As talked about briefly in my article about how fast passenger airplanes fly, generally speaking, there are two different types of speed when talking about airplanes – ground speed and airspeed.
While ground speed is the airplane’s speed relative to the surface of the Earth, airspeed – at least true airspeed – is its speed relative to the air it is flying in.
Below, I will explain the two types of speed in more detail, as well as talk about the four types of airspeed that are commonly used.
Airspeed vs. Ground Speed
As mentioned above, true airspeed is simply the speed at which an aircraft is moving relative to the air it is flying in. As such, it’s also the speed at which the air is flowing around the aircraft’s wings.
Ground speed, on the other hand, is the aircraft’s speed relative to the ground. One thing that should be noted here is that it’s its horizontal rather than vertical speed – an aircraft climbing completely vertically would have a ground speed of zero.
In other words, while airspeed is what determines whether there is enough airflow around an aircraft to make it fly, ground speed is what determines how fast an aircraft will get to its destination.
Wind’s Effect on Ground Speed
The relationship between airspeed and ground speed is fairly simple. Ground speed is simply the sum of airspeed and wind speed.
If the aircraft is flying in the same direction as the wind is blowing, the aircraft experiences tailwind, and its ground speed is higher than its airspeed. On the other hand, if the wind is blowing against the direction the aircraft is traveling in, the aircraft experiences headwind, and its ground speed is lower than its airspeed.
To give you an actual example, imagine an aircraft that cruises at an airspeed of 500 miles per hour that has to cover a ground distance of 2,000 miles.
If there is no wind at all, then both the aircraft’s airspeed and ground speed would be the same 500 miles per hour, and the aircraft would reach its destination in four hours.
If there was a 100 miles per hour headwind – wind blowing against the aircraft’s direction of travel – the aircraft would still be traveling at an airspeed of 500 miles per hour. However, its ground speed would be just 400 miles per hour (100 miles per hour slower than its airspeed). And as such, it would take the aircraft five hours to reach its destinations.
Finally, if there was a 100 miles per hour tailwind – wind blowing in the same direction as the aircraft’s travel – the aircraft would still be traveling at an airspeed of 500 miles per hour, but its ground speed would be 100 miles faster. And, at 600 miles an hour, the aircraft would reach its destination in just three hours and twenty minutes.
The above is the reason why some flights go “out of their way” to avoid headwinds or catch tailwinds. And, why some flights might appear to be traveling at “supersonic speeds,” even though their airspeed – the speed that would actually matter in determining whether or not the flight truly is supersonic – is subsonic.
The Moving Walkway Analogy
In case you are still unsure about the difference between airspeed and ground speed, here’s an analogy to bring the concepts “down to earth.”
Imagine a moving walkway going from point A to point B moving at 3 miles per hour.
And, imagine you are walking on the walkway at a speed of 3 miles per hour relative to the walkway. In other words, you are moving from point A to point B at a speed of 6 miles per hour – the sum of the speed of the walkway and the speed at which you are walking relative to the walkway.
What if, though, you tried to walk on the walkway from point B to point A?
Obviously, if you walked at 3 miles per hour relative to the walkway, you would be standing still. To move, you would have to walk faster than 3 miles per hour relative to the walkway. For example, if you walked at 5 miles per hour relative to the walkway, you would be moving from point B to point A at a speed of 2 miles per hour.
Finally, if the walkway was broken and wasn’t moving, your speed relative to it would be the same as the speed you would be moving at from point A to point B.
By now, you have probably figured out that in the analogy above, the speed of the walkway takes the place of wind speed, the speed at which you are moving from point A to point B (or vice-versa) is the ground speed, and the speed at which you are moving relative to the walkway is the airspeed.
The Three Types of Airspeed
When I talked about airspeed earlier in this article, I was talking about true airspeed. However, pilots commonly use three different types of airspeed: indicated airspeed, calibrated airspeed, and true airspeed.
Indicated airspeed is an airspeed that is calculated directly off an aircraft’s pitot-static system. It’s the calculated off the aircraft’s dynamic pressure – the difference between its total pressure and static pressure.
The dynamic pressure depends not only on the aircraft’s speed, but also on the density of the air it is flying in. As such, the higher the aircraft flies – and the lower the air density as a result – the bigger the difference between indicated and true airspeed is.
Calibrated airspeed is indicated airspeed adjusted for a variety of errors.
Just as an example, one of the things it’s adjusted for is the flap position. The reason for that is that at different flap positions, air flows differently around the pitot-static system and affects the indicated airspeed readings.
True airspeed is, as has been mentioned numerous times in this article, the actual speed at which an aircraft is moving relative to the air it is traveling in. It’s calibrated airspeed adjusted for the the exact conditions (altitude, air temperature, etc.) that the aircraft is flying in.
For pilots, both airspeed and ground speed are very important. While the first of those helps them make sure they are flying fast enough to take off, not to stall, and so on, the second one helps them figure out how long it will take them to get from one place to another.
On the other hand, if you are a passenger, you will only need to be concerned about the ground speed as that will tell you how fast are you flying from your origin to your destination – how fast you will be able to get to your meeting, meet your relatives, or do whatever else the reason for your travel might be.
17 thoughts on “What Is the Difference Between Airspeed and Ground Speed?”
On 39000ft Airspeed is always 0.25% higher as groundspeed even the wind speed is 0.
Great information and breakdown of the differences in airspeed. I’m actually going to save this article.
Thanks for excellent explanation
A possibly better explanation is that if You -on the automated walkway- bike with the three knots with which You need to travel to keep the balance when biking, then You will not fall (/stall), but will actually bike with respectively 0 or 6 knots speed (TAS), depending on direction.
-So TAS has nothing to do with airplane stall/aerodynamic performance, but is only about navigation….?
That’s a different concept when talking about bikes on a treadmill
Hello, Sir, I have a question. Is it possible to know the speed of the aircraft if we only know the distance and time that the aircraft will be arriving? Since my problem did not mention other conditions, can I just the formula V=d/t?
How does the earth’s rotation speed affects speed and time to reach destination when traveling east to west or west to east? After all, the earth rotates at an aprox speed of 1,000 m/h
The reason there is time difference when travelling has to do with Jetstream, instead of earth rotation. There are few key things about Jetstream, first they always travel west to east, second their speed is determine by temperature difference and usually it is between 129 to 225 kilometers per hour, as for you question let take example of plane flying at 900 km/h to east and jet stream is 200 km/h, in this case ground speed would be 900 + 200 = 1100 km/h, if travelling is towards west then speed would be 900 – 200 = 700km/h from ground perspective. That is the reason when you are flying east it take less time then if you were flying west, even though distance is same.
How is it that airspeed can be discussed without reference to the rotational speed of the ground (Earth)? 1,037 mph at the equator.
Because, in general, you are part of earth, and are rotating at the same speed as the earth.
If you jump up in the air, you land in the same spot, because you are rotating with the earth. Same applies on a plane. You’re still rotating with the earth.
Great article. Helped me figure out what the pilots were talking about just now, while watching “The Odyssey of Flight 33”, an episode of the original ‘Twilight Zone’.
Does airspeed take into account the fact that the circumference on the ground is shorter than the circumference at altitude? Or is there difference negligible?
okey so, i don’t understand this. for example, if the headwind would be 500 miles/h (doesn’t happen but theoretically), it cannot be possible for the plane to take off on an airspeed of 500 m/h, because the ground speed would be 0 and therefore it would stand still. but at the end of the article it is explained that a pilot looks at the airspeed to see if it’s going fast enough to take off, but isn’t it more important to know the ground speed because of what i said above?
Thom, he really didn’t go into what makes up air speed or how it’s calculated other than it’s the relationship between the speed of the plane and air around it. He held it constant and made variable wind speeds for his calculation.
I’m no expert but I think he’s emphasizing how air speed influences ground speed but not the other way around. I kind of doubt a good lane could take off in a 500mph headwind, but I think the idea is pilots are making head/tailwind adjustments to maintain their constant air speed and monitor that airspeed as a means of whether it stays in the air or not.
Also interesting is planes fly and perform better and generate more lift in headwinds and vice versa in tailwinds. So they make thruster and pitch adjustments to compensate
If the airspeed was “theoretically” as you put it 500 m/h at take off then the plane would fly away by themselves. It would be totally possible. In fact you would need to tie the planes to the ground so they didn’t fly away when parked.