ATA-29 Hydraulic power

Larger aircraft normally have a hydraulic services panel in the crew compartment, which contains indicators covering parameters such as fluid quantity, pressure and temperature, and switches to control operation of emergency pumps and valves.

The instruments and switches for each separate hydraulic system are normally grouped together, and the panel may be marked with a mimic diagram to assist the crew in transferring hydraulic power, or in overcoming an emergency situation.

Quantity Indicators: A clear window fitted in the reservoir provides a means of checking fluid level during

servicing, but the reservoir may also be fitted with a float-type contents unit, which electrically signals fluid quantity to an instrument on the hydraulics panel in the crew compartment.

Pressure Relays: A pressure relay is a component which transmits fluid pressure to a direct reading

pressure gauge, or to a pressure transmitter which electrically indicates pressure on an instrument on the hydraulics panel. In some cases both types of indication are provided, the direct reading gauge being fitted in the hydraulic equipment bay, adjacent to the relay. A typical pressure relay is shown in Figure 25. During normal operation the piston acts as a separator, transmitting fluid pressure to the gauge side. If a leak develops on the gauge side, the piston moves to the gauge end of the cylinder and the valve seats in the cylinder head, thus preventing leakage from the system. The valve also permits bleeding when a new gauge, or gauge line, is fitted.

Pressure Gauges: Electrically operated pressure gauges are fitted on the hydraulics panel, to register main

and emergency system pressures. Direct reading gauges are often fitted to the accumulators and reservoirs, to enable servicing operations to be carried out.

Pressure Switches.Pressure switches are often used to illuminate a warning lamp, and to indicate loss of

fluid pressure, or loss of air pressure in a reservoir. Such switches contain a diaphragm, which flexes under fluid or air pressure, this movement being transmitted to a micro switch, which, at the appropriate pressure, makes or breaks contact with the warning lamp.

Flow Indication: A flow indicator valve is often fitted in the outlet line from a pump, and is used to

provide warning of pump failure. The valve comprises a body, a spring-loaded plunger connected to an actuator arm, and a micro-switch. During normal operation, fluid pressure overcomes spring pressure, and the plunges moves to allow full flow through the valve. If pump output decreases below a predetermined minimum, the spring loading overcomes fluid pressure, moving Hit- plunger and actuator arm, and closing the micro-switch contact to illuminate the warning lamp.

Temperature Indication: Warning of fluid overheating is normally provided by a temperature sensing element in

the reservoir. Warning of overheating of electrical motors which are used to operate emergency pumps is normally provided by fitting a similar element in the motor casing. The sensing element tales the form of a bimetal strip or rod arrangement, which operates a snap-action switch when the warning temperature is reached. Operation of the switch closes the contacts to an associated warning lamp.

Hydraulic SystemsThe Boeing 777 has three independent 3,000-psi hydraulic systems; the left, right, and

center systems. Each system has its own reservoir, pumps and filters.The left system has one engine-driven pump (EDP) and one AC motor-driven pump

(ACMP). It supplies hydraulic power to the flight controls and to the left engine thrust reverser. The right system also has one EDP and one ACMP that supplies power to the flight controls, the normal main gear brakes and the right engine thrust reverser.

The center system has two ACMPs, two air driven pumps {ADPs) and one ram air turbine (RAT) pump. Pneumatic: power from the two engines or the auxiliary power unit (APU) operates the ADPs. The center system supplies power to:

• Flight controls• Leading edge slats• Trailing edge flaps• Alternate and reserve main gear brakes• Normal and reserve nose gear steering and nose gear extension-retraction:• Main gear extension-retraction• Main gear steering.

The ram air turbine deploys automatically during flight and supplies emergency hydraulic power to the primary flight controls if any of these conditions occur:

Both engines are shut down. Both AC buses’ are not powered All three hydraulic system pressures are low.

Only the flight controls use hydraulic power from the RAT.The EDPs in the left and right systems and the ACMPs in the center system are the

primary pumps. They operate continuously. The ACMPs in the left and right systems and the ADPs in the center system arc the demand pumps. They operate automatically when there are heavy demands on the systems. This part time operation increases pump life, system efficiency and reliability.Hydraulic fuses are installed in some of the fluid lines to protect against a loss of fluid in these systems:

o Main gear steeringo Brakeso Main gear actuationo Flight controlsAll of the electric pumps as well as all of the air driven pumps and engine driven pumps are

interchangeable. The hydraulic reservoirs and filters are all near the pumps they supply, and all of the reservoirs may be filled from a single-point reservoir servicing station.

Hydraulic System IndicationsEach pump has an amber fault light that illuminates when a pump overturns or is

producing low pressure, and there is a green light that shows high RAT output pressure and an amber light illuminates when the RAT is unlocked. The Engine Indicating and Crew Alerting System (E1CAS) displays a number of hydraulic system alerts and their levels, these are:

RAT unlocked Low system pressure Low pump pressure Pump overheat Reservoir low quantity

Reserve brake and steering failure HVDIM (Hydraulic Interface Module) failure.The status display shows the reservoir quantity and system pressure for each system. The

hydraulic synoptic display shows a real-lime diagram of the operational status of the entire hydraulic system, and the hydraulic maintenance page displays all of the pertinent hydraulic data needed by the maintenance personnel.

Automatic ControlThe Hydraulic Interface Module (HYDTM) cards are the computer components that control

the automatic operation and indication of the hydraulic system. There is one card for the left system, two for the center system, and one for the right system. These cards receive data from the various sensors and send it to the Airplane Information Management System (AIMS). They control these functions:

Demand pump AUTO operation RAT deployment Landing gear Auto-Off operation Center hydraulic system isolation

These cards also control these hydraulic system indications: System pressure Pump pressure Pump temperature Reservoir quantity Reservoir temperature Reservoir pressure

HYD INDICATING SYSTEMHydraulic Fluid Quantity Indicating System

The hydraulic fluid quantity indicating system gives indications of fluid quantity in the hydraulic system A, B and standby reservoirs. The system uses quantity transmitters at the system A and B reservoirs to send signals to the common display system. The quantity for system A and system B show as a percentage of full on the systems display.The system uses a quantity switch in the standby reservoir to send signals to the STANDBY HYD LOW QUANTITY light on the flight control panel.

Hydraulic Pressure Indicating SystemThe hydraulic pressure indicating system gives indication of system pressure in the flight

compartment. The system uses pressure transmitters on the hydraulic system A and B pressure

modules to send signals to the common display system. The pressure for each system shows on the systems display.

Hydraulic Pump Low Pressure Warning SystemThe hydraulic pump low pressure warning system gives indications when the pressure of

any hydraulic pump on the airplane decreases to less than normal. The system uses pressure switches on the hydraulic system A and system B pressure modules to send signals to the HYD PUMP LOW PRESSURE light on the hydraulic panel.The system uses a pressure switch on the standby hydraulic system module to send a signal to the STANDBY HYD LOW PRESSURE light on the flight control panel.

Hydraulic Fluid Overheat Warning SystemThe hydraulic fluid overheat warning system gives indications when the temperature of

hydraulic system A or system B electric motor-driven pump (EMDP) is more than normal. The system uses temperature switches in the pump case drain lines to send signals to the HYD PUMP OVERHEAT lights on the hydraulic panel.