Free sim
Density Altitude
Your airplane doesn't care what the altimeter says — it performs at the altitude the air feels like. Set the field elevation, temperature, and altimeter, and watch the ground roll stretch past the sea-level baseline: the engine makes less power, the prop grips less, and the same lift-off airspeed takes longer to arrive.
Field elevation 5,000 FT, 30 degrees Celsius, altimeter 29.92 inches: pressure altitude 5,000 FT, density altitude 7,800 FT, ground roll 1.60 times the sea-level standard-day book number.
The airplane performs as though it were at the density altitude, no matter what the field elevation says — thinner air means less power, less prop grip, less lift, and the same lift-off airspeed arriving later. The multiplier applies to your POH's sea-level standard-day number; the POH always wins. Charts don't include humidity, so a muggy day is worse than the math above.
Source. FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 11, Aircraft Performance (FAA-H-8083-25C), pp. 11-3 to 11-4, Density Altitude — the definition and its effect on performance; p. 11-5, Effects of Humidity (Moisture) on Density — no chart, expect a decrease; p. 11-14 to 11-16, Takeoff Performance — same IAS at altitude, acceleration varies with density. Retrieved from faa.gov 2026-09-08 and frozen in this site’s repository, where a test checks every quoted phrase against that copy. The ground-roll multiplier is derived from two of those statements (same indicated lift-off speed; acceleration varying with density) and is presented as a multiple of your own POH figure, never as a distance of its own.
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Next step
- Compute density altitude on the E6B
- Takeoff and landing performance tool
- Density altitude and performance — a verified checkride question
- Find your weak areas — the free 24-question Readiness Test, scored by knowledge area.
- Create a free account for the full bank: 2,074 verified questions with the FAA source on every answer, a timed practice exam, and a missed-questions drill.
The Private Pilot Quick-Review Study Guide covers this in chapter 4 (performance). It is in CFI review and not on sale yet; the guide page has the list for launch notes.