Page 772 - engelsk

Aircraft general knowledge

Let us now assume that Jens buys a UL aircraft with the intention of flying from

his own field. He has no control tower – he does not know at what height his

runway is located.

In such cases, you use what is called QFE (Field Elevation). What you do is turn

the adjustment knob until the altimeter shows zero.

You then know that when you come back to land, you are on the ground when

the altimeter shows 0 (assuming the pressure has not changed in the meantime).

With QFE set on With QNH set on
the altimeter, the the altimeter, the
height above height above
the runway is MSL is displayed
displayed

Most people probably know that the air expands when it is warm. So if you take

the altimeter up the mountain, it will not show the same on a hot day as on a

cold day.

When it is warm and the air has expanded, 1 hPa may correspond to a distance

of 31 feet, instead of the standardized 30 feet. This means that the altimeter

shows too low an altitude in the heat.

The situation is the opposite, but

worse, when it is cold, the altimeter

shows too much, which means that you

will be too low!

You have a factor of 2% per 5 degrees

deviation from the standard

temperature at that altitude.

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Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 772