low altimeter and FL180

Ron Natalie wrote:

"Frank Ch. Eigler" wrote in message news
"Michael 182" writes:


Was flying in the Denver area today and approach told a plane that FL 180
was unavailable die to low altimeter, which was 28.87 at the time. What is
the reason for this? Confusion with 29.92 readings?

FL180 is 18000 indicated feet with the altimeter set to 29.92. That
same airspace region is at 16950 ft indicated with the actual
altimeter setting. In order to keep traffic using the two altimeter
settings apart, FL180 is not used, nor is FL190 (17950) in this case.


To put it another way, flight levels don't exist below the transition altitude
(18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available.





I hope you'll forgive the intrusion from a lurker and non-pilot (well,
a wannabe-pilot). Would someone please explain a few points about this
to me? I understand that altimeters are re-calibrated to correct for
the current barometric pressure (extrapolated to MSL). My questions:

- Why would people be using two different altimeter settings anyway?
- Why would this problem affect just FL180 and FL200? Why not FL160, 220,
and every other altitude?
- Portions of other posts make it sound like there's something special
about calibrating ones' altimeter to 29.92, as if that's commonly done
in certain situations. Is there something different about that specific
number? or am I misinterpreting other posts?

I don't have any reference books mentioned in another post, so cannot
look up these answers myself. Thanks for any light you can shed.

- Bo

Robert Lyons wrote:

- Why would people be using two different altimeter settings anyway?

Because the regulations require it. In the States, all aircraft flying at or
above 18,000' ASL are required to set their altimeters to 29.92. Aircraft flying
below 18,000' are expected to set their altimeters to the local barometric
pressure.

- Why would this problem affect just FL180 and FL200? Why not FL160, 220,
and every other altitude?

It only affects flight levels at which the actual altitude of a plane using the
29.92 setting would be low enough to conflict with aircraft using the actual
pressure setting. In this case, a plane flying at 18,000' using 29.92 would be
at 16950 ft indicated with the actual altimeter setting. If another aircraft
using the local barometric pressure is flying at 17,000', there is a possibility
of collision. One pilot "knows" he's at 18,000' and the other pilot "knows" he's
at 17,000', but they're actually at nearly the same altitude. Since altimeters
can be slightly inaccurate, they could easily be at exactly the same altitude.

Since ATC is tasked with ensuring a certain amount of separation between IFR
aircraft, ATC will simply not assign 18,000' as a flight altitude.

FL 200 and higher don't present problems here. FL 210 would be an altitude of
19,950', and there will be no aircraft at that altitude using the local setting.

George Patterson
You can dress a hog in a tuxedo, but he still wants to roll in the mud.

Thanks VERY much, George and Peter, for your cogent answers to my
questions. I understand now.

- Bo

"G.R. Patterson III" wrote in message
...


Robert Lyons wrote:

- Why would people be using two different altimeter settings anyway?

Because the regulations require it. In the States, all aircraft flying at
or
above 18,000' ASL are required to set their altimeters to 29.92. Aircraft
flying
below 18,000' are expected to set their altimeters to the local barometric
pressure.

I have occasionally idly wondered about one little detail. When it's a
significantly low or high pressure day, when do you change the altimeter
setting while climbing or descending? Maybe as you pass through FL180
descending, and as you pass through 18,000ft climbing?

Or maybe you do it when you get the clearance through the transition
altitude, to make it easier to plan the climb/descent (assuming the computer
isn't doing all that for you). Or maybe it takes you so long to twist the
knob that it doesn't much matter. What do you oxygen-suckers actually do?

-- David Brooks

"David Brooks" wrote in message
...
I have occasionally idly wondered about one little detail. When it's a
significantly low or high pressure day, when do you change the altimeter
setting while climbing or descending? Maybe as you pass through FL180
descending, and as you pass through 18,000ft climbing?

Most of the planes flying in the flight levels, they climb quickly enough
that asking "when do you change the altimeter" is like asking "should I have
my dinner delivered at 5 minutes, 31 seconds past the hour, or 5 minutes 37
seconds past the hour".

For the rest of "us" (my plane is certified to 20,000', though it's not
worth the trouble to fly that high), I believe that you're supposed to
switch over to 29.92 during the climb when you actually reach FL180, and
switch back to the local altimeter during the descent when you leave FL180.

When you actually do switch is, of course, more a matter of practicality
than anything, since you won't be given vertical separation during the climb
or descent that might depend on your altimeter setting (not counting
separation at the top or bottom of the climb or descent, of course).

Pete

"David Brooks" wrote
Maybe as you pass through FL180 descending, and as you pass through
18,000ft climbing?

You got it!

Bob Moore

The US uses 18,000 or FL180 as it's "transition" altitude.. other countries
us other altitudes, some as low as FL040 or 4,000ft MSL.

In the US: reset to 29.92 when climbing upon passing 18,000ft and climbing
to a FL
reset to the assigned local setting as soon as starting a descent when the
"altitude cleared to" is below FL180.

BT

"David Brooks" wrote in message
...
"G.R. Patterson III" wrote in message
...


Robert Lyons wrote:

- Why would people be using two different altimeter settings anyway?

Because the regulations require it. In the States, all aircraft flying
at
or
above 18,000' ASL are required to set their altimeters to 29.92.
Aircraft
flying
below 18,000' are expected to set their altimeters to the local
barometric
pressure.

I have occasionally idly wondered about one little detail. When it's a
significantly low or high pressure day, when do you change the altimeter
setting while climbing or descending? Maybe as you pass through FL180
descending, and as you pass through 18,000ft climbing?

Or maybe you do it when you get the clearance through the transition
altitude, to make it easier to plan the climb/descent (assuming the
computer
isn't doing all that for you). Or maybe it takes you so long to twist the
knob that it doesn't much matter. What do you oxygen-suckers actually do?

-- David Brooks

3000ft here in the UK. Unless specified otherwise:

http://www.caa.co.uk/docs/64/DAP_ACD_8_Altimeter.pdf

Paul

"BTIZ" wrote in message
news:zqYnb.103692$La.22441@fed1read02...
The US uses 18,000 or FL180 as it's "transition" altitude.. other
countries
us other altitudes, some as low as FL040 or 4,000ft MSL.

In article , "David Brooks"
writes:

What do you oxygen-suckers actually do?

Reset the altimeter passing through FL180 going up or down. However, I hope I
am not sucking oxygen as this would be indicative of a major malfunction. :-)

G. Lee

In article ,
(GLPILOTSRV) writes:

Reset the altimeter passing through FL180 going up or down. However, I hope I
am not sucking oxygen as this would be indicative of a major malfunction. :-)


Ah, I forgot to mention that I do this in the continental US. The rules, of
course, are different alsewhere.

G. Lee