump could mask the sensor of the bad pump.
The system tries to maintain a pressure of 1350- 1550psi. A pressure relief valve automatically relieves excessive pressure caused by a malfunctioning pump. It begins to open at 1650psi and it’s fully open at 1850psi. Excess liquid is routed back to the reservoir.
If the Relief Valve is unable to control the pressure below 1850psi a Bypass Valve controlled by the HYD PRESS REL switch in the flight deck can dump the pressure by connecting supply and return lines. No indication except for gauge is provided for system overpressure.
If electrical power is lost, the hydraulic pressure gauge will indicate zero.
If the hydraulic system fails, braking and main gear door retraction can be accomplished with pneumatic pressure provided by an emergency nitrogen bottle located in the nose of the a/c pressurized 1350-1650 psi.
Brake System
Each main gear wheel is equipped with a hydraulically operated disc brake assembly.
Pressure for operation of the wheel brakes is provided by the aircraft hydraulic system and is initiated by the brake pedal master cylinders.
Braking is controlled by the pilot applying the most toe brake force.
The power brake anti-skid control valve is used to release and apply hydraulic pressure to the main gear brake assemblies. The control valve has three modes of operation: Manual, Power, Anti-skid.
If normal hydraulic pressure is not available to power brake anti-skid control valve, system hydraulic fluid pressure from the master cylinders will hold the shuttle valves open, thereby allowing hydraulic pressure to actuate the brake assemblies.
The manual mode of braking may have limited effectiveness and should not be relied on from high energy states. Anti-skid is non-functional in the manual mode.
With normal hydraulic pressure available to the power brake anti-skid control valve, the brake system functions in the power mode. An accumulator charged with 900 50 psi of nitrogen stabilizes hydraulic pressure during fluctuations of the hydraulic system. The power brake system will operate normally with or without anti-skid control.
An anti-skid control system is installed on the aircraft to electronically monitor and control the power brake system, providing maximum braking efficiency through the prevention of wheel skidding.
Wheel skid control is only activated above 14 knots tire speed during taxi. Do not test anti-skid in congested areas. After testing the anti-skid system, normal braking with anti-skid will not be available for approx. 4 seconds.
The brake assemblies on the main gear wheels are designed to operate on nitrogen pressure for emergency braking should the hydraulic brake system malfunction. The nitrogen cylinder provides sufficient pressure for at least 7 to 10 emergency brake applications or 7-9 brake applications if the pressure has been used to close the main gear doors. The emergency brake is controlled by a hand operated valve, as the valve is released, residual nitrogen pressure is vented overboard.
The parking brake is manually set from the pilot’s position. The parking brake should not be set if the brakes are hot. Do not set the parking brakes by using the copilots brake pedals.