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

Unfortunately not.

At Mach 1,0, the shock cone has apex angle of 180 degrees - it is a flat plane supported at the nose, and will not touch nose.

However, a big issue with sound barrier is backwards shift of the centre of lift. Concorde had to pump fuel backwards during acceleration to transonic. A subsonic airliner does not have such pumps. If it were to accelerate near the speed of sound, it would suffer a Mach tuck - as the centre of lift moves backwards, the plane drops nose and enters into dive, the elevator authority is not enough to raise nose, so the plane accelerates in a supersonic dive till it meets ground or is broken mid-air by excessive aerodynamic forces.

Basically, the drag (drag coefficient, C sub D, for the aerodynamic geeks among us) approximately triples as you go throught the speed of sound. Nobody will give an aircraft three times the power it needs otherwise unless going through the "sonic barrier" is a requirement. Once past it, it drops off again to about 50 percent higher than its subsonic value, so cruising at supersonic speeds is practical (though expensive, for a bunch of other reasons besides drag as well).

Ah right, I had forgot that part, about the cone opening angle being a function of v.

But interesting about the shifting center of lift!

Real supersonic (airspeed, not ground speed) has been done before in a 747. Coincidentally, also on the TPE-LAX route.
http://www.airliners.net/open.file?id=0067630&size=M

Glad I wasn't on that flight.:eek: :eek: :eek:

Scary, but funnily with very little harm done!

It is not known how many g-s the plane and passengers suffered, because the acceleration broke the acceleration sensors for the black box, but it was more than 5 g.

Out of over 270 passengers and crew, only two were injured - one crew and one passengers. Everyone else was completely unscathed.

I suppose when you are pulling >5 g's everyone pretty much remains in their seats whether they are wearing their seat belts or not.

Maybe there were few serious injuries to passengers but I bet their underpants were not so lucky.

I was one of the lucky ones at CO's Houston DO II that got to 'fly' the 777 flight sim. While in many ways it was no more difficult than driving a car, one of the things on my list that we never got to because everyone needed 2 shots a landing to succeed was to take it supersonic :(

My plan, if I could not pull out of a dive in a reasonable period of time was to do a mid-air engine reverse. It would have been interesting to see the result.

Maybe next time :)

FL33 is 3,300'. 33,000' is FL330.

FL's don't begin until FL180. There is no FL33.

Some allow GPS units, DL or CO, I can't remember exactly, plainly listed in the inflight mag.

I believe most ICAO countries use FL's at lower levels.

I was not aware of that. In the US it's only used above 18,000MSL. Also the altitude at which one sets the altimeter to 29.92... so in fact 18000' |= FL180 unless the actual altimeter reading is 29.92.

That's true in the United States but elsewhere the transition level varies significantly. But, that wasn't the point...

FL33 is how 3,300' would be written as a flight level. It is not 33,000' as the poster intended.

And they don't call them FL's, just 'F', as in F80.

I'm sure you are aware that a FL altitude is just a pressure level above some standard reference level of 29.92 inches. While the absolute altitude will vary w/ pressure changes, the relative part doesn't and todays instruments are very very precise. Witness the Gol and Embraer collision over Brazil.

I've used my GPS on loads of flights, including one at close to 700 MPH. Qantas was the only airline that has ever asked me to switch it off.

I use my GPS routinely on Delta flights. It is neither included or excluded in the magazine. No FA has ever even mentioned it and I have had it is clear plain view a lot. My top speed: 730 mph. I have it in my pocket tonight if I ever get around to flying.

On the way back from JFK to LHR in December on a BA747 we were riding the Jetstream at 690-700mph ground-speed. Only 70mph short ;)

Never mind.

Who does?

In aviation, it certainly is FL180, FL330, etc. In 26 years as a pilot I've never seen an altitude written as F80.

I don't understand you point. FL33 would never be an assignable altitude in US airspace. It indicates 3,300' above the standard pressure datum of 29.92" but the US transition level is always FL180 so no FL below FL180 will ever be assigned.

Sometimes the lower flight levels can become unusable due to low pressure. For example, if the local altimeter setting drops below 29.92 then FL180 becomes unusable as you'd no longer have 1000' separate between FL180 and 17,000' (or 500' separation between FL180 and 17,500' the highest VFR altitude). At 28.92 FL190 would become unusable, etc.

Never mind...

They are not. FL180 is the lowest flight level that can be assigned in the US. If FL180 is below 18,000' MSL due to low pressure then FL180 is not assignable.

Thanks, you're right. I should have done my research before speaking.

And what happens if, due to high pressure, FL180 is above 18 000 feet?

That's OK - you still have proper separation from aircraft below 18000MSL.