Emergency Power Supply
Most midsize and larger aircraft install an APU, which performs many functions. An APU is a small power plant, invariably a turboshaft engine that uses the same fuel (i. e., AVTUR). When ground facilities are not available, the APU can provide an emergency electrical supply and air-conditioning, and it can start the main aircraft gas turbines. It is interesting that an APU exhaust can reduce aircraft drag, regardless of how small. A typical example of an aft-mounted APU is shown in Figure 15.24 (i. e., a schematic layout). The APU and its installation weight range from 100 to 300 kg depending on the size. The size of an APU in a military aircraft depends on user requirements. An APU can be started using onboard batteries.
A ram air turbine (RAT) is another way to supply emergency power. This is a propeller-driven device mounted on an aircraft surface (at the fuselage underbelly) that operates when an aircraft is in motion. A RAT is retractable. Figure 15.25 shows the schematic layout.
Figure 15.24. Auxiliary power unit
15.9.2 Avionics Subsystems
A host of avionics “black boxes” support the flight deck and beyond. The black boxes serve navigation, communication, aircraft-control, and environment-control systems; and record and process important data to analyze and monitor malfunctions and so forth.
With increasing features, the electrical cable length is long and relatively heavy. Multiplexing of data transmission significantly reduces cable weight. Recently, fiber optics have been used for data transmission; when used with a FBW system, it is appropriately termed FBL (see Chapter 12). This section introduces readers to design features (i. e., hardware) that assist in a more accurate prediction of weight and cost.
Most avionics black boxes have microprocessors, which help to standardize connections for data flow. The connection of wires is called a bus. Following are the prevailing standards for a bus architecture.