True vs Absolute Altitudes

In one of the recent AOPA newsletters I get, there was training tip on remembering the different altitude terms. Indicated, pressure, and density altitude are all pretty easy to remember–and, since you use them all the time, they’re also easy to understand and apply, right? Density altitude is what you use on your performance charts, and it takes into consideration 2 things: pressure altitude at the landing site and temperature. Not an elegant definition, but a working one. Pressure altitude is corrected for…atmospheric pressure, or is what the altimeter shows when set to standard pressure. And indicated is…wait for it, wait for it: what you see on the altimeter.

The 2 that trip me up, especially on tests, are true and absolute. The best excuse I can come up with for this is that you just don’t use those specific terms day-to-day, even though you constantly apply the concepts of both true and absolute altitudes. What we (or maybe just “I”?) need is a memory aid to help us when somebody asks for true or absolute altitude, or we see those specific terms in our studies. I got all excited when I read that newsletter:

Memory aids may help you remember the meanings. Indicated altitude is just that—the altimeter’s indication at the current altimeter setting. Pressure altitude is what you get when you set standard pressure (29.92 inches hg) on your altimeter. Density altitude is an important calculation telling how air density at any level is affected by nonstandard temperature and pressure. True altitude is defined as the “vertical distance above sea level.” Think of it as the true yardstick measure. Absolute altitude is the vertical distance of an aircraft above ground level, and is an exception to the above rule that altitudes are usually referenced to sea level.

Well, those are a little helpful, but a memory aid should be something that really sticks with you. For example, during grad school, I walked into class one day, and the last lesson from the class before was still scrawled on the chalkboard: “Some Say Marry Money But My Brother Says Big Breasts Matter More.” Can’t tell you what my class was about, but never forgot that.* CAMAFOOTS, IP-TA-FER, 7-5/taken alive! are some of the ones I like for required equipment, position reporting, and squawk codes. Here’s what I came up with for Absolute and for True:

Absolute = Altitude Above ground, or just remember that the A in Absolute is for the A in AGL

True = altitude in the Troposphere

Not elegant either, but maybe they’ll provide a some simple terms that help you form a mental image of these two terms.

*(it was the med students trying to remember the order of sensory and motor neurons)