Altimeter question
 

I have just been reading about pressure and QNH and QFE and I understand 90% but what I don't understand is how does the pressure once reset on the altimeter automatically change the altitude

Kind regards
Adam

It doesn't. The altimeter is simply a barometer. It reads air pressure and expresses it as a height / altitude / flight level depending on the pressure set on subscale.

It has no idea it's doing this, of course. We have simply designated that if you set the sub scale to a value that will have the instrument reading zero on the ground, it is measuring height above that point on the ground (QFE).

If we set it so it reads (approximated) height above sea level (so it'd read zero at sea level) it is measuring altitude (Regional QNH).

If we set the subscale to 1013.3mb (fruit pastilles these days) it will measure flight level (FL).

...or to put it another way
As you go higher, the air pressure drops and the altimeter measures that drop and converts it to height - around 30 feet corresponds to a drop in pressure of one millibar (or one hectopascal - same thing).

If the general atmospheric pressure never changed, that's all we would need. However, the atmospheric pressure goes up and down with the weather. The QNH is used to set the altimeter so that it would be zero at sea level. Once that's set, the altimeter simply shows another 30 feet of height when the air pressure drops by a millibar.

In the UK we often use QFE and set the altimeter to show zero when at airfield level. If the airfield is 60 feet higher than see level then the subscale will show 2mm less than it would if you were setting the QNH.

Sit in an aeroplane and turn the knob. You will see.

No need to be rude to instructors.


Careful! You'll have the pedant police along to tell you that it's SPS and not flight level.

I meant to turn the altimeter's knob, not the instructor's.

I find it helpful to remember that the altimeter indicates the height that you are at, above the place where the actual pressure in the air is the pressure that the mb subscale is indicating.


So if the subscale is set to 1003mb (say) and that is the pressure today at sea level, then you are above sea level by whatever the altimeter reads.


If you set it to 1013mb so that you can say you are at a particular flight level, it is now telling you how high you are above 1013mb. Which could be below sea-level, depending on what the actual pressure is today.


And remember
QFE = above airfield = "height"
QNH = above sea level = "altitude"
and "elevation" means the height of the airfield above sealevel (which doesn't change of course!)

While all the replies are informative and correct, I don't think they have answered the original question:

"How does the pressure once reset on the altimeter automatically change the altitude?"

The barometric (pressure setting) knob has a direct mechanical connection to:

1. the subscale, or barometric setting; and
2. the altimeter pointers (or counter drum).

This means that when you turn the barometric knob in an 'increase' direction, two things happen simultaneously:

1. the subscale shows an increase in the barometric setting; and
2. the altimeter pointers (or counter drum) show an increase in the indicated altitude, at the rate of approximately 30ft per hPa of barometric pressure (or 10ft per .01 in Hg)

The similar but opposite actions occur if the knob is turned in a 'decrease' direction.

Note that the aneroid capsules which expand or contract with decrease/increase in static air pressure do not change their size during this process. In other words, the movement of the altimeter pointers or counter drum is achieved without expansion or contraction of the aneroid capsules.

Adam - Here. https://www.youtube.com/watch?v=gU4MkbK9Jy0

Pictures paint a thousand words.

https://www.youtube.com/watch?v=dLdsdQa6cmo

The altimeter is little different to a set of bathroom scales, you set it to read zero at whatever height you want, usually sea level or airfield level, just like your wife can dial the scales to read what she wants it to!:;)

The dial on ours doesn't go that far...:E

Thanks a lot guys really cleared the clouds away from my head from understanding but now I do understand
Many thanks
Adam