|By Russ Still
May 22, 2020
One of the most confusing issues for pilots involves pressure levels and the airplane’s height above sea level - its true altitude. When plowing through the books, every student encountered the sayings “hot to cold, look out below” or “high to low, look out below”. They understood the sayings, but didn’t quite get how, or why, it works.
Let’s take it apart and see what’s inside.
The context is that you are flying along with a constant indicated altitude and a constant barometric pressure setting in the Kollsman window. Everything you see on the altimeter remains unchanged.
As a reminder, the atmosphere tends to be cooler and less dense the higher we go. And conversely, it tends to be warmer and more dense at lower altitudes.
So we start out with some barometric pressure setting at the surface and as the air pressure around the airplane changes during our flight, the altimeter responds by either moving up or down.
Assume that we have a correct pressure setting in the altimeter and we’re flying along with 5,000 feet indicated. Over distance, if the pressure levels stay where they are, we cruise along and our height above the surface, or above mean sea level to be specific, remains constant.
If we intentionally descend from 5,000 feet to 4,000 feet, we are physically moving into air that is more dense, and our altimeter reacts by showing the change in indicated altitude.