QNH and altimeter question
Join Date: Oct 2002
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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
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
Last edited by Hempy; 14th Feb 2013 at 09:03.
From another thread!
Logging Command Time
and now....
22 posts on how to set the Altimeter
What are Flying Schools teaching these days?
Logging Command Time
Three pages on how to log flight time!! What is wrong with this picture?
22 posts on how to set the Altimeter
What are Flying Schools teaching these days?
Originally Posted by Fathom
22 posts on how to set the Altimeter
