Figure 1-8. Altitudes

AVIATION WEATHER

CHAPTER ONE

Figure 1-8 Altitudes

Density altitude (DA) is pressure altitude corrected for nonstandard temperature deviations. On a

hot day, air molecules are farther apart, decreasing the air density and increasing the density

altitude. In this situation, the DA of an airfield would be higher than both the published field

elevation and the pressure altitude. The opposite is true on a colder day: Increased air density

causes a decreased density altitude and a DA lower than the published field elevation and the

pressure altitude.

DA is not a height reference; rather, it is an index to aircraft performance. It affects airfoil,

engine, propeller, and rotor performance. Thrust is reduced because a jet engine has less mass

(air) to compress. Lift is also reduced due to thinner air. Additionally, higher density altitudes

result in longer takeoff and landing distances and a reduced rate of climb. Takeoff distances are

longer since reduced thrust requires a longer distance to accelerate to takeoff speed. Landing

distances are longer since a higher true airspeed is required to land at the same indicated airspeed.

Climb rate is decreased because of reduced available thrust. At certain high density altitudes,

takeoffs and/or single-engine flight (loss of one engine after becoming airborne) are not possible

due to limitations of thrust, lift, and runway length. Figure 1-9 summarizes the effects of

temperature on aircraft performance. Moisture affects aircraft performance in the same manner as

temperature, but to a lesser degree. No instrument in the cockpit displays density altitude, it

needs to be calculated.

General Structure of the Atmosphere, and Atmospheric Temperature and Pressure 1-11