SIGMET.siBack to briefing
INSIGHTS / PERFORMANCE

Density altitude and takeoff distance

The airplane performs as if it were at the density altitude, not the field elevation. On a hot day, that difference can decide whether the runway is long enough.

What density altitude is

Density altitude is pressure altitude corrected for non-standard temperature. The standard atmosphere assumes 15 °C at sea level, cooling about 2 °C per 1,000 ft. When the air is warmer than standard it is less dense, so the wing, propeller and a normally aspirated engine all produce less.

A rule of thumb

Density altitude rises roughly 120 ft for every 1 °C above the standard temperature at that altitude. A sea-level field at 35 °C is about 20 °C above standard, which puts density altitude near 2,400 ft. High humidity reduces performance a little further. Use the POH charts or an E6B for actual figures; the rule of thumb is only a cross-check.

Effect on takeoff

  • A higher true airspeed is needed for the same indicated rotation speed, so the ground roll gets longer.
  • Engine power falls with density, so acceleration is slower.
  • Climb rate drops, so the distance to clear a 50 ft obstacle grows faster than the ground roll.
  • Weight, runway slope, surface (grass, wet) and tailwind make it longer still. Apply the POH correction for each.

Using a margin

POH figures come from a new aircraft flown by test pilots in ideal technique. Many instructors and safety organizations recommend adding a margin, commonly 50%, to the book distance over a 50 ft obstacle. Plan to abort the takeoff if the airplane is not near rotation speed by a decision point you chose in advance.

How SIGMET.si shows it

The runway chart compares a modeled ground roll and 50 ft distance with the length of the selected runway. It turns amber only when the 50 ft distance plus 50% is longer than the runway. The model is illustrative, not POH data; see the methodology.

Sources

  • FAA-H-8083-25, Pilot’s Handbook of Aeronautical Knowledge, Chapter 11 (Aircraft Performance)
  • FAA-P-8740-2, Density Altitude (FAA Safety Team pamphlet)
  • The approved Airplane Flight Manual / POH for the specific aircraft

Educational summary reviewed 8 Oct 2026. Always use the current official editions and an official briefing; the Pilot in Command retains final authority under 14 CFR § 91.3. More guides in Insights.