I work for Fedex at SNA in Orange County, CA and last night, Feb 16 I was tracking our flight on its return trip to MEM. After it landed I looked at the flight log and noticed something a little odd. At exactly 10pm PT it reached a ground speed of 593 knots or 682 mph at 33,000 feet. I did a little research and found out that at that altitude the speed of sound is 669 mph, so it apparently reached Mach 1.018. I was wondering if you’ve ever heard of anything like this before. In case you want to look it up it was FDX Flight 1258 with an Airbus A300-600F.
Ground Speed is not considered when calculating Mach numbers, only indicated airspeed, which you can’t track unless you’re sitting in the cockpit. If that plane had a tailwind of 100 knots, for example, then the indicated speed of the aircraft is only 493 knots.
Also, you can’t always say that one MPH always equals a certain Mach number. Environmental factors (temperature, air pressure, etc.) have to be taken into account as well.
I think you met True Air Speed. An indicated of 493 at FL330 would make for a TAS of about 749kts, which would be about 1.29 mach.
A TAS of 493 would be about 305IAS.
Either way FA shows GS. So TAS + Tailwind = GS. Still 593 over the ground is pretty fast. But it is a winter jet stream.
Yes, that’s exactly what I meant, my mistake.
I was flying in that area the last week or so and the winds aloft have been screaming. I had one leg with a ground speed of well over 600 kts. My passengers loved that the airshow display was giving them a ground speed of over 700 mph.
It’s a good thing your passengers weren’t flying east-to-west, isn’t it?
I’m wondering if any of the GA prop planes managed to experience negative ground speeds east-to-west.
I’ve read a few threads over at airliners.net where pilots talked about going backwards in high winds.
I think cfijames alluded to flying “at a standstill” over an airfield into a hefty headwind. Maybe he’ll post a response…(cue) 
He’s busy flying today AFAIK. Perhaps he’ll check in tonight.
BTW, he passed his ATP ride here (N14) Friday!
I’ve flown backwards in a Pacer. I did have the power back a bit, about 1700rpm. I landed with close to 2000rpm and had an indicated of about 60 and a GS of (I’m guessing) 20. In the flare the aircraft DID STOP moving forward, 0 ZERO ground roll. I needed a lot of power to taxi and I was dancing for my life of the rudder pedals, there was no way I could have taxied in a crosswind or tail wind or had the wind been gusty.
Ya, I’ve done it plenty of times. I’d do slow flight over the airport facing north and end up at the south end of the field. I’d usually have one of the line guys go outside to watch, they always said it looked really weird.
As far as getting negative groundspeeds unintentionally? I dont think so. No one would even try to go somewhere if the winds were even close to the cruising speed of their aircraft.
Leardvr, nice job keeping the Pacer going straight, I’ve had my hands full flying a Clipper in even mild winds, so I gotta give you…ahem…“props.” 
On a scale of 1-10
Skill 3
Luck 9
Being a dumb butt 10
Going straight was easy, that little airplane wasn’t going to turn out of the wind. We thought about shutting down and getting out and wing walking the plane to the hanger.
I may be wrong but I believe that the speeds provided in the track log may be groundspeeds and not air or indicated airspeeds, since these position reports come from radar hits.
With enough wind on the tailcone the groundspeeds can be impressive when one has the jet stream on your tail.
Come on keep up, I’m pretty sure that’s what we said. 
Just kidding, your right FA shows GS.
Duhhh! Didn’t see that … all I saw was True and Indicated.