The FAA defines Density altitude (DA) as pressure altitude corrected for nonstandard temperature. When calculating aircraft performance we use density altitude.
As the density of the air increases (low density altitude), the aircraft performance increases. The same goes for high density altitude, as the air density decreases (thin air), the aircraft performance decreases.
Let’s simplify density altitude:
Aircraft performance, in respect to aspirated engine aircraft (non turbo charged or turbine engines) is affected by temperature, humidity, and elevation. If the density altitude is at 4,000 feet at an airport that is at sea level, the airplane will perform as if it is actually at 4,000 feet.
There are three factors that affect density altitude:
Low density air - means thick air, found near sea level or on a cool morning. The aircraft will perform great, better climb and engine performance.
High density air - means thin air, found on hot summer days. When the density altitude is high, the aircraft performs poorly, reducing power settings and lift. As if the airplane is flying at a higher altitude.
An example of poor aircraft performance on a high-density altitude day is, if we are taking off from Las Vegas (KLAS), the field elevation is 2,181 feet MSL. In August the altimeter setting is 29.76, and the OAT (outside air temperature) is 43⁰C, this would give us a DA of about 6,000 feet. With that said, the airplane would have a much longer takeoff roll and perform as if it were flying at about 6,000 feet MSL.
Humidity, also known as relative humidity is the amount of water vapor that can be held in the air, which is represented in percentage. As temperature increases the air can hold more water vapor Saturated air can no longer hold any more water vapor, this is known as 100% relative humidity. The more water vapor in the air the higher the density altitude. Water vapor molecules push the air molecules further apart, resulting in more thinner the air. The aircrafts overall performance produced by the wings and propellers is even less.
If high humidity does exist, it is a good idea to add 10 percent to your computed takeoff distance and anticipate a reduced rate of climb.
Humidity on Density altitude Example:
If the pressure altitude is 30.90” at Miami (KMIA), field elevation is 9 feet MSL, and the temperature/dew point is 27°C (80°F)/25°C, the relative humidity is 88%. The density altitude (DA) would be 700 feet.
The higher you go in altitude the thinner (less dense) the air is, the aircrafts performance decreases. This goes for flying in or out of high mountain airports like; Big Bear, CA (L35) with a field elevation of 6,752 feet MSL, or especially Leadville, CO (KLXV) with an elevation of 9,933 feet MSL. You are already starting out at a high altitude, then adding high density altitude on top of that.
How to figure DA:
First we need to find pressure altitude (PA). Finding pressure altitude is very basic, it is the altitude that is read directly from the altimeter when set to 29.92.
Now, if we don’t have access to an altimeter at the time, there is a quick easy formula available.
Formula: (29.92 – current alt. setting) x 1000 + field elevation = Pressure Altitude
PPL Test Supplement FAA-CT-8080-2H