different types of altitudes

FlyBoyJae

New Member
ok what is the difference between indicated altitude and true altitude?? in jeppesen's manuel, they say that indicated is what you read off of the altimeter and true is MSL...does indicated equal true then??
 

E_Dawg

Moderator
No, when temp is nonstandard and/or there are errors in the instrument, indicated will be different from true.

On hot days you'll be higher than the altimeter says and on cold days you'll be lower.
 

FlyBoyJae

New Member
[ QUOTE ]
No, when temp is nonstandard and/or there are errors in the instrument, indicated will be different from true.

On hot days you'll be higher than the altimeter says and on cold days you'll be lower.

[/ QUOTE ]

i guess that is the famous "high to low, look out below, low to high look to the sky"

so, i guess as long as we adjust the barometric pressure, we will be fine right?
 

fucius

Well-Known Member
indicated altitude is "approx." true altitude when the altimeter setting is set properly in the Kollsman window (correction for non-standard pressure). The indicated altitude will also vary from true altitude when flying in non-standard temps... but there is no correction for that, hence the "approx." true altitude will be shown on the altimeter. Indicated altitude is equal to true altitude only under standard temperature and pressure conditions.
 

FL270

New Member
[ QUOTE ]
so, i guess as long as we adjust the barometric pressure, we will be fine right?

[/ QUOTE ]You will be fine, to the extent that your altitude will be off by nonstandard temperature by the same amount as everybody else's. No altimeter corrects for non-standard temperature, but we do (except in the flight levels) all correct for the current barometric pressure. Make sense?

Russ
 

BobDDuck

Island Bus Driver
This leads nicely to a question I was going to ask. Indicated altitude = pressure altitude? And then if your correct for the tempature (using a chart of calculator) you have the density altitude? Do I have it right? Gleim doesn't do a great job on that chapter.

Ethan
 

FL270

New Member
First half wrong, second half right.

Indicated altitude does not equal pressure altitude UNLESS the altimeter setting is standard (29.92"). Otherwise, you obtain pressure altitude by correcting for the non-standard pressure.

You are correct that pressure altitude corrected for nonstandard temperature is density altitude.

Russ
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
Indicated altitude does not equal pressure altitude UNLESS the altimeter setting is standard (29.92"). Otherwise, you obtain pressure altitude by correcting for the non-standard pressure.

[/ QUOTE ] ...or by turning the Kollsman setting to 29.92 for a moment and reading pressure altitude right off the dial.
 

BobDDuck

Island Bus Driver
Got it... So pressure altitude is the altitude corrected for the pressure (duh) and if the pressure happens to be 29.92 then it will be true altitude (MSL). Thanks.

Ethan
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
Got it... So pressure altitude is the altitude corrected for the pressure (duh) and if the pressure happens to be 29.92 then it will be true altitude (MSL).

[/ QUOTE ]Maybe. Not sure exactly what you mean by "corrected".

At it's simplest, Pressure Altitude is what your altimeter would say if the sea level pressure were 29.92. Don't try to read too much more into it.

Let's step back a moment. Altitude in general has two purposes in aviation. Obviously, it's how we measure our height off the ground. But it's also used to measure performance. Changes in altitude affect how our engines, propellers, wings, and the whole aircraft perform. As we get higher and higher, the density of the air gets less and less (until we hit outer space). And we want to be able to measure those performance differences.

When we want to measure things, it's usually helpful to start with some kind of standard. A place to start. It could be anywhere. When we measure distance, we use a ruler and start with 0 as our standard. When drug companies test a new drug, they have a control group - a set of people who aren't taking the new drug - to they can compare results.

The standard we use for altitude comparisons is "Pressure Altitude". Someone along the way came up with the concept of a "standard day" also called the "standard datum plane" — 15°C and 29.92" at sea level. They could just as easily come up with 0ºC and 30.00". Don't read too much into it. It's really only a starting point for making other comparisons. A way of measuring other things.

And "true altitude" isn't simply what your altimeter reads when it's set at 29.92 on a day when the sea level pressure is really 29.92. True altitude is your actual height above sea level. Remember that the altimeter doesn't correct for nonstandard temperature? Well, in order for the altimeter to tell you true altitude, the sea level pressure has to be 29.92 =and= the real pressure at your altitude has to be standard for that altitude =and= the sea level temperature has to be 15ºC =and= the real temperature at your altitude has to be standard for your altitude.
 

MidlifeFlyer

Well-Known Member
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My head is spinning!

[/ QUOTE ]Exactly. As soon as you try to treat "pressure altitude" as anything other than a yardstick, it gets complicated.
 
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