QNH and altimeter question

[xxRED BARONxx]

When you turn the knob on the altimeter to decrease the QNH why does the altitude reading on the altimeter decrease as well? Im confused because a lower QNH would indicate a lower atmosphere pressure so wouldnt the altimeter read an increase in altitude?

[Arm out the window]

Say you're at sea level and the altimeter's set to read zero.

The pressure drops, so the altimeter reads high. You then reset the subscale by winding it down to the new lower pressure setting, which also winds the altitude reading back to zero.

[WhyByFlier]

You're confused! Don't over think it! You're not considering calibration or reference.

The pressure is less the higher up in the atmosphere we go (every 27 feet up its 1mb less).

We set a reference pressure setting on the altimeter.

We're at a beach airport and our chart says the airport elevation is 0 feet. The ATIS says the QNH is 1013.

A cold front approaches causing the pressure to drop.

The ATIS hasn't been updated.

The drop in pressure 'tricks' your altimeter into thinking you've climbed because, as you've correctly identified, higher in the atmosphere there's less pressure.

As the 'situationally aware' pilot you are, whilst performing a pre departure altimeter check you notice your altimeter reads 60 feet. Remember the chart says the airport elevation is 0 feet. This prompts you to ask ATC for the latest QNH and they advise it is now 1011 (1mb is 27 feet). You wind off two millibars and wind off 54 feet. Your altimeter now correctly reads actual elevation. Wind off millibars, wind off height.

[topdrop]

Look at pressure as a different way of measuring altitude, where the figures decrease with greater altitude. Today 1013 is sea level, 1012 is 30ft higher than sea level. If you are at altitude 1003 (10 hpa lower but 300 ft higher) and reset your base datum to 1012 you are now reading a smaller distance - 9 hpa difference equals 270ft.

[Wally Mk2]

There's some clever answers here :ok:
Simply put ( as has been mentioned) the millibar sub-scale & the Alt reading respond in the same sense as has been mentioned, wind off Millibars wind off the Alt reading or the reverse, add Millibars add Alt:-)

Wmk2

[Capn Bloggs]

Think of this as you being a distance (vertically, of course) from a pressure level. Take the 1020 pressure level. If you have 1020 set on your altimeter and it reads 300ft, then you're 300ft from the 1020 pressure level. Now set 1010. The altimeter reduces by 300ft (where'd this 27ft/mb come from??) because the 1010 pressure level is closer to you.

Sama Sama if you set 1030; you are measuring the distance between you and the 1030 level, which is further away than the 1020 level. This is indicated by an increase in "altitude".

[T28D]

Using the sub scale to set QNH or QFE only rotates the movement inside the case thus setting the position of the needles against the fixed card, the capsule stack is not changed at all it rotates with the movement as the base of the stack is fixed to the movement and theoutput comes off the top of the stack via the rocker shaft to move the pointers as the capsule stack expands with decreasing atmospheric pressure .

[Arm out the window]

Crikey, this thread has got legs.

[Con_G]

I remember having problems getting my head around the same thing. I had myself convinced that my instructor, the text books and even Microsoft Flight Sim were all wrong.

Just keep in mind that the sub-scale is a reference only that the altimeter works off to calculate your height AMSL. So if your at sea level and the current QNH is 1013 which is correctly set on the altimeter then it shows 0 feet. If you change the reference (i.e.the sub-scale of the altimeter) to say 1012, the altimeter still sees an actual pressure of 1013 but because its reference, or where it thinks sea level is, is now 1 hpa lower (or 30 feet higher) it in turns thinks you are at minus 30ft.

Hope this helps.

[Counter-rotation]

Or straight out of left field, I offer you this...

If you are trying to sight-in a rifle, and you have it in the stand and are consistently hitting high, and left of the mark - do you wind the sight down and right? Or UP AND LEFT?

:E

Not trying to be deliberately complex or confusing, but sometimes a bit of "lateral" explanation can break a simple subject that someone is stuck on, and the penny drops.

:)

CR.

Answer: This can be counter-intuative at first, until you realise that the rifle is FIXED, and the sight is VARIABLE, during this process (that is, the adjusting of the sight). Once you have wound your scope up and left, you can then adjust your aim with the rifle, and your rounds will be (should be) closer to the mark.

:ok:

[Homesick-Angel]

Jeepers!

Just like the DG which is "tied" to the magnetic compass, the altimeter needs to be set off data (either elevation or QNH if from the atis) to start with to give you the correct reading. Moving the dial moves all the pointers and the subscale in the same direction, but has no effect on the capsule. The capsule will only change with actual changes in pressure by either changing altitude or the natural pressure variations .

[Clearedtoreenter]

More millibars more feet. Less millibars less feet.

[Horatio Leafblower]

Did a BFR for a bloke recently who reset his DG to the track on his GPS "because GPS is more accurate than a compass..." :D :ugh: :uhoh:

...and they say you "can't fail a BFR"...

[MakeItHappenCaptain]

If you are trying to sight-in a rifle, and you have it in the stand and are consistently hitting high, and left of the mark - do you wind the sight down and right? Or UP AND LEFT?

Adjust towards the fall of shot. Up and Left.

But I digress,

A better way to look at it is that the altimeter is reading the difference between sea level pressure and the aircraft's elevation. Big difference means a big altitude. If you reduce the pressure setting the setting in the Kollsman window (lesson for the day #2, Kiddies:O), you are reducing the difference between "sea level pressure" and the ambient pressure, hence the altimeter "reduces".
Smaller difference, smaller altitude.:ok:

[T28D]

Rotating the movement to set QNH only changes the zero reference point, it has absolutely no effect on pressure.

Thus this is misleading : altitude. If you reduce the pressure setting the setting in the Kohlsman window (lesson for the day #2, Kiddieshttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/embarass.gif), you are reducing the difference between "sea level pressure" and the ambient pressure, hence the altimeter "reduces".

[waren9]

Misleading?

It is outright false.

An altimeter does only one thing. It tells you the height between you and the pressure you set in the window.

[MakeItHappenCaptain]

And I have used the quotation marks why???

If you reduce the pressure setting in the Kollsman window you are essentially reducing the difference between SLP and the ambient pressure,

Is that easier for you to understand? Do we need to dumb it down any further? Maybe we do.:rolleyes:

Did I say I was changing the actual pressure? Who do you think I am, the late JC?
I said you are changing the setting which a reasonable person would be able to interpret as changing the reference point. Thus is not misleading or false.

My explanation is just another way to skin the proverbial cat.
The OP's question was why does the altimeter reading decrease when the set QNH is also decreased.

T28D
Using the sub scale to set QNH or QFE only rotates the movement inside the case thus setting the position of the needles against the fixed card, the capsule stack is not changed at all it rotates with the movement as the base of the stack is fixed to the movement and the output comes off the top of the stack via the rocker shaft to move the pointers as the capsule stack expands with decreasing atmospheric pressure.

Fark me. 80 words in one sentence and it all makes as much sense as the fact I used to have a red one until the wheels fell off!:rolleyes:

[T28D]

Maybe you better put thewheels back on the Red one because thats how the Kollsman Altimeter works, the Smiths with counter Pointer is a little different but very few GA aircraft go to the expense of the Smiths.

[MakeItHappenCaptain]

T28,
I have no doubt you know what you're talking about. Happy to defer to your knowledge on the innermost workings of this instrument, but the guy is just asking for a SIMPLE explanation of why it behaves the way it does when you change the setting.

My explanation was NOT incorrect and didn't need to be picked apart because it didn't sound as confusing as yours.

[T28D]

So the simple reply is, you use the sub scale to set the reference point , either QNH or QFE.

QNH is elevation corrected for atmospheric pressure QFE is elevation uncorrected.

[Hempy]

An altimeter is a very simple instrument. It is a barometer. The enclosed capsules (fixed pressure) expand and contract depending on the air pressure at your current location. Therefore, if you know the elevation (AMSL) of your current location (e.g on the ground at an aerodrome), say via ERSA, and set the altimeter subscale so that the known elevation is the same as the needle, the subscale reading will be the QNH (assuming the altimeter is accurate).
So, now you climb a thousand feet. Yes, the outside air pressure is now lower than it was on the ground, but the altimeter is set for the ground pressure. As you climb, the lessing of the outside air pressure causes the altimeter capsules to expand (they have a higher pressure than ambient, and that air wants to get out but it cant...they are fixed capsules) As they expand, they wind the needle forwards i.e up...as they contract (i.e outside air pressure increases, reducing the air in the capsules need to 'escape') the needle winds down.
Best I can explain it

[Capt Fathom]

From another thread!

Logging Command TimeThree pages on how to log flight time!! What is wrong with this picture?

and now....

22 posts on how to set the Altimeter :ugh:

What are Flying Schools teaching these days?

[T28D]

Fathom, You really do have to wonder ???????? If they don't understand basic altimetry what hope for density altitude ??????

[Capn Bloggs]

22 posts on how to set the Altimeter
Actually, the thread is about why, now how. :cool: