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THE TURBINE PILOT’S FLIGHT MANUAL
package is commonly referred to as a “PACK.” Normally two are installed for capacity and redundancy. In most modern aircraft, comfort is controlled via automatic temperature control systems tied to cabin air temperature sensors. Manual control of the temperature mixing valves is usually available to back up the automatic systems.
One other advantage of a VCM is that it can be set up to provide cooling on the ground, without an operating engine, APU, or external high-pressure air source. While many VCM compressors are enginedriven, they can also be set up to operate from electric motors. That way, a crew sitting on a hot ramp can plug in ground power and cool down the passenger cabin before start-up. Given the different efficiencies and benefits of ACMs and VCMs, many aircraft have both systems installed.
Fuel Systems The fuel system in any aircraft is designed to store fuel for flight, then deliver it to the engines in the proper amounts and at the correct pressures. In turbine aircraft, this simple assignment becomes rather complex. Large (often huge) amounts of fuel must be carried and distributed, many tanks and engines may be installed, and allowances must be made for the characteristics of jet fuel and turbine engines themselves. Most civilian turbine fuel systems are designed for long-range operations plus IFR reserve capacity.
Aircraft Environmental System To control cabin temperature the cooled air from ACM or VCM is simply mixed with hot bleed air. Two or more temperature mixing valves are used for this purpose. In large aircraft, the whole environmental heating/cooling system is bundled together, including ACM, bleed heat source, VCM (if installed), and mixing valves (see Figure 5.17). This
cool air, to cabin
3. Refrigerant absorbs heat and changes back to a gas in the evaporator (another heat exchanger). Heat is absorbed from passing cabin air.
warm gas returns to compressor
evaporator warm, recirculated cabin air
cooler, liquid refrigerant
heat dumped overboard
cool outside air
compressor hot, pressurized refrigerant (gaseous) 1. The compressor (electric or engine-driven) pressurizes the refrigerant.
condenser
2. The condenser is a heat exchanger where heat of compression is absorbed and removed by cool outside air. The combination of high pressure and reduced temperature causes the refrigerant to condense into a liquid.
FIGURE 5.16 | Vapor cycle machine (VCM, or “Freon unit”).
