Density Altitude
Pressure altitude corrected for non-standard temperature; higher DA reduces performance.
Formula
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Example
On a hot day a runway can feel thousands of feet “higher,” sapping engine and wing performance.
How it works
Density altitude is a way of expressing how the air behaves for an aircraft by combining altitude and temperature into one performance number. Since warm air is less dense than cold air, an aircraft at a given pressure altitude will "act like" it is at a higher altitude when the outside air temperature is above the ISA standard, which reduces lift, engine power, and propeller or rotor efficiency. The 120 factor is an aviation rule of thumb that converts each 1°C deviation from standard temperature into about 120 feet of equivalent altitude change.
When to use it
Use this when estimating takeoff, climb, and landing performance, especially at airports with high elevation, hot weather, or both. It is also useful in flight planning to judge whether an aircraft can safely depart with the planned load.
Worked example, step by step
Given: PA = 5,000 ft, OAT = 32°C Step 1: Find the ISA standard temperature at 5,000 ft. ISAtemp = 15 - (2 × 5) = 5°C Step 2: Find the temperature difference from standard. OAT - ISAtemp = 32 - 5 = 27°C Step 3: Multiply by 120. 120 × 27 = 3,240 ft Step 4: Add this correction to the pressure altitude. DA = 5,000 + 3,240 = 8,240 ft Final answer: The density altitude is 8,240 ft.
Common mistakes to avoid
- Using true altitude or field elevation instead of pressure altitude
- Forgetting to subtract the ISA standard temperature before applying the 120 factor
- Assuming density altitude is only about altitude and ignoring the effect of temperature
Did You Know?
On a hot day at a high airport, density altitude soars and aircraft performance drops — a known killer in aviation.
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