How does an altimeter work?
ManhoodUSA
The only guess I have is air pressure. I know air pressure decreases as
the plane ascends. But pressure changes with the weather. I imagine
pilots need an awfully accurate figure for altitude, particularily at night
or in weather conditions that compromise visibility.
Also, different airports have different elevations, naturally. How does
the altimeter correct for a flight from Denver to Death Valley?
So how does the altimeter give the pilot accurate altitiude?
Thanks in advance,
Rich.
Will Flor
In article <rddEGp...@netcom.com>, manho...@aol.com (ManhoodUSA) wrote:
>The only guess I have is air pressure. I know air pressure decreases as
>the plane ascends. But pressure changes with the weather. I imagine
>pilots need an awfully accurate figure for altitude, particularily at night
>or in weather conditions that compromise visibility.
>
Yes; the tower, ground control or ATIS gives the pilot the pressure, in inches
of mercury in the US, kilopascals most other places. Each altimeter has a
little window called a "pressure window" with a movable card behind it, and
the card is calibrated in inches of mercury or kilopascals. By turning a knob
on the face of the instrument, the pilot sets his altimeter's pressure window
to the correct reading. If one flies from a field where there is not tower,
ground control or ATIS the pilot adjusts the altimeter so that it indicates
field elevation and more-or-less ignores the pressure window.
>Also, different airports have different elevations, naturally. How does
>the altimeter correct for a flight from Denver to Death Valley?
>
While en-route, the pilot periodically recalibrates the altimeter as described
above. Even in a flight that doesn't cover vast distances, this is necessary
since the pressure is normally constantly (but slowly) changing. Of course,
this doesn't have anything to do with the change in elevation between Denver
and Death Valley - the altimeter indicates that - "correcting for" it would be
a problem!
>So how does the altimeter give the pilot accurate altitiude?
>
That's how it's calibrated; the actual inner workings are those of an aneroid
barometer. Any standard introductory book on flying has more information; a
book on aircraft systems has a lot more.
-Will Flor wi...@will-flor.spamblock.com
Appropriately adjust my return address to reach me via e-mail.
eer...@nlr.nl
ManhoodUSA (manho...@aol.com) wrote:
: The only guess I have is air pressure. I know air pressure decreases as
: the plane ascends. But pressure changes with the weather. I imagine
: pilots need an awfully accurate figure for altitude, particularily at night
: or in weather conditions that compromise visibility.
: Also, different airports have different elevations, naturally. How does
: the altimeter correct for a flight from Denver to Death Valley?
: So how does the altimeter give the pilot accurate altitiude?
:
Basically, _pressure_altitude is computed from:
h = { (p/p0)^0.19 - 1 } * (T0 / -0.00198) ft
where p = pressure measured outside the aircraft
p0, T0 pressure and temperature at reference altitude (T in Kelvin, BTW).
Now, the altitude reading you get depends on the selection of reference
altitude, i.e., setting the pressure in a subdial on the altimeter. The
following settings are common:
QFE : subdial such that altimeter reads zero when on the ground (i.e. set
to local barometric pressure)
QNH : set to local sea-level pressure. The local barometer reading is
corrected for the local elevation, using the same formula. Altimeter reads
altitude above local sealevel.
QNE : set to 1013.25 mbar, International standard atmosphere sea level
pressure. Altitude reading = Flight Level, used for vertical separation at
higher altitudes.
The altimeter (almost) never reads the exact altitude in feet above ground
level, since the atmosphere does not behave like the formula. You must have
a radar altimeter for that. It's no problem, as long as you realise this,
and all aircraft in a flight control region use the same setting. Vertical
separation controlled by ATC is based on this latter assumption.
You could get in big trouble when trusting the altimeter, and flying long
distances through High/low pressure regions. There are some handy sayings
to warn you, like: "From high to low, look out below".
-----------------------------------------------------------------------------
Johan Eertink eer...@nlr.nl
All opinions in this post are strictly my own;
They do not represent NLR's view
-----------------------------------------------------------------------------
Don Stauffer
ManhoodUSA wrote:
>
> The only guess I have is air pressure. I know air pressure decreases as
> the plane ascends. But pressure changes with the weather. I imagine
> pilots need an awfully accurate figure for altitude, particularily at night
> or in weather conditions that compromise visibility.
>
> Also, different airports have different elevations, naturally. How does
> the altimeter correct for a flight from Denver to Death Valley?
>
> So how does the altimeter give the pilot accurate altitiude?
>
>
> Rich.
You are certainly on the right track. When a pilot wants to take of or
land, he needs to know the barometric pressure at the airport. He gets
this information by radio. At larger airports now a frequency
automatically gives this information along with the other weather
information. At smaller airports the information is available from the
control tower.
Aircraft altimeters have an adjustment knob and window that allows the
pilot to adjust to the current barometric pressure.
As far as for altitudes at different airports, the pilot adjusts the
altimeter to read altitude above sea level. He must then look up the
airport altitude at the airport at which he wants to land. He must
mentally subtract that altitude. For instance, he wants to fly the
pattern at 800 feet above ground. But the airport he is landing at is
at 500 feet elevation. Therefore he must fly the pattern at an
indicated altitude of 1300 feet.
--
Don Stauffer in Minneapolis
home web site- http://home1.gte.net/stauffer/
Mike Garrison
ManhoodUSA wrote:
>
> The only guess I have is air pressure. I know air pressure decreases as
> the plane ascends. But pressure changes with the weather. I imagine
> pilots need an awfully accurate figure for altitude, particularily at night
> or in weather conditions that compromise visibility.
>
> Also, different airports have different elevations, naturally. How does
> the altimeter correct for a flight from Denver to Death Valley?
>
> So how does the altimeter give the pilot accurate altitiude?
>
> Thanks in advance,
Airports measure the local air pressure at their airports. This is
converted to sea level pressure based on a "standard atmosphere." Then
it is radioed to the pilot, who calibrates his altimeter to the same sea
level pressure.
Now that his altimeter is corrected for the local air pressure, the
altitude it reports is calibrated for that location at that time. The
real elevation of the runway is published in the pilot's maps.
(More advanced avionics packages also have other ways to find out how
far the plane is from the runway, like radar altimeters which directly
measure the height above the ground, GPS systems, and microwave landing
systems.)
-Mike
Steve Gushwa
In article <rddEGp...@netcom.com>, manho...@aol.com (ManhoodUSA) says:
>
>The only guess I have is air pressure. I know air pressure decreases as
>the plane ascends. But pressure changes with the weather. I imagine
>pilots need an awfully accurate figure for altitude, particularily at night
>or in weather conditions that compromise visibility.
>
>Also, different airports have different elevations, naturally. How does
>the altimeter correct for a flight from Denver to Death Valley?
>
>So how does the altimeter give the pilot accurate altitiude?
>
>Thanks in advance,
>
>
>
Air pressure is the key. An altimeter is a sealed aneroid barometer with
a pointer attached. It measures air pressure (the weight of the air
column above it). Local pressure is affected by humidity and temperature
(density). A pilot sets the altimeter to the published local field
elevation by adjusting a knob which offsets the aneroid. The altimeter
setting is read from the Kollsman window (29.92 in. Hg on a "standard
day"). When flying from a field in Death Valley a correctly set altimeter
would read -100 feet (Mean Sea Level). While enroute the pilot regularly
resets the altimeter setting in the Kollsman window from radio updates
(this corrects for local pressure variations). Upon landing in Denver the
altimeter would read 5431 feet (MSL). You are correct in assuming that a
very accurate altimeter is necessary for safe flight but all altimeters
have a small degree of mechanical error. It is also interesting to note
that while all aircraft flying their assigned altitudes have safe
separation - their altimeters do not correspond to the "true altitude" or
geometric height above sea level except on a standard day (because of air
density changes from temperature and humidity).
For more information I would recommend any Instrument Rating manual.
Steve
Jürgen Tantau
ManhoodUSA wrote:
>
> The only guess I have is air pressure. I know air pressure decreases as
> the plane ascends. But pressure changes with the weather. I imagine
> pilots need an awfully accurate figure for altitude, particularily at night
> or in weather conditions that compromise visibility.
>
> Also, different airports have different elevations, naturally. How does
> the altimeter correct for a flight from Denver to Death Valley?
>
> So how does the altimeter give the pilot accurate altitiude?
>
> Thanks in advance,
>
> Rich.
Hi Rich,
whenever you request flight information services or you are aproaching
an airfield for landing, the ground station will give you the QNH which
is the air pressure at sea level or the QFE which is the pressure at the
airfield. This figure is dialled at the altimater, and the altimeter
then gives the correct readings for that specific area.
If the altimeter is set to QNH it will tell you on the ground the
altitude of the airfield above sea level. If ot is set to QFE it will
read zero on the ground of that airfield.
Regards
Jürgen
--
to reply, please remove the xxx at the front of my address
Jürgen Tantau (juergen...@t-online.de)
ECRS
In addition to the other interesting information, when flying above 18000
feet, altimeters are set to 29.92 and no longer reflect a relationship to
MSL but to the standard datum plane. Vertical seperation, which is the
point if you are high enough not to hit anything earthly, is maintained by
everyone using the same datum. when decending back through 18000, the
altimeter is again set to the local altimeter setting.
soma...@hotmail.com
ManhoodUSA wrote:
>
> The only guess I have is air pressure. I know air pressure decreases as
> the plane ascends. But pressure changes with the weather. I imagine
> pilots need an awfully accurate figure for altitude, particularily at night
> or in weather conditions that compromise visibility.
>
> Also, different airports have different elevations, naturally. How does
> the altimeter correct for a flight from Denver to Death Valley?
>
> So how does the altimeter give the pilot accurate altitiude?
>
> Thanks in advance,
>
> Rich.
ManhoodUSA wrote:
>
> The only guess I have is air pressure. I know air pressure decreases as
> the plane ascends. But pressure changes with the weather. I imagine
> pilots need an awfully accurate figure for altitude, particularily at night
> or in weather conditions that compromise visibility.
>
> Also, different airports have different elevations, naturally. How does
> the altimeter correct for a flight from Denver to Death Valley?
>
> So how does the altimeter give the pilot accurate altitiude?
>
> Thanks in advance,
>
> Rich.
You set your altimeter to 29.9 which is for sea level. At specfiec
airports, the control tower will tell you the current barometric
pressure, and you correct your alitmeter to correct one.
Judah Milgram
In article <rddEGt...@netcom.com>,
Steve Gushwa <Steve....@stpete.honeywell.com> wrote:
>Air pressure is the key. An altimeter is a sealed aneroid barometer with
>a pointer attached. It measures air pressure (the weight of the air
>column above it). Local pressure is affected by humidity and temperature
>(density). A pilot sets the altimeter to the published local field
>elevation by adjusting a knob which offsets the aneroid.
Next question:
does the adjustment knob shift the p vs. h curve along the altitude axis,
that is, add a constant delta-h so that it always indicates e.g. 100'
higher than the current pressure altitude.
or
does it shift it along the pressure axis by adding a constant delta-p,
so that the altimeter indicates the pressure altitude corresponding
to the current ambient pressure plus/minus a bit?
Or something else?
I'm not certain of the answer. Both can be implemented mechanically.
I once looked at the Altimeters TSO but it didn't say anything about it
(it was an ancient copy). I suppose taking apart a retired altimeter could
resolve this, but still you'd expect to see it written down somewhere.
Anyone know where?
Judah
--
Judah Milgram mil...@eng.umd.edu (301)405-1144
finger milg...@eng.umd.edu for address, pgp key etc.
or: http://www.glue.umd.edu/~milgram/
Trevor Fenn
eer...@nlr.nl wrote:
>
> ManhoodUSA (manho...@aol.com) wrote:
> : The only guess I have is air pressure. I know air pressure decreases as
> : the plane ascends. But pressure changes with the weather. I imagine
> : pilots need an awfully accurate figure for altitude, particularily at night
> : or in weather conditions that compromise visibility.
> :
> : Also, different airports have different elevations, naturally. How does
> : the altimeter correct for a flight from Denver to Death Valley?
> :
> : So how does the altimeter give the pilot accurate altitiude?
Well that's all very nice but it doesn't answer the mans question.
Trevor Fenn
trev...@erols.com