What Is Pressure Altitude?

Learn about the significance of this important measurement.

What is pressure altitude

An altimeter is a necessary tool required to fly which keeps track of how high the aircraft is.[Credit: Unsplash]

Aircraft have a lot of necessary tools required to fly, including an altimeter that keeps track of how high they are. However, pilots must use several different types of altitude readings. 

Pressure altitude is the altitude that your aircraft performs at on a non-standard day. The aircraft’s performance is based on this pressure reading, including how fast it will climb, how much runway it needs for landing or lift-off, how fast it will fly, and how much fuel it will use. 

The standard used for pressure at sea level is 29.92inHg, inches of mercury. You calculate using this standard and the actual daily pressure to find out how your aircraft is performing. 

How Do You Calculate Pressure Altitude? 

To be able to calculate your pressure altitude, you use specific aviation standards. These are held into consideration when building aircraft or operating altimeters. 

The standard is known as the ISA or International Standard Atmosphere. At sea level, the standard conditions are 29.92inHg/ 1013.4 miliBar and 59 Fahrenheit / 15 celsius. To calculate pressure altitude, you use the following formula:

{(sea level pressure - 29.92) x 1,000} + true altitude (or field of elevation in on the ground) *With sea level being the actual daily pressure dependant on weather conditions 

*Per 1000 feet you rise, your altimeter loses approximately 1 inch of mercury.

How Is Altitude Measured? 

Onboard an aircraft is a barometric altimeter that measures the pressure at your current height and uses a formula to translate this pressure into height measured in feet. Different areas have different standard pressures. You can always ask your air traffic control for an updated altimeter setting. 

Why Is Pressure Altitude Important? 

An aircraft's performance data is calculated based on the ISA. However, sometimes the actual pressure is different because of weather conditions. This differential affects air density and affects how the aircraft performs. 

If the air density is higher, the aircraft will perform like it is actually at a lower altitude. Conversely, if the air density is lower, the aircraft will perform as if it is flying at a higher altitude. 

After calculating your pressure altitude, you can look in the aircraft's manual to see how well it is performing.

When Is Pressure Altitude Used? 

Pressure altitude is used to determine aircraft performance calculations for flights. It is used when landing, taking off, and in flight. It also allows you to determine the amount of runway you need on a non-standard day. It is also used in high altitude flights to determine how high you are flying. 

What Is the Relationship Between Altitude And Air Pressure? 

Generally speaking, the lower you fly, the higher the density of air or air pressure. This also means the higher you fly, the lower the density of air or air pressure is. However, these factors can be influenced by weather conditions. 

For aircraft, this relationship directly affects how well they perform. But we as humans are also affected by air pressure. Lower density air means that there is a lower concentration of oxygen in the air. As a result, people can get altitude sickness when moving from sea level to a mountainous area due to this lower air density. The body needs to adapt to the lower concentration of oxygen in the air. 

At What Altitude Is Atmospheric Pressure the Greatest? 

Atmospheric pressure is the greatest at sea level. This is true because air pressure is the mass of air pushing down on the earth. Therefore, as altitude increases, there is less air pressing down upon the object, which reduces atmospheric pressure. 

Example of Pressure Altitude 

You are flying in an area where the pressure at sea level is currently 30.22. The standard pressure is 29.92inHg. 

So you want to subtract the standard from the current pressure. In this case, the difference is 0.3. 

You multiply this by 1000 because per 1 inch of mercury, the altitude changes by 1000 feet. Or 0.3 x 1000 = 300. 

This means that an aircraft flying at 5000 feet will behave as if it was flying at 4700 feet. 

The 5 Types Of Altitude 

Pressure altitude is an essential metric for pilots to understand. However, pressure isn’t that simple. Pilots deal with different types of altitude for flying, and these are dependent on flight and weather conditions and humidity and temperature in the area.

Pressure Altitude 

Pressure altitude is the altitude corrected for non-standard pressure. It is used to determine your aircraft's performance on a non-standard day. 

A non-standard day is a day where the pressure is not 29.92inHg. It is always important to adjust your altimeter during flight to make sure you fly at the right height. 

Indicated Altitude 

Indicated altitude is, as the name suggests, the reading of your barometric altimeter. When set to the local barometric pressure at sea level, this is what the altimeter shows. It is the uncorrected altitude measured.

True Altitude 

True altitude is the exact vertical height of your airplane above sea level. True altitude is expressed in feet MSL, which means feet above mean sea level. Objects on flight charts use true altitude and are commonly expressed in feet MSL. 

Density Altitude 

Density altitude is pressure altitude which is corrected for temperature. Your altimeter is designed to tell you your altitude based on pressure without taking into consideration temperature. 

On a hot day, air density is lower, and if you follow your altimeter, you will be flying higher than the altimeter indicates. On a cold day, this effect is reversed, and you will be flying lower than your altimeter indicates. 

The ISA states that the temperature at sea level is 15 degrees Celsius and reduces by 2 degrees celsius every 1000 feet you rise. To calculate density altitude, you must find the deviation from standard temperature. Per deviation of 1-degree Celsius, your actual altitude will deviate by approximately 120 feet. 

Absolute Altitude 

Absolute altitude is the distance between your aircraft and the ground. This number is ever-changing because the land is rarely evenly flat. Absolute altitude is measured in feet AGL, which means feet above ground level. 

So absolute height varies with the height of the terrain as well as the height of the aircraft. You can use a radar altimeter to measure absolute altitude up to 2500 feet AGL. Absolute altitude is calculated by measuring how long it takes for radio waves to reflect back from the ground. 

Pressure Altitude vs. Density Altitude 

The difference between pressure altitude and density altitude is that pressure altitude is corrected for different pressure levels but not temperature. Conversely, density altitude is pressure altitude corrected for deviations in temperature. 

Pressure Altitude: Flying High

There are many different altitude readings that pilots have to use to fly their aircraft safely. Pressure altitude lets them adjust their aircraft performance for higher or lower density air and accurately fly at higher altitudes. Meanwhile, density altitude allows pilots to adjust for temperature deviations. 

But don’t worry, you don’t have to be a rocket scientist to figure out these different altitudes. These days, most of the calculations are done by onboard technology.

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