There is, of course, a catch: that 801-mph Vmax was the plane’s ground speed, or the rate at which it was passing over the chilly landscape of PA below.  As far as the 787 was concerned, however, it was still well within the performance envelope it was designed for, zipping along close to its established cruising speed of around 560 miles per hour.

How? It’s a matter of airspeed versus groundspeed, of course.

Groundspeed, as it sounds like, is how fast a plane appears to be traveling from the vantage point of an observer on the solid, unmoving ground below. If you were the Flash and wanted to keep pace with the 787 above you yesterday, you’d have to be running along at 801 miles per hour to match it. Airspeed, on the other hand, is how fast a plane is traveling compared to the air around it—for example, how fast it might look to a balloon floating along in the same air. In the case of yesterday’s speeding Dreamliner, that wind was a jet stream current flowing from west to east at faster-than-normal speeds; as The Washington Post pointed out, a weather balloon lofted from central Long Island’s Upton, New York clocked the jet stream yesterday at a record 231 miles per hour. With wind that strong at the aircraft’s back, even a 225-ton 787-9 can pick up a nice speed boost—in this case, enough to push it to speeds never before seen for its type, at least from the ground.