Distance and Speed

The speed that an airplane travels with respect to the air mass is seldom identical to the speed measured with respect to the ground. This is, of course, due to the influence of wind on the airplane's movement. Similarly, the air distance and the ground distance that an airplane covers are rarely the same. The customary units that are used for the measurement of air or ground distance are nautical miles (nm), kilometers (km), and statute miles (mi). For navigation purposes, the true airspeed (TAS) is associated with the airplane's heading. An airplane's TAS will differ from its speed over the ground in the presence of wind. This chapter describes the different speeds that are encountered in airplane performance analysis namely: true airspeed and ground speed, Mach number, equivalent airspeed, calibrated airspeed, and indicated airspeed. It discusses the relationships between these speeds.