Density altitude

Thinner air hurts every part of airplane performance: the wings make less lift, the propeller makes less thrust, and a non-turbocharged engine makes less power. Density altitude is the altitude in the standard atmosphere where the air density would equal what you actually have. It's "the altitude the airplane thinks it's at."

What raises density altitude

• High elevation
• High temperature
• Low pressure
• High humidity (a smaller effect, but real)

"High, hot and humid" is the bad combination. Airports along Utah's Wasatch Front sit around 4,000 to 4,500 feet, and on a hot summer afternoon the density altitude can be thousands of feet higher than the field elevation.

Step 1: pressure altitude

Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000.

Example: field elevation 4,400 ft, altimeter 30.12. (29.92 − 30.12) × 1,000 = −200. Pressure altitude = 4,200 ft. You can also just set 29.92 in the altimeter and read it.

Step 2: density altitude

Use the chart in the PHAK or an E6B for the exact value. A rule of thumb: density altitude ≈ pressure altitude + 120 × (OAT − standard temperature). Standard temperature at altitude is about 15°C minus 2°C per 1,000 feet.

Continuing the example with an outside air temperature of 32°C: standard temperature at 4,200 ft ≈ 15 − 8.4 = 6.6°C. 32 − 6.6 ≈ 25.4. 25.4 × 120 ≈ 3,050. Density altitude ≈ 4,200 + 3,050 = about 7,250 feet.

What it means

• Longer takeoff roll and landing distance (your true airspeed is higher for the same indicated airspeed).
• Reduced climb rate, sometimes dramatically.
• Lean the mixture for best power before takeoff at high density altitude airports, per the POH.

Indicated airspeeds for takeoff, approach and stall don't change; the airplane just needs more ground to get there.