Altimeters (PPl training)
 

Hi

Question about how altimeters work, im new to this and couldn't find a thread for PPL training in the UK.

Im studying for Air Law and cannot get my head round whats probably a really easy to understand topic!!

Ive got the 'just' of how atmospheric pressures effect the altimeter readings, however there was a sample question I came across which was 'If a pilot changes the altimeter from 1012mb to 1011mb what would be the initial effect of the reading?' and the answer was 'decrease in altitude' now please correct me! but if you are telling the altimeter that the pressure is 'less' wouldn't you expect the thing to show a 'higher' altitude as pressure decreases with height?? :ugh::ugh::ugh:
Cheers!

Imagine if you left your aircraft overnight reading 0 ft at a pressure of 1012. Overnight the pressure changes to 1011, so the indicated elevation goes UP by 30 ft. So, to restore the zero reading you have to change the subscale reading from 1012 to 1011. That's the way I try to remember it - in re-setting the subscale reading to a lower value you are compensating for a reduction in air pressure, and this reducing the indicated elevation.

Hope that helps

Peg

I don't think I've really heard many people say Airlaw is a really easily understood topic, so just keep at it.

Your problem is that you're not really understanding the question, therefore the answer doesn't seem to make sense.
If the static pressure measured by the altimeter decreased from 1012mb to 1011mb, then yes, the indicated altitude would decrease. (Small point, pressure is now measured in hectopascals (hPa), not millibars, same thing just a new name, like Centigrade & Celcius).
However, what the question is actually saying, is that you are changing the altimeter sub scale setting from 1012mb to 1011mb, therefore you are moving the datum point from which your altitude is measured. As the point or level of 1011mb pressure is physically higher up than 1012mb, then if you measure your vertical position (which hasn't changed) from the new, higher datum, then it will be less.
You can try this quite easily on your next lesson, just sit in an aircraft on the ground, set the QNH on the altimeter subscale, it should then show airfield elevation (also your altitude). If you then move the datum on the subscale a mb or two either way, your indicated altitude will change accordingly.

But you are not telling the Altimeter that the pressure surrounding it is less. You are telling it that the pressure at the Base Line (eg Sea Level) against which it measures the difference in pressure is less.

If the pressure, at Sea Level, is 1012Hpa and the Altimeter is at 2,100' then the pressure surrounding the Altimeter will be approximately 942Hpa.(70Hpa difference as 1Hpa equates approximately to 30' difference in Altitude)

Then, if the Altimeter Pressure Sub-Scale is set correctly to 1012, the Altimeter will indicate 2,100' because it is measuring the pressure difference (70Hpa) between where it actually is and the Base Line YOU have input.

If the Altimeter is incorrectly set to 1011Hpa then it will indicate 2,070' because the difference in its surrounding pressure and the Base Line YOU have set is now only 69Hpa.

Hi Piper.

The altimeter just compares the pressure you set on the subscale with the actual pressure it is sensing. If they are the same, the altimeter will read zero.

If the actual pressure is less than the one you have set, the altimeter will show a positive reading.

If the actual pressure is more than the one you have set, the altimeter will show a negative reading.

So, if you decrease the pressure you have set, whilst the actual pressure remains the same, the altimeter reading will decrease. (By approx. 30 feet per hectopascal.)

Hope that helps;)


MJ:ok:

Though I agree with what has been written, to keep things in context, let's recall that the core of an altimeter is an absolute measuring instrument. It measures a pressure effect upon sealed cells, which are temperature compensated. It has no idea what the pressure nor altitude is, until you tell it.

You adjust the datum for the altitude measurement, which might be set as the altitude you know to be that for the altimeter, or the subscale pressure. (By the way they can be set relative to each other, but don't do that, unless you are so authorized - when you drop the screw to your surprise, and can't get it back in, even if you do find it, the maintainer will know you had it apart!)

With the altimeter in "operating" mode, when you turn the setting knob, you turn the entire altimeter mechanism inside the case (it'll go around as many times as you want), and you turn the subscale by gearing (it will not go around endlessly).

So your setting the altimeter subscale or altitude, does nothing other than to move the instrument datum to where it should be - it has zero idea of either value, beyond what you tell it, other than it keeps track of changes once you tell it.....

Happened to a "friend" did it? ;)

The simplest way to remember it is that the altimeter tells you how high you above the point at which the actual pressure is that which the sub-scale reading shows (which could of course be below the ground).


This will also help with more complex altimetery questions in met and nav exams.

A simple way remember which way it works is Wind Up Wind Down

What I read somewhere:

Wind on hectopascals, wind on height.

Wind off hectopascals, wind off height.

In air traffic we use "wind on pressure, wind on height". Works well :) the pooleys books will explain in detail how it all works. No.4 covers a great deal and I'd recommend reading it in conjunction with your studies as you'll find a lot of questions in the exams come from it in some way or another.

Its a Trick Question...
 

Of course this could be a trick question.... Changing just 1mB from 1012 to 1011, there is likely to be sufficient 'Sticksion' that the Altimeter would not change its reading at all.


In some aircraft it is not uncommon for the altitude to remain fixed at say 1000ft until you tap the glass, and then it shows you are really at 600ft as you turn onto final... One good reason to judge your landings by visual references rather than the instruments.
This hysteresis is more common on gliders where there is no associated engine vibration to keep the meter constantly shaken.


.

After 42 years working in ATC the following works for me...

Wind on Hectopascals wind on height

Wind off Hectopascals wind off height

So simple, no need to do silly calculations on a piece of paper :cool::cool:

For me personally, if you just consider that the altimeter always knows the correct outdoor pressure it all becomes a bit easier.

Say the pressure outside is 1010 - the altimeter knows this. All it doesn't know is what the sea level pressure is. Why would it, unless it's by the sea.

Once it finds out, it can calculate your altitude accordingly using the 30ft per hPa rule. Which is why the lovely people in ATC give you this in the form of the QNH.

If you tell it the sea level pressure is 1013, it will think it is 90 ft above the sea.

If you tell it the sea level pressure is 1014, it thinks it's 120ft above the sea and so on. Thus, as you turn the scale up, the indicated altitude will go up.

:ok:

luizmonteiro - Aviation Calculations Home
this page has quite nice altimeter (and some other avionics equip) simulation where you can setup anything including error situations like blocked pitot ram or drain or statics and how it will look comapring to normal system during speed or alt change..


quite handy

just deviating here a little.....

The better altimeters, deluxe models, will have an inbuilt vibrator to reduce the stiction in the gearing...

TW

Heyy thanks everyone!! all makes sense now! Think I am trying to think too deeply into things and was out to find out the whole inner workings of the thing!! when really all I needed to understand was what you guys have so kindly put! Thank you to everyone who has took time out to reply so greatly appreciated! Was really banging my head on the wall with this one!! :ok::ok::ok:

The better better models are electric and work off a CADC...;)

pipercherokee

Just one small point: In the real world no one uses hectopascal thingies !

It'll change again in a couple of years anyway no doubt. I vote for fathoms personally. 'Make circuit height 150 fathoms', 'Aye aye'. Got a certain ring to it I think.