Pressure & Density Altitude Study Lab

1. The five altitudes

• Indicated altitude: what the altimeter shows.
• True altitude: height above mean sea level (MSL).
• Absolute altitude: height above ground level (AGL).
• Pressure altitude: indicated altitude with the altimeter set to 29.92 "Hg.
• Density altitude: pressure altitude corrected for nonstandard temperature.

2. The standard atmosphere (the "yardstick")

Everything is measured against the International Standard Atmosphere (ISA):

• Sea level: 15 °C (59 °F) and 29.92 "Hg (1013.2 mb)
• Temperature drops about 2 °C (3.5 °F) per 1,000 ft, up to 36,000 ft
• Pressure drops about 1 "Hg per 1,000 ft, up to 10,000 ft

Anything different from these values is a nonstandard temperature or pressure. Aircraft instruments and performance charts are all calibrated to this standard. That's why we need corrections on a real day.

3. Pressure altitude

Pressure altitude is the height above the standard datum plane (SDP), a theoretical level where pressure is 29.92 "Hg. As actual pressure changes, the SDP can be below, at, or above sea level.

PHAK gives three ways to find it:

1. Set the altimeter to 29.92 "Hg and read the indicated altitude.
2. Apply a correction to the field elevation based on the altimeter setting (Figure 11-3 table).
3. Use a flight computer.

FAA example (Fig. 11-3): Field at sea level, altimeter setting 29.70. The table shows +205 ft, so pressure altitude is 0 + 205 = 205 ft.

Pressure altitude is the starting point for performance calculations. It's also how flight levels above 18,000 ft are assigned.

4. Density altitude

Density altitude is the altitude in the standard atmosphere where you'd find the air density you actually have today. The airplane performs as if it were at the density altitude, no matter what its real altitude is.

• Hotter than standard → air expands → less dense → DA higher than PA
• Colder than standard → denser → DA lower than PA
• Exactly standard → DA equals PA

FAA example: Pressure altitude 5,000 ft, temperature 20 °C above standard. Density altitude comes out above 7,000 ft, and a 790 ft takeoff ground run grows to about 1,000 ft.

Tools the FAA gives you: the PHAK density altitude chart (Fig. 11-4), a flight computer, and the FAA-P-8740-2 rule-of-thumb chart. Those are what this site uses. You may have heard a "120 ft per degree" shortcut. It doesn't appear in these FAA documents, so this site doesn't teach it.

5. High, hot, and humid

Three things raise density altitude (thinner air): high elevation (plus low pressure), high temperature, and high humidity.

• Pressure: density is directly proportional to pressure (at constant temperature).
• Temperature: density varies inversely with temperature (at constant pressure).
• Humidity: water vapor is lighter than air, so moist air is less dense than dry air.

The PHAK says there is no rule of thumb or chart for humidity's effect, but you must still consider it. FAA-P-8740-2 advises that if high humidity exists, add 10% to your computed takeoff distance and expect a reduced climb rate.

6. What high density altitude does to you

Thin air reduces power (the engine takes in less air), thrust (the propeller is less efficient), and lift (less force on the wings). In the cockpit you'll see:

• Increased takeoff distance
• Reduced rate of climb
• Same indicated airspeed (IAS) but higher true airspeed (TAS) on approach and landing
• Increased landing roll

What to do about it:

• Check the AFM/POH performance charts first. Use the Koch chart only if the AFM/POH isn't available.
• Fly during cool hours (early morning or late afternoon).
• Above 5,000 ft density altitude (or below 75% power), lean normally aspirated engines for maximum power on takeoff unless you have automatic altitude mixture control. Turbocharged engines don't need it.
• Consider reducing weight.

Koch chart example: 100 °F at 6,000 ft pressure altitude means add 230% to takeoff distance and reduce climb rate by 76%. So 1,000 ft becomes 3,300 ft, and 500 fpm becomes 120 fpm.

7. Reading the FAA test charts

The knowledge test hands you a figures booklet (FAA-CT-8080-2H). These are the charts it uses for density altitude and performance.

Figure 8: Density altitude chart

1. Find the altimeter setting in the table on the right and apply its conversion factor to field elevation. That's pressure altitude. Positive factors add; negative factors subtract.
2. Find the temperature on the bottom scale (use the °F row for °F, the °C row for °C) and go straight up to your pressure altitude line. Interpolate between lines.
3. Go straight left and read density altitude.

The red line is standard temperature. On it, density altitude equals pressure altitude. Right of it (hotter) density altitude is higher. Left of it (colder) density altitude is lower.

Figure 38: Landing distance table

Pick the column for your pressure altitude, then apply the notes: −10% per 4 kts of headwind, +10% per 60 °F above standard, and for dry grass add 20% of the "total to clear 50 ft" figure to both distances.

Figures 37 and 40: Landing and takeoff graphs

Work left to right through each section. Start at the temperature, go up to the pressure altitude, then go right. At each reference line, follow the slanted guide lines to your weight, then your wind, then your obstacle height, and continue across. The red arrows on each chart show a worked example.

Practice

Pick topics, then answer. If you're right, you'll see why it's right. If you're wrong, you'll get an explanation of what likely went wrong and how to work it out, with the FAA source.

Pressure altitude: worked calculator

Enter a field elevation and altimeter setting to see every step, using the PHAK Figure 11-3 correction table.

Is it standard? Temperature check

Enter a pressure altitude and the outside air temperature. You'll see the standard temperature at that altitude and which way density altitude moves.

PHAK Figure 11-2: Properties of standard atmosphere

PHAK Figure 11-3: Altimeter setting correction

Positive values: add to field elevation. Negative values: subtract from field elevation.

FAA-P-8740-2: Density altitude rule-of-thumb chart

Approximate density altitude (ft) for an airport elevation and temperature. Standard temperature for each elevation is shown.

Sources (FAA only)

• Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 11: Aircraft Performance (PDF)
• FAA-P-8740-2, Density Altitude (FAASTeam safety pamphlet)
• FAA-CT-8080-2H, Airman Knowledge Testing Supplement for Sport Pilot, Recreational Pilot, Remote Pilot, and Private Pilot (Figures 8, 37, 38, 40)
• FAA Private Pilot-Airplane (PAR) sample questions

This is a study aid, not a substitute for the FAA publications above, your aircraft's AFM/POH, or instruction from a CFI. For real flights, always use your aircraft's performance charts.