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(Disclaimer: I know nothing about aviation. It is not even a hobby to me. I'm sorry if this question sounds obvious or silly to experts.)

I was casually checking the arrival time of a flight on FightAware when I came across this strange speed profile, apparently for a routine flight between Korea and Japan:

Speed profile

(Reference: https://ja.flightaware.com/live/flight/AAR152/history/20171009/0035Z/RKSI/RJSS)

Indeed, it seems that the speed of the plane went slowly down, in a controlled way, and reached 380 km/h for about 5 minutes. Then it went up again, much faster, and came back to its normal value. The event had no influence on altitude.

As I said, I am no aviation expert, but... 380 km/h at 10,000 m seems SO SLOW to me! The speed was more than divided by 2 and actually came close to the speed of a train. To my knowledge, I’ve never been on a flight that slowed down so much in mid-air (and it would freak me out).

I then checked many other speed profiles to see if it was a common pattern, but I couldn’t find anything close to this. How could one explain such a steep decrease in speed? Is it a common occurrence or a rare safety issue? What usually causes this kind of event?

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    $\begingroup$ My guess would be for traffic separation at the destination airport. That seems a rather significant drop in ground speed, though, so I'd guess that there were also some errors in Flight Aware's data gathering and/or reporting. They're not perfect. $\endgroup$ – FreeMan Oct 19 '17 at 12:06
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    $\begingroup$ The perfectly straight lines would also suggest poor data acquisition during this stage. Suggesting that the drop is just two poor data points where the ground speed was being misreported. During the climb there is a similar but less obvious jump aswell to the same speed. $\endgroup$ – DJ319 Oct 19 '17 at 13:21
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    $\begingroup$ I think you've spotted the opposite effect of what @FreeMan posted. This is a possible duplicate: aviation.stackexchange.com/questions/16610/… $\endgroup$ – Jamiec Oct 19 '17 at 15:08
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    $\begingroup$ Errors in Flightaware. You are basically looking at an unofficial source for historical air traffic data $\endgroup$ – Steve Kuo Oct 19 '17 at 15:47
  • $\begingroup$ Funny, @Jamiec, that's the same Q&A I linked to... :) $\endgroup$ – FreeMan Oct 19 '17 at 17:39
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At that point, altitude shows about 11 km, or FL330. Up there, 380 km/h, or 205 knots true corresponds to something like 124 knots indicated, well below clean stall speed.

This is ground speed though, so with very strong head-wind it could be possible. But the only sufficiently strong wind is the Jet stream and flying eastward they would have it in their backs.

Also for westbound flight, the dispatcher would have to make rather big mistake to send them across the jet stream—they do take forecast winds aloft into account when preparing the flight plan.

Data error is the only plausible explanation here.

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You are right that this seems odd. An A320 would not be able to sustain flight at such a low speed at that altitude. Your reference data is erroneous. Don't trust everything you see on the internet.

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