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DENSITY ALTITUDE



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:

  • Temperature

  • Humidity

  • Elevation


Temperature:

  • 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:

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 thinner 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.



Elevation:

The higher you go in altitude the thinner (less dense) the air is, the aircraft's 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 density altitude?

There are two primary ways to calculate density altitude, using the following methods:

  • Density altitude chart

  • E6B flight computer

Keep in mind this are approximate calculation, when using either method.


Pressure altitude and outside air temperature.

To find density altitude we first need to find to numbers:

  • Pressure altitude

  • Outside Air Temperature (OAT)

Using the aircraft altimeter, which reads pressure altitude when in the Kollsman Window is set to 29.92'.

The OAT can be read from the thermometer in the aircraft or taken from the ATIS, AWOS. Remember, to use degrees Celsius.


Using the Density altitude chart:

Using the Density altitude chart can be done in a couple different ways.


For this example we read a pressure altitude of 2,500ft, and the Outside Air Temperature (OAT) is 30°C.

  1. Start by drawing a line vertically from the bottom of the chart at the corresponding OAT (30°C) up to the diagonal line representing the pressure altitude (2,500ft).

  2. Then draw a horizontal line to the left until you reach the density altitude indication.


The first example, the Density Altitude is approximately 4,800ft.



A different way to use the density altitude chart is a two-step process.


For this second example, we will use the altimeter setting of 29.80, OAT of 30°C, and now a field elevation of 1,349ft.

1. First find the pressure altitude using the altimeter setting. Either subtract or add to the airport elevation.

1,349

+ 112

PA = 1,461


2. Next, locate the given OAT on the bottom. Move up to the pressure altitude figured in step one. Then move across and find the Density altitude.


The Density Altitude is approximately 3,000ft


Note: To find the altimeter setting, OAT, and field elevation, you will need to reference the METAR and sectional chart, or any other sources available.




Calculating density altitude with the E6B:

First, we need to know the current pressure altitude and OAT. (NOTE: The same method can be used to get the pressure altitude as mentioned early, by setting the altimeter to 29.92).

  1. To calculate density altitude using the E6B, line up the pressure altitude window with the temperature scale above. Pay close attention to which side the positive and negatives numbers are located when setting temperature.

  2. Next read the altitude in the window above highlighted Density Altitude.


Using the same numbers for this example, pressure altitude is 2,500ft, and the Outside Air Temperature (OAT) is 30°C.


The Density Altitude is approximately 4,900ft.




Remember:

To find density altitude we first need to find to numbers:

  • Pressure altitude

  • Outside Air Temperature (OAT), remember to allows use Celsius.

Keep in mind this are approximate calculation, when using either method.




References:


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