Pressure Settings
David Moran
approximate change in indication?
Altimeter will indicate .15" Hg higher.
Altimeter will indicate 150 feet higher.
Altimeter will indicate 150 feet lower. <- correct answer
--
sorry, but this is really confusing me. If the pressure is 0.15'' less, then
surely you have gained 150ft since pressure drops as altitude is gained?
David
Robert M. Gary
Pressure stayed the same, you just told the altimeter the reference
was lower.
Topset
The way I remember it is as follows: The altitude indicated is directly
proportional to the pressure setting e.g. increase the setting, indicated
altitude increases. Decrease the setting, indicated altitude decreases.
You are right. Pressure does drop as altitude increases, but when adjusting
the altimeter pressure setting you are "synchronizing" the altimeter, not
experiencing a change in altitude.
I have zero tolerance for zero tolerance policies.
- Greg Gensicki
Herb Martin
three ways might give you a method that makes the
MOST SENSE to you.]
> If a pilot changes the altimeter setting from 30.11 to 29.96, what is the
> approximate change in indication?
> Altimeter will indicate .15" Hg higher.
> Altimeter will indicate 150 feet higher.
> Altimeter will indicate 150 feet lower. <- correct answer
Ok, the easy part: 1 inch Hg is about 1000 feet.
So, 0.15 inches is going to be EITHER 150 up or down, right?
You are going to hear "High to Low, Look out below" from others
trying to help you -- but this didn't really do it for me although once
you understand THIS issue, it WILL help you keep it straight.
Look out below, means that the INDICATED altitude will be
lower than the TRUE altitude (people below might be in danger
even though you THINK you are high ENOUGH.) So if YOU
CHANGE from HIGH setting to LOW setting, you will INDICATE
less than before.
This is a HIGH->LOW so it is a DECREASE -- or 150 feet
in altitude [indicated --> true]
[Here is another way to UNDERSTAND the issue -- but it is
a terrible way to REMEMBER it so you might want to re-read
what is above after you either "get this" or "decide it doesn't
help": When your altimeter is set to a LOWER VALUE, you
are moving the BASE pressure (sea level) UP and this is
LESS of a difference between that base and the current
altitude setting pressure. So you FEET read lower when
your PRESSURE setting is LOWER.]
Another thing to be careful about, SOME of the questions
will give you "true altitude read High/Low" while some like
this one will be based on "indicated altitude" (as yours above
is.) BE CAREFUL that you are reading it correctly.
> sorry, but this is really confusing me. If the pressure is 0.15'' less,
then
> surely you have gained 150ft since pressure drops as altitude is gained?
The PRESSURE you are setting is the BASE, and as you CLIMB
the pressure gets lower. The ALTIMETER really reads the DIFFERENCE
between these two pressures: Current altitude and the BASE sea level
pressures. So a lower base (pressure, not altitude) gives LESS DIFFERENCE
as the altitude continues to lower the pressure.
Since ALTITUDE gives a LOWER PRESSURE, reducing the
pressure of the BASE gives a SMALLER DIFFERENCE
...a smaller difference between the Sea Level Base pressure
and the Current Altitude Pressure.
Don't bother to apologize, I made an excellent score on my
Knowledge exam and THIS question (also other versions)
gave me trouble right up to the end.
Now, I can write this without even looking (assuming I got it
write <grin>)
Herb Martin
<student pilot>
"David Moran" <da...@hostireland.com> wrote in message
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Rad
confusion out...
"David Moran" <da...@hostireland.com> wrote in message
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Antonio
get my drift.
Imagine the slinky is the atmosphere. Attach little tags to the slinky
that correspond to different pressure levels. So at the bottom of the
slinky I attach a tag that says "29.92", and above that (since the
pressure will be lower) I attach a tag that says "29.01, etc. Note that
the pressure level of 29.01 will always be above 29.92 because pressure
decreases with altitude.
After you have attached all your tags strecth the slinky. All of the
pressure levels rise. This corresponds to a hot day when the atmosphere
has expanded. The altitude at which you will find 29.92 changes from day
to day depending upon temperature and pressure. With me so far?
Now there you are at 5000' flying along with 29.92 in the little window
on the altimeter (the Kolisman window) and the temperature drops. This
causes the slinky to contract, which causes the 29.92 level to drop
lower. This has all happened while you are flying along fat, dumb, and
happy as you chase the needle on the altimeter to make it say 5000'. You
have no idea that the temperature has dropped, or that the pressure has
changed. What has happened?
Well, the pressure level that you were using to measure off
of--29.92--moved downwards because the atmosphere shrunk. You chased the
needle to make it say 5000' and this put you closer to mother earth,
right? But you think "I am still at 5000', why does that mountain look
so high?"
As has been said, the altimeter measures or references off of whatever
you put into the Kolisman window. It tells you how high you are above
that level. Keep in mind that the pressure levels move up and down and
that you are measuring your distance above that level and I think you
will have your answer.
In your example the pilot changed the level which s/he was referencing
off of. That level is physically higher so, if I were standing at the
top of a 1000' tower and were measuring with my hand-held altimeter and
I was first measuring the distance between me and the the 30.11 level
and then changed the setting to measure between me and the 29.96
(higher) level the altimeter thinks the distance has shrunk, right? So
it will measure lower.
Antonio
Terry
think of the altimeter as just what it is , it senses actual pressure at
what ever height you are at but gives a response proportional to the
difference in pressure between what it senses and what you put in the
window as a reference
so lets say your altimeter is sensing 28 " Hg and you have set 30" as your
reference. this is a difference of 2" Hg which the altimeter converts to
2000 ft. ( 1"Hg is about 1000 ft ) but if you set the reference at only
29"Hg the altimeter still senses 28" but this is now a difference of only
1" Hg or 1000 ft from the reference.
so just remember reference lower in the window must mean a lower indicated
altitude.
My apologies if this futher adds to anyones confusion
Terry
Geelong , Australia
52 hrs 7.5 PIC
Patrick
you change the pressure on sea level.
Example:
You're flying at 1,000 ft MSL, pressure outside your aircraft is 28,92. So,
the pressure at sea level is 29,92 (that's what your dial would say!) -
that's exactly 1" per 1,000 ft.
Now, change the dial to 29,87. That makes the difference between "real"
pressure outside your aircraft and the one you dialled in SMALLER (now just
.95"). If the difference is smaller, so is the indicated altitude!
It's a simple rule: if the numbers in the pressure window get lower, so does
the indicated altitude!
Patrick
"David Moran" <da...@hostireland.com> schrieb im Newsbeitrag
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Stu Browne
analogy made it even clearer to me.
Cheers
Stu
Antonio <tap...@scattercreek.com> wrote in message
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TD
I like to think along the lines of what the instrument is doing. If you set
it at 30.11, it thinks there is a relatively high pressure and large amount
of air molecules pressing on it. If you suddenly change the setting to
29.96, it expects to feel less air molecules around it. However, there is
the same high number of air molecules as before, when it was set at 30.11,
so the altimeter will think it is at a denser and lower altitude. Note that
this is the reverse of what happens when you forget to change your altimeter
setting when going from a region of high to lower pressure. In this second
case, you altimeter will read artefactually high (meaning you are too low)
which is worse than scenario one, usually.
TD
David Moran
3.3.3.9.3.A.1 I22
Which condition would cause the altimeter to indicate a lower altitude than
true altitude?
A Air temperature warmer than standard.
B Atmospheric pressure lower than standard.
C Air temperature lower than standard.
I chose "C" because the altimeter reads a higher altitude in dense air (cool
air), so if the temperature was lowered, the air would be more dense and the
altimeter would feel closer to the ground?
David
"David Moran" <da...@hostireland.com> wrote in message
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Antonio
watch out for. The altimeter is *indicating* you are lower than you
actually are. Another way to say that is: your true altitude is higher
than what is indicated on the altimeter.
You can remember all of this by the old mnemonic: "High to low, lookout
below. Low to high, you're in the sky". When you go from a high to a
low you need to 'lookout below' because you are closer to the ground
than the altimeter says you are--that is-- the altimeter is *indicating
* higher.
The question asks,"What would cause the altimeter to *indicate*
lower..." which can be tricky.
The altimeter will *indicate* you are lower than you actually are when
the temperature or pressure has risen. "Low to high you're in the sky"
means that when the temperature rises your altimeter will *indicate*
that you are lower than you actually are.
Antonio (Wiping the sweat away...)
Antonio
illustrate this. It is nice to know that I can claim to have invented
something in the world of aviation! ;-)
Antonio
Charles Oppermann
(cool
> air), so if the temperature was lowered, the air would be more dense and
the
> altimeter would feel closer to the ground?
If you start over-thinking it you'll get mixed up. It's fine to understand
the ins and outs, but in the air just think, "high to low, look out below."
Going from high temp/low pressure to low means trouble - trouble means
you're closer to the ground (AGL) than you think. What makes you think
that? Indicated altitude.
So, the IA is higher, but the True altitude is lower.
When you take into account the fact that you'll be keeping the plane at the
same indicated altitude throughout, you're true altitude is now getting less
and less.
--
Charles Oppermann, Copper Software
Author, "Microsoft Windows 2000 Active Directory Programming"
http://www.coppersoftware.com
Charles Oppermann
should read "high temp/high pressure".
"Charles Oppermann" <coppers...@charles.com> wrote in message
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