JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
When aircraft fly pressure altitudes, they are assigned a flight level (FL) of three digits, representing
hundreds of feet above 29.92. As an example, an aircraft assigned FL250 (pronounced “flight level
two five zero”) would be flying a pressure altitude, and the pilot would fly the aircraft so that the
altimeter reads 25,000 feet with 29.92 in the Kollsman window. These above altitude definitions are
illustrated in Figure 1-5.
Figure 1-5 — 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.
Density altitude 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. Table 1-1 summarizes the effects of
temperature on aircraft performance. Moisture affects aircraft performance in the same manner
as temperature, but to a much lesser degree.
HIGH TEMPERATURE OR MOISTURE
Lower Air Density
LOW TEMPERATURE OR MOISTURE
Higher Air Density
1-7 Version 3.2/Dec 08