Density Altitude: The Invisible Runway Thief
Hot, high, and humid days move the airplane's true altitude thousands of feet upward — and every performance number moves with it.
A 3000-foot runway is a 3000-foot runway on Tuesday and a much shorter one on Saturday if Saturday is hotter, higher, and more humid. The dimension that changes is not the pavement — it is the density altitude: the altitude at which the airplane actually thinks it is.
Density altitude is pressure altitude corrected for non-standard temperature. The standard atmosphere assumes 15 °C at sea level, falling about 2 °C per 1000 ft. When the actual temperature is above standard, the air is less dense; the wings make less lift at a given IAS, the propeller bites into less mass, and the engine ingests less oxygen. A field elevation of 1200 ft on a 35 °C day can produce a density altitude above 4000 ft. The airplane behaves accordingly.
Every performance number in the POH is calibrated to density altitude, not to the field elevation printed on the sectional. A typical piston single might see takeoff distance grow 10% for every 1000 ft of density altitude above field elevation, and climb rate shrink faster than that. Above about 5000 ft density altitude, most fixed-pitch normally aspirated engines cannot hold full rated RPM at all — mixture leaning becomes a takeoff checklist item, not a cruise item.
The pilot's job is to compute density altitude before every summer takeoff and treat it as the real number. Add temperature, field elevation, and altimeter setting; run the density-altitude chart in the POH; then take the resulting takeoff, climb, and landing numbers seriously. The airplane will not surprise you with a bad number if you look up the number first.