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low altimeter and FL180



 
 
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  #1  
Old October 28th 03, 10:19 PM
Michael 182
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Default low altimeter and FL180

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?

Thanks,

Michael


  #2  
Old October 28th 03, 10:46 PM
Steven P. McNicoll
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"Michael 182" wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...

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?


Because there would be less than 1000 feet separation between aircraft at
17,000 MSL using 28.87 as an altimeter setting and aircraft at FL180 using
29.92. In this case, FL190 is also unavailable for the same reason.


  #3  
Old October 28th 03, 11:00 PM
Frank Ch. Eigler
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"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.

- FChE
  #4  
Old October 28th 03, 11:57 PM
Ron Natalie
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"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.


  #5  
Old October 29th 03, 04:15 AM
Michael 182
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Default

Thanks for the replies. Makes sense now.

Michael

"Ron Natalie" wrote in message
m...

"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.




  #6  
Old October 29th 03, 04:34 AM
BTIZ
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Default

AIM Section 7 Chapter 2

"Michael 182" wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...
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?

Thanks,

Michael




  #7  
Old October 29th 03, 05:57 PM
Robert Lyons
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Default

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

  #8  
Old October 29th 03, 06:42 PM
G.R. Patterson III
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Default



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.
  #9  
Old October 29th 03, 06:47 PM
Peter Duniho
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Default

"Robert Lyons" wrote in message
...
- Why would people be using two different altimeter settings anyway?


Well, the situation at hand is that one airplane is flying at or above
"flight level 180" while the other is flying below "flight level 180". By
definition, altitudes "in the flight levels" are referenced to standard
atmospheric pressure (29.92) even if non-standard conditions prevail. So
anyone flying at or above FL180 has their altimeter set to 29.92. However,
for all flying below the flight levels, the altimeter will generally be set
to the prevailing conditions (sea-level corrected, of course).

This is, in fact, the whole point with respect to the question at hand.

The reason for using a single altimeter setting for all airplanes at or
above FL180 is that most aircraft flying at that altitude travel so quickly
that it would be impractical for them to have to keep changing their
altimeter setting as they fly through different weather systems. They are
at a high enough altitude that the error between indicated and true altitude
won't cause any problem with terrain (at least in the US), which is the main
reason for using a local altimeter setting in the first place.

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


It affects all flight above FL180. There is no "FL160" in the US, and I'm
not sure why you got the impression that flights at FL220 are unaffected.

- 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?


29.92" is the pressure at sea level in standard atmospheric conditions. It
just happens to be a nice "middle-ground" number. Nothing special other
than that.

By the way, we typically reserve the word "calibrate" for an infrequent
operation in which the accuracy of the altimeter is ensured. Setting the
altimeter setting to a specific number in flight is generally referred to as
"setting the altimeter setting".

Pete


  #10  
Old October 29th 03, 07:15 PM
Robert Lyons
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Default

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

- Bo

 




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