# Altitude differences

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Sometimes whilst attempting landings i get the altitude on my instruments disagreeing with the altitude being called out when landing what is the cause of this is there something i have missed carrying out i have checked that the pressure is set to what active sky is saying cant understand why this happens can someone explain thanks
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Altitude being called out = Above Ground Level (local)

Altitude on instruments = Above Sea Level

The difference between the two is the airport's elevation.

Wim

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Altitude being called out = Above Ground Level (local)

Altitude on instruments = Above Sea Level

The difference between the two is the airport's elevation.

Wim

I don't know whether the airliners use another form of callout or not, but the callout will certainly be AGL if it is from the radar altimeter.

Altitude on instruments = Above Sea Level

Technically that is above Mean Sea Level (usually referred to as MSL, but almost the same thing), and (for the OP) is from the altimeter which reads barometric pressure and is set to the altimeter setting to compensate for barometer changes over time.

The difference between the two is the airport's elevation.

Again, technically, the difference is the elevation of the spot immediately below the aircraft where the radar hits. While not much of a difference in flat terrain, the difference when approaching a place like Telluride, CO (KTEX) can be rather considerable.

Edited by lnuss

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Just like both posts mention. But I'll try to simplify it.

You have two types of altitude in aviation: AGL and MSL.

AGL means Above Ground Level. That is what the radar altimeter is giving you. The altitude above the ground. So you hear 1,000, 500, 300, 200...50..10. That tells you how high you are above the ground. It works with a radar signal bouncing off the ground and the receiver measuring the time it takes for the signal to return. The moon landings probably used this.

MSL, or Mean Sea Level is the height above the sea. So if you're here in Colorado the altitude is about 5,000 feet which is above sea level and when you are on the ground and your pressure is set for the altitude gauge you should see something like 5,000 feet. If you're at around 7,000 feet your actually about 2,000 feet above ground. Understand?

When you fly over the ocean at about 2,500 or less and look at your altitude gauge and your radar altitude, they will be the same since MSL and AGL is coinciding exactly the same since there's no terrain.

London has an approximate altitude of 36 feet. So on the ground your altitude gauge should read about 36 feet. Throughout the entire world all areas have different elevations. That's MSL.

Edited by CRJ_simpilot
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Sometimes whilst attempting landings i get the altitude on my instruments disagreeing with the altitude being called out when landing what is the cause of this is there something i have missed carrying out i have checked that the pressure is set to what active sky is saying cant understand why this happens can someone explain thanks

Altitude being called is from the RADAR Altimeter which is AGL. See other posts about the difference between MSL and AGL. Airports are usually MSL.

Soa callout "50 feet" into a runway that is 150 feet above mean sea level means the aircraft height above the ground is 50` when it's altitude is 200 feet. Clear?

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No, technically you only have one! In aviation, altitude is the vertical distance of an object above mean sea level. If you're measuring the vertical distance above ground level (or any other specified datum, like a runway) then that's height, not altitude. QNH is used to indicate altitude, QFE (or a rad alt) is used to indicate height.

We don't use those designations in the U.S. And I'll disagree with you anyway, unless you're telling me that the definitions you use are defined by the UK aviation authority (CAA is it?), since altitude, elevation and height are, sometimes, synonymous and are always describing something similar.

All we're really doing is describing different ways of measuring something's distance above something else.

In aviation, altitude is the vertical distance of an object above mean sea level.

I don't think we want to get into the different things here, such as pressure altitude, true altitude, indicated altitude, etc., and in any case we're trying to keep things simple for the average simmer. In fact, in this thread we're just trying to clear up some confusion for the OP, not trying to confuse him more.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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I see those definitions, and by that document you are, of course, correct. Nor will I dispute your contention that it's an ICAO defintion, as well.

You might note that, though I started flying in 1969, and though I've done a lot of instructing over the years, the first time I'd ever come across the QFH/QFE terms is when some UK folks used it on this forum -- by then I'd been flying for over 30 years. These terms are not used in the U.S. because we always set our altimeters relative to MSL. Setting so that the altimeter reads zero on the ground when anywhere but exact sea level, just isn't done. Note that the altimeter probably couldn't be made to read zero in much of the western U.S., and even in some areas in the eastern U.S. It certainly couldn't at my house (5230 MSL) or airports in the area. Or try Leadville, CO around 10,000 MSL.

I expect that many (most?) other countries have the same problem that we in the U.S. do, that is, being unable to set the altimeter to zero at many airports. Nor do I understand a reason for doing that, though I know you do it in the U.K.

You might note, too, that not all things ICAO necessarily apply in the U.S. -- flight plan forms, just for one example (there are others, of course), though it seems to be drifting in that direction somewhat. We've even recently gone to METARS.

In any case, let's go back to trying to clear up the OP's confusion, rather than causing more confusion for him.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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How much of your flying was outside of North America?

None, though that is irrelevant, but I am very aware (as I'm sure you are) that much of South America, much of Europe and Asia, and parts of Africa have rather high mountains.

along with rad alt information, I used merely as an example of what constitutes height.

So why is it a radar altimeter, rather than radar height finder? Why is it radar altitude, rather than radar height.

Enough -- we disagree, and I'm done. No need to hijack this thread any further.

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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You can't just ask a question and go! It's called a radar altimeter because "altimeter" is a generic term for any instrument which measures vertical distance. We also use an altimeter for flying at flight levels but it doesn't change the name of the instrument. In my career as a RW pilot, I always considered information from a rad alt as height, not altitude (which I got from my pressure altimeter with QNH set).

In my career as a real world pilot, only rank amateurs ever refer to `altitude` as `height`.

Height only measures how far your head is away from your feet - and in aviating terms is only of use in telling you whether the fin will fit in the hangar..!

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Sometimes whilst attempting landings i get the altitude on my instruments disagreeing with the altitude being called out when landing what is the cause of this is there something i have missed carrying out i have checked that the pressure is set to what active sky is saying cant understand why this happens can someone explain thanks

Apart from previous posts above: I'm triggered by your remark "altitude being called out".

So I assume you have some addon (or built-in) gauge that implements these verbal callouts ??

If so:what might be wrong, is that the gauge you are using for "callout's" uses the basic FSX variable Radio Height, and is not corrected properly (for this aircraft) for the landing gear position.

I mean this: the verbal callout should be based on the distance between the main landing gear and the ground. whereas the Radio Height variable in FSX indicates the distance between the defined model datum and ground. As defined in the aircraft.cfg by "static_cg_height = "

Example: for the default B747-400, this value is 17 feet.

Meaning that (if Radio Height is not properly compensated in the gauge for the callout) you will get an altitude callout "20" while the actual distance between main wheels and ground is only 3 feet.

Not sure if this is what your mean, but this a problem with some old addon Altitude callout gauges.

Rob

Edited by rcbarend

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