10.pressure-altitude-vs-density-altitude. Pressure Altitude vs. Density Altitude
Altitude has several distinct meanings in aviation, and student pilots must clearly distinguish between pressure altitude and density altitude because both directly influence aircraft performance, instrument indications, and regulatory limits.
Pressure altitude is the height above the standard datum plane (SDP), a theoretical level where atmospheric pressure equals 29.92 inches of mercury (1013.2 hPa) and the temperature is 15 °C. Because the actual atmosphere rarely matches these standard conditions, pressure altitude is a reference value — not a true geometric height — used for performance computations, flight planning, and altimeter operation above 18,000 feet MSL (the floor of Class A airspace, where all aircraft fly on the standard setting of 29.92).
To determine pressure altitude in flight, the pilot momentarily sets the altimeter's Kollsman window to 29.92 "Hg and reads the indicated altitude. On the ground, the same procedure yields the pressure altitude of the airport. Alternatively, pressure altitude can be computed from field elevation using the rule of thumb:
Pressure Altitude = Field Elevation + (29.92 − Current Altimeter Setting) × 1,000
For example, at an airport with a field elevation of 4,000 feet and a current altimeter setting of 29.45, the pressure altitude is approximately 4,000 + (0.47 × 1,000) = 4,470 feet.
Density altitude is pressure altitude corrected for nonstandard temperature. It represents the altitude in the standard atmosphere at which the air has the same density as the air the aircraft is presently flying through. Because aircraft engines, propellers, and wings all behave according to the density of the air — not its geometric height — density altitude is the single most important atmospheric value for predicting takeoff distance, climb rate, true airspeed, and service ceiling.
Three variables drive air density and therefore density altitude:
- Pressure — as pressure decreases (higher altitude, lower altimeter setting), density decreases and density altitude rises.
- Temperature — as temperature increases above ISA, air expands, density falls, and density altitude rises.
- Humidity — water vapor displaces denser dry air molecules; high humidity further reduces density. Humidity is not directly factored into standard performance charts but should be considered qualitatively.
A practical rule of thumb is that density altitude increases approximately 120 feet for every 1 °C the outside air temperature exceeds the standard temperature for that pressure altitude. Standard temperature at sea level is 15 °C and decreases at the standard lapse rate of 2 °C per 1,000 feet. Therefore at a 5,000-foot pressure altitude, ISA temperature is 15 − (5 × 2) = 5 °C.
Example: An airport sits at 5,000 feet pressure altitude with an OAT of 30 °C. Standard temp is 5 °C, so the deviation is +25 °C. Density altitude ≈ 5,000 + (25 × 120) = 8,000 feet. The airplane will perform as though departing an 8,000-foot field on a standard day — significantly degraded.
Density altitude can also be read from the density altitude chart in Chapter 10 of the PHAK or computed using an E6B or electronic flight computer. Most POH performance tables ask the pilot to enter with pressure altitude and OAT, internally accounting for density effects.
Effects of high density altitude on aircraft performance:
- Reduced engine power (normally aspirated engines lose roughly 3% power per 1,000 feet of density altitude).
- Reduced propeller thrust (the prop bites less air per revolution).
- Reduced lift (wings produce less lift at the same indicated airspeed because fewer air molecules pass over them).
- Longer takeoff and landing rolls — sometimes dramatically so.
- Decreased climb rate and lower service ceiling.
- Higher true airspeed for a given indicated airspeed, which can lengthen runway requirements and increase touchdown groundspeed.
Summary of relationships:
- Indicated altitude — what the altimeter reads with the current local setting.
- True altitude — actual height above mean sea level.
- Absolute altitude — height above ground level (AGL).
- Pressure altitude — height above the 29.92 "Hg standard datum plane.
- Density altitude — pressure altitude corrected for nonstandard temperature (and, qualitatively, humidity).
Understanding these definitions and the math that connects them is required for accurate use of POH performance charts and for compliance with 14 CFR 91.103, which obligates the pilot in command to become familiar with takeoff and landing distance data for the planned flight. On hot summer afternoons at high-elevation airports, density altitudes of 9,000–11,000 feet are routine and have contributed to numerous accidents in which pilots accepted indicated altitude figures at face value rather than computing how the airplane would actually fly.