What is the difference between indicated altitude and true altitude

[hinhin]

hey guys,
I have just come across with the altitude definition....

Actually what is the true altitude?
Is it the indicated altitude when QNH is set ? or does it imply for a non-standard temperature correction as well?
Thanks in advance!!

[212man]

Indicated altitude is true altitude under ISA temperature conditions i.e 15°C at MSL reducing by 2°C per 1000 ft. When the Temp is above ISA the altimeter will under read, when lower than ISA the over read - the latter case is potentially dangerous and may require correction when flying a Bar-VNAV approach.

[Jan Olieslagers]

Isn't there some terminology confusion? There does exist true air speed vs. indicated airspeed but I only learned of altitude, unqualified.

[Above The Clouds]

The difference in temperature from ISA.

[ch.ess]

...confusion

As mentioned before - these are not standard terms
what you have is
-altitude @ ISA 1013 hpa - same as FL
-altitude @ current QNH (and ISA temp)
-temp corrected altitude at QNH

in addition, a lot of people use now:
- GPS altitude: GPS calculated altitude against the WGS84 model
mathematically more precise and no issues from changes in temp and pressure, but carries the issues of Mother Earth not fully conforming to the WGS84 ellipsoid

whatever you use, make sure there are at least 3feet between you and the nearest thing beneath you

[bookworm]

I don't think the phrase "true altitude" is unusual. It is used for example in Skybrary's Altimeter Temperature Error Correction article as well as in EASA's Learning Objectives for the PPL and professional licences.

[Jan Olieslagers]

I don't think the phrase "true altitude" is unusual.

I am willing to accept that - especially seeing who wrote it. But what then is the definition, and what authority gives it? Or would it be "just another UK peciuliarity" ?

[mm_flynn]

I was always taught that true altitude was indicated altitude, corrected for any altimeter error and then compensated for non-standard temperature, which should then be you actual altitude above local sea level.


Most GPS Altitudes are against the Geoid (probably EGM96 of Giod99) not WGS84 ellipsoid and as such are typically are pretty consistent with Altitude above local sea level and very consistent for altitude vs locally charted terrain on modern maps in Western countries (where the elevations will typically have been determined vs the Geoid).

[bookworm]

Or would it be "just another UK peculiarity"?

It's certainly not a UK peculiarity, and pilots tend to be more commonly aware of the concepts in countries which (a) get quite cold and (b) have large pointy bits. The UK doesn't really qualify on either criterion.

I don't have the document of ICAO definitions, but it does appear to be the term used, e.g. in PANS-OPS Vol I:

5.4.3.2 Descent below the fix crossing altitude/height should not be made prior to crossing the fix.

5.4.3.3 It is assumed that the aircraft altimeter reading on crossing the fix is correlated with the published altitude, allowing for altitude error and altimeter tolerances. See Part III.

Note.— Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any deviation from ISA will therefore result in an erroneous reading on the altimeter. If the temperature is higher than ISA, then the true altitude will be higher than the figure indicated by the altimeter. Similarly, the true altitude will be lower when the temperature is lower than ISA. The altimeter error may be significant in extremely cold temperatures.

[Mike Flynn]

In simple terms it is the space between the aircraft and the ground

[Jan Olieslagers]

Negative - altitude is never about "ground" - that would be "height" or perhaps "elevation".

[212man]

Originally Posted by Jay Sata

In simple terms it is the space between the aircraft and the ground

in simple terms, that's completely incorrect!

[Piltdown Man]

The gentlemen above have very kindky privided you with the reasons for the difference, but I'm going to guess that you don't know why you need this information (I certainly didn't have a clue when I first got a PPL). So I'll try answer it before you ask. The most important thing to realise is that on very cold days, your altimeter will over-read. You will be physically lower than your altimeter says. As most PPL flying is VFR over flattish ground, it shouldn't matter too much because you can visually avoid obstructions. But as soon as cloud gets in the way or you start messing around with mountains or tall structures, this becomes rather important. Not knowing your true altitude could result in you unintentionally too close (or worse) for comfort.

PM

[212man]

High to low, watch out below!

[Crash one]

Originally Posted by 212man

High to low, watch out below!

Expressions like that mean absolutely bugger all to some one who has no clue what it means.
Does it mean "If you descend from high to low you will maybe hit something?"
Is a student likely to imagine it refers to high to low pressure changes at the same level that if not corrected on the subscale will result in the altimeter progressively reading high causing the pilot to descend in order to maintain the altimeter reading?

[212man]

Originally Posted by Crash one

Expressions like that mean absolutely bugger all to some one who has no clue what it means.
Does it mean "If you descend from high to low you will maybe hit something?"
Is a student likely to imagine it refers to high to low priessure changes at the same level that if not corrected on the subscale will result in the altimeter progressively reading high causing the pilot to descend in order to maintain the altimeter reading?

Sorry to have muddied the waters with ancient adages. For those who don't know, what my expression relates to is if the QNH or temperature falls below ISA, you need to take care. But, as a 32 year, 11,000 hour commercial pilot, I'm happy to hand this discussion back to those who are clearly more knowledgable than myself. Happy altimetry campers....,

[Crash one]

Originally Posted by 212man

Sorry to have muddied the waters with ancient adages. For those who don't know, what my expression relates to is if the QNH or temperature falls below ISA, you need to take care. But, as a 32 year, 11,000 hour commercial pilot, I'm happy to hand this discussion back to those who are clearly more knowledgable than myself. Happy altimetry campers....,

I bow to your superior knowledge and experience, but to a far less knowledgable student it may come across as gobbledygook and not understood.
I don't think we, certainly not I, are claiming to be more knowledgable than such as yourself. Sarcasm is not required.

[Jan Olieslagers]

So what have we established? In extreme temperatures, either low or high, there can (and most likely will) be a discrepancy or deviation or difference between the real altitude and the altitude shown on the altimeter, even if set to the correct QNH.

Q1: how to determine said discrepancy?

Q2: why don't we fly to GPS altitudes? That would allow us to do away with the cumbersome QNH comm's and settings, including the (by some) hated regional QNH.

A2a: because all the procedures would have to be rewritten
A2b: because certain smart-asses would have less opportunity to show off
A2c: because it would make it harder to find 20 good complicated questions for the PPL theory exam

[blueandwhite]

Originally Posted by Jan Olieslagers

.............
Q2: why don't we fly to GPS altitudes? That would allow us to do away with the cumbersome QNH comm's and settings, including the (by some) hated regional QNH.

A2a:
A2b: because certain smart-asses would have less opportunity to show off
A2c:

Reply to Q2, Because GPS altitude is not very accurate.

Ref A2b are you trying to do yourself out of a hobby?

I think you may have trouble getting everyone to buy a GPS. There are a lot of people who still refuse to use one for horizontal navigation even.

[eckhard]

So what have we established? In extreme temperatures, either low or high, there can (and most likely will) be a discrepancy or deviation or difference between the real altitude and the altitude shown on the altimeter, even if set to the correct QNH.

Q1: how to determine said discrepancy?

A simple rule of thumb is that the error is 4ft per 1000ft above the airfield for every degree C deviation from ISA.

For deviations that are colder than ISA, subtract the correction from the indicated altitude.

For deviations that are warmer than ISA, add the correction to the indicated altitude.

From this, you can see that with a cold deviation, the altimeter will over-read, in other words, you will be lower than you think you are.

As a deviation in the cold sense is more dangerous, let's discuss that first.

The deviation from ISA, technically speaking, is the average temperature deviation of the airmass between the altimeter and the QNH reference. In other words, it is a measurement of how cold the air is below the aircraft. (The colder it is, the more 'compacted' the isobars will be in a vertical sense.) In most cases that can be translated more simply as taking the airfield OAT and calculating how this differs from ISA.

Example 1:
Airfield elevation 500ft
Airfield OAT 14C
QNH 1000hPa
ISA temp Dev 0

Altimeter set to 1000hPa
Indicated altitude 3500ft (3000ft above the airfield)
Calculated altimeter error is 4x0=0ft per 1000ft
True altitude 3500-0=3500ft

Example 2:
Same as example 1 except
Airfield OAT 4C
ISA temp Dev -10

Altimeter set to 1000hPa
Indicated altitude 3500ft
Calculated altimeter error is 4x10=40ft per 1000ft
True altitude 3500-(40x3)=3380ft

So, although you are actually just over 100ft lower than you think you are, this is not a huge problem and note that the error will reduce as you descend closer to the airfield, eventually reaching zero as you touch down. This is why altimeter temperature error is not considered important for most PPL VFR flight operations and hence most PPL holders have not heard of it.

For IFR operations, the error can be operationally significant. As the error increases with altitude, high ground close the airfield can be problematic when calculating MSA and minimum holding and procedural altitudes that are significantly higher than airfield elevation.

Example 3:
Airfield elevation 500ft
Airfield OAT -6C
QNH 1000hPa
ISA temp Dev -20
MSA 8700ft
MHA 9000ft

Altimeter set to 1000hPa
Indicated altitude 9000ft
Calculated altimeter error is 4x20=80ft per 1000ft
True altitude 9000-(80x8.5)=8320ft

So now you are nearly 700ft lower than you think. In fact, you are 400ft below the MSA. If you are in IMC, that is not a good place to be!

Again, in this example, the error would reduce as you descend, reaching zero as you land.

What can you do about it?

A/ Be aware that this is a real feature of all barometric altimeters.
B/ Be especially careful when the airfield OAT is significantly (e.g. 20 degrees C) colder than ISA.
C/ Remember that the error increases with altitude, so be more careful in areas where the MSA is much (e.g. 4000ft) higher than the airfield elevation.
D/ Ask ATC for a higher level/altitude. ATC may have issued their clearance having already allowed for cold temperature corrections.

What you must not do is arbitrarily fly at a higher altitude than that given by your ATC clearance. That could reduce vertical separation from other traffic.

If you are not happy with the cleared altitude, ask for a higher one, explaining your concerns if required. A simple statement like, "my operational minima require me to maintain _____ft" can work wonders.

For deviations that are warmer than ISA, the same arithmetic applies but the final correction is added to the indicated altitude. This has the effect of increasing your terrain clearance and therefore is not seen as a safety issue.

Warm deviations can cause confusion nevertheless.

Consider the following:

Airport elevation 500ft
QNH 1000hPa
Airport OAT 34C
ISA temp Dev +20
ILS glide slope intercept at 12nm and 4400ft

Altimeter set to 1000hPa
Indicated altitude 4400ft
Calculated altimeter error is 4x20=80ft per 1000ft
True altitude is 4400+(80x3.9)=4712ft

You are now just over 300ft higher than you think you are and will intercept the ILS glide slope at 13nm, instead of 12nm.

How relevant is all of this in the real world?

Cold
I have renegotiated higher holding altitudes at Innsbruck.
I have flown higher altitudes on a DME arc at Iqaluit.

Warm
I have had some issues with glide slope intercept at Mauritius and Mexico.
I have adjusted my altitude whilst flying aerial surveys in Saudi Arabia.