Knowing the speed of an airplane is very important, yet there are several different ways to measure it. Today, we will be discussing airspeed and groundspeed, the differences between the two, and the usages of each.

Defining Some Terms

Airspeed is the speed of the airplane traveling through the air. It is derived from a few handy little instruments aboard the aircraft known as the pitot tube and the static port. There can be several of each, depending on the type of airplane.

The static ports are small holes on the aircraft’s skin that measure the air pressure at the current altitude. Because these holes are so small, they can sometimes become clogged with debris, so we have alternate static ports as a backup.

The pitot tube is a small device that often hangs from the wing in general aviation aircraft. On large airliners, they are often mounted under the cockpit. They measure something called “ram air”. As the airplane flies through the air, a small amount of air enters the pitot tube, where its speed is calculated. Then, the ram air and air pressure are compared, and the airspeed is calculated.

Groundspeed is much more straightforward. It is the literal speed over the ground as would appear on GPS. This is the speed passengers care about, as it tells them how long it will take to arrive at their destination.

What sets them Apart?

So, that’s great to know… But what sets them apart? How can airspeed be different than groundspeed?

That’s a valid question.

Wind is the factor that distinguishes airspeed from groundspeed. Tailwinds increase groundspeed, while headwinds increase airspeed and decrease groundspeed.

Let’s dive into more detail.

Tailwinds are winds that come from behind the aircraft. They push the aircraft forward, thus causing it to move faster over the ground. Tailwinds do not affect airspeed.

Headwinds are much more complex. Sometimes they’re favorable, while at other times, they are not.

Headwinds are favorable on takeoffs and landings, as they reduce the required runway length for each manoeuvre.

Headwinds increase airspeed, but decrease groundspeed. Remember how airspeed is derived? With a headwind, ram air increases, thus making the differential pressure between the ram air and air pressure greater, which increases airspeed.

But they also decrease ground speed. Think about it, will you go faster or slower when biking against the wind? The latter, you go slower. It works the same way with an airplane, the faster the headwind, the slower the groundspeed.

So how does this make headwinds both favorable and unfavorable?

Headwinds are favorable on takeoffs and landings, as they reduce the required runway length for each manoeuvre. Because headwinds increase airspeed, we will be able to achieve takeoff speed over a shorter distance. As for landings, the headwinds will decrease the groundspeed, allowing us to stop over a shorter distance.

The unfavorable part about headwinds comes during the cruise phase of flight. During a cruise, headwinds will require us to burn more fuel and travel slower over the ground. A tailwind will do the exact opposite.

When Can They be Used?

While flying, pilots are more concerned with airspeed. Airspeed is so crucial for operation because of how it affects aerodynamics. If the airspeed is too high, it could cause damage to the aircraft. If the airspeed is too low it could put the airplane in a potentially dangerous position.

Groundspeed is used for estimated arrival times and fuel planning. Knowing how long it will take to arrive at the destination with wind accounted for is important. This allows the pilots to accurately plan for fuel.

Summary

Airspeed and groundspeed are two different yet important ways of measuring an aircraft’s speed. Groundspeed measures the aircraft’s speed over the ground, while airspeed measures the airplane’s speed through the air. The effects of wind distinguish the two measurements from each other. Airspeed is used by pilots for aerodynamic concerns during the flight, while groundspeed shines during the planning phase.