AFH 4-18

Review of Terms and Definitions The terms and definitions specific to this chapter appear below. Aircraft Energy Management The process of planning, monitoring and controlling altitude and airspeed targets in relation to the airplane’s energy state. Note that this definition is concerned with managing mechanical energy (altitude and airspeed) and addresses the safety (flight control) side of energy management. It does not address the efficiency (aircraft performance) side of energy management, which is concerned with how efficiently the engine generates mechanical energy from fuel and how efficiently the airframe spends that energy in flight. Energy System A flying airplane is an open energy system. That means that the airplane can gain energy from some source (e.g., fuel) and lose energy to the environment (e.g., surrounding air). In addition, energy can be added to or removed from the airplane’s total mechanical energy stored as altitude and airspeed. Total Mechanical Energy Sum of the energy in altitude (potential energy) and the energy in airspeed (kinetic energy). Kinetic Energy Amount of energy due to the airspeed, expressed as ½mV², where m = airplane’s mass, and V = airspeed. Potential Energy Amount of energy due to the altitude, expressed as mgh, where m = airplane’s mass, g = gravitational constant, and h = altitude. Energy State The airplane’s total mechanical energy and its distribution between altitude and airspeed. Energy Exchange Trading one form of energy (e.g., altitude) for another form (e.g., airspeed). Energy Balance Equation According to this equation, the net transfer of mechanical energy into and out of the airplane (a function of thrust minus drag) is always equal to the change in its total mechanical energy (a function of altitude and airspeed). Note that this simplified definition does not account for long-term changes in total mechanical energy caused by the reduction in aircraft weight as fuel is gradually burned in flight. Power Available The airplane’s rate of energy gain due to maximum available engine thrust at a given airspeed. Expressed as TV, where T = engine thrust and V = airspeed. Usually measured in horsepower, foot-pound per minute, or foot-pound per second. Power Required The airplane’s rate of energy loss due to total drag at a given airspeed. Expressed as DV, where D = total drag and V = airspeed. Usually measured in horsepower, foot-pound per minute, or foot-pound per second. Specific Excess Power (PS) Measured in feet per minute or feet per second, it represents rate of energy change—the ability of an airplane to climb or accelerate from a given flight condition. Available specific excess power is found by dividing the difference between power available and power required by the airplane’s weight.