# Is the point of “groundspeed” in aircraft just to have a second reference of speed but two totally different measurements of speed? [duplicate]

From not understanding groundspeed will always be significantly higher than airspeed, unless the plane is pitching up and down. Is the ground speed just there for pilots to look at and say “okay we have x amount of airspeed and x amount of ground speed, so we’re okay”?

To explain a bit better, I’ll give a scenario I’m thinking of when something would be confusing or just off to the pilots:Airspeed is decreasing rapidly but groundspeed is staying the same, the planes pitch is also the same.

Would this be an indicator of unreliable airspeed or something different? Or is there an actual phase of every flight where the pilots needs groundspeed?

• Your understanding is wrong. Ground speed can be much higher than air speed, or much lower. It depends entirely on the wind. With a small plane in a stiff headwind, you could even have negative groundspeed. You need groundspeed for figuring out when you'll be getting to your destination, and if you have enough fuel. With a stiff headwind, it will take much longer than if you calculate the time from your air speed, while with a tailwind, you might overshoot your destination and have to turn and fly back. Dec 3, 2019 at 4:35

Airspeed dictates how (and if) a plane flies because it relative to the air that the plane is flying through, the only thing that matters for aerodynamics.

Groundspeed can be greater or less than the airspeed depending on which way the air is moving over the ground, i.e. wind.

The latter is only important for figuring out when you'll arrive at your destination (and related things like ensuring you have enough fuel to keep the engines running that long); for everything else, the former is what matters.

• I would argue that groundspeed is more important than just knowing when you arrive at your destination. If you don’t monitor your ground speed, and the resulting fuel consumption, you won’t be arriving at your destination at all. Dec 3, 2019 at 16:30
• Indeed, I'd argue that (except for takeoff and landing) airspeed is not all that relevant for normal flying. You take off, climb to your cruise altitude, set your throttle to desired RPM/power, and it stays there until you're ready to descend for landing. Ground speed determines whether you make it to your destination before you run out of fuel, overshoot your destination and have to turn back, or (with crosswinds) wind up well to one side or the other of your destination. Dec 3, 2019 at 17:27

The groundspeed can be more, less or the same as the airspeed.

Go back to boats. I'm driving around in my boat on a big river with a 5 mph current. My boat goes 30 mph. If I'm going with the current, my boat is going 35mph relative to the river bottom and the shoreline. If I turn around and go the other way, my boat is going 25mph relative to the river bottom and the shoreline. If I go across the current, it's a wash and my boat speed and speed over the river bottom is the same, as if there was no current.

My boat's water speed is 30 all this time. My boat's "river bottom speed" is water speed plus or minus the effects of the current. As far as the boat itself goes, it doesn't know or care what direction its going relative to the river bottom, it's just concerned with the speed relative to the water surface, which is moving relative to the bottom.

An airplane is flying along a "river" of air and it has its airspeed, the speed through the air mass, and it's speed relative to the surface the air mass is flowing over, the "river bottom", or the ground in our case.

The pilot's main concern with groundspeed is the same as the boat captain's concern with boat speed added or subtracted from the current (say when navigating in the ocean). It affects how long it will take to get from A to B, and how much fuel will be required, and related things.

It doesn't have much effect on the flying of the plane itself except in an extreme case like a windshear encounter, where the inertia of the airplane itself can be a factor. The case you describe could be what is called a "reduced performance shear", which is a very sudden and extreme tailwind, which makes airspeed drop while groundspeed stays the same or increases fairly slowly. An airliner on approach that ends up with a downburst behind it, and is caught in the high velocity outflow of the downburst from behind, would see that happen due to the inertia of the aircraft until it's able to recover it's original speed through the air mass. While it's doing this it would have to pitch up and use some of its inertial energy to keep from descending below the glide path.

Even there though, you aren't really concerned with the groundspeed itself, except to the extent that the groundspeed affects how quickly go from A to B on the approach. You are totally concerned with recovering the lost airspeed that the airplane needs to fly.

• I pocket-dailed the downvote button, sorry. If you make an edit I can revert it to an upvote. Dec 3, 2019 at 6:26

Groundspeed is the MOST important speed to know when it comes to flight planning, and flight monitoring. If you don’t know how much time it will take to arrive at your destination, you may never arrive.

There is is only a finite amount of fuel in your tanks. Every flight has to use estimated groundspeed before the flight, and actual groundspeed during the flight, to know if you have enough fuel to reach your destination.

If you fail to monitor your groundspeed, you may run out of fuel short of your destination. In that case indicated airspeed would become important as you try to glide your aircraft to a safe landing.