Higher altimeter errors at higher altitudes

[sorrygottarunway]

Okay, I'm going through my company ops manual today (yes, I'm that bored) and am reading through the portion that explains altitude correction factors with low-pressure altimeter settings. Why, in meteorological/mechanical terms, do we make this correction? What is the theory behind it that affects my altimeter causing me to have to make a correction. Not that I'm up past 18,000 that often, but I'm seeing things like "Altimeter Setting 28.41-27.92, correction factor: 2000 ft."

I understand that ATC restricts the use of these flight levels during low settings, but why?

[sorrygottarunway]

Okay I just scratched a few things out on paper and this is what I came up. In the US, we reset our altimeters to 29.92 passing through 18,000, regardless of the actual altimeter setting we had at 17,999. Therefore, if it was actually 29.41, and we then dial in 29.92 in the kollsman window, it will show us actually climbing a fair amount without the aircraft physically moving UP in space. So we'll now descend back to where our altimeter reads 18,000 and be happy.

Now, take an aircraft that wants to cancel IFR from 18,000, and start a VFR cruise at 17,500. UH OH!!! He descends and sets his meter to 29.41, which on our altimeters lowers the indicated altitude. He then climbs to maintain that 17,500 for his cruise.

You can see where this is going- the two aircraft are no longer separated by 500ft, but rather have moved closer to each other, simply by doing the correct thing by setting the required kollsman values.

Sorry for the confusion, answered my own question!

[sorrygottarunway]

I guess this stuff would be perfect for the curriculum of mountain flying, since you'll run into problems with extreme cold temperatures as well as pressure variations, in addition to the good mix of IFR/VFR traffic in these areas.

[OldTownPilot]

And if the pressure was less than 29.38 or so a VFR airplane at 17,500 feet would physically be at the same altitude as a IFR airplane at "18,000 feet".

When its less than 29.38, the VFR plane would be above the IFR traffic. From what i understand they just don't use FL180 in this situation, as you cant really restrict the VFR altitudes. Therefore the person could not cancel IFR at FL180, because he couldn't get there in the first place.

[GUNIT]

Yea, there's a section in the air traffic controller's manual (7110.65) that gives a table of at what certain altimeter settings are the lowest usable flight levels.

[ppragman]

its in the far/aim too. 29.41 and the 180 is not the lowest useable flight level

[B767Driver]

Originally Posted by sorrygottarunway

Sorry for the confusion, answered my own question!

You got it. It's not an altimeter error...it's a flight level that impedes upon an altitude. Therefore certain flight levels become unusable at low altimeter settings.

Occasionally, with an altimeter setting of 29.90 you will hear a pilot request FL180. A controller will then state "FL unusable." FL180 is only usable for altimeter settings of 29.92 or higher.

[ORDinary]

Sounds like you did answer your own question. FYI: the altimeter setting at JFK at one point this past Sunday was 29.05, imagine forgetting to reset it in that case. (It was the lowest I've seen, except maybe during Katrina).