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Joined: Wed Nov 17, 2004 4:35 am

Altimeter Setting

Sun Dec 12, 2004 7:10 am

This is something that has confused me since I began flight training. Why is it that when you set the altimeter to a lower setting, the indicated altitude decreases, but yet when you fly from high pressure to low pressure, the indicated altitude increases. This seems inconsistent for some reason. By setting the alitimeter to a lower value, aren't you in fact telling the airplane that it is higher ? So I would think that lowering the setting would indicate increased altitude.
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RE: Altimeter Setting

Sun Dec 12, 2004 7:47 am

It has to do with how the altimeter works. It is measuring the difference in pressure between what you set in the kollsman window and the actual pressure of the air around the aircraft. For example, on a day with standard conditions you would have 29.92 set into the altimeter, and it would read zero at sea level. If you climb up to 1,000 feet, the air pressure around the aircraft would be about 28.92. The altimeter is measuring that difference in pressure and displaying it in feet of altitude. As you lower the altimeter setting, the difference between what is in the kollsman window, and the actual pressure decreases, causing the altimeter to show a lower setting.

This confused me while I was a student pilot too. I hope this helps.
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RE: Altimeter Setting

Sun Dec 12, 2004 7:48 am

The answer is Set vs. Actual.

When you set a lower reference pressure the machine will now read the correct altitude based on that existing lower pressure.

When you fly from a high to a low without resetting the altimeter thinks the pressure is higher than it really is. So, the air is thinner than the altimeter is biased for - therefore you are lower than it says you are.
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RE: Altimeter Setting

Sun Dec 12, 2004 9:03 am

The altimeter shows you the difference between the set pressure and your current (lower) ambient pressure.

Pressure alt = set pressure - current ambient pressure,

where set pressure > current ambient pressure

The rest is fiddling with scalings and units to get the alt in reasonably believable feet rather than pascals.

If you lower the set pressure, the difference decreases. Alt goes down.

If your ambient pressure decreases, the difference increases. Alt goes up.
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RE: Altimeter Setting

Sun Dec 12, 2004 9:25 am

Thanks guys !! This has helped a lot...good way of explaining it.
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RE: Altimeter Setting

Sun Dec 12, 2004 7:54 pm

A little saying I learnt when doing my PPL

From high (temp. or press ) to low go slow.
As explained in the previous threads, basically what it means is that if you go from one to the other, you should always reset your altimeter due to the fact that you are actually indicating a higher altitude than your actual altitude.
This will decrease your AGL, and needless to say would lead to dangerous situations.

If it's not broken, don't fix it !
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RE: Altimeter Setting

Sun Dec 12, 2004 10:46 pm

The way that helps me to think of it is that all that air is pushing down on itself. As you go up, there is less air pushing down and the pressure gets lower.

Honestly though, the old pros around here on Tech/ops explained it better than I could.
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RE: Altimeter Setting

Sun Dec 12, 2004 10:47 pm


Others have explained correctly what is happening, but may I just pick up on something you said?

...yet when you fly from high pressure to low pressure, the indicated altitude increases...

No, it doesn't. That's the danger!

The indicated altitude stays the same, assuming that you, like the rest of us, fly "level" by maintaining a constant altitude reading on your altimeter.

What happens is that you are actually descending in order to maintain that constant altitude reading, quite possibly dangerously eroding your ground clearance.

If I take off from a sea level airport, where the altimeter setting is 1025 Mb, for an IMC flight over the sea at 1,000 ft indicated, I will initially be 1,000 ft AMSL.

If then, during the course of my flight, the local pressure should gradually fall to 987 Mbs, and I do not update my altimeter setting, then, whilst appearing to maintain 1,000 ft (because that's what the altimeter is reading) I will actually be slowly descending, flying ever lower over the sea.

Eventually, I will hit the water, wondering what went wrong, because my altimeter still indicates 1,000 ft!

That is the real danger of this situation.


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RE: Altimeter Setting

Mon Dec 13, 2004 12:24 pm

Just to carry this just a bit further, this is how piston airliners years ago navigated across oceans, using pressure pattern navigation.
A high range radio altimeter was used in the aircraft to keep track of the absolute altitude, and by doing so, the navigator could stay on a line of equal barometric pressure, thereby following around a high or low pressure area, to obtain the minimum time track (maximum tailwind/minimum headwind) between departure and arrival gateways.

Old time guys knew their stuff, and INS or GPS was not required.

Of course this would not work today because in many areas, fixed (daily) tracks are used, thereby rendering useless pressure pattern navigation, as the track was always curved using same.

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RE: Altimeter Setting

Wed Dec 15, 2004 6:25 am

Yep it all makes sense now guys...thanks ! So how often should you get a local altimeter setting in flight to update yours in the air ? Such as matching your heading indicator with the magnetic compass approx. every 15 min. I know for local flights with little variation in terrain it wouldnt make to much of a difference, but what about you guys that fly over various regions with mountains, etc. How do you manage that ?
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RE: Altimeter Setting

Wed Dec 15, 2004 7:32 am

It's recommended that you set your altimeter to a local station within 100nm of your location. If you're talking to approach or center, as you get handed off from one sector to another you get told the altimeter setting. Otherwise it's up to you to obtain the altimeter setting by listening to AWOS/ASOS within range or contacting Flight Watch to get the altimeter setting.

But if you fly high enough, 18,000ft in the US, you just set it to 29.92 and fly flight levels.

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RE: Altimeter Setting

Wed Dec 15, 2004 9:08 am

Sometimes thats why its good to be talking and squaking. Cause every time you get handed off, you get a new altimiter setting. Look out on the written...there may be a question or two about altimiter setting in different situations. The saying goes: From high to low look out below.

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RE: Altimeter Setting

Wed Dec 15, 2004 9:44 am

You've probably got quite a bit of advice already, but I always remembered it like this: the higher you go, the lower the pressure. So when you fly into an area of lower pressure, your altimeter thinks it's higher because the pressure is lower (i.e. imitating higher altitude).

Hope that made sense,

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