Airplane performance

Density altitude and the POH charts: how weight, wind, and altitude change takeoff, climb, cruise, and landing.

17 min read · The airplane: engine and systems

After this lesson you can

  • Compute pressure altitude from field elevation and altimeter setting, and density altitude from pressure altitude and temperature.
  • Read and interpolate a takeoff distance table using real PHAK figures.
  • State the approximate effect of a 10 percent increase in weight, and of headwind and tailwind, on takeoff distance.
  • Distinguish Vx from Vy and explain when each is used.

A light airplane's performance is not a fixed number printed on a placard -- it is the result of the day: weight, air density, wind, and runway condition. The same airplane that clears a 1,300-foot strip on a cold winter morning at sea level might need twice that distance on a summer afternoon at 3,000 feet.

Nearly everything in this lesson traces back to one physical quantity: air density. Less dense air means less power from the engine, since it draws in less air per stroke; less thrust from the propeller, since it moves a smaller mass of air per revolution; and less lift at a given indicated airspeed.

The AFM/POH numbers behind all of this come from flight tests with a new airplane, average piloting skill, and a well-maintained engine. Treat them as a best case to be adjusted downward, not a guarantee.

The rest of this lesson

  1. 1.The standard atmosphere
  2. 2.Pressure altitude
  3. 3.Density altitude
  4. 4.Reading a combined AFM/POH performance graph
  5. 5.A worked example: interpolating a takeoff distance table
  6. 6.Correcting for weight and wind
  7. 7.Climb: Vx, Vy, and the two ceilings
  8. 8.Landing distance, runway slope, and hydroplaning
  9. 9.Range, endurance, and the long-range cruise compromise
  10. 10.Pilot technique, configuration, and the airport itself

These 10 sections, their figures, the checks along the way and the end-of-lesson quiz come with the PPL track. Paid once, yours for good.

See what the PPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.