"High to low, look out below!"

Njonl

Njonl

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Njonl
Is there a simple way of understanding these altimeter errors? I'm seeing "low to high, clear the sky" and "high to low, look out below." I'm really overthinking these concepts, and I'm wondering if anybody has a simplified way of understanding it. Thanks!
 
Flying from a high pressure area to a low pressure area WITHOUT changing your altimeter setting or your indicated altitude will result in you descending to maintain that indicated altitude. Hence "look out below".
 
If you're flying IFR and go from a high pressure to a low pressure and the pressure differential is enough to put you off by 200' and you end flying an approach into an airport and your DH is 200' and you have not adjusted the altimeter, the results are not likely to be good. ;)
 
How about this:

Imagine that instead of flying along at a given altitude, you are following a given air pressure. Instead of an altimeter in your panel, you have a plain old barometer. You know that air pressure decreases by about 1" Hg for every 1000' so if are flying by your barometer at 25" you would be at, say, 5000'.

With me so far? (edit: Note that we are disregarding temperature for now, I covered that in my next post)

Now assume that the air pressure is constant - no high, no low. So long as you kept the barometer at 25", you remain at 5000', right?

But let us say we fly into an area of low pressure. That means that the pressure on the surface is lower, right. You could imagine that all the pressures have "moved down". And that puts your 25" Hg pressure lower in altitude also, right? Think about it, if you took it to some extreme, impossible low pressure then the sea level pressure might be 25" and you would hit the ground flying at 25" Hg. That is an impossible situation but just to clarify which direction things go.

Following?

Well, our altimeter is a barometer that is marked in feet, not inches of mercury. So when you fly into a lower pressure area holding a constant altimeter reading without adjusting the altimeter for that lower pressure (getting the local altimeter setting and putting it into the Kollsman window) then you will descend to follow that pressure line.

And the opposite holds if you fly to an area of higher pressure.

So it is "High to low, look out below" meaning that the ridge that you thought you were going to clear by 1000' might be a lot lower if you just flew into a low pressure area from a high pressure area without adjusting your altimeter.

edit: and the comment about the instrument approach is even more apropos.

Does that help or did I just confuse you more?
 
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Also, the above was a discussion of non-standard pressure. The "high to low" rule on non-standard temperature should be easy to visualize. What happens to a gas when you heat it? When you cool it? So if you are riding on top of a column of air (a given pressure), what will happen when you heat the column? When you cool it?
 
Isn't a simple way to just remember high to low, look out below to mean

high altitude to low altitude, look out below (because your altimeter is telling you you're higher than you really are)
or
high pressure to low pressure, look out below (because your altimeter is telling you you're higher than you really are)

This is assuming you didn't correct your altimeter of course.



Njonl said:
Is there a simple way of understanding these altimeter errors? I'm seeing "low to high, clear the sky" and "high to low, look out below." I'm really overthinking these concepts, and I'm wondering if anybody has a simplified way of understanding it. Thanks!
Click to expand...
 
pilotod said:
Isn't a simple way to just remember high to low, look out below to mean

high altitude to low altitude, look out below (because your altimeter is telling you you're higher than you really are)
or
high pressure to low pressure, look out below (because your altimeter is telling you you're higher than you really are)

This is assuming you didn't correct your altimeter of course.
Click to expand...

Not everyone processes information the same way.

In regards to viewing the altimeter as a pressure gauge. The higher the altitude, the lower the pressure. The lower the altitude, the higher the pressure.

If there was just your airport alone. You listen to the ATIS, you receive an altimeter setting of 30.00 as an example. You go flying to your practice area. You come home and now the ATIS altimeter setting is 29.95. The pressure is now lower. If your altimeter did not have altitude readings, and instead had pressure readings, you now would have to fly to a lower altitude in order to increase the pressure reading back to 30.00. Just like deeper in the ocean there is more pressure. Since you did not adjust your altimeter to the new setting of 29.95, you are about 50 feet lower. Meaning the original altitude of your airport according to your altimeter is now 50 feet under ground. When you adjust your altimeter to the correct setting, you are now synchronized with the pressure, and your altimeter will view the altitude of your airport what it should be.

Need more help, starting to gel?
 
Thanks for all the replies. It is now starting to make sense to me. I'm not sure why I couldn't understand it..... I seem to be overthinking the whole concept. Your descriptions really helped though, thank you!
 
pilotod said:
Isn't a simple way to just remember high to low, look out below to mean

high altitude to low altitude, look out below (because your altimeter is telling you you're higher than you really are)
or
high pressure to low pressure, look out below (because your altimeter is telling you you're higher than you really are)

This is assuming you didn't correct your altimeter of course.
Click to expand...

Sure, that is a simple way to remember it but the OP was looking for help in understanding it. That requires a bit more verbage. :D
 
This is how I understand it:
(I hope I do anyways, because I take my PPL written in about 3 hours, LOL!)

Imagine a balance scale, one side is the Kollsman Side, the other is the outside air pressure side, and id the Kollsman side is higher than the altitude side, the scale goes up, aka the altimeter, and you are physically sitting at sea level (0 MSL) on a standard 15C/29.92 day.

Scenario 1:
-------------
So if you turn up the Kollsman from 29.92 (balanced at 0) to 30.92, the scale says 1000', even though the scale hasn't moved and is at sea level.

Indicated: 1000' MSL
Actual: 0' AGL

You'd die if you were landing thinking you were at 1000' AGL.


Scenario 2:
-------------
So you turn the Kollsman back to 29.92, but ironically , the outside air pressure changes to 28.92, again the altimeter goes up 1000' even though you are still 0' AGL.

Indicated: 1000' MSL
Actual: 0' AGL

Again, dead if you were landing thinking you were 1000' higher than actual.


Scenario 3:
-------------
Ok, so now the outside air pressure goes up to 30.92. The altimeter would read -1000' MSL and you are still at 0' AGL. So the altimeter would show you lower than you actually are. So thinking you were at 0 feet, but still having 1000' untill your wheels touched the ground, you would go on flying.

Same thing with temperature. Lets say the air pressure stays the same, but only the temp changes from 15C to 0C. That 1000' of air outside at that was 15C just shrunk because cold makes things contract, and so 1000' MSL (@ 15C which is what the other scale the altimeter is calibrated to) indicated, now translates to 970-ish' actual. So subtract 1000' from each:

Indicated: 1000' MSL- 1000' = 0' MSL
Actual: 970' AGL - 1000' = -30' AGL
Again, dead if you were landing from 1000'

"High (pressure/temp) to low, Your AGL is below"
 
Why is everybody making it so complicated?

When you fly from a high pressure to a low pressure area, the altimeter will read higher because of the lower pressure. If you don't adjust the altimeter setting, and you continue to fly the same indicated altitude, you will be lower than you think, so "look out below."
 
Dana said:
Why is everybody making it so complicated?

When you fly from a high pressure to a low pressure area, the altimeter will read higher because of the lower pressure. If you don't adjust the altimeter setting, and you continue to fly the same indicated altitude, you will be lower than you think, so "look out below."
Click to expand...

Why? Because they don’t know any better. Sort of like replying to a necro post from nearly six years ago.
 
SkyDog58 said:
Why? Because they don’t know any better. Sort of like replying to a necro post from nearly six years ago.
Click to expand...
note to self. sometime next year ask SkyDog what 'necro post' means, lol. Anyway, it's a good enough subject. @PatrickK just got here. Welcome to the joint Patrick. The posts have dates. When you see something old that interests you, you can start it out with sumpin like "I know its an old thread but.................
 
Air pressure differences and changes is something I wish ground school and instruction spent more time on in laying a strong foundation during primary training. So many core concepts tie back to air density.. just to name a very few

*TAS vs IAS
*take off performance
*climb performance
*engine performance
*human performance!
*atmospheric conditions (clouds, moisture, ice, rain, fog, etc.)

A simple way of understanding it is to picture a ball on the ground with a toy plane sitting on top of it. As the pressure in the ball is increased the plane goes up, and as the pressure is decreased the plane goes down.. IE, high to low look out below. The plane rides along this "column of air" so to speak.

But I also realized after writing this that the post is from 2012(!) so this was kind of a waste, but I already wrote it so I'm not deleting it!

PS - imagine if water (liquid) was not a constant density and varied like air? How trippy would it be to have mountains and valleys of water...
 
Necro or not, alfadog's explanation is the clearest one I've heard in a long time. That doesn't mean I'll necessarily remember it right, but there's hope.
 
Dana said:
Just go out onto the ocean in a small boat on a rough day...
Click to expand...
true... those small craft advisory days can be a rough ride even in a 40' foot boat
 
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