Density Altitude & Air Density

International Standard Atmosphere (ISA)

29.92 "Hg (1,013.2 mb) @ std. temperature of 59 °F (15 °C) at sea level with a lapse rate of 2 °C per 1,000ft

Indicated Altitude

Altitude shown on the altimeter

True Altitude

Height above the mean sea level (MSL)

Absolute Altitude

Height above ground level (AGL)

Pressure Altitude

The indicated altitude when an altimeter is set to standard datum plane of 29.92 in Hg

Define Datum

Imaginary vertical plane or line from which all measurements of arm are taken

Density Altitude

Pressure altitude corrected for nonstandard temperature

• As temperature and altitude increase, air density decreases.

  • Basically it's the altitude at which the airplane "feels" its flying at

Air Density

It is the mass of air molecules within a given volume or set space

What 3 factors affect Air Density (DA)?

• Pressure

• Temperature

• Humidity

How does air pressure affect Air Density (DA)?

Density increases as air pressure increases, because higher pressure pushes air molecules closer together.

By approximately how much does air pressure decrease as altitude increases?

-1 inch of mercury (Hg) per 1,000 ft. of altitude gain.

How does temperature affect Air Density (DA)?

Hotter air = Less dense.

Cooler air = More dense

• With heated air, the molecules have more NRG which causes them to move around more & spread apart

How does Humidity affect Air Density (DA)?

For a given volume of air, you have more H2O molecules with increased humidity, which means there is more mass per volume of air (compared to dry air)

High VS Low Density Altitude?

High density altitude = thinner air

low density altitude = denser air

How does a low air density affect performance?

It reduces

• Power: due to less air for the engine

• Thrust: prop is less efficient in thin air

• Lift: you'll have to fly faster to get the same amount of generated lift

If air gets colder as you go up in altitude, wouldn’t the air be more dense & give us better performance?

No

• While it does get colder the higher you ascend, Air pressure also drops while ascending

How to calculate Pressure Altitude?

PA = 1,000 (29.92 - Current Altimeter Setting) + Elevation

How to calculate Density Altitude?

DA = 120 (Current Temp - ISA Temp) + PA