When it comes to flying airplanes, there are numerous types of altitudes you, as a pilot, must be aware of to ensure flight safety. If you’re new to flying, think of piloting as being equivalent to what baking is to cooking.
A chef can play around with different recipes for his or her Bolognese sauce, but a pastry chef (much like a chemist) must follow exact instructions to bake a souffle, or else it will fall. And there are five types of airplane altitude. It’s not just about setting the correct pressure and reading your altimeter.
1) INDICATED ALTITUDE
Let’s start with the easiest – It is simply the altitude you read directly off your altimeter. If uncorrected for pressure changes, your altimeter won’t be very useful.
2) PRESSURE ALTITUDE
When you set your altimeter to 29.92, you’re flying at standard pressure. This is the altitude of the aircraft above the standard datum plane, the theoretical location where at 15 degrees Celsius the altimeter setting will equal 29.92 inches of mercury.
Many of the calculations you’ll find in your POH require knowledge of what pressure altitude you’ll be flying at.
All aircraft flying above 18,000 feet MSL are required to set their altimeters to 29.92 inches Hg. This means that all aircraft flying in the flight levels will have the same altimeter setting.
3) DENSITY ALTITUDE
It’s a pressure altitude corrected for non-standard temperature. Now that summer is almost here, you probably noticed your airplane not performing as well. That’s because, with hot temperatures, the density increases, and your airplane “feels” like it’s flying at a higher altitude.
Less air mass flowing over your wings prevents you from generating as much lift, and less oxygen mass in your cylinders prevents you from burning as much fuel, meaning less power. Decreasing air density decreases performance, so be careful on hot days at high altitudes.
4) TRUE ALTITUDE
It’s the vertical distance of your airplane above sea level. Commonly expressed as “feet MSL” (feet above mean sea level), many of the airspace altitudes, terrain figures, airways, and obstacles you’ll find on aeronautical charts are expressed in (MSL), feet above sea level.
5) Absolute Altitude
Constantly changing, absolute altitude is the distance measurement of your airplane above the ground. Expressed in “feet AGL” (above ground level), you can also find many obstacles and airspace classifications that exist in feet above the ground.
A radar altimeter (or radio altimeter) measures altitude above the terrain presently beneath an aircraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane. Radar altimeters generally give readings up to 2,500 feet AGL.