There’s far more to altitudes and altimeters than most people think. When you equip yourself with the proper knowledge of the various types of altitudes, you’re allowing yourself to pilot some of the safest flights. True altitude is a frequent reference altitude for many aeronautical charts, obstacles, and other points and is critical to know when piloting any aircraft. With five types of altitudes, there are questions that arise and this article aims to answer all of them!
True altitude is defined by the FAA as the height above mean sea level (MSL). This means that true altitude refers to the distance between your plane and the sea level. Using the mean sea level is important as the tides and waves are constantly changing and this in turn keeps the sea level and your distance above it changing as well.
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How Do You Calculate True Altitude?
To properly calculate true altitude, you have a couple of options. Over water, the easiest way is to use a radar or radio altimeter and simply measure the altitude over the water. This would be similar to your absolute altitude and may not always be completely accurate.
The other main way to calculate true altitude involves a formula using your indicated altitude and the temperature. The indicated altitude would be the altitude shown on the altimeter. Here is your True Altitude from Indicated Altitude formula, True Altitude = Indicated Altitude + (ISA Deviation × 4/1000 × Indicated Altitude). This equals a loss of 4 feet of altitude for each 1°C deviation for every 1,000 feet. The “ISA Deviation” is referring to the atmospheric pressure. There are many factors that can affect the true altitude and knowing each can greatly improve the safety of your flying endeavors.
How Do Pressure Variations Affect True Altitude?
The atmospheric pressure decreases as you increase in altitude and with the altitude changes, there are often temperature changes as well. Your plane’s altimeter was designed to perform at its best at standard pressures and temperatures. When temperatures or pressures are more drastic one way or the other, this could affect the reading of your indicated altitude. Because you use your indicated altitude to calculate your true altitude, this can cause the true altitude to be inaccurate.
Under Which Condition Will Pressure Altitude Be Equal to True or Indicated Altitude?
While it is rare that indicated or true altitude and pressure altitude could be equal, if the local barometric pressure is 29.92, then they could be the same. This is the reason that all aircrafts (in the U.S. and Canada) flying 18,000 feet above the mean sea level are required to have the same altimeter settings which is 29.92 in Hg. This allows them to be on the same flight levels, even though ground and sea levels are constantly varying.
How Is True Altitude Affected by Temperature?
Temperature affects your true altitude in the same way as pressure does. Temperatures that are too hot or too cold will cause your indicated altitude to be off and this in turn affects your true altitude calculations.
Under What Condition Will True Altitude Be Lower Than Indicated Altitude?
Colder temperatures could cause your true altitude to be lower than your indicated altitude. The lower the temperature, the more likely your true altitude will be lower. Adding in the proper temperature corrections could be crucial to ensuring you can clear all necessary obstacles and terrain on your flight path.
What Is the Difference Between Absolute Altitude and True Altitude?
While similar, there is a major difference between absolute altitude and true altitude. The source you use to measure from is quite different. Absolute altitude is the height above ground level (AGL), while true altitude is height above mean sea level (MSL). While vertical distance is what you measure for both of these altitudes, your absolute altitude will be constantly changing with the ground levels they are associated with.
Example of True Altitude
A great example of calculating true altitude comes from the resources at https://ppla.education/.
True Altitude = Indicated Altitude + (ISA Deviation × 4/1000 × Indicated Altitude)
Example: Indicated Altitude is 20000 feet. SAT is -35°C. What is True Altitude?
ISA at FL200 is -25°C, so our SAT of -35°C is ISA -10°C.
Substituting into the above equation:
True Altitude = 20000 + ( – 10 × 4/1000 × 20000)
True Altitude = 20000 + ( – 800 ) = 19200 feet
The 5 Types of Altitude
Being able to safely pilot an aircraft requires both a lot of knowledge and experience. Know the 5 different types of altitudes and how they can affect your times in the air.
Indicated Altitude
Indicated altitude is the altitude shown on the altimeter. This means that when you are reading your plane’s altimeter and it is set appropriately, you are reading the indicated altitude. This altitude is named because your altimeter “indicates” the altitude.
True Altitude
True altitude is the height above mean sea level (MSL). This means that the vertical distance between your aircraft and the average sea level is what is measured. The average, or mean, sea level is used to allow for better accuracy as the sea level is constantly changing with tides, etc.
Absolute Altitude
Absolute altitude is similar to true altitude. However, instead of being measured from the mean sea level, the absolute altitude is measured from the ground level. Defined by the FAA as the height above ground level (AGL), this altitude is always changing.
Pressure Altitude
Pressure altitude is the indicated altitude when an altimeter is set to 29.92 in Hg (1013 hPa in other parts of the world). It is primarily used in aircraft performance calculations and in high-altitude flight. The FAA has defined this as such to ensure all aircraft can fly safely at the same flight level.
Density Altitude
Density altitude is pressure altitude corrected for nonstandard temperature variations. Density altitude is important to consider as your aircraft will not perform the same in varying density altitudes. Density altitude is not an actual altitude, but it is referring to how your aircraft’s performance will decrease as the air density decreases as well.
Indicated Altitude vs True Altitude
While you do use your indicated altitude to calculate your true altitude, it’s also true they can be the same in rare and perfect scenarios. If your local barometric pressure is 29.92, then they will be the same, but otherwise, don’t expect to see them equal. Simply setting your plane’s altimeter to the proper setting will help you avoid many issues, but ensuring you’re prepared to make corrections for temperature variations is key to your safety and success as well.
Be Confident of Your Aircraft’s Altitude, No Matter What’s Below You!
Wherever you plan to fly, knowing the difference in the types of altitudes and how to accurately calculate them is a necessary piece to being a great pilot. No matter the temperature, with the right information, you can be sure of your true altitude in any environment. If you’re new to aviation and learning all the ins and outs of becoming a knowledgeable pilot or if you’re touching up as an expert, FLYING Magazine has it all. Subscribe today and always have the formulas and information you need for a safe flight every time you take off.
