I've noticed that (long-haul) airliners sometimes travel at as high as 1000 km/h (I believe I've even seen 1040 km/h), but usually they fly closer to 800 km/h, for most of the trip. This seems odd to me.

I would understand this if it was due to wind speed; however, usually those that travel slowly seem to do so for most if not all of the journey, regardless of the direction of travel (e.g. even if they are going up to the north pole and back down, and whether the net direction is west or east). I find it nearly impossible to explain this using wind speed.

Similarly, given that the speed of sound is around 1,060 km/h at 12 km above sea level, I find it equally uncompelling to reason that they need to go at less than mach 0.8 due to the speed of sound. I would understand it if they limited themselves to (say) mach 0.95 (obviously you don't want to do mach 0.99 due to variability etc.), but I routinely see top speeds that are around mach 0.8 for cross-continental flights, and I don't understand why.

So, what's the real reason?

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    AFAIK the 747 is still the fastest airliner in service at 0.85 Mach. Even at 0.8 M some parts of the aircraft experience supersonic flow. – TomMcW Jun 12 '16 at 5:10
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    BTW, where are you seeing these speeds? – TomMcW Jun 12 '16 at 5:11
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    Related question: aviation.stackexchange.com/questions/11936/… – Andrew Grimm Jun 12 '16 at 9:32
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    This is not duplicate of the marked question. That question is about what the top speed is, but this one is about why it is sometimes indicated as high as 1,060 km/h and sometimes as low as 800 km/h, which that one does not even touch. – Jan Hudec Jun 13 '16 at 12:52
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    @Federico, why they limit themselves to 0.8 Mach is part of the question, but clearly the variation is part of it too and must be explained to answer it, even with the current wording. The suggested duplicate does not do that. – Jan Hudec Jun 13 '16 at 14:05

There are three different speeds that are of relevance here:

  • Groundspeed- This is probably the speed indicated to the passenger. For them, this is the most relevant as it determines the time taken for the trip

  • Airspeed- This is the speed of relevance to the flight crew and is used for flight.

  • Local airspeed- This determines the maximum speed of (subsonic) airliners. The reason is that the local airspeed over the wings are higher compared to the undisturbed flow- a result of the air being accelerated over the wing. As a result, the local airspeed over the wing can exceed Mach 1 quite a bit before the airspeed of the airliner goes anywhere near that.

The result of such local supersonic flow would be a rapid rise of drag at the drag divergence Mach number (which is greater than the critical Mach number).

Drag Divergencce

Image taken from notes of Advanced Aerodynamics by Professor H.M. Atassi of University of Notre Dame

In order to avoid the drag penalty, the airliners fly at speeds below the drag divergence Mach number. The reason is reduced speed is not the free-stream Mach number- it is the local Mach number, which should be kept below the Drag divergence Mach number.

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    +1 so I guess the important point here is that even an airliner at Mach 0.8 probably has airflow somewhere near Mach 1... is that correct? (just checking, because it's more than I intuitively expected) – Mehrdad Jun 12 '16 at 6:20
  • @Mehrdad Yes. Also remember that the Mach numbers depends on, e.g., umidity and temperature. so the same speed could be Mach 0.8 in a certain condition and Mach 0.95 in an other. – Bakuriu Jun 12 '16 at 10:25

Information displayed to passengers through the entertainment system often gives the aircraft ground speed rather than airspeed.

Wind affects the ground speed, you're right about that.

Few things out of the way first.

  • Pilots do not use ground speed for flying, instead they use indicated airspeed.
  • Mach number is not derived from the ground speed, as it is dependent on air temperature, instead it is derived from the true airspeed. Notice we already have two kinds of airspeed. There are more still.
  • Aircraft type (for example 737 vs. 777) determines "top speed". Not all airliners are the same. Those two types range from Mach 0.77 to Mach 0.85, roughly.

In less words than Wikipedia which I linked:

  1. Indicated airspeed tells the pilot the aerodynamically relevant speed to be used for flying.

  2. True airspeed is the actual speed relative to the surrounding atmosphere.

  3. From the true airspeed, add or subtract tail or headwind, and you get ground speed from that. Typical wind experienced by the regular subsonic airliner is fast (+100 km/h). The tail or headwind component would vary depending on the plane's direction and the wind's direction.

  4. Finally, Mach number is a function of temperature and true airspeed (notice, not ground speed). The higher you are, the colder it is. Also very important, Mach number is a ratio, never a speed. Since the speed of sound varies as the atmosphere changes density and temperature at the different altitudes.

Here's a nice story, a 1200 km/h subsonic flight.

Summary:

Pilots don't use ground speed (for flying), mach number is not a speed.

  • A couple examples of different aircraft: 777's published cruise is 0.84 M with am MMO (highest certified speed) of 0.87M. A320 and 737-800 have an MMO of 0.82M – TomMcW Jun 12 '16 at 5:39
  • I just grabbed a couple off the web for examples to illustrate your point – TomMcW Jun 12 '16 at 5:44
  • +1 but the information is sort of tangential and not directly answering my question. – Mehrdad Jun 12 '16 at 6:23
  • You wrote "pilots use airspeed for flying". That's true for the "use the controls" part of flying. I would say that a commercial jet pilot spends more time thinking about fuel than using the controls, and for fuel management the relevant speed is ground speed. – Martin Argerami Jun 12 '16 at 14:15
  • As far as I can tell, that "only" can be hair-raising. Years ago there was this blog "FL390" from a USAir pilot flying A319/20/21, and it was a constant topic how fuel and wind were on his mind as he dodged storms all over the continent. – Martin Argerami Jun 12 '16 at 14:30

It is all about the wind. If you check schedules, you'll see that for example USA-Europe flights are way shorter (about an hour, say) than Europe -USA. This is due to the jetstream. I found it even more significant in the Southern hemisphere, when flying Australia-New Zealand (and back) or Santiago-Buenos Aires.

As mentioned in the other answers, no commercial jet goes anywhere near mach 1. The ground speed you are shown may be close to the speed of sound (it could even be higher, if the wind were strong enough), but the actual airspeed of the aircraft (its speed relative to the air mass) is way less than that (around mach 0.8 for the bigger jets, and less for many smaller comercial aircraft).

Similarly, given that the speed of sound is around 1,060 km/h at 12 km above sea level, I find it equally uncompelling to reason that they need to go at less than mach 0.8 due to the speed of sound. I would understand it if they limited themselves to (say) mach 0.95 (obviously you don't want to do mach 0.99 due to variability etc.), but I routinely see top speeds that are around mach 0.8 for cross-continental flights, and I don't understand why.

What matters is the speed of the air passing the plane. As the other answers have explained, the numbers you see in the cabin are typically speed over the ground, rather than speed through the air, so a big component of the difference is whether the plane has a headwind or a tailwind, which can easily add or subtract 100km/h at cruise altitude.

The other thing you need to look at is not just the plane's speed through the air but the speed of the airflow over the plane. How are those things different? The point is that, as a plane pushes through a mass of air, air has to move out of the way of the big metal thing. This means that the air passing over the plane will be moving quite a bit faster than the plane's speed relative to the air. Thus, even a plane moving quite a bit below Mach 1 can have supersonic air flowing over it. The reason you don't fly at Mach 0.99 is not because a gust of headwind might take you over the speed of sound, but because you'd already be experiencing supersonic airflow 100% of the time, at that speed.

There are other factors that people already mentioned.

On modern jets the flight computer can be programmed for efficiency during the route.

One of the reasons why mach 0.6-0.8 is used is it is sufficiently fast without burning a lot more fuel. The 777 if I remember correctly uses about 30% more fuel fully loaded to accelerate to 0.87 (edited) mach than at 0.7 mach at around 11.5 km. That's a lot of fuel to save yourself less than 20% of time. That extra fuel would be the result of more expensive tickets, in an industry which is trying to keep ticket prices to the minimum to compete.

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    I don't think the last part of your answer makes any sense. – SMS von der Tann Jun 12 '16 at 15:56

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