What do pilots use the Density altitude for?

Question

As a controller we do have the density altitude available if it is requested and we include it in our ATIS brodcast but it is not issued with the numbers, Runway, wind, altimiter. I am wondering if this is something that is relatively important and what it is used for.

Answer 1

Density altitude is the altitude the A/C thinks it at:

You set the Kolsman to 29.92 and whatever the altimeter reads, that's the altitude the A/C thinks it's at and you base your performance on that and not the actual altitude.

Example: Your at an airport which is at Sea Level, you set your Kolsman to 29.92 and let's say it reads 750 feet, you then base your performance (Take off roll, etc) on this figure.

Answer 2

It affects many things--especially takeoff roll distance and climb performance.

Answer 3

It's usually only mentioned if it is significant. So for example when I'm flying through a place like Gallup NM (elev. 6472) and it's really hot, they'll mention density altitude as a reminder that I'm going to climb a lot slower than normal. In a light twin when it's hot and high, I might already be above my single-engine service ceiling before I take off!

So while I always try to be aware of density altitude, having it included in ATIS is a gentle reminder that the "true" airport elevation may not be what it says on my chart.

Answer 4

so they can figure out there altitude,

the altimeters are dependant on air pressure,

which as we know changes with the weather, so to get an accurate reading they need to know that :D

Answer 5

Density altitude is a direct measure of the density of the air relative to the density at sea level under standard conditions.  Density altitude is the operant quantity in calculations of relative takeoff and landing distance requirements, engine power, stall speeds, and other critical variables in the performance of an airplane when you are hot, high, thin, or any combination thereof.

So it is critically important in flight planning under many conditions.

Answer 6

On small aircraft they use it to measure their performance.  On certificated airliners, the charts they use take care of all of that.  We have charts that we go to the specific runway and temperature and it tells us what the maximum takeoff weight will be for those conditions.  Smaller aircraft usually figure out how much runway they will need and then decide if they can takeoff that day.  Since airliners usually go no matter what the temp is, they just want to know what the maximum weight for takeoff will be.  Of course, the charts we use also take into account obstacles and other factors that have to be runway specific.

Answer 7

To figure out how dense the altitude is silly!!

Answer 8

Yes it's important in regard to expected aircraft performance because the Flight Manual's numbers are based on the standard atmosphere. This provides for easy comparisons between different aircraft. Any deviation from standard (temp, barometric pressure, elevation, humidity and lapse rates for pressure and temperature), affects the performance numbers, such as acceleration, climb rate, and maximum operating altitude.

For example, the standard temp at sea level is 59 degrees F, but the actual temperature is 30 degrees above standard or 89 degrees. The aircraft will perform as if it's already about 1800 feet (the density altitude). The performance at 1800 feet is less than at sea level because the air is less dense.