Supersonic in a 777
 

Well, not really. ;)

I noticed the other day we were doing a ground speed of 763mph over the Pacific. Since the speed of sound at sea level is 761mph, I figure that made us supersonic ;) ;) . OK,,,, so I presume in actuality there was a very favorable tailwind. I wonder if the pilot was having some fun and maybe pushing it just a little too since I watched the speed gradually increase over a period of time until the ground speed peaked out exactly 2mph above the speed of sound at sea level. Y’all suppose a pilot would crank up the speed just a little more than normal just to be able to say he had a supersonic ground speed. ;) To bad it was not a 747. I bet they were easily making 800mph.

Alas we tracked pretty far from a great circle route and were almost all the way down to 30° N latitude much of the way between TPE and LAX which is wayyyy off the shortest route. Distance traveled came out almost 500 miles more than the number of FF base miles I will receive for the flight so I guess we traveled pretty far to catch that jet stream. And in the end we landed early at LAX, didn’t have an open gate because the plane at our gate hadn’t left yet, had to wait 20 minutes before finally taxiing to a remote pier and being bussed about two miles back to immigration. Did you know they can fit the entire contents of a fully loaded 777-300ER into only two buses? Well apparently they can.:td: :td: :td:

The speed of sound falls significantly with temperature. Down to 660mph at cruising altitude according to http://en.wikipedia.org/wiki/Speed_of_sound

"there's more than one way to skin a cat"

See http://www.flyertalk.com/forum/showthread.php?t=656467 for a similar discussion. Ground speeds up to 845mph have been documented.

The sound barrier is also relative to the air around the airplane. In other words, at cruising altitude, a jet would need a ground-speed of 660mph PLUS the speed of any tailwind. (IOW, 550 MPH speed + 120 MPH tailwind = 670 MPH ground speed, but < the 660 MPH sound barrier at that altitude).

I hope this doesn't require too techinical an answer, but why is this? The sound waves don't travel at a different speed depending on the winds, do they? So if, with the wind, the plane is moving 763 mph, isn't it moving faster than the speed of sound relative to the ground? Doesn't this mean that on the ground a sonic boom would be observed?

No. You don't measure mach number relative to ground speed. Shocks, which lead to sonic booms on the ground, are produced when the local mach number, the speed of the air over parts of the aircraft, exceed mach 1. At 35000 feet, the speed of sound at standard temperature is 576 knots which works out to 663 mph. Ignore the wind speed.

OK, so doing the math..........

We were doing a ground speed of 763MPH

We were at FL33 (lower than usual for this flight too)

Mach 1.0 at FL33 is ~670MPH (depending on temperature)

777-300ER's cruise at .84 Mach

So airspeed 'probably would have been' ~563mph (670x.84)

Thus tailwind must have been ~200MPH (763-563):cool:

Was this TPE-LAX on BR?

I can't speak to the flight conditions (temperature/windspeed) for your flight, but no sonic booms will reach the ground unless the velocity of the aircraft through the air is greater than mach 1. There are even some cases if the aircraft is travelling slightly faster than mach 1, no sonic boom will reach the ground. This is dependent on atmospheric conditions. Almost certainly parts of the aircraft experienced local supersonic flows over their surfaces (cockpit) that would result in increased boundary layer noise, but not shocks that would propagate to the ground.

763 mph is an abnormally high groundspeed. On several flights I have seen the groundspeed well in excess of 600 mph, but never 700. In fact at http://www.groundspeedrecords.com/ you can see unverified ground speed records accompanied by photos of cockpit displays with tailwind componants of well in excess of 200 knots (230 mph). Notice the TAS values being a more pedestrian ~500 kts.

Several posts around these forums have noted diversions due to unusually high headwinds flying east to west lately. I would not be surprised to see off track deviations of 500 miles (as your original post indicated) if a pilot could get 200 knot tailwinds on a flight such as TPE to LAX.

To clarify, in case the "Well, not really ;)" wasn't clear. Yes, indeed we were nowhere near really supersonic. :D No sonic booms :( :( .84 Mach is .84 Mach is .84 Mach regardless of ground speed. But the 763mph was kinda cool. I started getting excited when I noticed we were at 720 because I hadn't ever noticed a speed that fast before. And it just kept climbing from there. Surprisingly a very smooth flight too. You normally hear about allot of turbulence with such a strong tailwind. Not so this flight.

BR16

Pretty cool, eh? The time honored lament though is that the strong winds are always on your nose. Nice to have the converse for once.

I bet there was some good turbulence as the edges of the jet core were penetrated.

I remember reading that the older 747's can break the sound barrier (and not in a controlled descent).

;

My understanding is that a normal jet aircraft will have a real hard time approaching and breaking the speed of sound at whatever density, temperature it's at. This is because at the speed of sound, the cone shaped shock wave would intersect the relatively wide wings causing drag and mechanical shock. And that is why supersonic aircraft are often Delta wing shaped, ie. they have very triangular swept back wings that fit inside the shock cone so this doesn't happen. Also for the same reasons the shape of the nose in supersonic aircraft is very different from the subsonic rounded shape of normal passenger jet airplanes.

But I'm not an expert, just repeating what I've heard vaguely...

;

It would be fun to...
 

pull out your handheld Garmin, illegally turn it on while in the plane, and record the 800+ GS on it. That would really impress your friends!

Swimmin' M