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Spinrecovery created the topic: ALTIMETER question
If you dont know the QNH at the departure aerodrome, you can set the airport elevation to get an approximate QNH but what about setting your altimeter to zero feet, what do you get from that setting?
If you zero the altimeter, the altimeter sub-scale will read the QFE which is the atmospheric pressure at your current position. The "FE" stands for "Field Elevation" so QFE is the code they used when querying the pressure at field elevation. QFE isn't used operationally in Australia any more. All operations will be on QNH or 1013 depending on your altitude.
Could I please clarify, where you will read QFE from if you set zero on the altimeter?
Will the sub scale read elevation, or do you need to set zero on the sub scale to get the elevation from the altimeter?
An altimeter simply measures the difference between the pressure around you at the moment and the pressure you have set in the sub scale. This difference is then displayed in feet where 1 hPa of difference is shown as 30 ft of altitude difference. In other words, it shows you how many feet there are between the altimeter and the point in the atmosphere where the pressure is what you have set on the sub scale.
So, if we set 0 ft on the altimeter, the altimeter is simply saying "There are 0 ft between you and the pressure you set on the sub scale". If there are 0 ft between us and the sub scale, then the sub scale must be the pressure right here, right now. Therefore the sub scale is telling us our QFE.
Similarly, if we are at an airfield with an elevation of 1200 ft (for example) and we we wind the hands of the altimeter to show 1200 ft on the altimeter face, we can look at the sub scale reading and see the pressure at sea level. The altimeter in this case is saying "There are 1200ft between you and the pressure set on the subscale". If we are 1200 ft above the sea and the altimeter says we are 1200 ft above the pressure set on the subscale, then it follows that the pressure on the subscale must be the pressure at sea level at the moment. The sub scale setting would therefore be the current QNH.
Elevations are surveyed heights above sea level. Altitudes are heights above some datum, typically sea level. QNH and QFE are pressures where QFE is the pressure at Field Elevation and the QNH is the pressure at Nil Height.