Imagine this, you’re out for an afternoon flight, a few hours and you return to the airfield to realise that the chap behind the A/G or tower has gone home for the night.
No big deal right? We just land.
However, we’ve been flying on regional QNH for umpteen hours … how do we figure out the QFE so we can fly the correct circuit height and safely know how high above the airfield we are?
Calculate QFE using the map
So, let’s say you’re flying at 2,400 feet on a QNH of 1024, kindly provided by your basic service with the local LARS.
You get the map out (you shouldn’t be flying without one) and you can see that the airfield is 550 feet above sea level.
Simply wind back the altimeter 550 feet and you’ll have yourself the QFE.
You can read off the QFE millibars from the millibar window on the altimeter and the height set is the height above the airfield, your QFE.
However, sometimes winding off 550 feet from the altimeter in choppy conditions, while maintaining altitude steadily, is easier said than done!
So, to get an accurate figure, we need to use a bit of maths!
To get to the figure mathematically, we need to remember that 1 mb = 30 feet.
Take the airfield elevation which in this example is 550 feet (for Popham airfield).
You then find divide that elevation, by 30.
550 ÷ 30 ≈ 18
Then, you take the 18 and take it away from the current QNH.
1024 – 18 = 1006 mb
That will give you your QFE.