(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:
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?