Mach 1.26

In 1975, on the flight of the Los Angeles to New Orleans leg of the trip to Mobile in which John W. and the Frog took place, the pilot came on the speakers to announce the reason for the departure from the usual flight plan. Normally, the trip included a "tour" of the Grand Canyon, however on this one, we were far out over the Gulf. We had also climbed to 43,000 feet which I thought was out of the flight envelope of a 727 (it was only 1,000 feet too high according to Wikipedia).

It seemed the jet stream had dipped unusually far south and was carrying a speed of something greater than 250 mph. With the engines cranked up, the height which placed us in the middle of the jet stream, and the ferocious tail wind, we were traveling at a ground speed of 820 mph!

This is from Altitude variation and implications for atmospheric acoustics :

290 meters per second @ 43,000 feet = 649 mph

340 meters per second @ sea level = 761 mph

820/761 = mach 1.08 @ sea level

820/649 = mach 1.26 @ 43,000 feet

820 - 250 = 570 mph actual air speed

570/761 = mach 0.75

570/649 = mach 0.88 maximum aircraft speed

...so under no case did the aircraft break the sound barrier. Neat, huh?! BTW, we made New Orleans 45 minutes ahead of schedule.