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JET STREAM CLAIMS

Old 22nd Feb 2019, 11:49
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JET STREAM CLAIMS

Currently on PPRuNe and Social Media, claims of an impressive Ground Speed achieved in the Jet Stream by a Virgin 'Dreamliner' twin has triggered an significant interest.
One or two British Tabloid newspapers, aided by sub-editors with a tenuous grasp of basic physics have confused a few of their readers, who's fanciful comments on 'Facebook' and
similar about Supersonics are thus understandable. However, as someone who has never clapped eyes on one of these recently introduced 'Dreamliners', my question is: What was/is Virgin's economy Long
Range Cruise Mach Number on this Aircraft? I assume thrifty twins are slower, but I imagine also that the pilots with an eye on the opportunity, might have nudged it up a notch. How close to 0.9
can a 'Dreamliner' slide along at in smoothish air? I recall vaguely reading about its earliest post production days and some new composite materials that were used in construction, so perhaps instead
it has a redline on skin temperature in case it might risk becoming unglued like Concorde's leading edges might have. ( as told me by a Concorde colleague in BA.....might have been a wind-up though )
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Old 22nd Feb 2019, 14:05
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M0.84 to 0.85 is normal cruise speed
M0.86 to 0.88 is possible but heavy on fuel. OK for a short while.
M0.90 is Mmo.

Although it is possible to fly at Mmo, any exceedence (caused by windshear, wave or auto flight misbehaviour) would trigger an over-speed warning. The crew would then have to report it and explain what they were doing.

The crew may have pushed up the Mach number briefly, maybe up to M0.88 to M0.89, just to see a high ground speed readout.

There is no readout of skin temperature, and therefore no limitation.
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Old 22nd Feb 2019, 17:32
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Commercial airliners do have to take into account commercial viability. Therefore speeds are not exactly the fastest available ones. The currently fastest civil jets are business jets, i believe the Cessna Citation X+ with M 0.935 MMO and the Gulfstream G650 with M 0.925 as MMO are pretty much the fastest ones available, both apparently able to cruise quite comfortably at 0.92 if one sacrifices a bit of range for speed. And quite honestly, the owners are not really looking for every penny in fuel saving...
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Old 23rd Feb 2019, 00:01
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Seen here, Dreamliner 787-9 from LA to London reaches 1289km/h as a furious jet stream packs record-breaking, supersonic speeds, published in Australia but derived from the Washington post:
"Dreamliner 787-9 from LA to London reaches 1289km/h as a furious jet stream packs record-breaking, supersonic speeds"

"The ordinary cruising speed of a Dreamliner is 561 mph, with a maximum propulsion of 945 kmph." (What?)

"And you might notice something suspect about the 1289 kmph reading - it's above the speed of sound (1234 kmph). Commercial aircraft ordinarily can't break the sound barrier, because they're not designed to handle the sudden increase in drag and other aerodynamic effects associated with those speeds. Despite a ground speed that high, the plane didn't come close to reaching that threshold because it was embedded in the swiftly moving air." (OK, better...)

You rather get the impression that the journo is a little further beyond their own understanding than usual.
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Old 23rd Feb 2019, 05:41
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The jet stream is exactly where clear-air turbulence (CAT) is shown on aviation meteorological charts (sigwxh). Smooth air is least likely under these conditions (outside of thunderstorms). As others have noted, this would not be the time for messing around with cruise speeds and MMO.

BTW, all the media reports seem to be focused on ground speed, nothing from onboard the aircraft, or passenger comfort. Imagine alternative headlines: "It was so bumpy I thought we were going to die". Unless it was freakishly smooth.

Originally Posted by nonsense View Post
"And you might notice something suspect about the 1289 kmph reading - it's above the speed of sound (1234 kmph). Commercial aircraft ordinarily can't break the sound barrier, because they're not designed to handle the sudden increase in drag and other aerodynamic effects associated with those speeds. Despite a ground speed that high, the plane didn't come close to reaching that threshold because it was embedded in the swiftly moving air." (OK, better...)
Edit: The speed of sound varies with temperature (altitude), at cruising level it would be1062 km/h, which is very different from sea level. Not that the average journalist would know this. This would emphasise that the numbers are actually more impressive than the article states.

Last edited by GordonR_Cape; 23rd Feb 2019 at 06:05.
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Old 23rd Feb 2019, 06:49
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If the tailwind component briefly reached 200 kts, then only about .86 - .88 mach (depending upon temp) would be required to achieve 500 KTAS and a GS of 700 kts. The highest GS I ever saw was 605 kts. And that was at 425 KTAS (about as fast as a typical Westwind is likely to go) The UNS 1 showed the wind right on the tail at 182 kts.

200+ knots jet stream velocities are not so uncommon as to be particularly surprising. Turbulence should be expected whenever there is a large difference in wind velocity or direction over a short distance. Unless it's a particularly lucky day, you'll find it!
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Old 23rd Feb 2019, 14:43
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I've only seen 200 kts once, and not for long. It was over the uk, and we made our way south quickly.
Avoided it on the way back, and the difference in block times was less than you'd imagine.
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