Aviation Weather Hazards: Density Altitude “Don’t let the DA get you.”

Density Altitude Several FAA Handbooks define DA as “pressure altitude corrected for non-standard temperature.” S271, Chapter 5 defines DA as “pressure altitude corrected for temperature and humidity.” Problem: FAA handbooks, pilot training and testing materials, and manufacturer flight manuals DO NOT provide a chart, equation, or method by which to calculate the effects of RH. In S271, we generally teach that, since the effects are minimal, and we don’t have the charts or tools to factor in RH, we can just ignore it.

Density Altitude is sometimes taught as “the altitude the aircraft thinks it is at, and therefore performs accordingly.” To determine DA, we need to know only three items of information: Elevation of the airport, helibase, or helispot. Current Barometric Pressure (in order to calculate Pressure Altitude) Temperature If you know your location elevation, the Barometric Pressure and Temperature information can be acquired with ONE phone call.

Who are you going to call?

What is “Pressure Altitude?” Pressure altitude is the altitude displayed on an altimeter when it is set to Standard Pressure as adopted by the International Civil Aviation Organization (ICAO) in 1964. 29.92 is the STANDARD pressure in the world of International Standard Atmosphere (ISA).

better for aircraft performance than LOW pressure. When barometric pressure is above or below the “standard” 29.92 inches of Hg, Pressure Altitude (PA) will be lower or higher than field elevation. This table may be used to calculate Pressure Altitude. ADD 28.3 1535 feet 28.4 1435 28.5 1340 28.6 1245 28.7 1150 28.8 1050 28.9 955 29.0 865 29.1 770 29.2 675 29.3 580 29.4 485 29.5 390 29.6 300 29.7 205 29.8 110 29.9 20 SUBTRACT 29.92 0 feet 30.0 - 75 30.1 - 165 30.2 - 255 30.3 - 350 30.4 - 440 30.5 - 530 30.6 - 620 30.7 - 710 30.8 - 805 30.9 - 895 31.0 - 965 Note: HIGH atmospheric pressure is better for aircraft performance than LOW pressure.

What is “Standard” vs. “Non-Standard” TEMPERATURE? The International Civil Aviation Organization (ICAO) baseline for all aircraft performance data is a “Standard Temperature” of 15 deg. Celcius (or 59 deg. F) AT SEA LEVEL, with zero RH. The global mean standard atmospheric lapse rate is 2 deg. C (or 3.5 deg. F) per 1,000 ft. of altitude (up to 36,000 ft.)

Standard Temperature at MSL Altitudes Altitude Deg. (C) Deg. (F) 14,000 -13 10 13,000 -11 13.5 12,000 -9 17 11,000 -7 20.5 10,000 -5 24 9,000 -3 27.5 8,000 -1 31 7,000 1 34.5 6,000 3 38 5,000 5 41.5 4,000 7 45 3,000 9 48.5 2,000 11 52 1,000 13 55.5 Sea level 15 59 To determine DENSITY ALTITUDE, add 117.4 ft. to the Pressure Altitude for each degree Celcius above standard temp.

Solve for. Airport elevation: 7,420 ft. Density Altitude Solve for Airport elevation: 7,420 ft. Density Altitude Altimeter/Barometer: 29.3” Temp. 80 deg. F or 27 C (Std. temp. at 7,500 = 0 deg. C) 28.5 1340 28.6 1245 28.7 1150 28.8 1050 28.9 955 29.0 865 29.1 770 29.2 675 29.3 580 29.4 485 29.5 390 29.6 300 29.7 205 29.8 110 29.9 20 29.92 0 feet 30.0 -75 30.1 -165 30.2 - 255 30.3 - 350 30.4 - 440 Airport Elevation (MSL) 7,420 Correct for non-std. pressure + 580 Pressure Altitude (PA) = 8,000’ Current temp. is 27 deg. warmer than std. temp. Calculate for non-std. Temp. 27 X 117.4 ft. = 3,170 ft. PA 8,000’ Add temp. factor to PA: + 3,170 DENSITY ALTITUDE = 11,170’

The non-standard temperature conversion is usually calculated via the Density Altitude chart shown below. A copy of this chart can be found in Chapter 10 of the FAA “Pilot’s Handbook of Aeronautical Knowledge” available at www.faa.gov and in several Aviation and Weather publications.

What about Humidity? Is the effect significant?

ASOS and AWOS Recordings are updated every minute. Temperature and Dew Point are factored into the Density Altitude given at the end of the report. High RH can contribute over 500 ft. to Density Altitude.