In this article we’ll explore the different types of altitude every pilot needs to understand.
Most ground-pounders think of altitude in black-and-white terms—how high the plane is off the ground.
But pilots know there’s a little more nuance to it. Like everything worth doing, flying is complicated. There are actually five types of altitude that pilots look at and think about every flight.
Here are the most used types of altitude in aviation.
Indicated altitude is what you read off of the altimeter. It may be fairly accurate depending on how well the instrument is calibrated and if you have the proper altimeter setting put in.
Remember, when you see the word “indicated” in aviation, it means it’s not always correct, and there’s more to the story!
Absolute altitude is your height above terrain or obstacles. It is abbreviated AGL, or “Above Ground Level.”
Knowing you will be high enough not to hit anything is vital. But, generally, everything a pilot does is done in terms of above sea level, not the ground.
True altitude is your height above mean sea level, or MSL. Altimeter settings in aviation are corrected to sea level, so if you put the current altimeter setting into your altimeter, the instrument should indicate true altitude.
But this only works if you keep updating the altimeter setting as you fly.
There’s a little more to the story, however. The changes in air density that your altimeter measures are also affected by temperature. So, if it is very hot or cold, your altimeter will not indicate correctly.
If it’s very hot, your altimeter will indicate lower than your actual height—which is not dangerous because you’d just be flying higher than you thought you were.
On the other hand, if it’s very cold, the altimeter will indicate high, and you’d be flying lower than you thought you were. In this case, pilots must manually add a correction to their altitude when operating below safe altitudes.
This is an important consideration for instrument pilots when flying approaches in cold conditions.
Altitude Performance Calculations — Knowing How Well Your Plane Flies
The next types of altitude aren’t read off of your instruments or even considered while you’re sitting at the plane’s controls.
Instead, these are theoretical numbers used to calculate how the plane will fly. You usually calculate them and think about them during your pre-flight planning.
The wings, propeller, and engine don’t care how high they are above sea level or the ground—they care about how much air there is. That comes down to pressure and temperature, the two things that change the density of the air.
With more air, the wings can make more lift, the propeller more thrust, and the engine more power.
For the pilot, it’s important to understand when there’s less air because that means the plane has less performance.
It will take a longer takeoff roll to get off the ground and climb slower. This must be considered daily because pressure and temperature changes can make it impossible for small planes to perform in some circumstances.
Many crashes have resulted from pilots overestimating their plane’s performance and underestimating the effects of air pressure and temperature. This is most common at high elevations on very hot days.
Pressure altitude is the plane’s height above (or below) the standard datum plane, which is 29.92 inHg.
So, instead of measuring the distance in feet above sea level or the ground, it’s measuring it from wherever standard pressure (29.92 inHg) is that day.
If it’s a standard day and sea level pressure is 29.92 inHg, then true altitude will equal pressure altitude. Also, if you set 29.92 into your altimeter, it will indicate pressure altitude.
The standard lapse rate for pressure is 1.00 inHg for every 1,000 feet. This is how altimeters are calibrated. So if the air outside is 25.92 inHg, the pressure altitude is 2,000 feet.
As pressure decreases, pressure altitude increases. Remember, it is a measurement of altitude, so a higher number indicates thinner air.
What if you’re at sea level and the altimeter setting is higher than standard, say 30.92 inHg? In that case, the pressure altitude would be -1,000 feet. Yes, pressure altitude can be a negative number!
A Sidebar About Flight Levels…
Above the transition level, which is 18,000 feet in the US, all planes set 29.92 inHg into their altimeters and fly based on the pressure altitude. For clarity, this is called a flight level and is measured in hundreds of feet. For example, a plane flying 35,000 feet above 29.92 inHg is flying at Flight Level 350 (FL350).
By using flight levels, pilots avoid having to change their altimeters every few minutes. Plus, all planes operating above the transition level will use the same reference point, increasing safety by ensuring everyone’s altimeter is accurate.
Pressure altitude gives you an idea of the air pressure outside the plane, but it’s not the whole picture.
What if the pressure levels aren’t exactly 1,000 feet apart for every 1.00 inHg? That happens when the temperature is colder or warmer than standard (15º C at sea level).
In these cases, a more accurate measurement is needed, and this is called the density altitude. Density altitude is pressure altitude corrected for non-standard temperatures. If it’s hot outside, the density altitude will be higher than the pressure altitude; if it’s cold, it will be lower. Humidity also affects density altitude.
However, its effect is negligible and usually disregarded for a pilot’s purposes.
Density altitude is the ‘gotch ya’ of aircraft performance calculations. If you fly from high-elevation airports, you already know that the pressure altitude will be high—it’s usually fairly close to the field elevation. But on a hot summer day, the density altitude may be several thousand feet higher than that.
Here’s an example.
If you’re operating at a field with an elevation of 6,000 feet, and it’s 32º C outside, then the density altitude would be approximately 9,500 feet. In that case, your Cessna or Piper might not have enough power to make it off the runway.
And even if it did make it off the ground, it might not have the climb rate to clear obstacles after takeoff!
And if you need help, here’s an article on how to calculate density altitude.
Types of altitude are discussed in the FAA’s Pilot’s Handbook of Aeronautical Knowledge, Chapter 8: Flight Instruments.
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Brian is an experienced digital marketer who joined Thrust Flight in 2022 as the Chief Marketing Officer. He discovered a passion for aviation at 10 when he went for his first flight in a Piper Cherokee and enjoys helping others discover a career path as a professional pilot. He is an experienced marketing consultant helping brands with a variety of marketing initiatives. Brian received a bachelor’s degree in Communications from Brigham Young University.