By Jordan Bullock CFI, FO B737
Most people think that altitude is simply your plane's height above the ground. Most people aren't pilots, though. Pilots know that weather has an intense impact on flying. This includes our altitude! In aviation, altitude isn’t a simple blanket term to be thrown over our height above the ground. It can mean an abundance of different things.
Altitude (without sounding like Captain Obvious) might just be the most important thing to a pilot. As an inexperienced, aspiring pilot, you’ve surely heard the saying that “the least important thing in aviation is the runway behind you, the fuel left in the truck and altitude above you”. But, altitude is all the same, right? It’s just how high from the ground you are… While that's somewhat true, there are a few different types of altitude every pilot should be familiar with.
Before discussing the different types of altitude, let's discuss the one common thing every pilot uses in reference to altitude: The altimeter. An altimeter is standard in every aircraft, and its sole job is to inform the pilot of how high in the sky they are. The altimeter is NOT a barometer, as it does not measure the barometric pressure of the air. It simply measures the static air pressure and translates it to an altitude setting. That might sound confusing, but think of it like this: The altimeter gives an indication of the altitude, using static air pressure.Let’s take a look at the different types of altitude:
Pressure altitude is altitude corrected for non-standard weather. What is standard weather? Sunny and 75 right? Wrong. Standard weather is 59 degrees celsius and 29.92 inHG at sea level. Confusing? Don’t worry about the metrics, just remember 29.92 is the setting that is input into the Altimeter window when weather is standard at sea level. Every airport with weather reporting devices in the world will report a 4 digit altimeter setting, which the pilot will then input into this window. So if you fly one day and the weather is ‘standard’, you will probably hear “altimeter setting 29.92” on the weather reporting frequency. When setting this, you are sure to check what the altimeter is actually showing. If you set 29.92 per the weather station, and you’re in say Miami, the altimeter should read close to zero (or whatever the Miami airport elevation is). If it reads 5,200 ft (Like Denver), then your setting is wrong or your altimeter is broken.
All this sounds like a lot, but if you take anything away from it, take this: Pressure altitude is what your altimeter setting reads when you input the pressure setting into the window. It’s really that simple. This is why when crossing flight level 180 (18,000 ft), all airplanes revert their altimeter settings to 29.92. This ensures everyone is reading the same altitudes.
Density altitude is my personal favorite, and the reasoning is simple. It’s the altitude at which your airplane thinks it's at. Sounds odd right? Airplanes think? When checking your ATIS (or AWOS/ASOS), sometimes you’ll hear at the very end “Density Altitude ___ feet”. I hear it all the time flying out of KSPG (Albert Whitted airport in Saint Petersburg, Florida). Density altitude is very simple. It’s pressure altitude, corrected for temperature and humidity. Florida is obviously very hot and humid, so we hear this just about every day.
This is extremely important as it has a direct impact on your performance. If the density altitude is, for example, 3,000 ft in Miami, which is basically sea level, I can expect degraded performance. With warmer temperatures, the air molecules are thicker, resulting in a negative impact on my engine performance. Thus, my takeoff roll will be negatively affected, meaning I would need a longer runway to takeoff on compared to somewhere with a density altitude of say 1,000 feet. Remember, Density altitude is the altitude at which my plane will perform at, regardless of my actual altitude.
The last altitude we’ll look at is True altitude. True altitude is your actual vertical height above sea level. It is measured in MSL, or mean sea level. If I’m in Denver, my true altitude could be 7,000 feet and my pressure altitude could be similar, but I could only be 2,000 feet above the ground since Denver sits at 5,200 feet above sea level. It’s the easiest to understand as it's simply just how high above sea level you are.
In instrument flying, you have to be careful as some numbers are represented in AGL (above ground level), or MSL (mean sea level). As you go through your training you’ll get proficient at deciphering when you will use these two altitudes.
Altitude can be a difficult thing to grasp, especially when you throw in the formulas and how each is mathematically figured. However, the first step is truly understanding the difference between all. Remember when you’re talking to your non-aviation friends, that there is more than one type of altitude!By Jordan Bullock CFI, FO B737
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