What am I missing?

Fast Leroy

Fast Leroy

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Fast Leroy
Question about setting the altimeter while in the air...

If I set the altimeter to the pressure at my home field and read the field elevation +- 75', then as I approach my destination (at 3,500') that's say 2,000' higher than my home field elevation and reset the altimeter to pressure that ATIS gives me, won't my altitude read that field elevation, all the while I'm still at 3,500'?
 
When you change to the baro of the destination, it doesn't change anything until you arrive. You said you're still at 3500'

The altimeter only tells you field elevation while you're on the ground, right?
 
Fast Leroy said:
Question about setting the altimeter while in the air...

If I set the altimeter to the pressure at my home field and read the field elevation +- 75', then as I approach my destination (at 3,500') that's say 2,000' higher than my home field elevation and reset the altimeter to pressure that ATIS gives me, won't my altitude read that field elevation, all the while I'm still at 3,500'?
Click to expand...
The baro pressure is corrected for elevation. So, if you took the same barometer from the first airport to he second, an adjustment will be made for the change in elevation.
 
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The altimeter reading is the actual altitude +/- the barometer setting. When you’re adjusting it, you are just tweaking it a little. Setting the barometer for the destination tells you what adjustment is needed for the altimeter to be “accurate”.

The little window where you set the barometer is called the Kollsman window. All it does is spin the needle a little up or down. Changing the reading by a full inch (29.00 to 30.00) changes the altimeter 1000’. So changing it .01 is a 10’ change.
 
The barometer setting is the number that should give you purd near the field elevation when you are on the airfield.
 
The barometer setting you enter in the Kollsman window should be the barometric pressure corrected to the reference point (mean sea level). This is referred to as the QNH setting. If you have the correct QNH setting for your current position, your altitude reading should closely correspond to your elevation above mean sea level. Other aircraft will also be using the same settings so your relative altitudes will be comparable. QNH is what you get when you read METARs or listen to ATIS or ATC. It is NOT the actual barometric pressure at the station, which will be considerably lower than the QNH value unless you are at sea level.
 
I think we've got it now.

Question - how far off is too far off? Say if you adjust the barometer exactly right, but the altimeter is showing 200 ft above or below the field altitude?
 
bflynn said:
I think we've got it now.

Question - how far off is too far off? Say if you adjust the barometer exactly right, but the altimeter is showing 200 ft above or below the field altitude?
Click to expand...

I don't know exactly what the legal limits are, but that's gotta be to much
 
bflynn said:
I think we've got it now.

Question - how far off is too far off? Say if you adjust the barometer exactly right, but the altimeter is showing 200 ft above or below the field altitude?
Click to expand...

AIM says the altimeter should read the field elevation +/- 75 feet with the proper altimeter setting to be appropriately accurate. It's not a hard and fast legal requirement, but if you fly with it that far off, you can always be potentially tagged with the "careless and reckless operation" bit.
 
Fast Leroy said:
Question about setting the altimeter while in the air...

If I set the altimeter to the pressure at my home field and read the field elevation +- 75', then as I approach my destination (at 3,500') that's say 2,000' higher than my home field elevation and reset the altimeter to pressure that ATIS gives me, won't my altitude read that field elevation, all the while I'm still at 3,500'?
Click to expand...

The altimeter setting is the hypothetical sea-level barometric pressure*, that's why the altimeter setting in Denver 30.10" right now, not something like 24.5".

*Not completely accurate but good enough to clarify the concept to OP.
 
Fast Leroy said:
If I set the altimeter to the pressure at my home field and read the field elevation +- 75', then as I approach my destination (at 3,500') that's say 2,000' higher than my home field elevation and reset the altimeter to pressure that ATIS gives me, won't my altitude read that field elevation, all the while I'm still at 3,500'?
Click to expand...
I suppose its possible but not likely. As mentioned, 1" of barometric adjustment (i.e. 29.00" to 30.00") is equal to 1000' of change on the altimeter. If the difference in altimeter setting is 0.20" off from where you set it when you took off, then your altimeter will read 200' higher or lower when you pick up the ATIS and put in the new setting.

Put in the new setting, adjust your altitude and continue on as normal.
 
Rgbeard said:
Let us know when the lightbulb goes off in your head.
Click to expand...

Asked another way... I set my altimeter to the B pressure that's given to me by ATIS for whatever airport. So now my altimeter reads that fields elevation, yes? But I'm say at 5,000 AGL?
 
Fast Leroy said:
Asked another way... I set my altimeter to the B pressure that's given to me by ATIS for whatever airport. So now my altimeter reads that fields elevation, yes? But I'm say at 5,000 AGL?
Click to expand...

Why and how would it do that?

Even if that's how it worked, which it's not, how would the ATIS know what altitude you were at?
 
dmspilot said:
Why and how would it do that?

Even if that's how it worked, which it's not, how would the ATIS know what altitude you were at?
Click to expand...


That's my question...

Let's try this.

ATIS gives the pressure at the airport reporting for Yes?
That pressure setting should cause your altimeter to read field elevation +- 75' Yes?
You reset your altimeter to that pressure setting yes?
Your altimeter now reads that field elevation +- 75' Yes?
But lets say your at 5,000' AGL?

Where am I going wrong?
 
Entering the current baro setting (in flight) will tell you your current MSL altitude of the plane. When your tires are on the ground it will show you the msl altitude which will happen to be the field elevation...since your tires are on the field.
 
Fast Leroy said:
Asked another way... I set my altimeter to the B pressure that's given to me by ATIS for whatever airport. So now my altimeter reads that fields elevation, yes? But I'm say at 5,000 AGL?
Click to expand...


Okay - here we go.


At Airport ABC, you set the altimeter to either field elevation or to the ATIS-dictated setting. In any case, let's say its 29.92, and shows field elevation of 1,500 feet above sea level. Your altimeter is gold-standard as far as tolerances go, fresh from an overhaul and calibration.

You climb to 5,000 feet to fly to airport XYZ, whose field elevation is 2,000 feet MSL.
You fly your altitude dead-on at 5,000 feet, you're an awesome pilot!

10 miles away from XYZ you learn their altimeter setting for that moment is 29.95. This is a small difference of 0.03"/hg barometric pressure.

Going from a lower pressure to a slightly higher pressure, the altimeter will read a little lower than what you're actually flying.

You correct the window from 29.92 to 29.95 (just a small adjustment. Use squinting as necessary.)

This causes your altimeter to raise by about 30 feet. It now shows you flying at 5,030 feet.

When you land, the altimeter reads field elevation of 2,000 feet.
 
Fast Leroy said:
ATIS gives the pressure at the airport reporting for Yes?
That pressure setting should cause your altimeter to read field elevation +- 75' Yes?
You reset your altimeter to that pressure setting yes?
Your altimeter now reads that field elevation +- 75' Yes?
But lets say your at 5,000' AGL?
Click to expand...

It will read field elevation within 75 feet if the airplane is on the ground at the field.

If entering the altimeter setting caused the altimeter to read the field elevation universally regardless of the location of the airplane, altimeters would be pretty useless, no?
 
Rgbeard said:
Okay - here we go.


At Airport ABC, you set the altimeter to either field elevation or to the ATIS-dictated setting. In any case, let's say its 29.92, and shows field elevation of 1,500 feet above sea level. Your altimeter is gold-standard as far as tolerances go, fresh from an overhaul and calibration.

You climb to 5,000 feet to fly to airport XYZ, whose field elevation is 2,000 feet MSL.
You fly your altitude dead-on at 5,000 feet, you're an awesome pilot!

10 miles away from XYZ you learn their altimeter setting for that moment is 29.95. This is a small difference of 0.03"/hg barometric pressure.

Going from a lower pressure to a slightly higher pressure, the altimeter will read a little lower than what you're actually flying.

You correct the window from 29.92 to 29.95 (just a small adjustment. Use squinting as necessary.)

This causes your altimeter to raise by about 30 feet. It now shows you flying at 5,030 feet.

When you land, the altimeter reads field elevation of 2,000 feet.
Click to expand...


That's it, that's what I needed. Thank you! See the the light bulb!
 
Twenty plus posts in: Please discuss this with your CFI, since we clearly aren't getting through.
 
Fast Leroy said:
Asked another way... I set my altimeter to the B pressure that's given to me by ATIS for whatever airport. So now my altimeter reads that fields elevation, yes? But I'm say at 5,000 AGL?
Click to expand...
No.
 
Remember that field elevation is measured at the highest point of the field. Your wheels on the ground may or may not reflect that elevation even WITH the +-75'. For example, my field has an elevation as 2704' measured at the approach end of runway 30. At the other end the elevation is 2589'.
 
Another way to think about it, if you are on a cross country flight or the pressure is changing rapidly due to weather, changing the setting allows all planes in the area to have the same definition of a particular altitude. A couple hundred feet of error isn't going to put you in the ground (VFR) but two planes with 250 feet of opposite error could put a VFR and an IFR plane at exactly the same altitude. That's why on Flight Following or IFR, one of the the first things you hear when you pick up a new controller is the Altimeter setting for his area.
 
Tom-D said:
never was concern I've adjusted the Kolsman window to match the field altitude, because that's all we are allowed ta do.
Click to expand...

But what if your altimeter is out of calibration?
 
Fast Leroy said:
That's it, that's what I needed. Thank you! See the the light bulb!
Click to expand...
I'm not convinced. The altimeter is really just a bathroom scale that measures the weight of the column of air above it all the way to the top of the atmosphere. You zero the scale and an altimeter with an adjustment knob the same way, more spring tension is needed to offset higher pressure and zero the needle. "Zero", in the case of an altimeter, is field elevation when at the field with the existing weight of the column of air. Above the field, the column weighs less (hands show altitude). When you adjust the knob for more pressure, the hands indicate higher and vice versa.

There.:)
 
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Fast Leroy said:
But what if your altimeter is out of calibration?
Click to expand...
It requires an instrument repair station to repair instruments.
anyone can adjust.
Adjust the Kolsman window to the correct atmospheric pressure, disconnect the Kolsman window, adjust the altimeter to the correct altitude, reconnect the Kolsman window.
 
Wait for it ----- (the "it" is why does the transponder show a different altitude than the altimeter)
 
Kinda simple. In flight you get an altimeter setting from wherever you intend to land once you get near. If where you want to land doesn't have a setting, you get one from someplace nearby. Should be close enough.
 
steingar said:
Kinda simple. In flight you get an altimeter setting from wherever you intend to land once you get near. If where you want to land doesn't have a setting, you get one from someplace nearby. Should be close enough.
Click to expand...

Exactly, IFR approach plates lists alternates sources for baro-correction if the prescribed one is not available.
 
Tom-D said:
Adjust the Kolsman window to the correct atmospheric pressure, disconnect the Kolsman window, adjust the altimeter to the correct altitude, reconnect the Kolsman window.
Click to expand...

:confused: I had no idea you could do that :cool:
 
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