Over the Airwaves
If we were to rank order the following weather factors in terms of risk (high to low), what would they be? While we might generate a debate, most pilots would likely agree on the following:
1. Thunderstorms and associated events, e.g., tornadoes, microbursts;
2. Icing and freezing rain;
3. Winds, e.g., gusts and crosswinds;
4. Density altitude.
Likely the least understood among these weather factors and its effect on aircraft performance and safety is density altitude. Why? Because, unlike the other weather phenomenon, we cannot see or observe density altitude. Instead, it's just there!
What do we know about altitude in general?
Even the most novice flight student knows that air becomes "thinner" the higher in the atmosphere we go. Similarly, he knows that non-turbocharged aircraft perform less well the higher we climb. This is because the number of molecules of air per unit of volume decreases as altitude increases. The "volume" we referred to here is either a given area of the wing that produces lift or the size of the cylinders inside the engine that produce power. Thus, less lift and power is produced as altitude increases. This, in turn, occurs because the amount of pressure (coming from the "weight" of the atmosphere) decreases as altitude increases. In other words, pressure altitude is simply a measure of the weight of the atmosphere. This weight constantly changes because of the unequal heating and cooling of the earth's surface. As the air heats, it lifts. As it cools, it descends. This heating and cooling produces high and low pressure areas. We measure pressure altitude in inches of mercury, e.g., 29.92 inches of mercury is standard pressure at sea level.
How temperature affects pressure . . .
The accompanying chart illustrates the effect of temperature on pressure altitude. For example, look at the 7,000' pressure altitude line slanting upward from left to right. At zero degrees C, 7,000' pressure altitude equals 7,000' density altitude. At -20 degrees C, our 7,000' pressure altitude equals 4,600' density altitude. At +40 degrees C, this same 7,000' pressure altitude equals 11,400' density altitude. In short, density altitude equals pressure altitude corrected for temperature.
How does density altitude affect aircraft performance?
Let's say we're operating our airplane at an airport whose field elevation is 7,000' above sea level (MSL). Let's also say that our airplane has a maximum service ceiling of 10,000' MSL. A quick glance at the above chart reveals that our airplane simply cannot take off if the temperature is 80 degrees F. or higher!!! By referencing the performance tables in our aircraft's POH we can see just how much density altitude affects our total takeoff distances.