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Questions on high altitude pressures



 
 
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  #1  
Old November 24th 08, 06:40 AM posted to rec.aviation.piloting
es330td
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Posts: 96
Default Questions on high altitude pressures

At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA
Ads
  #2  
Old November 24th 08, 07:25 AM posted to rec.aviation.piloting
[email protected]
external usenet poster
 
Posts: 2,889
Default Questions on high altitude pressures

es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA


The relation between altitude and pressure is actually logrithmic, but
up to about 10,000 feet or so the rule of thumb is pretty close.

For a chart of altitude versus pressure up to 100,000 feet, see:

http://www.engineeringtoolbox.com/ai...ure-d_462.html

Everybodies altimeter is the same and the nonlinear nature of the
atmosphere is one of the reasons everyone sets the altimeter to
29.92 in the flight levels.


--
Jim Pennino

Remove .spam.sux to reply.
  #3  
Old November 24th 08, 10:59 AM posted to rec.aviation.piloting
terry
external usenet poster
 
Posts: 215
Default Questions on high altitude pressures

On Nov 24, 5:40*pm, es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. *Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. *Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?


Heres an equation which gives the official ISA atmsopheric pressure
(mbar) as a function of altitude in km. you can put this equation in
excel and plot it yourself. When you plot it you will see the pressure
fall off pretty much linearly until about 15000 ft then it curves
upwards.

Press (mbar) = (101325*(1-6.5*altitude/288.15)^(9.80665*28.9644/
(8.31432*6.5)))/100

at 12000 ft the rule of thumb of 30 mb per 1000 ft gives you 653 mb
while the official ISA value from the above equation is 644 mb which
is pretty damn close. But then when you get to say 30000 ft there is
a big error in using the rule of thumb ( 113 mb vs 301 ISA)

Heres the reference if you want to look it up.
http://wahiduddin.net/calc/density_altitude.htm

Terry
PPL Downunder
sorry about the units. where metricated downunder.
  #4  
Old November 24th 08, 01:37 PM posted to rec.aviation.piloting
es330td
external usenet poster
 
Posts: 96
Default Questions on high altitude pressures

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.

Thanks for the answers.
  #5  
Old November 24th 08, 02:38 PM posted to rec.aviation.piloting
Robert Moore
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Posts: 134
Default Questions on high altitude pressures

es330td wrote
Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and
another that is right at actual altitude.


Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.

Bob Moore
  #6  
Old November 24th 08, 03:58 PM posted to rec.aviation.piloting
a[_3_]
external usenet poster
 
Posts: 562
Default Questions on high altitude pressures

Not nearly as helpful as some of the other posts, but to give you a
sense of how things change, the pressure at 18,000 feet is about half
of that on the surface (30 inches of Hg on the surface, 15 at 18,000
feet). It's halved again at twice that altitude (7.5 inches at
36,000) and halved again at twice that altitude (3.7 inches at 72,000
feet).




On Nov 24, 1:40*am, es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. *Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. *Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA


  #7  
Old November 24th 08, 09:27 PM posted to rec.aviation.piloting
Darkwing
external usenet poster
 
Posts: 604
Default Questions on high altitude pressures


"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.


Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

In theory you could use GPS altitudes when flying MSL/QNH setting,
because both systems measure absolute altitued, so you would expect
them to be the same. If GPS could be relied upon that is. Unfortunately
it cannot, GPS altitued measurements are unrealiable and can drift
wildly (as opposed to GPS 2D positioning which is quite accurate).
Traditional altimeters are much better.

--
I'd like to jump right on the floor



If GPS altitude is unreliable how do you shot a GPS approach or is this why
WAAS was implemented?


  #8  
Old November 24th 08, 09:50 PM posted to rec.aviation.piloting
Mike
external usenet poster
 
Posts: 573
Default Questions on high altitude pressures

"Darkwing" theducksmail"AT"yahoo.com wrote in message
news

"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.


Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

In theory you could use GPS altitudes when flying MSL/QNH setting,
because both systems measure absolute altitued, so you would expect
them to be the same. If GPS could be relied upon that is. Unfortunately
it cannot, GPS altitued measurements are unrealiable and can drift
wildly (as opposed to GPS 2D positioning which is quite accurate).
Traditional altimeters are much better.

--
I'd like to jump right on the floor



If GPS altitude is unreliable how do you shot a GPS approach or is this
why WAAS was implemented?


Exactly. WAAS corrects GPS errors and makes the receiver accurate enough to
perform the equivalent of a Cat I ILS approach. If LAAS is ever
implemented, it will allow the GPS equivalent of Cat III approaches.

  #9  
Old November 24th 08, 09:55 PM posted to rec.aviation.piloting
[email protected]
external usenet poster
 
Posts: 2,889
Default Questions on high altitude pressures

Darkwing theducksmail"AT"yahoo.com wrote:

"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.


Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

In theory you could use GPS altitudes when flying MSL/QNH setting,
because both systems measure absolute altitued, so you would expect
them to be the same. If GPS could be relied upon that is. Unfortunately
it cannot, GPS altitued measurements are unrealiable and can drift
wildly (as opposed to GPS 2D positioning which is quite accurate).
Traditional altimeters are much better.

--
I'd like to jump right on the floor



If GPS altitude is unreliable how do you shot a GPS approach or is this why
WAAS was implemented?


GPS specified altitude accuracy with Selective Availility (SA) turned
off is +/- 150 m.

WASS specified altitude accuracy is +/- 7.6 m.

Typical actuals are usually +/- 4.7 m and 1.3 m respectively, but not
guaranteed at any particular place and time.

The goal of LAAS is to provide a guaranteed accuracy of less than 1 m.


--
Jim Pennino

Remove .spam.sux to reply.
  #10  
Old November 24th 08, 10:53 PM posted to rec.aviation.piloting
Allen[_1_]
external usenet poster
 
Posts: 252
Default Questions on high altitude pressures


"Darkwing" theducksmail"AT"yahoo.com wrote in message
news

"Wolfgang Schwanke" wrote in message
...
es330td wrote in



In theory you could use GPS altitudes when flying MSL/QNH setting,
because both systems measure absolute altitued, so you would expect
them to be the same. If GPS could be relied upon that is. Unfortunately
it cannot, GPS altitued measurements are unrealiable and can drift
wildly (as opposed to GPS 2D positioning which is quite accurate).
Traditional altimeters are much better.

--
I'd like to jump right on the floor



If GPS altitude is unreliable how do you shot a GPS approach or is this
why WAAS was implemented?


Standard GPS approaches a flown with the altitudes from the altimeter. WAAS
approaches rely on GPS position correction from a local transmitter.


--

*H. Allen Smith*
WACO - We are all here, because we are not all there.


 




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