Aircraft Hydraulics

Hydraulic systems made their appearance on aircraft in the early 1930s when the retractable undercarriage was introduced. Since that time an increasing number of tasks have been performed by the application of hydraulic power and the power demand has consequently increased greatly.

The hydraulic system today remains a most effective source of power for both primary and secondary flying controls, and for undercarriage, braking and anti-skid systems. However more-electric systems are being considered to replace hydraulically powered systems in some areas.

In its simplest form a hydraulic system is as follows. The primary source of power on an aircraft is the engine, and the hydraulic pump is connected to the engine gearbox. The pump causes a flow of fluid at a certain pressure, through stainless steel pipes to various actuating devices. A reservoir ensures that sufficient fluid is available under all conditions of demand.

To achieve the levels of safety requires at least two hydraulic circuits as shown in the figure. The degree of redundancy necessary is very largely controlled by specifications and mandatory regulations issued by the national and international bodies charged with air safety. The requirements differ considerably between military and civil aircraft. Military aircraft frequently have two independent circuits, large civil transports and passenger aircraft invariably have three or more.

Under normal working conditions hydraulic fluid needs cooling and cleaning. Occasionally it is necessary to de-aerate by the connection of ground equipment, although increasingly modern systems are being produced with devices to bleed off any air accumulating in the reservoir.

For cooling purposes the fuel/hydraulic heat exchanger is used. This ensures that cooling on the ground is available. Further air/fluid cooling may be provided once the aircraft is in flight. Since heat exchangers are low-pressure devices they are normally situated in the return line to the actuator/service.

When a pump is running off-load, all the heat generated by its inefficiencies is carried away by the pump case drain line. The heat exchanger should therefore be positioned to cool this flow before its entry into the reservoir. Care must be taken to determine the maximum pressure experienced by the heat exchanger.

Electricity for Aircraft

All modern aircraft and spacecraft depend on electricity and electronics. Electronics is a special application of electricity. Avionics is the term for electronics when it is applied to aircraft.

We know electricity performs many functions in the airplane. Electricity supplies light, heat and power. It ignites fuel-air mixture in the engine. It operates light, retractable landing gear, wing flaps, engine cowl flaps, radio and navigation equipment. We use electricity for ground control, operation of different mechanisms, computers, communication, etc.

Nowadays the pilot has airborne avionic system with improved reliability and performance. Reliability is increased by a combination of improved components and better system design. Performance is improved primarily by the large-scale use of on-board digital computers. On-board equipment processes data from various sources and performs computations for navigation and flight control.

Reliable air-to-ground communication in remote areas is provided by a satellite system.

An aircraft complexity increases, the number and complexity of instruments and equipment with electronic control also increases. A great number of various display devices is and will be widely used for multiple applications.

Aircraft Instruments

Avionics is a term used for electrical, electronic, radio and other aircraft instruments, which assist the crew in controlling the machine and monitoring the operation of various aircraft systems

According to their function aircraft instruments are traditionally divided into flight and navigation instruments and engine control instruments. The former provide data for the flight control and navigation, while the latter secure reliable operation of the power plant.

Instruments are needed to measure pressure, temperature, altitude, velocity, rate of flow and numerous other conditions and parameters affecting the flight and operation of aircraft. The pilot must be provided with accurate information on all these factors.

Most of these instruments contain special types of sensors receiving input data, circuits for their transmission, and indicators for the information display. The information obtained from various sensors can also be used for the purpose of automatic flight control In this case output signals are transmitted to the servomechanisms, which actuate the necessary controls.

Flight instruments can be grouped according to the type of sensors they use. ,

There is a large class of instruments based on aneroid sensors. The instrument that indicates the altitude of the aircraft above the ground is called an altimeter. Another very important device is an airspeed indicator. The vertical speed indicator serves to measure the rate of the aircraft climb and descent.

Electricity and Avionics

All modern aircraft and spacecraft are very dependent upon electricity arid electronics. Avionics is the term for electronics as applied to aircraft.

One of the prime functions of the electrical equipment in an airplane is to ignite the fuel -air mixture in the engine. In addition the electrical system provides light, heat and power. Besides it operates different lights, retractable landing gear, wing flaps radio, instruments and navigational equipment. Electricity and electronics are needed for ground control computer, communication, etc. in short we employ electricity to perform multitude of tasks.

Modern planes contain many miles of electrical wiring and hundreds of electrical and electronic components. Without electricity arid electronics there would be no modern airplanes, missiles or spacecraft. Today modem aircraft vehicles are equipped with scores of different electrical systems. To supply the power for these systems, the vehicles arc equipped with generating equipment sufficient to provide light for a town.

It is obvious that one of the most important factors in reliability of any aircraft is reliability of its electrical system. It is electrical system that is needed for the control of propulsion system, for control of guidance and navigation, for communication.

The operation of modern aircraft could not exist without the use of instruments. All instruments are usually operated electrically or electronically. There is no quicker, easier or more reliable method of conveying information from every section of airplane to the flight crew than by means of electric circuits and devices.

Cockpit and its Equipment

The cockpit is where the crew members sit. It is a portion of the fuselage front where the aircraft is taken to the air, controlled in flight and landed.

The cabin is equipped with a great variety of instruments, devices and equipment. The efficient utility of modern aircraft depends largely on the ability of instruments to depict accurately what the aircraft is doing in flight and how well its power plant and components are functioning. In addition to instruments a variety of radio equipment is required to provide efficient navigation. The combination of aircraft instruments, radio equipment and the use of electronics in its instrument design is called avionics.

Basic flight instruments are the instruments which a pilot uses to fly an aircraft. To understand their role better they are divide into three categories: control instruments, performance instruments and navigation instruments.

Control instruments. Primary control instruments are of great importance. They are tachometer, temperature indicator, fuel quantity indicator, cabin pressure indicator, the undercarriage position flag.

Performance instruments. Primary performance instruments on the instrument panel of the front and rear cabins are: the combined airspeed indicator, the Mach meter, vertical velocity indicator, altimeter (radio altimeter), attitude indicator, the clock and many other instruments.

Navigation instruments. Primary navigation instruments are the radio compass and distance measuring equipment: distance ground speed indicator, distance indicator and some other instruments.