dan@fluke.UUCP (Dan Everhart) (03/20/85)
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Can anyone tell me a formula relating atmospheric pressure to altitude?
Thanks
Dan Everhart
John Fluke Mfg. Co.
{ decvax!microsof, uw-beaver, allegra, lbl-csam, ssc-vax } !fluke!danwolit@alice.UUCP (Jan Wolitzky) (03/21/85)
As I recall, for each 18,000 (approx) of altitude, the pressure drops by half. Thus, the pressure at an altitude of X feet, in atmospheres, is something like: 2 ^ -(X / 18000). Multiply by 14.7 for PSI, 29.98 for inches of Hg, etc. Of course, temperature, humidity, latitude, and plain old deviation from ideal gas behavior all have their effect. -- Jan Wolitzky, AT&T Bell Laboratories, Murray Hill, NJ; (201) 582-2998
mlf@druxn.UUCP (FontenotML) (03/23/85)
*********************************
The dependence of pressure on altitude can be derived from the
hydrostatic equation:
(dp/dz) = - rho g
and the ideal gas law: p = R rho T, together with the assumption
that temperature decreases 2 degrees C per 1000 feet of altitude increase
(the "standard atmosphere" assumption). The result is that
(p/pzero) = ((288-2z)/288)**5.2072,
where p is the pressure at altitude z (with z expressed in thousands
of feet) and pzero is the sealevel pressure. The constant 288 in the
equation is the standard sealevel temperature (15 degrees C), expressed
in degrees kelvin. The exponent 5.2072 is g/(2R), converted to units
of degrees per thousand feet. Once this equation is available, TAS
can be easily obtained from CAS by dividing CAS by
((p/pzero)(288/T))**(1/2),
where T is the temperature at altitude z, expressed in degrees kelvin.
(and degrees kelvin = 273 + degrees C).
Mike Fontenot
Denver, Colo
*****************************jpj@mss.UUCP (J. P. Jenal) (03/24/85)
Just one more quick fact to add here - standard lapse rate is roughly 1" of Hg for each 1000'. That was the information that was most useful to me in terms of passing the written - hope it helps! Cheers... Jim Jenal (aka ...!scgvaxd!mss!jpj) Mayfield Senior School ( " ...!ihnp4!mss!jpj)
mat@amdahl.UUCP (Mike Taylor) (03/26/85)
Useful approximation at low altitude is 1 in. Hg = 1,000 feet. In
case of altimeter failure without backup, break glass on MP indicator
and use it as a substitute altimeter, with the above relationship.
--
Mike Taylor ...!{ihnp4,hplabs,amd,sun}!amdahl!mat
[ This may not reflect my opinion, let alone anyone else's. ]doug@terak.UUCP (Doug Pardee) (04/01/85)
> In case of altimeter failure without backup, break glass on MP indicator > and use it as a substitute altimeter, with the above relationship. Huh? Although the Manifold Pressure gauge can indeed provide some gross altitude info, I don't see how breaking the glass is going to help. What you need to do is to get the manifold vacuum to zero, for example by shutting down the engine and stopping the prop. With a non-turbo engine, firewalling the throttle will also bring the vacuum to near zero (but set your prop to fine pitch first :-) If you're VFR, looking out the windows will provide a clue as to your altitude. If you're IFR, ask Center what your Mode C readout is. (I presume if you're IFR in a plane with a C/S prop, you've probably got Mode C). Besides, you have to tell them about your altimeter failure anyway... -- Doug Pardee -- Terak Corp. -- !{hao,ihnp4,decvax}!noao!terak!doug
mat@amdahl.UUCP (Mike Taylor) (04/08/85)
> > In case of altimeter failure without backup, break glass on MP indicator > > and use it as a substitute altimeter, with the above relationship. > > Huh? Although the Manifold Pressure gauge can indeed provide some > gross altitude info, I don't see how breaking the glass is going to > help. What you need to do is to get the manifold vacuum to zero, > for example by shutting down the engine and stopping the prop. With > a non-turbo engine, firewalling the throttle will also bring the > vacuum to near zero (but set your prop to fine pitch first :-) > Shutting down the engine seems an extreme solution to the problem. Mode C is derived from an encoding altimeter (usually). If the altimeter failed (the original problem) then your mode C won't be working. While VFR, eyeball is an obvious solution (unless VFR on top, of course). Breaking the glass on the altimeter allows cabin air to enter the body of the MP gauge, providing a static source which replaces the line to the intake manifold. The MP gauge contains an exhausted bellows and a spring. This will react and measure the cabin pressure instead of the manifold pressure. The manifold pressure will only equal the static pressure when the engine is stopped. The flow through the ductwork and manifold will cause a loss of 1 to 2 inches at least, equivalent to a 1,000 to 2,000 foot error. That's why you break the glass. It's a good idea to always have two altimeters, or one altimeter and an independent encoder ... a so-called "blind encoder". With all due respect, I suggest that breaking the glass is a much better solution than stopping the engine if you don't have backup - my original premise. -- Mike Taylor ...!{ihnp4,hplabs,amd,sun}!amdahl!mat [ This may not reflect my opinion, let alone anyone else's. ]
rick@cadtec.UUCP (Rick Auricchio) (04/08/85)
another way to guess at altitude is to reverse-compute pressure
altitude from the TAS/IAS differential. Many airspeed indicators have the TAS
scale: you probably know OAT, IAS (if pitot is ok) and a "usual" TAS. Set the
"usual" TAS opposite IAS and read pressure altitude next to the OAT.
It's crude, and probably only good to +- 500 feet.
==============================================================================
Opinions expressed have been generated solely by a stuck mike...
{cbosgd,decwrl,hplabs,ihnp4,seismo}!nsc!cadtec!rick N1150G (408) 942-1535
"You know you've made a gear-up landing when it takes full power to taxi."scw@cepu.UUCP (Stephen C. Woods) (04/08/85)
In article <463@terak.UUCP> doug@terak.UUCP (Doug Pardee) writes: >> In case of altimeter failure without backup, break glass on MP indicator >> and use it as a substitute altimeter, with the above relationship. > >Huh? Although [...](but set your prop to fine pitch first :-) > >If you're VFR, looking out the windows will provide a clue as to your >altitude. If you're IFR, ask Center what your Mode C readout is. >(I presume if you're-- IFR in a plane with a C/S prop, you've probably >got Mode C). Besides, you have to tell them about your altimeter >failure anyway... > I beg your pardon, loss of altimeter should also mean loss of Mode C altitude information (where else does the X-ponder get it's information from?). -- Stephen C. Woods (VA Wadsworth Med Ctr./UCLA Dept. of Neurology) uucp: { {ihnp4, uiucdcs}!bradley, hao, trwrb}!cepu!scw ARPA: cepu!scw@ucla-cs location: N 34 3' 9.1" W 118 27' 4.3"
doug@terak.UUCP (Doug Pardee) (04/09/85)
> Mode C is derived from an encoding altimeter (usually). If the altimeter > failed (the original problem) then your mode C won't be working. Ooops. I'm so used to blind encoders I plumb forgot about the existence of encoding altimeters. Score another point for blind encoders. -- Doug Pardee -- Terak Corp. -- !{hao,ihnp4,decvax}!noao!terak!doug
dgb@mgwess.UUCP (04/10/85)
aa Most Mode 'C' systems use an independant blind encoder which will continue to report via mode c/transponder to center or other facility your altitude even if your visual altimeter fails. Donald G. Beahm AT & T - Montgomery, Il. s| q c C q! ! q r
ark@alice.UUCP (Andrew Koenig) (04/10/85)
> Most Mode 'C' systems use an independant blind encoder which will > continue to report via mode c/transponder to center or other facility > your altitude even if your visual altimeter fails. > Donald G. Beahm AT & T - Montgomery, Il. Some do, some don't. An airplane with a separate blind encoder is usually placarded as such, presumably so the pilot will realize there is a way to control altitude even if the primary altimeter quits. The airplanes I fly do NOT have blind encoders. If I flew a lot of IFR, I would probably invest in a backup altimeter.
doug@terak.UUCP (Doug Pardee) (04/12/85)
> If I flew a lot of IFR, I would probably invest in a backup altimeter.
And a backup altimeter needn't be expensive. I mean, this discussion
started with a description of using the MAP gauge as a crude altimeter.
I would think that the $5 automotive altimeters you find in the auto
section at K-Mart would be as accurate. Toss one in your flight bag,
along with some tape or something to stick it to the instrument panel
when the primary altimeter takes a powder. Maybe a mag compass, too?
--
Doug Pardee -- Terak Corp. -- !{hao,ihnp4,decvax}!noao!terak!dougpeterb@pbear.UUCP (04/24/85)
Most altitude encoding trasnponders that I know of use a seperate bellows to drive the transponder. When you upgrade your transponder to include altitude encoding information, you DO NOT replace the altimiter you have already. A black box is added that has a connection to the static air vent. It is fixed at 29.95 inches (I think) and the ATC adjusts for local barometric changes to come up with your real altitude. Therefor your mode C information is still reliable. However if both fail then you got troubles, but I would never break the MP glass since I would have no indication of engine performance. The MP guage can predict engine failure in advance of it actually happening. Peter Barada ima!pbear!peterb ihnp4!inmet!pbear!peterb
mat@amdahl.UUCP (Mike Taylor) (04/29/85)
> > > > Most altitude encoding trasnponders that I know of use a seperate > bellows to drive the transponder. When you upgrade your transponder to > include altitude encoding information, you DO NOT replace the altimiter you > have already. A black box is added that has a connection to the static air > vent. It is fixed at 29.95 inches (I think) and the ATC adjusts for local > barometric changes to come up with your real altitude. > > Therefor your mode C information is still reliable. However if both > fail then you got troubles, but I would never break the MP glass since I > would have no indication of engine performance. The MP guage can predict > engine failure in advance of it actually happening. > > Peter Barada > ima!pbear!peterb > ihnp4!inmet!pbear!peterb What you say is true for many older airplanes that predate altitude encoding requirements. However, most newer airplanes use encoding altimeters which provide electrical input to the transponder. This is a cheaper arrangement when you're starting from scratch. However, I really don't care to argue the point - I am just trying to pass along an obscure safety tip that might help somebody someday. Accurate altitude information is essential for safe IFR, in my opinion. On the other hand, timed climbs and descents can be used to make altitude changes. I've done enough of them to know that they require much practice. Personally, I'm not inclined to fly IFR without a backup altimeter because I don't want to guess at DH on a nasty day. -- Mike Taylor ...!{ihnp4,hplabs,amd,sun}!amdahl!mat [ This may not reflect my opinion, let alone anyone else's. ]
marcum@sun.uucp (Alan Marcum) (12/23/85)
Anyone have an algorithm for converting between PSI and pressure
altitude (both directions)?
--
Alan M. Marcum Sun Microsystems, Technical Consulting
...!{dual,ihnp4}!sun!nescorna!marcum Mountain View, California