Density Altitude

DENSITY ALTITUDE 

Dear future private pilot (PPL) and Air Transport pilot (ATPL). In This post we are going to talk about the most important concept in the aircraft performances, which is Density Altitude (DA)

We introduced the concept of density altitude in the post : TYPES OF ALTITUDE. With this post we will talk deeply about the density altitude concept and the factors that affect the density altitude.

¿ What is the Density altitude (DA)?

It is an altitude that it is only used for calculations issues. As you know depending of the air density, the engine efficiency changes, because it can move more particles backwards to go forwards (third Newton´s law). The effect is similar as if we are swimming. While swimming we are sending water backwards to move. We all know that swimming with a swimfins makes us move faster. Have you ever wonder why?

The answer it is easy as with the swimfins we are moving more particles of water backwards so we move faster forwards.

I would like to remind that the density is directly related with the mass of the fluid. So the higher the density is the more efficient is the engine.

¿Why it is only used for calculations issues?

The engineers calculate the behaviour of the aircraft in every value of the density, always referred to the ICAO International atmosphere.

For example, As it is shown in the table above at 1000 feet in the ICAO International atmosphere we have a density of 0.9711. So the engineers calculate the aircraft behaviour (efficiency of the engine, flight controls …. ) and them assign all those values to a 1000 feet Density altitude. The same procedure they do with the other density values.
Imagine that is calculated that with a density altitude (D.A) of 1000 feet we need a runway with a minimum distance of 1100 meters, and with a density altitude of 2000 feet we need 1500 meters.
If we are in a field with a real altitude of 1000 feet and this day we are in I.S.A conditions, the density altitude (D.A) is 1000 feet, so we need 1100 meters of runway. However not always we have the I.S.A conditions.

What would happen if we are not in ISA conditions?

Lets say that the temperature is higher than I.S.A. If we take a look to the article Atmosphere Characteristics, and remind this formula: P=ρRT. If temperature raises leads on less density for keeping the same pressure (we are just explaining the variation of temperature). As you can see in the table above the less density corresponds to the higher density altitude.
For training purposes lets say that the new density is 0.9428 which corresponds to a density altitude of 2000 feet. So in this case our real altitude is 1000 feet, but our plane behaviour is as it was at 2000 feet ( in this conditions it would need 1500 meters runway).
To summarize higher temperature leads on less density and higher density altitude, so our aircraft performance is worse.

Now the pressure is lower than ISA. Following the formula, explained in Atmosphere Characteristics, P=ρRT. It results in a lower density, as we have less density we have higher density altitude (DA).

Higher humidity makes the air to be lighter (weigh less) causing the density to decrease and the density altitude (D.A) to increase.

To conclude everything that makes the density dropping leads on an increment of the density altitude (D.A), which reduces the aircraft performances.