Why is calculating DENSITY ALT important

[BoDEAN]

Had a student ask me this today.
We figure out Pressure altitude, because our performance charts are based on pressure altitude.

I don't really use the e6b to find this out.
When I teach X-Countrys, I have them take 29.92 and subtract the local altimeter setting. Then this gives us what we need to correct our altitude to pressure altitude. Don't know if this is the "best" way of teaching it, but it works for me.

Just curious on how important the density altitude is for the student to calculate (late, having a brain fart)

 

[TEXAN AVIATOR]

Just curious on how important the density altitude is for the student to calculate (late, having a brain fart)

My opinion: It really depends on the area. I learned to fly in the mountains, so in the summer calculating Density altitude was a must. 9,000-10,000' density altitude on the field was not uncommon. So calculating this was a good idea. Especially because on days like this the PA28R-201 would climb at around 200fpm at vx. This will also get the student into the habit of checking for density altitude. So if they are based somewhere it’s typically not a factor but go to an unfamiliar area where it is, it’s regular practice.

It really just depends on the location and type of flight, local or XC.

TA

 

[pilotman2105]

Had a student ask me this today.
We figure out Pressure altitude, because our performance charts are based on pressure altitude.

Pipers performance chart will also allow for a calculation using density altitude (ie: it factors in the temperature as well)

Just curious on how important the density altitude is for the student to calculate (late, having a brain fart)

From Albuquerque FSS website, "What does [density altitude] mean to you? To a pilot this means that normal horsepower output is reduced, propeller efficiency is reduced and a higher true airspeed is required to sustain the aircraft. It also means an increase in runway length requirements for takeoffs and landings, and decreased rate of climb."

http://www.abqafss.jccbi.gov/Density.htm

 

[GFRacerX]

Density altitude is Pressure altitude corrected for non-standard temperature. The charts you see in your P.O.H. have different pressure alltitudes for different temperatures thus density altitude. The performance for you plane is based on Denity Alltitude, On a cold day with a high pressure alltitude your plane may perform better that a hot day with a low pressure alltitude. Makes a big diierence on takeoff roll, landing distance and climb performance.

 

[GFRacerX]

Oh yeah, the formula is 29.92 minus current altimeter setting multiplied by 1000 plus your field elevation. Thst is your pressure altitude. IF you don't have a current altimeter setting on the field you can go out to the plane, dial in 29.92 and look at your altimeter. That will also give you pressure altitude.

 

[labbats]

To demonstrate a high density altitude in the midwest, just don't allow them to go past a given manifold pressure on a constant speed prop plane. For instance, if your altitude is sea level and you get 28" of MP, then taking off at a density altitude of 8000' would give you 20" of MP. (1" per 1000'). Have the student take off on a looooong runway and only allow them to bring the power to 20" MP. While this won't completely mirror a high density takeoff, it's pretty close. That or try taking off from Cheyenne in summer. It's scary to see an airplane that jumps off the ground in the midwest struggle to get off in time up there.

 

[midlifeflyer]

Had a student ask me this today.
We figure out Pressure altitude, because our performance charts are based on pressure altitude.

1. Not all do. Older PA28s use density altitude calculations. There are likely others as well.

2. AWOS gives you the density altitude. Would be nice to know how to use it, wouldn't it?

3. The charts end. For example, the highest number in the CE-172 charts I've seen is for 8000' and 40ºC. So, what do you do when thinking about taking off from Leadville CO (9927 msl) when the temperature is a balmy 20ºC?

4. The lean for takeoff above 3000' checklist item is based on density altitude. You don't have to start out that high. Take PDK in "Hotlanta" on a 90ºF day, and you are there.

I don't really use the e6b to find this out.
When I teach X-Countrys, I have them take 29.92 and subtract the local altimeter setting. Then this gives us what we need to correct our altitude to pressure altitude. Don't know if this is the "best" way of teaching it, but it works for me.

Why not just go to the airplane, put 29.92 in the Kollsman window and read the pressure altitude off the dial?

 

[underdog]

Just curious on how important the density altitude is for the student to calculate (late, having a brain fart)

Sarcastic answer="Because crashing at the end of the runway is not good" .

Don't have the numbers for training type aircraft, but consider the following BFL numbers for a G4.

Takeoff weight=74500
Temperature= 15c
Altimeter=29.92
Sea level airport
BFL will be 5308'

Change temp to 90f (32c), the BFL is now 6100'. If your available runway was only 5300', you would have to reduce takeoff weight by more than 4000 pounds.

Failure to understand and apply the correct settings for density altitude will kill you.

 

[pilotman2105]

Re: Re: Why is calculating DENSITY ALT important

Failure to understand and apply the correct settings for density altitude will kill you.

And people don't believe me when I say that Darwinism exists in airplanes.

 

[greyhound]

DA...

I would explain like this....

You must know the DA of the field in order to get more accurate performance figures.

Just because you are at an airport at 1500 feet msl.
Doesn't mean you should use 1500ft/msl figures from your performance charts.

When the temp. is non-standard (Almost always is),
You need to know DA so you use that figure to calculate performance.

Example;
Elevation 2000 ft. MSL
Temp.80 F

Gives: DA 3900 ft.

When you go back to calculate your performance figures.
you should use 3900 ft. as your reference.

Not 2000.

 

[Timebuilder]

The short answer to your student's question, "why," is this: because this is the one figure that will be the main indication of the kind of performance the airplane will give you under the given circumstances.

It isn't a stretch for most students to grasp that cooler air at a higher pressure means more molecules of air available in any particular space or volume, whether those molecules are being allowed to enter a cylinder by means of an engine intake valve, or being met by a propeller or a wing. By the same token, most "good flying weather" happens at a higher temperature than standard, so the "standard" figure almost always must be discounted to a level of lessened performance.

It's a valuable means of making a very good guess about all of the values you need to know to take off, fly, and land the airplane safely.