What Is MSL in Aviation?

Pilots · 5 min read · Nov 29, 2022
msl in aviation

Aviation is one of the industries where acronyms are used the most. This is easy to understand given the number of terms that pilots, air traffic controllers, and other aviation professionals must memorize. And the MSL in aviation is a good example.

So, if you want to discover the meaning of this acronym in aviation, and how it is used within the industry we invite you to keep reading as we disclose the details.

What is MSL in aviation?

MSL stands for Mean Sea Level, a measurement of altitude that is used by pilots for both aircraft and drones. According to the Pilot Institute, a more accurate term would be Above Mean Sea Level, and this measurement could be defined as a particular altitude that usually stays constant regardless of the terrain below the aircraft and that is measured against the sea level, hence its name.

Mean Sea Level altitude is often regarded by pilots as elevation, but it is not to be considered the exact height of the aircraft above the ground since it does not take the different ground elevations into account.

An airplane landing on a runway close to the sea, very close to MSL.

How is MSL calculated?

Truth be told, calculating the MSL is not an easy task since the sea level changes over time with the tides, the wind, and other weather conditions such as atmospheric pressure.

So, how is MSL measured? MSL measurements have been taken for 19 years using the seasonal and tidal rise and fall of the ocean on the seacoast, and pilots normally use a datum by choosing a certain location and measuring the level of the sea at that certain point.

In short, the average MSL is the zero point from which a plane’s height is calculated. So, a plane flying at 10,000 ft MSL stays at 10,000 ft MSL regardless of the ground’s elevation.

This is because for the pilots is more practical to express the correct altitude by measuring it from a fixed point on the ocean surface instead of the varying, uneven ground, especially when maintaining altitude at cruise speed.

An aircraft flying high above MSL.

How is MSL Used in Aviation?

There are three different altitudes that will be mentioned when flying. These are pressure altitude, indicated altitude, and true altitude.

Indicated altitude is what you would see if you looked at the altimeter in an airplane. It will show your height above a predetermined zero point or Mean Sea Level (MSL).

Pressure altitude is used in the computation of density altitude and true altitude. It requests the current altimeter setting so that it may be set to 29.92” Hg or 1 atmosphere to calculate your height above MSL.

True altitude takes into account barometric pressure and temperature changes that will affect density altitude. It represents your height above MSL regardless of what your altimeter reads.

Most airports list their field elevation above MSL on their website or in other public documents. For calculating takeoff and landing distances, always use the pressure altitude that corresponds to the reported airport field elevation unless instructed otherwise by Air Traffic Control (ATC).

An aircraft taking off from a runway close to water.

Since all other airport elevations will reference back to MSL, setting your 29.92” Hg altimeter to the airport’s reported field elevation will put you at pressure altitude which can be used for density altitude computations. If using electronic devices during flight, make sure they are also set to 29.92” Hg or 1 atmosphere.”

Another use given to MSL in aviation is to determine the ceilings of controlled airspace for both aircraft and drone pilots. For example, in Chapter 15 of the Pilot’s Handbook of Aeronautical Knowledge, the FAA defines controlled airspace Class B as the airspace from the surface to 10,000 feet MSL surrounding the nation’s busiest airports in terms of airport operations or passenger enplanements.

However, some airspace classes are also defined with AGL values, which takes us to the next point.

What is the difference between MSL and AGL?

We already said that MSL stands for Mean Sea Level, and we made clear that MSL is the average height of the surface of the ocean relative to a fixed point on land, calculated based on actual tide measurements over a period of 19 years and used as a global reference point for measuring absolute altitude. Also remember that MSL refers to their height relative to sea level, regardless of geographical location.

Now, AGL stands for Above Ground Level, and while both describe the altitude of an aircraft in relation to a reference point on the ground, they measure different things.

Above Ground Level is the altitude of an aircraft relative to the terrain immediately beneath it, ground level. AGL values take into account local terrain features including hills and valleys that may be present around an airport or when flying over certain areas.

In other words, pilots use AGL when determining how high they are above obstacles such as buildings or mountains whereas MSL measures a pilot’s altitude relative to sea level from anywhere in the world.

Whereas MSL altitudes remain constant regardless of geography, AGL altitude can change dramatically depending on where an aircraft is located within a given region or air space.

This makes AGL particularly useful for providing accurate readings when flying around smaller bodies of land, as well as during low-level flights where small differences in altitude could make a big difference in safety procedures.

An aircraft taking off above water, MSL in aviation.

Transition Altitude

Transition altitude is a concept used in aviation that indicates the point at which aircraft flying at high altitudes begin transitioning from their current standard pressure setting to a standard pressure setting of 29.92 inches of mercury, also known as “one atmosphere.”

This altitude is set by each country’s aviation authorities to serve as a reference level for ATC operations and ensure that all aircraft on the same flight path fly at different altitudes while remaining within their respective airspace boundaries.

For example, in the United States, ATC sets the altitude at 18,000 feet above mean sea level (MSL).

An aircraft taking off in sunset.

Some final remarks on MSL in aviation

MSL and AGL both provide important information for pilots during flight; however, they should not be confused with each other. Knowing both numbers accurately can help ensure that planes fly safely while minimizing their risk of crashing into obstacles or other aircraft in the air at any given time.

Additionally, an aircraft or a drone pilot must always consider weather conditions since changes in pressure can affect both readings significantly depending on location and time of day. Having an understanding of MSL and AGL will help keep all aircraft safe by providing accurate readings that can be referenced during flight planning or while navigating through the airspace.

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Carlos Collantes
Carlos Collantes
A mechanical engineer and aviation enthusiast dedicated to share some knowledge by creating top-notch content, especially in engineering and aviation topics.

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