68 cockpit controls and flight control surfaces. In case of complete hydraulic system failure, the RAT then supplies necessary hydraulic pressure to actuate the flight controls. In order to produce the required hydraulic pressure to do this, the aircraft must be flown at a high enough airspeed to generate sufficient air pressure to turn the air turbine. (For example, the Boeing 767 requires a minimum airspeed of 130 KIAS be maintained when using the RAT.) RATs are typically installed in the underside of the aircraft fuselage, where they can be easily extended by gravity alone. RATs may be extended manually or automatically depending on the installation (Figure 4.23).
Hydraulic System Characteristics One interesting characteristic of hydraulic systems is that most of the work is done with relatively little movement. Continuous-operation systems are kept constantly pressurized by hydraulic pumps, but fluid movement occurs only when something hydraulic is operating. Even then, fluid flow is minimal when small actuators and cylinders are being powered. In these cases, the work is done more by pressure than by flow. Compare a hydraulic system to a flexible rod being pushed through a tube—small movement at the driving end of the system is transferred hydraulically to push a faraway button, to move a valve, or to deflect a control surface. Since hydraulic pressure is kept bottled up between the pump and devices it powers, hydraulic return lines generally carry little pressure or flow back to the reservoir. Exceptions occur when rotary hydraulic motors are powered to operate high-load devices such as landing gear and flaps. Then, both hydraulic flow and pressure are significant.
Pneumatic Power Systems Pneumatics provide yet another method for transmitting engine power to various aircraft systems. In this case, the medium for power transmission is compressed air. Since air, being a gas, is compressible, pneumatic power is far less efficient than hydraulic power for heavy-duty jobs. On the other hand, pneumatic systems are much lighter than hydraulic systems, need little maintenance, and require no special fluids.
THE TURBINE PILOT’S FLIGHT MANUAL
On piston airplanes, pneumatic power comes from pressure or vacuum pumps driven mechanically off the engines. These systems are basically pretty simple. Rotary pneumatic pumps correspond to the pump in our reference water power system. The ready source of atmospheric air eliminates the need for any type of reservoir, and control is relatively simple through a series of valves. Pneumatic systems in piston aircraft typically operate gyro instruments, pressurization, and deicing boots. Turbine-powered aircraft use pneumatic systems for the same types of applications, plus many more. The reason is that turbine engines are essentially giant pneumatic pumps. As you remember, the engine’s gas generator compresses huge amounts of air to support combustion. It’s a relatively simple matter to draw this “bleed” air from the engines and use it to power all sorts of things (see Figure 4.24). High-pressure bleed air is drawn from the compressor section of a gas turbine engine. Some systems draw bleed air from two or more “stations” on each engine, yielding different pressure and temperature outputs. (These differing bleeds are sometimes identified as “high-pressure bleed air” and “low-pressure bleed air.” In this book, the terms “high-pressure bleed air” and “bleed air” are used interchangeably to refer to all such variations.)
High-Pressure Bleed Air High-pressure bleed air has many applications straight out of the engine, including engine and wing thermal anti-ice. It is also used, via some combination of air cycle machines (ACMs, or Packs) and heat exchangers, for cabin pressurization, heating, and cooling. (See “Pressurization” and “Environmental Systems” in Chapter 5.) On large turbine aircraft, high-pressure bleed air also powers the engine starters. Large engines such as the CFM 56 are started pneumatically, as an alternative to the heavy electrical loads and motors required for electrical starting of turbine engines. Typically for engine starting, the aircraft APU is started electrically to provide bleed pneumatic power, or else a ground pneumatic power source is used. When the captain calls for starting, a start valve is opened, sending compressed air to spin up a small turbine in the pneumatic starter. This, in turn, spins up the engine core compressor. Fuel is
