I’m surprised they didn’t slow down to conserve fuel and arrive on-time. In the USA, if your flight is more than 20 minutes early, you are usually stuck on the tarmac waiting for a gate.
80 minutes of fuel for that plane must be very expensive. Is it possibly to fly that plane, at say 200 knots ground speed, and save on fuel? Or are they stuck at their speed they filed in the flight plan, to keep traffic separation?
I once took a Boston -> London flight that arrived significantly early thanks to the arctic jet stream. But that meant we arrived before the landing curfew ended at 6am so the aircraft was put in a holding pattern above central London for 30 mins or so, and I flew more or less over my home in East London a few times (I could see it out the window as we flew overhead).
We landed a few minutes after 6am, and by the time I got through immigration and customs and rode home during the morning rush hour on the tube it would have been 8 or 8:30am.
Super frustrating that the last mile took 3 hours to complete!
Back in 2002 a flight I took from Boston to London arrived into LHR airspace about 45-50 mins earlier than it should have due to a decent tailwind from the jetstream. Sadly we stacked for an hour and a half and ended up arriving later than the scheduled arrival time....because fricken Heathrow bottlenecks, and we almost missed our connecting flight back to Scotland. Grrr.
It doesn't always work to go slower. The speed of least drag is an airspeed, not a ground speed. And commercial jet engines are most efficient around 90% thrust, not at lower settings. So going slower can make you less efficient.
Can't really tell without knowing their weight and available cruising levels. But in general you should not run a commercial jet engine at lower thrust settings. The optimum altitude as calculated by the flight management system is essentially the altitude at which 90% thrust gets you the best cruise speed.
That's also why flying low is really bad for fuel consumption, you're going to have to decrease thrust and you'll be aerodynamically less efficient at the same time.
I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow?
In general turbine engines are limited mostly by the temperature at the first stage of turbine blades after the combustion stage. It's hard to get reliable instrumentation in this zone so a proxy temperature is often taken from a later stage.
In older airliners there was a table of max ITT values varying with altitude and outside air temperature. In modern FADEC engines the computer does the same calculations. It's not unusual for smaller jets to be set at their max continuous thrust setting shortly after takeoff and be left there until the beginning of the descent. Modern airliners will be flying a cost optimised Mach number.
That Mach number is entered on their flight plan and is used as a basis for ensuring they are separated from other flights on the north Atlantic tracks so they wouldn't be able to change it readily.
The interesting part of all of this is that they would have been assigned an altitude for the crossing, at high altitudes maximum speeds drop (because flutter margins are proportional to velocity rather than effective pressure) but stall speeds increase so the range of viable flying speeds is actually quite limited at cruise altitudes. They probably couldn't have slowed down if they wanted too.
Yes, but to the previous posters point, turbofans operate much less efficiently at lower altitudes. They are optimized to run the majority of cruise at a very specific set of altitude and power output settings. This is compounded by the fact that fuel load also assumes a set of assumptions of the flight plan
Aircraft performance metrics of all sorts are generally U shaped. There will be an optimum altitude and airspeed for a chosen regime and deviating from it in either direction will result in lower performance.
So if you are shooting for miles per pound of fuel, there is a best. You will lose total trip fuel if you throttle up and go faster and you will lose total trip fuel if you throttle down and go slower.
Air resistance goes up by the square of the velocity, so it takes more and more thrust to go faster. The most efficient thrust per unit of speed is therefore somewhat less than the most efficient thrust per unit of fuel.
I've only had some college physics and no aeronautical engineering so I could be way off. Of course there are other factors like the amount of lift per unit of velocity and so on...
> Air resistance goes up by the square of the velocity
This is true of form drag. Airplanes are also subject to a somewhat counterintuitive induced drag that is inversely proportional to airspeed. The minimum total drag is therefore somewhere between a slow speed and a fast speed.
There are three optimal speeds depending on what you're trying to optimize. Maximum range (distance per unit of fuel) is best glide speed, which would be a painfully slow way to get somewhere. Maximum endurance (time per unit of fuel) is roughly max endurance divided by 1.316—even slower. "Optimum cruise," or Carson's speed (max speed per unit of fuel) is roughly max endurance times 1.316.
> the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph
> "In the USA, if your flight is more than 20 minutes early, you are usually stuck on the tarmac waiting for a gate."
I've experienced this in the US and Canada too, but it doesn't seem to be a problem at Heathrow in my experience. LHR is slot constrained, but not gate constrained. Worst case, you end up on a remote stand with a bus ride in to the terminal.
Agree that it’s a pain when it happens (the gate delay) but Heathrow closes at night so these are some of the first flights to land so it might not be much of a problem.
All the morning (evening departures) transatlantic flights I’ve been on west -> East have arrived suspiciously early too so I suspect there’s some schedule padding going on...
They schedule the flights months out. The know how fast the jet stream is going maybe a couple days out. So yes, schedule padding to make sure a weak jet stream doesn't cause people to miss connections.
I asked similar question about angle of attack (both 737 max and AF447) - can't they just look at a glass of water to see they are at wrong angle.
But with planes it doesn't work that way - everything is relative to the air outside, not the ground. With good tailwind you still have to go with your cruising speed relative to the tailwind. Your ground speed will be insane. The opposite is with headwind.
As other comments have mentioned, a glass of water tells you nothing about the conditions outside the plane, which are the ones that matter.
But also, it is sometimes useless at telling you about the conditions inside the plane too. If you're rolling and yawing at the same time (known as a coordinated turn), the water in the glass will stay level (relative to the bottom of the glass), and the glass will not slide along its surface, even in a 20 degree bank.
But if you’re in straight and level unaccelerated flight, and the wind outside has no vertical component, the glass of water would allow you to deduce your angle of attack (namely, the angle of the water plus the angle of incidence, if I’m not mistaken).
Of course, that doesn’t help you at all during dynamic or unusual situations.
No. Angle of attack behaves in some very non-obvious ways-
For example, if you maintain the same attitude, but decrease thrust so that your forward airspeed drops, and you begin descending... your angle of attack increases, despite the aircraft not actually rotating.
Why? When you start descending, the air below now seems to be "coming up at you"- you go from
---> (===
to
7 (===
/
/
if that ASCII art helps at all (equals signs being the wing)
Now consider how roll, yaw, and crosswinds affect this- as the AoA becomes variable across the wing...
Doesn't work. The stall is about the angle at which the airflow meets the wing. The gyroscope (with or without glass of water) will tell you the attitude of the plane, but nothing about the direction from which the air meets the wing.
You can stall in a nose down attitude if you're descending fast, the air comes from "below" and you try to pull out ift the dive thereby increasing the angle at which the air meets the wing. All the while your nose is still pointing below the horizon.
Not only that, but Heathrow I understand has heavy curfew / noise fines for arriving before 6am. Maybe to the tune of several tens of thousands of GBP. I figure you would wait to takeoff just to avoid those -- but maybe equally on the departure end they're paying for occupying a gate...
There is also the question of relative airspeed. Planes need to create enough lift to stay in the air. With a large tailwind they need to go faster to keep enough air flowing over the wings thus higher ground speed to keep the same relative air speed.
This is a common misconception, but very much incorrect. I am a private pilot.
When an airplane is in a body of air which is moving across the ground, the airplane moves within that body of air with no knowledge that it is in a strong wind. Airspeed is unaffected, except for gusts or shear events. The airplane maintains it's normal airspeed within that body of air, even though that body of air is moving very quickly in relation to the ground.
This is evident in all phases of flight. For example, landing into a strong headwind does not change the approach speed required or the thrust required to obtain that approach speed. The only allowance is for gusty conditions, during which a gust will momentarily affect airspeed due to the inertia of the airplane. The heavier the airplane, the more time it takes a gust or shift to defeat the airplanes inertia.
I'm confused. Are you saying that it'll maintain its normal ground speed despite flying through fast-moving air? Or that it maintains the same air-speed?
The former doesn't make any sense, but the latter is what the GP was saying.
The original comment seems to mean, that the plane has to (actively) keep a larger ground speed in high winds while the reply meant that this just naturally happens as the plane is only "aware" of its relative air speed.
The ground speed will vary, but the airspeed will not be affected by wind (except momentarily for gusts or shears).
The poster I replied to was implying that due to the strong tailwind, they had to maintain an increased airspeed. That is incorrect. The airplane has no idea it is in a tailwind, and airspeed will be unchanged.
I think the root of disagreement here is that to you, as a pilot, "speed" means airspeed. But to a layperson, "speed" means groundspeed. So saying that an airplane with a tailwind "must go faster to keep enough air going over the wings" reads very strangely to you, as (although it is technically correct) it belies a groundspeed-first type of thinking that is a poor intuition for staying alive in a plane.
I think the confusion here is stemming in part from the fact that "relative airspeed" isn't a well-defined term. In aviation airspeed means the speed of the airplane through the air (modulo specifiers for true vs. indicated vs. calibrated airspeed).
The original was "relative air speed", which I would simply take as the way a person not familiar with aviation terminology would say "airspeed"--speed relative to the air.
I did. Saying “everyone” when it’s just probably one person is not usually ok. It’s fine to say if you interpreted the comment in a certain way, and ok to be the outlier. But assuming your interpretation applies to others more broadly than it does, is counterproductive.
Both comments were significantly downvoted at the time of my comment, which was what I was referring to. The original comment explicitly mentioned "relative airspeed", which I thought was pretty clear (it's the speed of the plane relative to the air that matters).
I think calling it a "common misconception" is also assuming a lot.
I teach people how to fly. Trust me when I say that this is an extremely common misconception. It's one of the concepts students have the greatest difficulty wrapping their heads around.
The first answer is correct, but I think the misunderstanding is because it can be read as the higher ground speed being the cause for the same relative air speed. Whereas it’s technically the opposite.
This is further compounded when they say “they need to go faster”, as though the pilot needs to push the throttle harder. In actuality, the pilot doesn’t do anything differently to maintain their normal cruising speed.
The correct phrasing would have been the relative air speed and high wind speed combined, to result in a higher ground speed.
And Heathrow has some sort of "AI" system for scheduling landings and take offs. Even telling pilots of aircraft exactly when they can start their engines and when to start to taxi.
It was a great documentary I think a math program it wasn't all about the Heathrow system.
That's something pretty much all airports do, just that normally the air traffic controller schedules that. With an airport the size of Heathrow and at capacity, they just use more automation to optimize for throughput and reduce fuel consumption.
In Europe, a lot of this is done by EUROCONTROL's Central Flow Management unit. They'll issue a "Calculated Take Off Time" and the ground/tower controller at the airport will get the plane to the runway at the correct time to meet the slot.
There were extreme wind conditions on that day and the wind was expected to get worse through the day. The pilots wanted to arrive as early as possible to beat the storm.
Source: I was on a Norwegian flight at exactly the same time and the pilot explained this logic to us and also mentioned that we were on pace to break this record. Unfortunately, the pilot couldn't land in the wind conditions and we had to divert. Good thing we had that extra fuel too as the diversion was to Copenhagen which is quite far.
Same in Europe, it's happened to me multiple times. It really riles people up for some reason, with people just flat out ignoring crew orders to remain seated etc.
They probably couldn’t slow down at that altitude without stalling. The window of possible air speeds at that altitude is rather small. Further, fuel efficiency would drop off dramatically too, but I feel the major contributing part is likely the stall speed.
Good question. Maybe (just speculating here) because it would be dangerous to land with too much fuel on board? Planes dump fuel for that reason when aborting shortly after takeoff, but that may only be a problem with tanks close to full.
After a transatlantic flight it's highly unlikely you'll land over your maximum landing weight. That situation usually happens only when they have to land within an hour of takeoff.
AIUI you can only save fuel by not loading it, they can't land with substantial fuel remaining onboard without damaging the airframe. They'll just circle the destination to use up the excess fuel.
80 minutes of fuel for that plane must be very expensive. Is it possibly to fly that plane, at say 200 knots ground speed, and save on fuel? Or are they stuck at their speed they filed in the flight plan, to keep traffic separation?