Density Alt

DENSITY ALTITUDE

Experienced pilots sometimes get into trouble with density altitude. It's not that they don't know what it is, it's just that they become complacent.

Federal Aviation Regulation 91.116 Pre-flight Action requires that a pilot check the density altitude ...

(b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information ... other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.

Density altitude is a term that sometimes causes confusion. A high density altitude is NOT a good thing. Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH.

Density altitude can be computed on a density altitude chart, flight computer, electronic flight calculator or by rule of thumb. Density altitude gives us some idea about the expected performance of the airplane, but only if you apply the information to the performance charts. The DENALT performance computer gives values to be multiplied by the sea level takeoff and sea level rate-of-climb to predict the expected performance under current density altitude conditions.

This special DENALT calculator has information for both fixed-pitch propeller or constant-speed propeller airplanes. The sea level performance can be written at the bottom. The temperature is selected at the top center, then the pressure altitude is selected along the center, to the left for take-off factors and to the right for rate-of-climb percentage. Multiply the sea level performance by the factor and obtain the expected performance of the airplane.

An accurate rule of thumb (usually any error will be less than 300 feet) for determining the density altitude is easy to remember. For each 10-degrees Fahrenheit above standard temperature at any particular elevation, add 600 feet to the field elevation. (And, conversely for each 10-degrees F below standard temperature, subtract 600 feet.)

Standard temperature at sea level is 59-degree Fahrenheit. For elevations above sea level, subtract 3.5 degrees per thousand feet of elevation from the sea level temperature of 59 degrees. For example, at Jackson, Wyoming the elevation is 6,444. Multiply 6.444 times 3.5 for 22.55. Subtract this from 59 (59-22.55) for 36.45. The standard temperature at Jackson is 36.5 degrees. If the existing temperature is 80 degrees, subtract (80-36.5 = 43.5). Divide this difference by 10 degrees (for each 10-degrees F above standard), and multiply 4.35 times 600 (600 feet per 10 degrees) equals 2,610. Add 2,610 to the field elevation (6,444) for a density altitude of 9,054. Under the existing conditions (of our example), the airplane will perform as it would on a standard day at 9,054 feet elevation.

Density altitude not only affects the takeoff distance and rate of climb, but also applies to the service ceiling of the airplane while en route.

A simple rule of thumb for determining takeoff distance exists that helps you deal with density altitude during takeoff. The only problem is that it does not guarantee rate of climb after takeoff, but it insures that you will be able to takeoff in the distance available for the runway involved.