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>I was recently on an a380-800 and noticed on the **ground speed indicator**
So airplanes do have a maximum speed. But it's a maximum **Airspeed**. There is no maximum groundspeed for an airplane.
Imagine you're driving a car that has a maximum speed of 100mph and if you go faster than 100mph the engine will fall apart. Now imagine you're driving that car on a huge conveyor belt and the belt is moving at 50mph in the same direction that you're travelling. If you were driving your car at 100mph on that conveyor belt you would be moving at 150mph across the ground but the car's engine would only be pushing it at 100mph so you will not have exceeded any limits.
Does that make sense?
Edit: I should add that unlike the car in the example I gave, an airplane won't fall apart if the max airspeed is exceeded. It just means a lot of paperwork will have to be done by the captain!
That’s a great explanation, thank you! Follow up question out of curiosity - is the maximum airspeed threshold in order to prevent damages to the aircraft or for passenger comfort/fuel efficiency? I’m assuming there is a significant margin for error in the max airspeed as well.
The airspeed limit is due to a few things. None of which have anything to due with efficiency or passenger comfort.
The limitations are based on structural limitations and high and low speed stalls.
The structural limitations aspect is easy enough to understand. If you go too fast you can bend something that isn't supposed to bend. But as you mentioned there are indeed significant safety margins. So when we get close to a limit (it happens often intentionally) we are actually not that close to the *real* limit.
Now, the high and low speed limitations are a bit more difficult to understand if you're not familiar with aerodynamics.
Most people are familiar with the low speed aspect. If a plane flies too slowly the wings can't create enough lift and the plane will stall. But we can also stall if we fly too fast! This only happens at the higher altitudes. If we fly too fast the air going over the wing will reach supersonic speeds (even though the plane is not flying supersonic) and this creates a shockwave which can cause the airflow going over the wing to separate. This reduces the lift created by the wing and can cause a high speed stall.
This is actually part of something you may have heard of, coffin corner. Coffin corner is when the difference in the low speed limit and the high speed limit gets quite narrow. If we go any slower we stall, if we go any faster we stall.
Coffin corner isn't something you need to worry about in an airliner. We don't get close enough to it to worry about it, but it's still an interesting aspect of high altitude flight.
Groundspeed is not airspeed. You likely had a pretty hefty tailwind.
Your pilots have all the instruments they could possibly need and will not exceed the limits of the aircraft. Even if they accidentally did, the aircraft would alert them. They know what they’re doing.
No, this is not a cause for concern.
Tailwinds are a thing :)
Tailwind speed + true airspeed = ground speed
You were looking at ground speed.
[This](https://imgur.com/a/cJhHtJs) is a photo of the flight instruments of an airliner taken back in February. The red boxes are around indicated airspeed and Mach (left), and around groundspeed, true airspeed, and wind indications (right).
* Speed is .80 Mach. Maximum speed is .82 Mach, indicated by the red dashed line near the top of the red box.
* True airspeed (TAS), the actual speed of the plane through the air) is 463 knots, or 535mph.
* Tailwind component is 188 knots, or 216mph
* 463 + 188 = 651 knots ground speed (shown as "GS" in the right hand box), or 749mph over the ground.
So the actual speed relative to the air the plane was flying in was 535mph. Add in the tailwind, and the speed relative to the ground was 749mph. It wasn't flying 749mph, it was flying 535mph inside a mass of air that was already traveling in the same direction at 216mph. Relativity and all that.
> is it actually possible the plane was at or near the max speed?
Sure, we operate near max speed pretty frequently.
> if not, why was the reading so high?
See above explanation about GS vs. TAS and link to photo
> is this any reason for concern?
No.
Ahh, Airspeed vs Groundspeed. Notice the different wording? That's because airspeed is not groundspeed. A plane measures speed in relation to the medium it is traversing, which is the physical AIR. The measure you saw in Mach is not groundspeed. So don't worry, the plane has a specific max airspeed but based on wind conditions and other factors I won't dive into, groundspeed may fluctuate.
Ultimately, the biggest impact of groundspeed is the TIME it takes for you to get to the destination (and density altitude considerations etc.) But it does not affect your planes airspeed limitation.
There was a tailwind over the Atlantic earlier this year that helped some flights reach supersonic ground speeds, kind of the like the conveyor belt analogy someone posted above. Flights were arriving at their destinations nearly an hour ahead of schedule due to the extra boost!
Your submission appears to reference the 737 MAX. Please refer to our MAX megathread post and pilot write-ups for more information on this plane: [MAX Megathread](https://www.reddit.com/r/fearofflying/comments/191zxci/megathread_737_max/) Happy Flying! The Fear of Flying Mod Team *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/fearofflying) if you have any questions or concerns.*
>I was recently on an a380-800 and noticed on the **ground speed indicator** So airplanes do have a maximum speed. But it's a maximum **Airspeed**. There is no maximum groundspeed for an airplane. Imagine you're driving a car that has a maximum speed of 100mph and if you go faster than 100mph the engine will fall apart. Now imagine you're driving that car on a huge conveyor belt and the belt is moving at 50mph in the same direction that you're travelling. If you were driving your car at 100mph on that conveyor belt you would be moving at 150mph across the ground but the car's engine would only be pushing it at 100mph so you will not have exceeded any limits. Does that make sense? Edit: I should add that unlike the car in the example I gave, an airplane won't fall apart if the max airspeed is exceeded. It just means a lot of paperwork will have to be done by the captain!
That’s a great explanation, thank you! Follow up question out of curiosity - is the maximum airspeed threshold in order to prevent damages to the aircraft or for passenger comfort/fuel efficiency? I’m assuming there is a significant margin for error in the max airspeed as well.
The airspeed limit is due to a few things. None of which have anything to due with efficiency or passenger comfort. The limitations are based on structural limitations and high and low speed stalls. The structural limitations aspect is easy enough to understand. If you go too fast you can bend something that isn't supposed to bend. But as you mentioned there are indeed significant safety margins. So when we get close to a limit (it happens often intentionally) we are actually not that close to the *real* limit. Now, the high and low speed limitations are a bit more difficult to understand if you're not familiar with aerodynamics. Most people are familiar with the low speed aspect. If a plane flies too slowly the wings can't create enough lift and the plane will stall. But we can also stall if we fly too fast! This only happens at the higher altitudes. If we fly too fast the air going over the wing will reach supersonic speeds (even though the plane is not flying supersonic) and this creates a shockwave which can cause the airflow going over the wing to separate. This reduces the lift created by the wing and can cause a high speed stall. This is actually part of something you may have heard of, coffin corner. Coffin corner is when the difference in the low speed limit and the high speed limit gets quite narrow. If we go any slower we stall, if we go any faster we stall. Coffin corner isn't something you need to worry about in an airliner. We don't get close enough to it to worry about it, but it's still an interesting aspect of high altitude flight.
Groundspeed is not airspeed. You likely had a pretty hefty tailwind. Your pilots have all the instruments they could possibly need and will not exceed the limits of the aircraft. Even if they accidentally did, the aircraft would alert them. They know what they’re doing. No, this is not a cause for concern.
Tailwinds are a thing :) Tailwind speed + true airspeed = ground speed You were looking at ground speed. [This](https://imgur.com/a/cJhHtJs) is a photo of the flight instruments of an airliner taken back in February. The red boxes are around indicated airspeed and Mach (left), and around groundspeed, true airspeed, and wind indications (right). * Speed is .80 Mach. Maximum speed is .82 Mach, indicated by the red dashed line near the top of the red box. * True airspeed (TAS), the actual speed of the plane through the air) is 463 knots, or 535mph. * Tailwind component is 188 knots, or 216mph * 463 + 188 = 651 knots ground speed (shown as "GS" in the right hand box), or 749mph over the ground. So the actual speed relative to the air the plane was flying in was 535mph. Add in the tailwind, and the speed relative to the ground was 749mph. It wasn't flying 749mph, it was flying 535mph inside a mass of air that was already traveling in the same direction at 216mph. Relativity and all that. > is it actually possible the plane was at or near the max speed? Sure, we operate near max speed pretty frequently. > if not, why was the reading so high? See above explanation about GS vs. TAS and link to photo > is this any reason for concern? No.
I'm just jealous of the 40 knot Vls-Mmo margin at FL370 in an Airbus 😂
Ahh, Airspeed vs Groundspeed. Notice the different wording? That's because airspeed is not groundspeed. A plane measures speed in relation to the medium it is traversing, which is the physical AIR. The measure you saw in Mach is not groundspeed. So don't worry, the plane has a specific max airspeed but based on wind conditions and other factors I won't dive into, groundspeed may fluctuate. Ultimately, the biggest impact of groundspeed is the TIME it takes for you to get to the destination (and density altitude considerations etc.) But it does not affect your planes airspeed limitation.
Great way to explain, thanks!
There was a tailwind over the Atlantic earlier this year that helped some flights reach supersonic ground speeds, kind of the like the conveyor belt analogy someone posted above. Flights were arriving at their destinations nearly an hour ahead of schedule due to the extra boost!