How to use the altimeter?
#1
1. The flight is under "normal" day-to-day operations, below transition altitude, and in all the following I will use QNH
2. I am the 1st guy to fly the aicraft in the day
3. I start the engine, listen to ATIS or contact the tower, it says 30 InHg, so I set the altimeter accordingly
4. The alimeter would then display the altitude of the departure airport (possibly with a max error range of 75 ft) and that altitude would be consistent with what I can read on a aviation map
5. I take off
6. During the cruise, I need to reset the altimeter as frequently as possible by listening the ATIS of the closest station
7. Before landing I need to listen to ATIS/contact the tower of the destination airport and reset the altimeter accordingly
8. After landing, the altimeter would display the altitude of the destination airport (possibly with a max error range of 75 ft) and that altitude would be consistent with what I can read on a aviation map

Attention point: the error range (75 ft allowed):
- pressure is not static
- temperature plays a role
- airport is not perfectly flat
- controlled airports don't publish the weather more than once an hour
- there can be a difference between ATIS and airports published weather

Temperature does not affect your altimeter reading at the QNH source. If your airfield from which you get your local QNH is at 700 ft, your altimeter would indicate approximately 700 ft at the airfield regardless of the temperature. However, if you get higher or lower, then your altimeter will show an erroneous altitude when the temperature deviates from the standard. If it was very cold, your altimeter set to QNH from airfield situated 700 ft above sea level would indicate negative altitude at sea level. Similarly, it would indicate too high when you fly some height above the airfield. In hot conditions, the effect is reversed. Farther away you are from the field elevation of your QNH source, larger the error.




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#2
A good way to check that we correctly understood the concept is to answer the following quiz...

An airport is 1700 ft above sea level, and temperature reading gives 10C.
From the barometer readings made all at the same time, QFE, QFF and QNH are computed.
Which of the following concerning air pressure is correct ?
A) QFF > QNH
B) QFF = QNH
C) QFF = QFE
D) QNH = QFE

The base is actually understanding the well known aviation adagio: 

"Fom High to Low, watch out bellow!!! From low to high, no fear... Fly!..."

and that it applies equally to pressure and temperature.

For VFR / IFR area settings are also used, a pilot just having to set the altimeter to the broadcast "Area QNH", which is kind of an average / corrected for MSA / obstacles clearance QNH over a given area.

In Europe METARs are issued every 30 min.
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#3
Thank you coach  Wink for your inputs.

I was aware of QNH and QFE but first time I ever read about QFF!

Need to study that.

For simplification purpose and because I cannot learn all at the same time so far I focused on QNH and flying under transition altitude.




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#4
Don't worry, QFF isn't even reported IRL - would be too complex to use…

But from a theoretical PoV if one understands it, one understands also the "from high to low…" in all of it's aspects.

The reasoning is more or less easy to understand, and a simple mnemonic is that of "the same signal direction", like in:

For ISA atmosphere, QNH equals QFF for a given location/airfield.

If temp > ISA then QNH > QFF

If temp < ISA then QNH < QFF

The ISA lapse rate is aprox 2º C / 1000 ft at low / mid levels ( actually more like 1,98º C / 1000 ft ) so, in the above example since the aerodrome reference point is at 1700 ft we should expect a T variation of -3,4º C, which summed to 15º C gives us 11,6ºC ISA.

Now if T(observed) = 10,0 ºC it is actually < ISA, so QNH < QFF, and A) in the above quiz is the correct answer.


Just out of curiosity, very few flight simulators ( desktop ) actually simulate this effect, or geopotential height variation, Aerowinx PSX, DCS World and IL-2 Great Battles being remarkable exceptions, although I've read that Flight Gear's advanced weather model also copes with it, and even War Thunder appears to model it, jus as simplistically as DCS and IL-2 do, based on the mission weather / season.


MSFS and derivates, as well as X-Plane, default or even with their best weather injectors do not model this effect.
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