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I kind of always assumed that jets like the Boeing 737 and Airbus A320 family travelled at their manufacturer's listed "cruising speed", because the manufacturer describes that speed as the speed at which it is "most economical" to fly them.

However, doing some rough calculations, that assumption does not seem to be playing out very well. For example, for flights between Boston and Miami the typical flight time by airlines is about 3.5 hours for both the 737 and A320 series. Since these times include taxi time, let's assume it is 15 minutes of taxiing at both airports. That means actual flight time (aloft) is about 3 hours. There are about 1100 nautical miles between Boston and Miami airports. If we divide 1100/3 = 366 nm/hr cruising speed. However, the A321 is advertised as having a cruising speed of 450 nm/hr. That's a pretty big difference.

I have heard rumors that airlines fly their planes extra slow to save fuel, but that kind of defies the "most economical speed" description of the cruising speed.

I guess another possible explanation is that the "cruising speed" listed is for an EMPTY plane (no passengers, luggage or other cargo), which of course would be pretty bogus, if true.

So, what's the story here? How fast do these planes actually go in the air on average (ignoring wind) in reality? Has anyone on this forum actually used a GPS on their phone or something to compute this?

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    This might be a better fit for aviation.se. But just a few of the things you might not have accounted for: planes don’t just fly in a straight line, especially at departure and arrival; time accelerating to/decelerating from cruise speed; preferred speeds vary with altitude; schedules often have a buffer built in so it’s easier to be “on time”.
    – Chris H
    Jun 26 at 20:05
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    nm/hr is nanometres per hour. If you meant nautical miles, it should be NM/h or nmi/h, which has a specific name: knot (kn or kt).
    – Zeus
    Jun 29 at 0:58
  • @Zeus: Wikipedia says this: "nm is a non-standard abbreviation used in many maritime applications and texts, including US Government Coast Pilots and Sailing Directions." So nm is widely used and understood, especially where the possibility of ambiguity is small :-)
    – TonyK
    Jun 29 at 13:51
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    I am actually a virus, so I do move in nanometers per hour. Jun 29 at 13:58
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You've made a number of incorrect assumptions in the question, which leads to your calculations being incorrect.

Firstly, the speed a plane flies at varies during different parts of the flight. It will be slower shortly after take-off and as the aircraft climbs to its cruising altitude, and then slower as it starts its decent and comes in to land.

Secondly, the flight distance will never be the same as the distance between the two airports. The "great circle" (basically "straight line", taking into account the world being roughly round) distance between Boston and Miami is 1,258 miles. Picking a random A321 flight from Boston to Miami, today's AA747 flight, the total distance flown was actually around 1,397 miles, including turns after take-off and before landing.

enter image description here

To put these two concepts into terms we're more familiar with, the straight-line distance between SFO airport and LAX airport is 337 miles, with much of the road being 70 mph speed limits. Thus we could conclude the trip will take a little under 5 hours (337/70). However plugging this route into Google Maps gives us a distance of 378 miles and over 5 1/2 hours - because roads aren't straight, and we won't be driving at 70 mph the entire distance.

Another assumption you've made is the phrase "ignoring wind". Aircraft cruise speeds are actually stated in terms of "air speed", which is the speed relative to the air around them. A flight flying at (say) 500 miles/hour will actually cover ground faster if it has a tailwind than if it has a headwind. E.g. if the aircraft is flying at 500 miles/hour, and has a 100 mile/hour tailwind, then the speed over ground will be somewhere around 600 miles/hour (this is an oversimplification, but close enough to make a point).

There are numerous web sites that will track commercial aircraft whilst they are in flight and show you their ground speeds (i.e. including the impact of winds) over time. The best of these for this is likely FlightAware.

FlightAware only makes data available for around 2 weeks without an account, but if you're reading this before early July 2021 you can view the details for the flight I mentioned above (including its track and speed plots).

That page shows the ground speed of the aircraft over time as: enter image description here

Viewing the "track log" shows that the flight actually spent a fair amount of its time above the standard cruise speed for an A321 (as you mentioned, 450 knots, or 518 miles/hour): enter image description here

If we jump over to Windy.com we can get a good idea on why it could do this. Setting the time to that flight's time, and looking at the winds at 34,000 feet we can see that the winds were blowing in roughly the same direction as the plane was. In other words, it had a tail-wind and thus on this occasion was able to reach a ground speed higher than the cruising speed. FlightAware doesn't report the actual air speed of the plane, but odds are it was around (or below) the normal stated cruise speed for the an A321.

enter image description here

If all of that wasn't enough, there are multiple other criteria that can affect the actual speed the pilots fly at. The most economical cruise speed actually varies with altitude, with aircraft weight, and with a number of other criteria - so it can actually vary even over a single flight as the altitude changes, and as the plane gets lighter due to fuel burn. Additionally, aircraft can be slowed down or sped up by factors such as air traffic control in order to maintain proper spacing, or to try and get a late-running plane back on schedule.

As for your question about using a GPS, outside of looking at sites like FlightAware, yes, I have indeed done this. In fact, I've measured a Singapore Airlines Boeing 747-400 with a GPS travelling at 1297 km/hour (806 mph). This is actually faster than the speed of sound, and clearly the B747 is not a supersonic aircraft, so your first thought might be that this wasn't accurate - the the truth is that it was extremely accurate (the in-flight entertainment reported a speed 2km/hour different!).

The reason we were able to travel so fast is down to what I mentioned above - the combination of ground speed (what the GPS is measuring), airspeed (what actually matters), and tailwind. In this case we had a ~300km/hour tailwind, so whilst our ground speed was almost 1,300 km/hour, our actual speed through the air was only around 1,000km/hour (620 miles) - slightly above the normal cruise speed for a 747-400, but well below the speed of sound and within the normal speed range for a 747. (In this case we were travelling above normal cruise speed due to a medical emergency on board that resulted in the pilots wanting to get to the destination as quickly as possible).

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    Also schedule padding, where airlines add extra time to an aircraft schedule to make sure the plane isn't late even if conditions are suboptimal. Which makes figuring out the true expected time of the flight quite hard.
    – niemiro
    Jun 27 at 6:30
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    Heh. So comprehensive & well written about ground vs airspeed! Depending on time of year, at the latitude of Perth, Western Australia, travel time from here to Africa (Mauritius) is 1.5 hrs shorter in one direction vs the other. Similarly, flying to Sydney can vary by up to an hour even on nearly identical reciprocal courses! Both nearly "straight" East-West.
    – Andy Dent
    Jun 27 at 8:52
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    "This is actually faster than the speed of sound" – Without any additional information, this statement is non-sensical. Sound can only propagate in a medium and the speed of sound is relative to that medium and its temperature, and in fluids and gasses also its pressure. The speed of sound in steel, for example, is much higher than in air (hence the Western movie trope of pressing your ear against the tracks to hear a train coming). Also, the speed of sound in air at 10km altitude is much higher than the speed of sound at ground level in standard atmospheric conditions. Jun 27 at 11:19
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    @JörgWMittag No, the speed of sound in the atmosphere is reduced at higher altitudes. Here is a chart: fighter-planes.com/jetmach1.htm
    – MTA
    Jun 27 at 13:06
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    For anyone curious, note that modern "flight durations" quoted by airlines are often much longer than they previously quoted. I think they do this so they can still claim flights are "on time" even if they are actually late/delayed. Jun 27 at 22:40
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There a bunch of factors that make it appear longer

  1. Besides taxiing it also takes a while to get to cruising altitude and cruising speed. Often takeoff and landing directions are completely opposite of where you are trying to go the way in and out of the airport is determined by wind, noise ordinance, traffic situation etc. There is a non-trivial amount of time in the air where you are not at cruising speed or going the wrong way.
  2. Prevailing wind makes things asymmetric: BOS to MIA is 3:30h but MIA to BOS is only 3:15h
  3. Airlines "pad" the schedules. Flight times have a natural variability to them. The published flight times are not the median but more of a 75% percentile. A "normal" flight arrives earlier than scheduled. This also makes their "on time" performance record look better.
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How fast do these planes actually go in the air on average (ignoring wind) in reality? Has anyone on this forum actually used a GPS on their phone or something to compute this?

You can view the plane speed information on some flight trackers, e.g. https://flightaware.com/live/flight/JBU416/history/20210624/1910Z/KSFO/KJFK (mirror 1, mirror 2):

enter image description here

https://flightaware.com/live/flight/JBU416/history/20210624/1910Z/KSFO/KJFK/tracklog (mirror 1, mirror 2):

enter image description here

The speed graph is based on ADS-B data from FlightAware. More information here.

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