Originally Posted by
john_tullamarine
An aside -
The majority of folks aver to the use of 30 ft/mb and 120 ft/C deg deviation when playing with pressure and density heights.
That's fine, providing that you keep in mind that these constants, in general, are wrong. For lower levels, they are adequate and fit for purpose. However, if you are playing with clearance considerations from the hard bits, the variation in pressure height calculation errors really need to be kept in mind.
A link to an explanation Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums
Originally Posted by
FullWings
Another important point is that things like airfield and terrain elevation in aviation are measured in feet (in most places) and these feet are always the same absolute length. A “foot” on your altimeter is only on very rare occasions equal to a foot as you would measure it with a ruler, due to the fact that pressure and temperature have a role to play.
Great for separating aircraft on the same pressure setting from each other, but not so great at separating aircraft from terrain which is why there are safety margins built in, plus corrections for atmospheric conditions significantly below ISA.
Very useful in case they ask further questions, thanks !