Understanding Altitude

Understanding Altitude

Much has been taught about pressure altitude, true altitude, absolute altitude, indicated altitude … did I forget something ?

I took some pictures inside Cessna 172 C-GNDP last winter, including two that really illustrate this point. On the first one, we see the indicated altitude using the barometric pressure … on the next, I set the calibration to 29.92, so what we read now? The pressure altitude.

Next, I went cross country skiing with my handheld GPS. I skied down and then climbed up the same hill several times. Now if we look at the indicated altitude of the GPS, we see that the top of the hill seems to be sinking. This GPS requires calibrating the altimeter regularly to account for barometric pressure variations (unless you turn the WAAS mode). The pressure increased through the afternoon … my indicated altitude was lower as the day went on!

And what about the absolute altitude? It’s pretty simple … zero, on top as well as the bottom of the slope. I did not jump sufficient high to increase it significantly. The absolute altitude … is the distance between me and the ground.

We are in winter. It is about 0° C. The density altitude, which takes into account the temperature, falls below the sea level on the flight computer. It is now almost 2000 feet below sea level, 1700 feet using the electronic computer. This is where the airplane “thinks” it is. That explains its strong rate of climb!

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