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flylku
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757 And Speed

Wed Jul 11, 2012 9:18 pm

Crossed the Atlantic today on UA 127 (DUB-IAD), a 757. About an hour west of DUB is noticed an AF 77W overtaking us at a pretty good clip. I think we were at 380 and he must have been 360. He stayed in view for no more than two minutes so I am guessing he had a 20 kts speed advantage on us. Later, over Canada, a UA 767-400 passed us from below with the same speed differential.

I thought, with the exception of the 737, the Boeings all cruised at around the same mach number. It looks like I was wrong.

Also, what other overtakes have others experienced over the last year or so? Who are the slow pokes and who are the speed demons.

Finally, how is spacing over the Atlantic handled with regards to speed?
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Polot
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RE: 757 And Speed

Wed Jul 11, 2012 9:30 pm

Quoting flylku (Thread starter):
Also, what other overtakes have others experienced over the last year or so? Who are the slow pokes and who are the speed demons.

Typical cruising speeds:

737NG: 0.78 Mach (Older 737s are slightly slower)
747: 0.84-0.855 depending on version (Classics are slower, -8 slightly faster than -400)
757: 0.80
767: 0.80
777: 0.84
787: 0.85

A300: 0.78
A310: 0.80
A320: 0.78
A330/A340: 0.82
A350: 0.85 (Obviously an estimate)
A380: 0.85 (It might be 0.855, not sure exactly)

Obviously the actual cruise speed will differ depending on the flight, operator and specific conditions, with the plane sometimes traveling faster or slower but the list gives you a general idea of where today's airplane's speed fall.
 
BoeingGuy
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RE: 757 And Speed

Wed Jul 11, 2012 9:34 pm

The 777 cruises at about .84 Mach. The 767 and 757 at about .80 IIRC. I would expect the 77W to overtake you but not the 767-400.

You were incorrect that all Boeing's cruise at the same mach number. The 747 is the fastest at around .87; 787 at about .85; 777 at about .83-.84 and the 757 and 767 at about .80.

The 777 is also much faster than the A340-200/-300. I had the opposite experience in a AA 777 after leaving JFK. I could look down and see this A340 with a purple tail come into view; pass under us backwards; and then drop out of sight behind us. We overtook it like it was standing still; and I'm sure we arrived in LHR at least 40 minutes earlier.
 
AADC10
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RE: 757 And Speed

Wed Jul 11, 2012 9:49 pm

Quoting poLOT (Reply 1):
Typical cruising speeds:

737NG: 0.78 Mach (Older 737s are slightly slower)
747: 0.84-0.855 depending on version (Classics are slower, -8 slightly faster than -400)
757: 0.80
767: 0.80
777: 0.84
787: 0.85

A300: 0.78
A310: 0.80
A320: 0.78
A330/A340: 0.82
A350: 0.85 (Obviously an estimate)
A380: 0.85 (It might be 0.855, not sure exactly)

Do most airlines still cruise at those "typical" speeds? Several airlines have mentioned that they cruise at slightly less than they used to in order to save fuel. Is that a load of bunk? Would the aircraft descend at cruise trim if they were below typical speed?
 
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kc135topboom
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RE: 757 And Speed

Wed Jul 11, 2012 9:49 pm

The B-764 could have been on a much shorter great circle routing, and that's why it passed you. Also, the headwinds at FL-380 and FL-390 could have been 20 knots, or so faster (stronger) than the headwinds at FL-350 or FL-360.
 
Viscount724
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RE: 757 And Speed

Wed Jul 11, 2012 10:01 pm

Quoting BoeingGuy (Reply 2):
The 777 cruises at about .84 Mach. The 767 and 757 at about .80 IIRC. I would expect the 77W to overtake you but not the 767-400.

Look at the block times of DL 764s LHR-JFK. They're the longest of all flights in that competitive market (about 20 daily nonstops). DL is the only 767 operator on that route and their block times are up to 40 minutes longer than other carriers departing about the same times using 744s and 777s. DL is the only carrier with block times of 8 hrs+ LHR-JFK.

Example:

BA177 (744) LHR 1300 JFK 1530 Block time 7:30
DL6 (764) LHR 1305 JFK 1615 Block time 8:10
 
vikkyvik
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RE: 757 And Speed

Wed Jul 11, 2012 10:57 pm

Quoting KC135TopBoom (Reply 4):
The B-764 could have been on a much shorter great circle routing, and that's why it passed you.

Not if it was visible from the thread starter's airplane! At close range, speed differences will play a MUCH larger role than any great circle differences. Besides which, they may not have been on the same route.

Winds could certainly have played a factor, though I don't know why the 757 wouldn't try and take advantage of them. Perhaps not enough of a gain to offset the higher fuel burn at lower altitude for their weight?

I was also surprised to read that your aircraft was at FL380 after only an hour of flight. Granted, I don't really know the real performance of various airplanes, but does anyone know if that's reasonable for a 757 on DUB-IAD?
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flylku
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RE: 757 And Speed

Thu Jul 12, 2012 12:55 am

Quoting vikkyvik (Reply 6):
I was also surprised to read that your aircraft was at FL380 after only an hour of flight. Granted, I don't really know the real performance of various airplanes, but does anyone know if that's reasonable for a 757 on DUB-IAD?

Honestly I think we were at 360 that early in the flight and 380 as a final. When I saw AF I was fumbling for my tablet to video him passing us and I never looked at the flight map after that.

I think the second sighting was a UA 767-400 (do they run them TATL? If not it was Delta) and it was definitely on the same track as we were. It looked just like AF passing us just a bit smaller.

I had not thought about differences between winds aloft although the sightings were identical and five house apart.

My take away is that the 757 is slower than I thought.
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Polot
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RE: 757 And Speed

Thu Jul 12, 2012 2:17 am

Quoting flylku (Reply 7):
I think the second sighting was a UA 767-400 (do they run them TATL? If not it was Delta) and it was definitely on the same track as we were. It looked just like AF passing us just a bit smaller.

UA runs the 764 TATL all the time. Are you sure that it was a 764 and not a 772? I don't know how good of a look you got, and the two are similar lengths (the 772 is only 8 feet longer).
 
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flylku
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RE: 757 And Speed

Thu Jul 12, 2012 11:32 am

Quoting poLOT (Reply 8):
UA runs the 764 TATL all the time. Are you sure that it was a 764 and not a 772? I don't know how good of a look you got, and the two are similar lengths (the 772 is only 8 feet longer).

I had a pretty good look; enough that I thought I could make out the blended winglets and I am fairly certain of the paint scheme also. I just did not know they were using them on the Atlantic so I second guessed myself.
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smittyone
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RE: 757 And Speed

Thu Jul 12, 2012 12:43 pm

Quoting flylku (Reply 9):
I had a pretty good look; enough that I thought I could make out the blended winglets and I am fairly certain of the paint scheme also. I just did not know they were using them on the Atlantic so I second guessed myself.

The 767-400ER has raked wingtips instead of winglets. So the mystery continues  
Quoting flylku (Reply 7):
My take away is that the 757 is slower than I thought.

I'm not convinced that it is specifically a 757 issue here, especially if you're saying a 767 passed you. Your flight might have been using a different cost index in the FMC or following slower traffic at that FL.
 
united319
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RE: 757 And Speed

Thu Jul 12, 2012 3:01 pm

Could it be that the 757 was cruising at a lower speed to conserve fuel? Maybe you guys were only set at .78 or .79. I know it can make DUB-IAD comfortably but TATL is still a longer hike on that aircraft. Also the winds yesterday I believe weren't too bad heading westbound.
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flylku
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RE: 757 And Speed

Thu Jul 12, 2012 4:36 pm

Quoting united319 (Reply 11):
Could it be that the 757 was cruising at a lower speed to conserve fuel? Maybe you guys were only set at .78 or .79. I know it can make DUB-IAD comfortably but TATL is still a longer hike on that aircraft. Also the winds yesterday I believe weren't too bad heading westbound.

Also an interesting thought. We arrived on-time so it sounds like they flew it as planned and maybe that calls for a lower mach number than say IAD-SFO.
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FlyHossD
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RE: 757 And Speed

Thu Jul 12, 2012 7:21 pm

Quoting flylku (Reply 7):

My take away is that the 757 is slower than I thought.
Quoting united319 (Reply 11):
Could it be that the 757 was cruising at a lower speed to conserve fuel? Maybe you guys were only set at .78 or .79. I know it can make DUB-IAD comfortably but TATL is still a longer hike on that aircraft. Also the winds yesterday I believe weren't too bad heading westbound.

Mach .78 isn't as uncommon on a 757 as some seem to think. I'd say the majority of my 757 Atlantic crossings were at that speed. The 757 is capable of higher speeds, but why "spend" the fuel if you're running on time?

And if Shanwick or Gander need a faster speed for the Atlantic crossing (for traffic separation), your clearance would state as much.
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