Would someone help verify that I have an understanding of these altitudes?

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StoodentPilöt

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StoodentPilöt
**Indicated altitude**
Indicated altitude is your altitude indicated on your altimeter when set to the local barometric pressure. Just as much as you can say your indicated _airspeed_ tells you how fast you're going, your indicated altitude tells you how high you are above MSL. But just like indicated airspeed, we all know there are some corrections that need to be made before you can trust your indicated altitude.

Indicated altitude is expressed as feet MSL

**Local Barometric Pressure**
The local barometric pressure that you put in the kollsman window is what your pressure _would be_ at sea level. My airport for example is roughly 400' above sea level. If I simply put in the kollsman window the pressure at the height of my airport, my altimeter would read zero.

**Pressure Altitude**
Pressure altitude is expressed as feet above the standard datum plane. If we come up with, and use, a single reference (datum), then planes can all avoid each other much easier. Above 18,000' MSL, everyone follows a decided-upon 'zero point' which is the datum plane.
Pressure altitude is indicated altitude corrected for non standard pressure

**Standard Datum Plane**
The standard datum plane is decided to be 29.92 inches of mercury, based off the average air pressure at sea level. While 29.92 is based on the average pressure at sea level, the reality of it is, the air pressure is very rarely standard 29.92 inches of mercury. The standard datum plane can be thought of as an imaginary line at sea level that dips and rises. The starts datum plane does not remain as a constant imaginary line that rests at MSL around the whole earth.

**Question #1:**
Why use pressure altitude for performance calculations if it doesn't even include temperature?

**Density Altitude**
Density altitude is pressure altitude corrected for non standard temperature. It is expressed as height in feet of what the plane is going to behave like at that altitude. Density altitude is where the plane feels like it's at. Density altitude is used for performance calculations. You could be literally flying at 5000 feet, but your plane is behaving like it's at 8000 feet.

**True Altitude**
True altitude is where the airplane actually is above the MSL. It is indicated altitude corrected for temp and pressure. Much like indicated airspeed has faults that are corrected to give us our true airspeed, indicated altitude has faults that are corrected to give us our true altitude

**Question #2**
How are true altitude and density altitude different? If they are both indicated altitude corrected for non standard pressure and temperature?

**Summary**
Indicated altitude when set to local barometric pressure is raw data. It is only accurate on the ground and set correctly. When you fly upwards, there are new temperatures and pressures to correct for. When you correct for them you get true altitude.

**Question #3:**
What happens if you take off, and every single foot you climb has the exact standard temperature it is supposed to be? Would true altitude and indicated altitude always be the same?

Thank you for your help
 
Why use pressure altitude for performance calculations if it doesn't even include temperature?
Performance charts sometimes start with pressure and then compensate for temperature because pressure altitude is easier to determine in the cockpit (set kollsman window to 29.92). Somethings are based on pressure only. Your lungs, for example only care about pressure.

How are true altitude and density altitude different? If they are both indicated altitude corrected for non standard pressure and temperature?
True altitude is not something you can measure easily. Without precise surveying equipment, or a long tape measure that reaches down to sea level, you will never know what your true altitude is. Setting the correct altimeter setting gives you indicated altitude (it is making some assumptions about how pressure changes with altitude). Then add nonstandard temperature to get density altitude. But neither of these are true altitude because of the assumptions being made.

What happens if you take off, and every single foot you climb has the exact standard temperature it is supposed to be? Would true altitude and indicated altitude always be the same?
See above answer. true altitude != density altitude
 
StoodentPilöt said:
**Question #1:**
Why use pressure altitude for performance calculations if it doesn't even include temperature?
Click to expand...
The airplane’s charts include temperature.
StoodentPilöt said:
**Question #2**
How are true altitude and density altitude different? If they are both indicated altitude corrected for non standard pressure and temperature?
Click to expand...
An altimeter measures the weight of the atmosphere above you. If the atmosphere was the same thickness over, say, a 2000 ft ground elevation as it was over something close to sea level, with the same temperatures you’d get the same density altitude but very different true altitudes.
 
FAA's definition of density attitude is trash and leads to a lot of confusion. You calculate DA by taking pressure altitude and adjusting for temperature, but you're not "correcting" anything, you're converting it. DA is not a measurement of altitude at all, but rather a measurement of air density, expressed in terms of altitude in the International Standard Atmosphere where that same density exists, instead of lb/ft3, kg/m3, or whatever other unit.
 
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I still don't understand pressure altitude.
I can recite the definition but I don't get it. Never have.
 
SixPapaCharlie said:
I still don't understand pressure altitude.
I can recite the definition but I don't get it. Never have.
Click to expand...
Its Easy, we all decide to stop working about the local pressure and dial 29.92 into the meter. THen we get assigned altitudes to avoid each other. We don't have to avoid obstacles "in the US" because we are over FL180.

And then for pressure altitude, we adjust for the actual pressure ;)
 
The goal is to determine how the aircraft will perform at a given altitude under the conditions that exist that time.

If the pressure altitude is 900 ft above the data plane at sea level, it is also 900 ft above the data plane at 6000 ft. It is the simplest conversion and the pilot is only required to know one value for sea level and the altimeter does the math for you for higher altitudes.

The altimeter is basically a scale. It can determine the weight of the air, it’s has no way to determine how dense the air is and does not account for non standard temperature. The pilot must apply an another correction to determine what physical altitude above sea level they would be at standard pressure and temp..
 
dmspilot said:
Yet you use it every day at your new job
Click to expand...

I just don't have a practical understanding of it.

MSL easy - feet above the ocean. I've seen the ocean
AGL easy - feet above the rocks. I have fallen on the rocks.
Density - thinner air up high = must move faster, Ice is less dense than water. It floats.

Pressure altitude is the height of an aircraft above a theoretical standard datum plane.
Dafaq is a theoretical standard datum plane? How do I swim in it or fall on it or float it in my Sprite Zero?
 
SixPapaCharlie said:
MSL easy - feet above the ocean. I've seen the ocean
Click to expand...
It's not height above the ocean, which is variable, it's height above Mean Sea Level. See below.

SixPapaCharlie said:
Dafaq is a theoretical standard datum plane?
Click to expand...
The standard datum plane for pressure altitude is mean sea level (ETA: at 29.92 inHg and 15 degC). With an altimeter setting of 29.92 your altimeter reads pressure altitude (disregarding things like parallax, instrument errors and position error)

Nauga,
and his tower flybys
 
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StoodentPilöt said:
**Local Barometric Pressure**
The local barometric pressure that you put in the kollsman window is what your pressure _would be_ at sea level. My airport for example is roughly 400' above sea level. If I simply put in the kollsman window the pressure at the height of my airport, my altimeter would read zero.
Click to expand...
Try this experiment:
Set the altimeter to your local airfield elevation, then note the value in the Kollsman window. Compare this setting to the local barometric pressure reported from the airport AWOS or ATIS. The values should be close.
 
dmspilot said:
FAA's definition of density attitude is trash and leads to a lot of confusion. You calculate DA by taking pressure altitude and adjusting for temperature, but you're not "correcting" anything, you're converting it. DA is not a measurement of altitude at all, but rather a measurement of air density, expressed in terms of altitude in the International Standard Atmosphere where that same density exists, instead of lb/ft3, kg/m3, or whatever other unit.
Click to expand...

True, density and pressure have different units. But pressure altitude and density altitude are both measured in ft, so it is not technically a unit conversion. We could use psi for pressure and lb/ft3 for density, but then it would become extremely cumbersome to use these values in aviation. However, I somewhat agree that correction does not mean we are correcting for an error. It is not an error, just accounting for additional factors in the calculation.
 
SixPapaCharlie said:
I still don't understand pressure altitude.
I can recite the definition but I don't get it. Never have.
Click to expand...

No, you do. You are flying at 30,000 ft, but your lungs feel like they are at 8000 ft. That's pressure altitude. 30,000 ft outside, 8,000 inside.
 
Cap'n Jack said:
Try this experiment:
Set the altimeter to your local airfield elevation, then note the value in the Kollsman window. Compare this setting to the local barometric pressure reported from the airport AWOS or ATIS. The values should be close.
Click to expand...

With engine off should also be close to what you see on the the engine manifold pressure gauge.
 
SixPapaCharlie said:
I just don't have a practical understanding of it.

MSL easy - feet above the ocean. I've seen the ocean
AGL easy - feet above the rocks. I have fallen on the rocks.
Density - thinner air up high = must move faster, Ice is less dense than water. It floats.

Pressure altitude is the height of an aircraft above a theoretical standard datum plane.
Dafaq is a theoretical standard datum plane? How do I swim in it or fall on it or float it in my Sprite Zero?
Click to expand...
The data plane
SL =29.92.
1000 = 28.92
2000 = 27.92

Non Standard.

SL= 28.92 which is where 1000 ft exists on a standard day. At sea level, your pressure represents the standard at 1000 ft. It’s where the aircraft is on a standard day, which could be < SL.
 
Clip4 said:
The data plane
SL =29.92.
1000 = 28.92
2000 = 27.92

Non Standard.

SL= 28.92 which is where 1000 ft exists on a standard day. At sea level, your pressure represents the standard at 1000 ft. It’s where the aircraft is on a standard day, which could be < SL.
Click to expand...
Also Hmmmmmm
 
sarangan said:
No, you do. You are flying at 30,000 ft, but your lungs feel like they are at 8000 ft. That's pressure altitude. 30,000 ft outside, 8,000 inside.
Click to expand...
If your altimeter is set to 29.92 like it should be your pressure altitude is 30000 ft MSL, your cabin altitude is 8000 ft.

Nauga,
not sure if serious
 
sarangan said:
No, you do. You are flying at 30,000 ft, but your lungs feel like they are at 8000 ft. That's pressure altitude. 30,000 ft outside, 8,000 inside.
Click to expand...
nauga said:
If your altimeter is set to 29.92 like it should be your pressure altitude is 30000 ft MSL, your cabin altitude is 8000 ft.

Nauga,
not sure if serious
Click to expand...
What if I'm in a newer passenger jet such as the Boeing 787? I think those are set to 6000' (not being serious):)
 
Bill said:
With engine off should also be close to what you see on the the engine manifold pressure gauge.
Click to expand...

The ATIS pressure is corrected to sea level, your MP gauge is not. So it'll be 1" lower per 1k feet, roughly.
 
I never loved the terms "pressure altitude" and "density altitude", but they're the terms we have.

tl;dr first you correct for nonstandard local barometric pressure, then you correct for nonstandard temperature.

You can also correct for humidity, but it only has a small effect.
 
I have always just assumed the purpose of pressure altitude is that we need it as a reference point for performance charts/calculations/etc.
 
The "mean" in "MSL" is why it a theoretical plane. The ocean isn't anywhere close to flat. You've got tides, gravity anomalies, salinity and weather all affecting where the surface of the sea is. As an example, the absolute height of the ocean on the west coast of the US is higher than the east coast. Similar to why planes use a standard altimeter setting to make sure they are the same altitude relative to each other, the "base" of that height needs to be standard. Hence the Standard Datum Plane.
 
The altimeter setting corrects for non-standard pressure and temperature only up to field elevation. With a non-standard atmosphere, which is almost always, the pressure gradient rate-of-change, above field elevation with be other than standard so the altimeter is accurate at field elevation but the errors increase as you climb above it. That is why when making cold weather corrections you only correct for the height you are above the reporting field elevation.

An easy way to calculate the difference between indicated and pressure altitude at a particular altimeter settings is to drop the decimal point and add a zero. 30.15 becomes 30150 and 29.92 becomes 29920. 30150 - 29920 = 230' difference between the two. When the altimeter setting is increased, the indicated altitude increases, and vis versa, so pressure altitude is 230' lower than indicated altitude with an altimeter setting of 30.15.
 
density altitude is likely the most misunderstood concept and the most important to grasp. here's my way of teaching it....

the complete standard atmosphere chart not only gives the temp and pressure at each altitude, but it also gives the resultant air density (expressed in units of density as slugs/cubic feet, but not important to know this , but helpful in understanding). this density is the most important column on chart as it gives the info needed to directly calculate performance (both thrust and lift). ...the greater the density (slug/cu') the greater aircraft performance (aka the thicker the air, resulting in engine ingesting more potential energy, more air for prop to grab. and more air for wings to produce lift at a given airspeed, etc.).

in test flying. if I have actual performance data for different densities, I can mathematically calculate what performance will be at other densities (which I will later compare to actual flight data. but it will be very close). so the aircraft does not care about PA or temp or humidity, but the resultant air density.

so, each altitude has a standard density. density altitude is simply where that same density exist in the ISA. It's a practical concept to express air density.

so if I say you have a DA of 8k ft. even though airport is at 6k'. it simply means the air density (slugs/cu') is exact same as 8k in SA....so performance will be the same as at 8k' in standard atmosphere conditions. performance charts reference ISA, so you maybe unknowingly calculate your DA when using the performance charts when entering PA and temp.

one more pilot point,...all performance charts us DA.....if not. your POH performance section would be size of a Chinese phone book as there would be a different charts for every combination of air pressure and temperature.

some older aircraft performance charts have you first calculate DA then use the chart....most now incorporate you first selecting your pressure altitude and temp (which is basically the air density slug in SA, and thus DA)

so here's my check ride answer if asked what is DA....."it's the altitude in the ISA where the air density is the same for my pressure altitude and air temperature conditions".

why do we need to know DA?.... because all performance data uses ISA conditions. and a significant number of accidents result due to not understanding or miscalculating performance at higher DA conditions.

how does DA affect performance?..... higher DA means less dense air resulting in less thrust and lift, resulting in longer takeoff and landing distances, decrease climb rates on takeoff and go arounds, lower max cruise altitudes.

how do you know you have higher DA than standard conditions at sea level? ....if temperature is above 15c and/or pressure altitude is less than 29.92"....

does humidity effect DA?....yes, but insignificant and performance charts for small aircraft don't consider it so there is no way to calculate its effect.

as I mention in prior post re keeping check ride answers short and articulate...such brief answers will show u have understanding of topic and no need for dpe to dive in deeper.

hope this helps. good luck on ride.
 
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