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This question is in the context of being a passenger on a commercial airliner, where you're in the cabin and the cockpit door is locked.

In another question, someone posited that if an airliner had reduced its airspeed by 60%, he'd be quite concerned. I asked "How would he even know?"

How would a passenger on a commercial airliner, who is reasonably technically savvy but not a pilot, be able to discern airspeed?

This is old school. The person does not have any modern smart devices. Or does, but their battery ran down. (Not looking for a product reco for a gadget or app, there's another answer for that.) The person could probably improvise a plumb bob, stuff like that.

Would anything change if the passenger has a clear view of the ground, relevant maps with scales, and a stopwatch (but not any wind data)?

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    $\begingroup$ Possible duplicate of How to determine aircraft altitude and speed, as a passenger? $\endgroup$ – Pondlife Oct 19 '17 at 22:25
  • $\begingroup$ @pondlife that one is asking about mobile apps, and is seeking a product reco which is off-topic. I'm asking old-school. $\endgroup$ – Harper - Reinstate Monica Oct 20 '17 at 1:38
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    $\begingroup$ With some experience, isn't it possible to form a rough idea from engine and wind noise? I could certainly tell when an airliner reduced speed significantly. It does work pretty well for me, in the roughly 50-150 kt range of single-engined prop planes. $\endgroup$ – jamesqf Oct 20 '17 at 4:17
  • $\begingroup$ @Harper OK, thanks for updating the question to make that point more explicit! $\endgroup$ – Pondlife Oct 20 '17 at 12:53
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It is impossible for a passenger to determine the airspeed to any reasonable accuracy (otherwise Birgenair 301 and Aeroperu 603 would not have crashed).

From my personal experience, it is not possible to receive GPS signal using a handheld-device in the passenger cabin. Even if it works, winds aloft at cruising altitude are often in the range of 100~200 knots. That throws the calculation completely off even if ground speed is known.

The only reliable way is to refer to the entertainment system, which obtains its data from the cockpit and is therefore accurate. However, the interface is usually programmed to display measurements in other units; the indicated airspeed has to be calculated from the displayed values manually.

Here are some ways you can guess the airspeed to a certain extent:

  • If you do not notice any significant airframe vibration, you can assume the airspeed is somewhere between the stall speed and the max airspeed.
  • If you can tell the flaps position from observation, you can also correlate that with the usual flaps extension speed of the model you're flying, which should give you ~20 knots of accuracy.
  • If you are familiar with the SID or STAR of the airport, and you know planes normally pass a certain point at a certain speed.
  • If you have a handheld ATC receiver, sometimes ATC will issue speed instructions to planes during approach.
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    $\begingroup$ If you do not notice any significant airframe vibration, you can assume the airspeed is somewhere between the stall speed and the max airspeed. - this is the best sentence I've read today $\endgroup$ – Steve V. Oct 19 '17 at 23:34
  • $\begingroup$ Isn't there something about consumer GPS receivers being required to be artificially limited in how high speeds they function at? (I don't remember what the cutoff speed was supposed to be.) That could conceivably cause difficulties even if you could get a good signal within the airplane. $\endgroup$ – a CVn Oct 20 '17 at 7:22
  • $\begingroup$ Michael - CoCom limits civ gps to about 1200mph. Probably not an issue on most airliners :-) $\endgroup$ – Rory Alsop Oct 20 '17 at 9:52
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    $\begingroup$ I've recorded my flights with all my handhelds with GPS, and it works perfect on a window seat and good enough on the next. my first phone sometimes had trouble with altitude, but my current one gives accurate data (validated with on board entertainment system) $\endgroup$ – sweber Oct 20 '17 at 15:48
  • $\begingroup$ Same for me, as sweber says. I use Maps Me on my phone, on a window seat, very often, and never had an issue. And the track is very precise, as I was able to confirm many times when the map was showing me the aircraft approaching the runway during landing. $\endgroup$ – Martin Argerami Oct 28 '17 at 5:29
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With no wind data and no knowledge of how the aircraft is configured or being flown I don't see how you realistically can.

In theory, if you could guarantee the aircraft was at a constant airspeed you could monitor groundspeed across legs in different directions - but in reality you aren't going to be on an airliner that's flown like that.

For example, if you went North at 500mph (Ground Speed) and South at 550Mph then we know the wind had a tailwind component on the second leg and I'm sure there's some clever maths somewhere to help you work some degree of it out but it'll be inaccurate by some margin.

You can't even really tell from the engines without access to instruments because you're not going to have pitch data.

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It's pretty much impossible

We can sense changes in our velocity - accelerations - but unless we were equipped with sensors and instrumentation (and sadly, we are not) able to calculate velocity from cumulative changes in velocity, the best we can do is note that we are speeding up and slowing down.

I say "at best", because even that is hard. If you're strapped into your airliner seat and the is pitched up or down, you will often have a subjective experience of speeding up or slowing down, simply because of the way your body shifts in its seat.

(Motion simulators make use of this, tilting the platform to provide a sensation of acceleration.)

And, in any case - it's useless. You want airspeed, and you'll have absolutely no way of knowing that just by working out what your ground speed is unless you also know what the wind is doing (i.e. to answer your final question, no amount of map and stopwatch cleverness is going to help you either).

But wait!

However: depending on your aircraft, it might be that in order to sustain level flight at a certain altitude in a certain configuration, it must fly at a certain minimum speed.

So, if you were armed with some flight envelope data for the aircraft, and you had altitude information (that would have to be barometric altitude, and I'm not sure where you're going to get that from), and you were able to discern the plane's configuration (deployment of flaps and so on) you might possibly be able to judge that the plane's airspeed must be at least such-and-such.

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  • $\begingroup$ [...] and you were able to discern the plane's configuration (deployment of flaps and so on)[...] really? you would guess the lift of the airplane by checking your flaps? Can you tell the degrees? and how you know the downforce on the tail? (Yes I am just poking you) $\endgroup$ – ares Oct 20 '17 at 4:06
  • $\begingroup$ @,Alexandros The degrees are often marked on the flaps themselves, see e.g. goo.gl/images/goS9pi $\endgroup$ – Sanchises Oct 20 '17 at 6:42
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Airspeed is going to be hard, if not impossible: you cannot stick your hand out of the window and don't have a connection to an outside transducer like the cockpit instruments have.

Ground speed is a different matter. I have successfully used the GPS transducer on my phone to measure ground speed on board of an aircraft while travelling as a passenger; there was a question about this on this forum some years ago. And ground speed will give at least an order of magnitude, yes you need to superimpose jetstream velocities which you can't measure, only estimate.

And accelerations can be detected by the biological transducers inside our head. They have limitations, tend to be desensitised after some time of perceiving a constant acceleration, don't detect small rotational accelerations etc. Too confusing and inaccurate to use as a reference on board an aircraft, which is what all IFR instructors keep saying as well.

So the best bet is to measure ground speed and add an educated guess on wind velocity.

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For a constant load, air density, Cl and wing area (no flaps deployed...), AoA and airspeed are correlated. If the horizon is visible, a passenger might (in theory...) determine the angle between the fuselage axis and the horizontal and thus estimate the airspeed.

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You could use a sextant! Your airspeed would be more averaged as it takes minutes to even start getting a bead on your current position.

There is an article in Air Space Magazine that explains the basics. Note that to get even 1 position with all the movement of the aircraft takes a few minutes

But acceleration of the aircraft and turbulence frequently deflect the true vertical; therefore, a single reading may not be accurate. For that reason, the bubble sextant also has a mechanical averager with a wind-up clock. It takes 60 altitude readings over a two-minute period, using a little counter that looks like a car’s speedometer to average and display the measurements.

Once I know the the star’s altitude, I can find where I am on an imaginary line extending from me toward the star’s geographic position. But finding the altitude of the star is the second step in plotting my position.

The first step is to assume a position, a set of coordinates I determine by deduced, or dead, reckoning. Based on three pieces of data—this assumed position, the altitude of the star, and its azimuth (its angular distance from true north), I calculate where I am by proving that I’m not where I assumed!

There is also this other stack question which has a neat explanation of the sextant port in the older VC10!

Now you say, " I can see the stars but we're flying over clouds!" well a captain in London came up with an artificial horizon attachment for his sextant described in Scientific American

An artificial horizon is provided which consists of two bars that exactly span the periphery of the sun's image in the horizon glass, and are maintained in horizontal position by means of a pendulum forming part of the attachment

though as the one commenter noted the graphic was not attached to the article...

Now would you know immediately about the power change? Probably not without using the other answers listed. BUT you may be able to confirm your suspicion over the course of say, 20-30 minutes if you were getting a position every 2-3 minutes.

Caveat: Also the star you'd be using would have to be on the horizon or risk poking a hole in the ceiling of the plane

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    $\begingroup$ That method would allow you to determine the ground speed, not the airspeed. For an airliner flying at cruise altitude, the difference can be very large... $\endgroup$ – xxavier Oct 20 '17 at 13:00
  • $\begingroup$ @xxavier absolutely right. I hadnt thought of that distinction when writing it. Maybe I'll update to include a weather balloon for winds aloft... $\endgroup$ – Bageletas Oct 20 '17 at 14:06

protected by Farhan Oct 20 '17 at 15:28

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