Teaching Pressure Altitude

Pressure altitude is a difficult concept to explain. While there are many ways of teaching the subject, here's an alternative explanation that you might find useful.

I begin by explaining that an altimeter needs two things before it can show the airplane's height above sea level. First, it needs to measure the outside atmospheric (static) pressure. It does this via the airplane's static port. Next, it needs to know the pressure at sea level. Pilots can tell the altimeter what this pressure is by placing the altimeter setting in the Kollsman window. Now the altimeter shows the height above sea level by calculating the difference between these two values.

With this understanding, explain to your students that the engineers who made the airplane's performance charts did so on a day when the pressure at sea level was 29.92 inches of mercury. (This is a slight distortion of the truth but it keeps the idea in story form, which makes the concept easier to remember.) Since all performance charts show altitudes, it's reasonable to assume that all of these altitude values were read from an altimeter with 29.92 set in its Kollsman window.

Therefore, if you're planning on using a performance chart, you need to know two things: the outside air temperature and an altitude. What altitude? Your height above sea level? No. You need to use the engineer's altitude (which is another term for pressure altitude). This is the altitude that an engineer would see on his or her altimeter with 29.92 set in the Kollsman window.

To find the engineer's altitude, set 29.92 in your Kollsman window. Now read the altimeter. Take this altitude value to the performance chart along with the air temperature to get an estimate of airplane performance. Once you've done this, set the current altimeter setting back in the Kollsman window to read your height above sea level.

Remember that good instructors must have more than one way of explaining difficult concepts. Their explanations are their tools-the more, the better.