Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds | Hacker News Reader

Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

latimes.com

207 pointsilamont180 comments

Note to readers, land speed not equal to air speed. As the article discusses it doesn't break the sound barrier.

That said, the news here is that the jet stream is carrying more energy than it has in the past (air mass is moving faster, kinetic energy content is 1/2mv^2). I read a paper a long time ago which talked about the impact of thermal 'tubes' where denser cold air travelling at speed would "punch holes" in high pressure systems leading to more complex weather patterns. Sadly I cannot find it! The question I think about when reading this is the impact on the duration and temperature swings of the winter months in the north east of the continent. In particular, atmosphere so cold as to support hurricane formation over land. Most famously exploited in a fairly mundane movie "the day after tomorrow" but the modelling works if the temperatures meet certain conditions.

I don't know if winter "super storms" are possible due to other moderating influences but if they are, having the jet stream behave in a new way is a prerequisite (feeding very cold air into the system).

Do you know if the rides in higher energy jet streams are less comfortable, or is it undetectable by the passengers?

As far as I have been able to tell in flying across the continent is the change in flight time (arriving early or late after departing on time).

I was on a flight to London that caught one of these tailwinds last year. I recall a little buffeting mid–Atlantic, don't know if that was in the jet stream or not. Only problem really was that we arrived too early in London and had to circle for half an hour before landing and then wait for immigration & customs to open.

It would be as undetectable as the motion of the earth. It's the change in velocity (not the velocity itself) that causes you to feel something. Moving from one air current to another, with a large change in velocity, would be very noticeable.

They could be more turbulent. I presume they aren’t, though.

I’ve had a SFO -> London flight once where the pilot intentionally flew into a very strong jet stream current (saved us a lot of flight time, think it was close to an hour). Experienced a short burst of mildly rough turbulence entering and exiting, but otherwise smooth sailing.

Just curious... How do you know that? I wonder how some nervous passengers might react if they learned the pilot decided to enter a turbulence area on purpose.

The pilot got on the intercom and specifically stated he was doing it. He assured everyone there was zero risk outside the brief bumpiness and it would get us there much faster. He got loud cheers of support from the cabin, so I guess those nervous just kept to themselves.

I wonder if the pilot got any compensation for the fuel savings? I could see that being a dangerous incentive though, forcing pilots to opt for more risky conditions for the fuel savings bonus.

> Note to readers, land speed not equal to air speed. As the article discusses it doesn't break the sound barrier.

Yes, but the first sentence is not why it did not break the sound barrier. They state it is due to being in swiftly moving air. The speed otherwise, even at airspeed (which is a negligible difference to ground speed) is fast enough to break the sound barrier.

This is probably a stupid question, but how is "ground speed" measured?

Is it how quickly the airplane moves past a fixed point at it's altitude, or along the earth's surface? I.e. the plane is effectively moving along the surface of a sphere, but if the plane is 30,000 ft up, then the radius of the plane's sphere is 30,000 ft greater than the radius of the "ground" so traveling at the same angular velocity will require a higher speed the higher you go.

If I'm thinking about this right, across their respective "planes," the airplane will be moving faster than it's shadow. So maybe a better way to ask this is: is ground speed the speed of the plane, or the speed of its shadow?

Hm, I think the difference is marginal because of the large radius of the Earth, at least for planes.

The Earth has almost a 4000 mile radius; 30,000 feet is less than 6 miles. The difference between movement over ground and movement with respect to the top of a 30,000 foot pole stuck in the ground is pretty negligible.

30k ft is nothing compared to the radius of the Earth which is 21M ft. The measurement instrument has larger error sources, so the answer is it doesn't actually matter.

https://www.grc.nasa.gov/www/k-12/airplane/move.html

ground speed is the speed of the plane, relative to an observer on the ground, air speed is how fast the people on the plane are going relative to themselves.

Airspeed = Ground Speed - Wind Speed (wind blowing towards front of plan is positive, tailwind is negative)

I’m a bit under the weather, so my comprehension may be lacking, but I think you’ve got your signs reversed (or your head/tail terminology flipped).

If tail wind (moving with the plane) were negative, then air speed would exceed ground speed. Intuitively, that doesn’t make sense. Air speed can only exceed ground speed if the wind is blowing in an opposing direction, i.e., the plane must travel faster against the wind to achieve the same speed relative to a ground observer.

Wind is "named" after where it comes from. If you fly north, and the wind is a north wind, you fly slower. (This terminology is in line with what GP said.)

Then, ground speed = air speed - wind speed.

Thus, GP seems to have either the sign wrong, or ground and air speed mixed up (but not both :-)

> Wind is "named" after where it comes from.

This is absolutely true, but has always infuriated me because it seems needlessly contrary as compared to the way we speak about anything else travelling in a direction as read from a compass. Does anybody know why it's the case?

Probably because it matters where the air was before. Stuff like: north wind is could, south wind is warm, west wind is humid, etc

Doesn't that very much depend on where you are though? E.g., with respect to the equator, major bodies of water and landmasses, mountain ranges, prevailing weather conditions. Also perhaps on seasons.

I can understand that it might be just an ancient tradition originating from a particular part of the world, that we've turned into a convention, but it loses meaning when you go elsewhere.

It rather depends on where you are, but so does language. The words we use were cast locally. If you speak English some words were cast in the Ukraine, some in Germany, some in Rome, some in France, some in Britain, but the people living in those places weren't obliged to make the words globally meaningful.

If you go to Cape Town, a "north wind" presumably doesn't mean cold, because the area north of the city isn't colder than the city itself. This didn't matter to the people who first said "north wind" and I don't think it means much nowadays either. Just another little thing to watch out for in intercultural communication.

Can you name anything else named after where it's going, not where it comes from? I can't think of anything named like that, but I guess there must be some.

Within a marine context, current is typically given as direction to while wind and waves are direction from.

Or when you're out walking you might say that you're "heading north". Granted if you wanted to report your rough position relative to someone else you might say something like, "I'm 500m north of you."

No you are right. In case of tailwind, it is KIAS + WS whereas headwind is KIAS - WS

KIAS: Knots Indicated Airspeed (i.e. the speed at which air is hitting the pitot tube) WS: Wind speed

Slight, but important (if you don't want to run out of fuel) nit, indicated airspeed (KIAS) is not the same as true airspeed (KTAS).

To calculate ground speed (as required for navigation and fuel planning) you need true airspeed (not indicated) as well as wind direction and speed.

See some discussion here: https://www.quora.com/In-aviation-what-is-the-difference-bet...

Not so slight. At airliner altitudes they can differ by 50% or so.

IAS is what the pilots care about while flying because it’s what matters for aerodynamics (ie stall speed)

> are going relative to themselves.

You mean relative to the air outside the window. Relative your yourself you're always standing still.

Most people on the plane were probably sitting still.

I'll show myself out.

For a given altitude, trim, and power, the airspeed is always the same no matter the wind. Wind only affects groundspeed.

Ground speed = true airspeed + wind component (where you're computing a vector based on wind direction to determine what amount of it is either tail wind (+) or headwind (-).

> For a given altitude, trim, and power, the airspeed is always the same no matter the wind.

If acceleration is zero.

(Internet, therefore pedantry).

> air speed is how fast the people on the plane are going relative to themselves

That makes no sense. How can people be moving relative to themselves?

relative to their own location Δt ago?

Location with respect to what?

Themselves meaning the ground observer as that was the point of view GP was speaking about.

The difference is about 1.1 mph (edit: out of 800 mph) at 30,000 ft compared to ground, so I don't think it matters too much.

The radius of the earth + 30,000 ft is about 0.14% more than just the radius of the earth. Basically ignore it, unless you are programming guidance control systems.

0.14% is enough to throw the number off by 1mph so it isn’t irrelevant.

lol, ok. Make that 800 MPH then.

It’s likely less than the reading error in most speedometers including those which are used for autonomous vehicles unless you are writing a guidance system for bombs or attempting to land on another planet that’s pretty negligible.

Given that the ground speed was likely drawn from gps data, which should have velocity readings more accurate than 1mph (particularly for an airborne gps unit), the choice of calculation method for ground speed does make a difference.

That's less error than slopes and oblateness introduce, so I think it is acceptable.

That is also an interesting question. If they’re measuring speed as if the airplane were at the surface rather than at 30,000 feet, is it speed along the WSG84 earth model?

This reminds me of a math puzzle I read many years ago. It went something like this:

"Assume the Earth is a perfect sphere, and its circumference is 25,000 miles. You have a 25,000 mile long rope looped all the way around the Equator, so it is tight against the surface. Now you want to raise the rope one foot in the air, all the way around. How much more rope do you need?"

I thought about this and said, "You need a lot more rope! I don't know how much, but it will be a lot. It has to reach all the way around the Earth, only higher!"

Of course the real answer is "You only need about three more feet. To be specific, you need another 3.14159... feet."

That's all you need to raise a rope a foot in the air, all the way around the Earth.

circumference = 2 pi r, so increasing the radius by 1 foot will increase the circumference by 2 pi feet, not pi feet.

Thank you! I had a feeling something must be off in my 50-year-old memory of this puzzle.

I'd better buy an extra 3-4 feet of rope before I try this experiment.

I had a science test in high school with a question that involved something to do with calculating the air pressure inside an airplane at some high altitude. I went up to to the teacher and said "is this a trick question? Airplanes are pressurized at that altitude" He looked around the room nervously and then told me to assume the airplane wasn't pressurized.

Ah yes, the notorious spherical cow!

https://en.wikipedia.org/wiki/Spherical_cow

A related one is to add a second section to the question asking how much extra rope would be needed to go round a tennis ball the same 1ft further out from the ball.

For someone who doesn’t know, or hasn’t calculated/thought about it that is pretty astounding.

It's measured by repeatedly taking GPS coordinates divided by time.

GPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface.

EDIT: assuming a constant altitude flight

>> GPS measurements are comprised of a 2D surface coordinate system

GPS is inherently 3 dimensional.

> and an altitude above mean sea level

Nah, you're clearly misunderstanding how GPS works.

GPS allows you to directly measure displacement of any 2 points in 3 dimensions.

You're speaking of the altitude + lat/lon instead, which is in polar coordinates.

There's some interesting stuff going on behind the scenes. GPS inherently gives you a 3-dimensional coordinate, which is useless for most applications. So the coordinate gets converted to a 2D surface coordinate (latitude/longitude) plus an altitude, which is what you generally want. However, since the Earth isn't a perfect sphere, this conversion is not well-defined.

The solution is a datum, a model of the Earth's surface that is used for the conversion. WGS84 is the datum used by GPS, and approximates the Earth as a specified ellipsoid. However, maps can use use different datums (data?), so there can be a discrepancy of many meters between what GPS says the location is, and what the map says the location is, and they can both be right relative to their datum.

No, GPS measurements are taken as receiver distance from orbiting satellites whose orbits are calculated in a 3-D+t inertial reference frame (ECI), which is then converted to an earth-centered-earth-fixed (ECEF) coordinate. From there an end user devices can transform to desired Datum reference (a model of the Earth's surface) depending on the application. Everything about GPS is inherently 3D+t.

Guys I am talking about how commercial aircraft GPS reports speed. I understand how it gets its fix from satellites; what it reports to the human is speed along the reference horizontal datum AKA "the ground."

If you fly from point A to point B in one hour at 10,000 feet, and then fly from point A to point B in one hour at 30,000 feet, GPS will report the same speed for both flights. It does not bother to correct for the slightly longer path at 30,000 feet because that level of precision is simply not needed for commercial aviation.

Ground speed is what most people would think of when they think of a speed. How fast is it moving from Point A to Point B.

Airspeed takes into account how fast you are going you are going AS WELL as how fast and in what direction the air around you is going.

Another example would be a person walking on a motorized walkway (like at an airport.) Person A is going WITH the direction of the walkway like someone normally would. Person B is going AGAINST the direction of the walkway but still making progress forward. How fast the person is going when observed from off the walkway is ground speed. To maintain the same ground speed, Person A needs to only walk slowly (less effort/lower airspeed) while Person B needs to be sprinting all out (more effort/high airspeed.)

* Here is an interactive example from NASA: https://www.grc.nasa.gov/www/k-12/airplane/Animation/airrel/...

kgermino is asking about the difference between (1) the actual speed (relative to the Earth) at the altitude where the plane is flying and (2) the speed of the path of the airplane when it is projected onto the ground. These are extremely close but not the same (~0.15%) because the Earth is a sphere and the airplane is at a slight further distance from the center of that sphere than it's projected point on the ground.
You are explaining the difference between airspeed and ground speed, which can be much more substantial (e.g., ~10%, or ~%100 in extraordinary situations).

Private pilot here. As others have pointed out, the difference is mostly negligible, but ground speed is the speed of the vertical projection of the aircraft's location on the earth's surface.

So essentially the speed at which the plane's shadow on earth travels while projecting on level ground.

Mh no, the shadow's position depends not just on the planes movement, but also on earth's rotation and trajectory.

Is that not also true of the plane's projection onto the Earth's surface?

I think "trajectory" refers to the position of earth relative to sun, which affects the shadow but of course has no effect on the projection.

I don't think so, since projection would be like taking the line between the center of the earth and the plane and finding where that intersects the surface of the earth. That doesn't depend on the trajectory of the earth through space.

Whoops, I totally ignored the trajectory aspect and was just thinking both would be affected by the rate of rotation of the earth

No. If the sun were always directly overhead this would be correct, but it isn't, so it isn't. The difference depends on the angle of the sun. In the middle of the day it's negligible, but at sunset and sunrise it becomes very large.

In the old days, you'd take the time between two "fixes". A fix could be a visual reference, or a NAVAID. That'd result in your shadow's speed.
Then came DME, distance measuring equipment. I don't know if DME ever took altitude into account or if the error resulting from altitude was just ignored, but analog DME radios would just tell distance at first and then digital versions had a groundspeed feature. At least instrument rated pilots were aware of the increasing error the closer to the NAVAID.
At some point in the ATC radar system, a controller's screen could show groundspeed. Pilots can ask ARTCC what ground speed is, although I would never ask approach or departure control such a question. This would have the same error as DME.
Today it's a value reported by GPS. I don't know if this is based on the plane's movement, or its shadow, or if there's a regulation that dictates it for aviation certified GPS. From a pilot perspective we don't distinguish between kinds of ground speed, whereas we do distinguish between kinds of airspeed: indicated, calibrated, and true. Mainly groundspeed is important navigationally to know if there's an unexpected headwind that'll lengthen enroute time and thus impact fuel management. And while there can be big enough error between estimated winds aloft and actual that this computation will affect enroute planning, I tend to only care about tens of minutes or hours, not seconds.

DME equipment explicitly does not take altitude into account.
It's colloquially called "slant range" for this reason, and if you care about actual ground distance you have to use trigonometry.

Ground speed is measured by GPS on modern aircraft.

Which is probably measured in it's trajectory and then recalculated as ground speed, which is sea level? Wonder if it accounts for sea levers being higher at certain parts of planet?

This makes me think that some time in the distant future, when we're really great a terraforming, we could influence the weather of the planet specifically to create functional jet streams that we could use to make travel faster, and then plan air routes specifically to use them.
A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspeed to avoid stalling, right?

Since they arrive faster they will spend less time in the air. Less time in the air means less fuel burnt. They are probably still burning fuel at a similar rate though.

> A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspeed to avoid stalling, right?
If a plane maintains the same airspeed, but gets extra ground speed thanks to a tailwind, it'll complete its journey in less time and therefore should save fuel. (Unless it ends up having to circle the destination airport while it waits for its original landing slot!)
Of course, planes going the other direction will use extra, so overall we don't win.

Just make a jet stream going the other way at a different altitude

Or just fly with the jetstream in all cases, all the way around the earth if necessary. Some trips would be quite long, but think how much fuel they'd be saving!

A 200 MPH tailwind doesn't compensate for a 20,000-mile-longer flight.

Plus, what airline is going to want to pay out those miles?

> Of course, planes going the other direction will use extra, so overall we don't win
Aren't some(all?) long-haul routes chosen on a flight by flight basis to take advantage of favourable winds where possible?
Compare yesterday's eastbound BA11 (LHR-Singapore) vs the westbound BA12 (Singapore-LHR) flights.[0] Neither route looks like a great circle.
Eastbound routing: London - north of Berlin - Minsk - Voronezh - Volgograd - cross Caspian sea - Turkmenistan - Lahore - New Delhi - KL - Singapore.
Westbound: Singapore - KL - south of Jaipur - Iran - just touched Turkmenistan - south of Baku - Tbilsi - Turkey (just) - south of Prague - south of Dortmund - LHR
[0] flightradar24.com or similar

Yes, most of the time. There is actually Winds aloft data that is made available to pilots to select routes. And in lots of cases IFR routes are designed keeping these speeds in mind.

Typically yes. Right now there is a general avoidance of Ukrainian (Crimean and nearby) and Syrian airspace so the flight plans will take that into account, the winds aloft, and the airways available.

> Of course, planes going the other direction will use extra, so overall we don't win.
The jetstream is small. You can avoid it and not have to pay extra. So yes, it can be an absolute win.

> Of course, planes going the other direction will use extra, so overall we don't win
It's even worse than not winning. You actually lose if you have to fly in the same wind both ways, once as a headwind and once as a tailwind. The extra time in the into the headwind direction is more than the time saved in the with a tailwind direction.
If the ground distance each way is D, airspeed is V, and wind speed relative to the ground is w, total time is D/(V-w) + D(V+w) = 2DV/(V^2-w^2) = 2D/(V-w^2/V).
For 0 < w < sqrt(V), this is more than the 0 wind case. (For w >= sqrt(V), the headwind is so high that you can't make any progress toward the destination).

Just a small fix for those that were checking as I was
If the ground distance each way is D, airspeed is V, and wind speed relative to the ground is w, total time is D/(V-w) + D/(V+w) = 2DV/(V^2-w^2) = 2D/((V^2-w^2)/V).
For 0 < w < V: this is more than the 0 wind case. For w >= V: the headwind is so high that you can't make any progress toward the destination).
The last result being somehow obvious ;)

Where it's possible they use a jet stream route going one way, and a great circle route on the return.
So yes it's a win, but a little more subtly.

If I'm in a tailwind of 100mph and have a cruising speed of 200mph, then I'm essentially travelling at 300mph relative to the ground, but using as much fuel as if I were travelling at 200mph. On the return leg, I would hit a head wind, and be travelling at 100mph while still using as much fuel as if I were travelling at 200mph.
So yes this does save fuel.

It saves fuel for the downwind leg of the trip.
However, it's worth noting that if the outward (downwind) and return (upwind) legs are affected by the same wind, the outward savings are more than offset by the extra fuel needed for the return. Suppose the journey in your example is 200 miles. With no wind, that's an hour each way, so 2 hours total cruising time.
With the 100mph wind, the outward journey only takes 40 minutes (200 miles at 300mph), but the return takes 2 hours (200 miles at 100mph) for a total cruising time of 2:40, and a significantly greater total fuel burn.
(In practice, airlines may route the two legs quite differently to optimize better, taking advantage of the tailwind in one direction while avoiding it as much as possible when going the other way.)

I distinctly remember my CFI in flying school brow beating it into us rookie pilots that "what you lose in a headwind, you never regain in the tailwind on the way back", and explaining it to us long handed on the blackboard. Seems illogical, but the math stacked up as in your illustration above.

My standard way to think about stuff like that is to extrapolate to infinity: even if your tail wind were infinite and you’d arrive in 0 time, your way back would have infinite headwind and you’d never make it back.
Conclusion: increased wind increases the round trip time.

I know this principle quite well from... riding a bike. Wind resistance dominates as soon as you get above a casual riding speed.

On a round trip, with a constant wind velocity, you use extra fuel overall - consider the extreme case where the wind speed is equal to the cruising airspeed.

No, it saves fuel. They're probably going faster AND using less throttle. Cruise speed is WAY above stall speed.

No. When you are flying high (at the route flight level), air becomes less dense and the stall speed increases a lot (expressed in true air speed). When a flight is at its highest available flight level, the range of available speeds is very narrow.

1. The plane has to fly sufficiently fast (to avoid stall), and sufficiently slow (to avoid "Mach tug"). As one flies higher (air density sinks), these two speeds converge, forming the dangerous "coffin corner". Now, in general, planes don't fly as high as possible, I'd say, so that there is some leeway in terms of chosen airspeed.
2. For given airspeed, the plane flies less time with a tailwind than a headwind.
So, this does save fuel, I'd think.

You have a typo here (it's "Mach tuck").

Oh thanks! I seriously always read it as your nose being pulled/tugged down...

Reading your comment, I thought "how bad is a high-altitude stall anyway? it's obviously undesirable, but what makes it dangerous? possibly entering a flat spin?"
Doing a little digging, it seems that there's also a significant risk of entering an overspeed condition during recovery, and inducing a structural failure! Makes sense considering that we're talking about speeds near Mach 1 to begin with.

Air France 447.

That's... a bit different? AF447 is a classic case of human factors in aviation accidents. The stall would have been completely recoverable if the pilot had nosed down.

True, but it started as a high speed/high altitude stall and ended because of poor pilot choices.
It was in and out of the coffin corner before the pilots realized what was happening.

> "coffin corner"
I watched a TV programme a couple of years ago talking about this on the U2 as the speeds were only a few knots apart and also varied with altitude and other factors.
The image at https://i.stack.imgur.com/Lu2Xt.jpg shows the airspeed corridor the pilots have to fly through. It's constantly flying on a knife edge.

This makes me think that some time in the distant future, when we're really great a terraforming
We're great at accidental terraforming now, we're actually warming the temperature of the planet.
But based on global reaction to the current terraforming I'm skeptical that we'll ever reach the point to where we can intentionally terraform the planet.

Where air masses, that are travelling at different velocities, meet you get shearing. Shearing causes turbulence. So even if we could create two jet streams moving in two different directions the turbulence in the zone where you transfer between the two would probably be too severe to actually fly through it.

For those of you who find this interesting, there's a whole site dedicated to this: https://groundspeedrecords.com/
801 MPH is not even close to the record, even for a 787.

865mph in a 744 is something.

Interestingly the National Weather Service in Wilmington, OH posted about the jet stream and its affect on their weather balloon yesterday [0] (please forgive the link to Facebook)
[0] https://www.facebook.com/NWSWilmingtonOH/photos/a.1920894441...

So what if they suddenly left the jet stream, for example flew a bit below or besides... If that's possible... and happened to be in normal wind speed, the plane would disintegrate due to sonic boom?

It wouldn’t disintegrate due to a sonic boom, because this extraordinary speed is due to the plane being helped along by tailwinds. If the tailwinds disappear, the plane slows down; it doesn’t speed up.
What I’m not clear on is, would the passengers feel like they just decelerated by 200+ mph in an instant?

But doesn't the plane have any inertia?

Yeah. Good question. It would really seem to depend on how fast the air speed changes.
I mean, the opposite effect is well attested to - you fly along with a headwind (and sufficient airspeed), then comes along a sudden tailwind, and your airspeed drops sufficiently that you stall, or at any rate descent. Wind shears, often associated with cold fronts or thunderstorms.
Maybe a) mach 1 is further from cruise speed than cruise speed from stall speed, and b) drag slows down the plane more effectively and quickly than the engines can accelerate it.
That could explain why wind shear has and does lead to stalls, but not to sonic booms/structural disintegration.

Fluid dynamics would spread that deceleration over a short period of time, so it wouldn't be like 'hitting the brakes in a car' type sensation. Passengers may feel a slight shift, but overall, as you explained to the previous poster, the speed is all relative to the air around the airplane.
More likely the turbulence and vortices in the demarcation zone between the jetstream and the different air current the plane enters would jostle and toss the passengers around more than the deceleration would.
From the article, it said the 787 was cruising at 35,000 feet. Seeing as the 787 was built to cruise close to 40,000 feet (above most traffic and weather) I daresay the pilot or company flight planners deliberately asked for a lower altitude based on expected wind conditions. I bet many other airlines were doing the same, leading to a very busy jetstream highway that day!

The jetstream, like all weather and winds, is on a continuum. There is no barrier where outside of the jetstream the air is static. See the principles of flight and how air functions VERY close to a wing for an example. This is actually a good example, though a bit unrelated.
https://news.ycombinator.com/item?id=13829625

It clearly must not be a dramatic effect, as the plane must have left the jet stream in order to land :)

I would expect the jet stream not to end abruptly, but there is likely a gradient you have to cross before reaching "still" air. When leaving the stream, airspeed will increase, because of inertia, and so will drag. This will quickly bring the plane back to its cruising speed.
I had a debate with a friend on this one the other day, but I am pretty sure something like 80% of the engine power is there to combat drag, which increases with v² (if you discount lift from that, otherwise it' obviously 100%).

When cruising, in level flight by definition, 100% of the engine power is there to combat drag, right?
That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.

Depends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up.
The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce.
That's what the GP meant by "discounting lift".

There are two types of drag, parasitic drag and lift-induced drag, but they are both drag.

Some of that thrust has to be used to generate lift.

If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).

That's just semantics; you can split the drag generated by the wings and fuselage into different components. Of course, 100% of a constant-speed plane's thrust is necessarily used to counter all the other forces acting on the airplane :)

The engines provide the energy to keep the plane flying. It must overcome two forces: Drag and gravity.
The energy requirements are split between those two.
If the plane is not generating lift then it's on the ground or about to end up in the ground.

After some clarifications from replies to my sibling post, the answer is "definitely more than 50%" because the lift-induced versus parasitic drag curve is at its minimum when the two are equal, but the efficiency curve is a relatively flat-bottomed bathtub shape, so the cruising speed is selected to be faster than where this point is as you get significant reductions in travel time for relatively small reductions in total efficiency when you are near the 50% point and increase airspeed.

Google for Clear Air Turbulence.

A lot of the times, when you hit turbulence at altitude in an airliner, it's because of this. If you watch the map display closely on your seatback you can actually see it happening: you'll feel a big bump or jolt and then your speed readout might be 50mph less and your altitude might be 2000 ft less.
As other people mention, it doesn't happen immediately - it's like merging onto a very bumpy freeway.

>The ordinary cruising speed of a Dreamliner is 561 mph, with a maximum propulsion of 587 mph. Any speed gained on top of that is thanks to Mother Nature's helpful boost.
Wow, cruise speed is that close to 'max' speed? Or maybe that's just a max suggested cruise speed.

I'm not an expert. But my understanding is that the "maximum propulsion" speed (587 mph) is limited by the "critical mach number" of the airframe, which for jetliners is something like 0.85.
Faster than that, air around portions of the plane may start moving faster than the speed of sound (even though the airframe as a whole is going slower than the speed of sound), and this wreaks havoc in terms of lift, drag, controllability, and structural integrity ("mach buffet").
I think cruise N1 is something like 80% (that is, of full power). If you pushed the engines to 100% power at cruise altitude, they could probably get you past 587 mph / mach 0.85, but you'd be having a really bad day.

When I was getting my pilots license I stumbled into a really cool video NASA did showing exactly what happens when you reach a fluttering condition like what you described. This is a link to the NASA recording if anyone is interested.
https://www.youtube.com/watch?v=pEOmCkZyXzk
https://www.youtube.com/watch?v=OhwLojNerMU (Older with music, kinda funny old-school science video from NASA)
Edit: formatting.

Similar video with a dg300 17m glider (modified by the akaflieg).
https://www.youtube.com/watch?v=qhJ1tVTtfPw

In practical terms, flutter due to approaching transonic conditions is probably pretty rare. That kind of flutter is (mostly or most often) due to the interaction of normal shocks with the control surfaces, or coupling with an elastic mode of the wing or tail surface.
The flutter in the first video is occurring at very much subsonic speeds, and looks to be either the result of flying a purposely underdesigned tail surface, or flying a properly built one beyond its rated flight envelope. The second video contains a wide variety of flutter instances, some of them aeroelastic, some of them transonic, and so on.
One way to get into a transonic flutter, however, is to be a hotshot business jet pilot who flies higher and higher and faster and faster. The higher you go, the lower the density, so your minimum speed increases. Also, the temperature goes down, so the speed of sound decreases. Where these two meet is called "coffin corner," and you don't always have to fly yourself into it by increasing your speed and altitude; you can fly close to coffin corner, and then fly into colder air or less dense air. No matter how you get there, you're stuck. Slow down, and the wings stall, the nose drops, you pick up speed, and hit transonic flutter. Speed up to stay in the air, by dropping the nose, and you hit transonic flutter directly.

Interesting.
Do you have any thoughts on composite-based aircraft like a Cirrus and flutter? A few times I've had a Cirrus SR22 into a pretty steep descent with poor controller sequencing for an approach into busy terminal space and had to push it down, but the plane felt solid even at 180-190kts TAS. I backed it off only because I get nervous with any unexpected turbulence which is not uncommon in Florida.
The Piper Saratoga I flew for a bit didn't seem to like the speed as much, that or the toga was a bit more vocal than the Cirrus in what it was feeling with regards to airspeed.

Only in the most general terms, and from first principles: Composite structures will have a higher stiffness per unit mass, which will cause the fundamental frequencies to be higher both for the pure structural modes and the control surface interaction modes. It's therefore likely that you'd only begin to encounter these aeroelastic modes at higher speeds. In other words, if you take the driving frequency as something like the inverse of the time it takes for air to pass over the wing, that may match the metal wing more closely than the composite wing. Modeling that stuff in a wind tunnel is very tricky, and you used to end up with these not-at-all-realistic-looking models that, nonetheless, captured some aspect of the full-scale aircraft being modeled.
Don't die. Stay in the envelope. Flutter is only the quickest way to ruin your airframe and day, not the only one.

An incredible visualization of winds aloft (and several other parameters can be seen at https://earth.nullschool.net/. Currently the jetstream (i.e. winds at 250 hPa altitude) are hitting around 230mph (360km/h).

When I first heard about global warming, the physicist in me said, hmm, the major effect of this will be energetic weather systems will become more energetic.
Now put that together with my personal experience of extreme weather events both in NZ and ZA, the jet streams has _always_ been involved.http://squall.sfsu.edu/scripts/shemjetstream_model.html
ie. If I see a major stream coming near me... Big Stuff happens with the weather. If something extreme is happening with the weather, I check and a big stream is going very near by.
ie. I expect we will see more interesting stuff happening with the jet stream as the climate cooks.

Apparently, climate change brings more irregularities to the jet stream
https://insideclimatenews.org/news/31102018/jet-stream-clima...

Is this dangerous for the plane if the wind hits it sideways or something? Does this mean the flights will be extremely bumpy?

No it's not. The airplane just has to do wind correction by either crabbing into or away from the wind depending on the wind direction.

If you stay within the jetstream, it is a surprisingly smooth ride - a bit like being in a canoe in a middle of a fast flowing current in a river. Jetstreams tend to be fairly long and continuous.
Where it gets bumpy is if you are transitioning in or out of the jetstream zone - a bit like the abovementioned canoe hitting some rapids in a shallow part of the river.

I remember once being on an almost empty flight from SFO to ATL a few years ago. I checked the flight information on the seatback screen in front of me and was startled to see we were going well over 700 mph ground speed. That was pretty much the quickest flight I’ve ever been on; I think the whole trip was well under four hours.

Would it be nice? Flying is already a tremendous environmental burden with the focus on fuel economy. I'm happy to sit in my seat a few minutes longer if it means taking away some of that emissions impact

For me on a trip right now, I have:
2h drive to ORD 2h airport security Theater 4h Flight to SFO 1h to deplane get an Uber to someplace
So 9 hour total journey on a direct flight. A 50% faster flight would make it be 8 hours instead of 9? I don't think I would pay anything for that.

2h airport security? In Zürich im usually done in 10 or 15 minutes...

My experience at ORD is that it's usually done in under 45 minutes, but takes longer often enough for me to budget ~90 minutes for security to be confident of making my flight.

Then you aren’t properly groped for freedom.

Well, parking to at my gate.
If you park at the airport - It's about a 20-30 minute journey from parking lot to the front door of the airport. If parking offsite it can even be longer.
Then going through TSA, even with pre-check, can be from 20-60 minutes. 20 minutes is about the minimum time, just due to how BIG the airport is. You can end up having to walk a mile door to gate. I can run a mile pretty fast, but that is generally frowned upon.
And if you need to somehow deal with an agent for checking bags or anything else? That can easily be another 30 minutes down the crapper.

I was in ORD a week ago and went from my hotel (Hilton, connected to the airport) to my gate in under 15m. I think my time going through security was around 45s, give or take. Anecdata caveats apply.

Yeah. I flew from JFK to Glasgow in April of last year and IIRC we arrived around 2 hours early. It was early enough that there were no passport control agents around, and they had to be called in.

The tweet from Peter James: https://twitter.com/jetpeter1/status/1097682898013044736

Last time I flew from SFO to MUC, we had almost 100mph tail winds, that was already very impressive and shortened the flight time considerably, but nothing compared to this.

Gee, maybe in an extreme case, to fly from NYC to Tokyo, fly east out of NYC and take the jet stream and save time and fuel!

That is 1289.085 kilometres per hour! The title should be updated on to include a measurement used by the rest of the world including the scientific community.

That would be editorializing.

They have already editorialised by converting the speed from knots
Anyway, still doesn't set an unofficial record by variant or type:
https://groundspeedrecords.com/wof-top-3/?aircraft-manufactu...
Absolute fastest I can find on that site is a 747-400 at 752 knots / 866 mph but I do have a memory of one of that type breaking 800 knots over the Pacific within the last few years.

That site only includes submissions where the pilot took a photo of the instrument panel, so yeah, definitely not the true top speed record as I’m sure that not every pilot takes a photo, let alone knows of the site.

If you're being pedantic about science-related matters, it's 1290 km/h. Don't give false precision with your conversion; there's no way the original measurement has >3 significant figures.
Even better yet, 361 m/s. km/h is a derived unit. ;)

You’re right I did just paste that figure from Google conversions.
Using the metric system over the imperial system isn’t being pedantic, it’s just courtesy in 2019

It's from a USA-based newspaper. I hate imperial with a passion, but it is still their backarsed standard. Surely anyone with a scientific bent can do a rough calculation in their head. For me, it was "oh, around 1300 km/h, cool" and moved on...

In many countries, miles are as obscure to people as leagues, perches and gills probably are to you.
In others, the word refers to a significantly different measure. For example, Norwegian and Swedish people often use the word to mean 10km, and can misunderstand a phrase like "it's just five miles outside London".
There's a list of possibilities here: https://en.wikipedia.org/wiki/Mile

If you want to get pedantic then you and the article are both wrong! Navigation, marine and aeronautical, is done in Nautical Miles, called Knots when used to measure speed. 1NM used to be exactly 1 minute of latitude but is now defined as very closely approximate to this as earth is now understood to be not a perfect geoid. There are 60 Nautical Miles to a degree of latitude. This allows distances and courses to be measured and plotted easily on charts which are Mercator projections.

The aircraft is rated in knots equivalent air speed. Everywhere but China and Russia tend to call out speeds in knots. If anything, it should be in knots. So, about 696 knots.

Does the Supermoon affect the Jet Stream?

Ground speed

Isn’t that the one that truely matters some you want to go from A to B?
Genuinely curious I know nothing about aviation.

Yes but it won't make a sonic boom.

If the wind is unstable, and slows down, can't the plane break it just out of inertia?

Not really - the airplane would still be doing around 500 knots (mph) relative to the air around it. The only way the scenario you mention could happen is if the airplane hit a sudden headwind of around 300 knots in an instant.
Fluid dynamics basically means that it is impossible for any parcel of air to have an instant demarcation from nil wind to 300 knot wind without going through a (very very turbulent) transition layer. And 300 knot winds?? Maybe routine on Venus or Jupiter, but very very rare if not nonexistent on Earth.

I suspect that a 787 traveling at 801 MPH airspeed might suffer some damage to the control surfaces. 787s aren't designed to break the sound barrier.
And I would argue that not damaging the plane matters more than how long it takes to get there.

Just an interesting fact that the 787 has been tested to m0.97 in a dive, pretty impressive! But not near 801 MPH airspeed.
https://web.archive.org/web/20110408073905/http://www.flight...

Yes, for those readers who may not be familiar - the speed of sound changes with altitude. At sea level, it is around 750mph but at 40,000 feet it is around 620mph IIRC. It is affected by the density of the air (which is a combination of the air pressure and temperature) - hence why they use Mach numbers to denote airspeed above certain altitudes as that takes into consideration the air density to give you a speed relative to the speed of sound at that altitude.

> [The speed of sound] is affected by the density of the air (which is a combination of the air pressure and temperature)...
This is incorrect. The speed of sound in an ideal gas is a function of temperature only, and not its density. Namely, it is sqrt(gamma x R x T), where R and gamma are unchanging properties of the gas.

Thank you. I stand corrected.

It's travelling at 801mph relative to the ground.
If I'm swimming down stream at 1 mph in water moving at 4mph, relative to the bank I'm doing 5mph relative to the water 1mph.
No sonic booms where involved.

I'm quite well aware of that. I'm not sure how you read my post as saying anything different.
boyter asked how anything but time from A to B could be relevant. Well, not damaging the plane is relevant. And therefore the airspeed not being 801 MPH matters.

Yes, correct!
And because the wind is not constant, airline dispatchers will generally try to pick the route and altitude with the most favorable wind direction and speed.

Yes. Ground speed is basically airplane speed + wind speed.

<pedant mode>Technically it would be airplane speed + wind speed vector for the same direction the airplane is heading.</pedant mode> :)

Hey, I said 'basically'! :P
But yes, you are correct. However, the person I was replying to already said they were ignorant of this stuff, so I didn't want to get too crazy.

So why do planes get slowed down by a jet stream? Can’t they simply stay out of it?

can any pilot here please comment on the danger of this?

> danger of this
Essentially zero.

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