Flying faster because of decreasing winds
 

Guys,

flying the 737. On final approach you have at 2500ft AGL a wind ahead with for example 35kt. The wind at touchdown zone will be the same direction with only 10 kt. thats only an example. so many guys say you can fly a constant ground speed. but how? do i have to fly a faster indicated airspeed at the upper level?

Thanks for your help!

OD

Me thinks you are trying to copy the Airbus 'ground speed mini'


In a nut shell:

-Work out your g/s on touch down using tower wind and Vref.

-Then maintain the g/s during the approach (add on difference between tower wind and wind aloft to your normal Vref)..... Speeds may look strangely high but works nicely on the bus (automated).


** Be warned, you will not be stable at 500ft in most windy conditions**

Good Luck

Olendirk - you certainly keep on coming up with strange questions.:eek:

FORGET ground speed. On the 737 (all types) we fly IAS. Adjust power to maintain the correct/desired IAS with changing wind - it is not difficult and we have been doing it for over 100 years. No 'black magic computers', just basic flying skills. - ignore them - they are mad.:)

IF you choose to do it, the info is on your EFIS, but I suspect any Captain with half a brain would then take control and have you sectioned - I would.

ERADICATE Airbus from your vocabulary until you need to speak it.

Yes, yes and yes.

No G/S during approach! Fly IAS as calculated!

BOAC,

That was an uncharacteristically tetchy and provocative response to a fair question.

And, in your case, unexpectedly ill-informed.

We have all been taught to fly approaches purely on IAS in the way you describe, and many of us have used the technique on a variety of jet aircraft. It is well known, however, that a 25-knot headwind at 100ft can disappear to nothing at the threshold, particularly at night and/or when the airfield is surrounded by trees. Unlike most propeller aeroplanes, increasing the power on a jet does not in itself generate extra lift from the wing; it may provide a small vertical component of thrust. And jet-engine response is slower than pistons and turbo-props.

Airbus, for all its virtues, did not invent the concept of calculating a minimum acceptable GS on finals – they merely introduced it to line pilots in automated form on the A320 in 1988. With the advent of INS in the 1970s, giving a reliable (unlike Doppler in the 1960s) and fairly accurate GS at all speeds, the concept was soon pioneered by crews. It was first explained to me by a flight engineer on the DC10, and I wouldn't be surprised if it was an American idea. Perhaps someone will enlighten us.

The popular us-and-them attitude between Boeing and Airbus pilots is ill-considered. Both manufacturers produce conventional aeroplanes with minor differences. All their products are well-built and fly well, using similar aerodynamics.

You know only too well that aeroplanes have to operate within the laws of Newtonian physics, one of which involves inertia. Inertia is a function of GS, not IAS. But lift requires IAS. As every schoolboy knows: IAS = GS plus headwind-component (sea-level/ISA). Shortage of inertia (GS) can only be corrected by applying extra thrust; for a period of time. On a bad day at the office, that time may not be available.

I would be very surprised if no Boeing pilot on this forum has ever applied the principle of working out a minimum acceptable ground-speed. You might even consider it yourself.

Chris

- I would not and NO!

I must admit that I stall at an IAS and not a G/S, old-fashioned as that may be.

PS No 'us and them' - merely sound advice to a 737 pilot. Of course, if olendirk's airline starts teaching G/S approaches, wipe all of that.

Whilst it doesn't alter the thrust of your post, that isn't strictly(or to be more precise, necessarily) true.

Inertia, also known as momentum, is Mass * Velocity. Velocity, however, is relative to whatever frame of reference you choose. A body has a certain velocity reference the ground, therefore a certain momentum relative to the ground, but messuring it's velocity reference the air around it is just as correct.

Newtons (and Einsteins) laws are Universal. They don't somehow only apply to movement relative to out very small Earth.

Only if that air is not accelerating relative to an inertial frame. Because air is itself dynamic, that is not generally the case. In windshear your airspeed changes instantaneously, your groundspeed cannot.

True enough, bookworm, and that is why windshear is an issue, though whether it is the air or the aircraft that is changing velocity is ALSO a matter of what frame of reference you are using.

The misconception, however, that velocity is mesurable only reference the earths surface is what leads to faulty concepts like the infamous "downwind turn" myth.

But momentum is not conserved in non-inertial frames. Thus the frame of the air is about as much use as a chocolate teapot when it comes to working out why your aircraft is sinking into the weeds because you failed to take account of the 30 knot drop in headwind in planning your approach! Minimum groundspeed looks very sensible to me.

I don't agree with that. It's perfectly possible to debunk "the infamous downwind turn myth" in any inertial frame -- you just have to remember that velocity, and therefore momentum, is a vector not a scalar.

Booky,

I concede both points.

In the first, I was only speaking about the entirerly hypothetical. GS v IAS is indeed a sensible measue of the effects of windshear.

In the second, certainly true and I have done just that, but the initial misconception usually comes from people saying that momentum is speed (scalar) over the ground times weight, when velocity (vector and reference ANYTHING) times mass in in fact the case.

Damn it Wiz, so what am I going to do with the rest of my Saturday evening if I can't even pick an argument on Tech Log? ;)

Hmmm,

OK, try this one:-

They shouldn't call it a stall turn, because the aircraft never stalls.....

Strange. When I started to write this, I was convinced that getting a groundspeed as an actual figure, was a total waste of time. By the time I had made my comment, I wasn't so sure. It went something like this.


Hah! I thought I was the one that always wanted to analyze things down to relativistic accuracy. But in this case, it seems that we've always had that ground speed thing ticking away in the back of our minds anyway.

Knowing the 2000' winds and surface winds, we then only have to factor in gusts. Doesn't that cover everything that's being said? This is assuming of course we're flying the correct range of IASs.

We get back to that 'It looks wrong' issue if the ground speed seems visually low. Just sitting here in my dotage, I get an very uncomfortable feeling when I visualize a surface-wind-induced crawl over the last half mile before touchdown. It just smells dangerous, but I could never imagine wanting to know my groundspeed in kts.


Okay, now we're in a modern electronic flight deck that might be landing in CAT several. Getting groundspeed is a touch of a button. Maybe, if the PF can get the feel from that figure that I used to get from the visual image, then that could only be to the good. I would suggest however, that the full meaning of that visual image should be well ingrained first.

But as for flying a groundspeed per se, that's just lost on me.

Horlicks.

To stay on topic Boeing didn't offer GS mini as an option and still don't to this day afaik, nor would airmanship suggest you try to imitate it, hence why deviate from what FCTM tells you to do with the command speed? They probably know best, when you get a bus job then let autothrust faff with your speed until you're content.

Wizofoz,
I see that you are a purist, like me. As you say, no velocity is absolute; it has to be relative to something. However, astronauts excepted, pilots and humans usually measure it in relation to the local surface of mother earth. Although the latter is revolving at up to 900kt about the Earth's axis, and the Earth is making its rapid journey around our sun, and our sun is travelling around the Milky Way galaxy, etc.; this reference is steady enough – and therefore useful – for the purpose of defining the "V" in the kinetic-energy equation.
The same cannot be said for the atmosphere, I can assure you, particularly when you are descending through divers layers of same.

Port Strobe,
If it's "Horlicks" to state that, for a given mass, inertia (kinetic energy) is a function of GS, perhaps you can tell us what else? IAS?
Although I was trying to avoid formulae, it is actually proportional to the GS (velocity) squared, which makes a shortage of it even more difficult to correct.
You also suggest: "...when you get a bus job then let autothrust faff with your speed until you're content."
A/Thr is not required, unless demanded by an airline's SOP. During 14 years on the A320, 90% of my manual approaches were flown with manual thrust from 1000ft, and 99% with "managed" IAS indicated as the target speed on the ASI. "Managed" IAS always provides GS-mini protection on Airbuses since the A320.
The great thing is: you don't need so many thrust changes – see (4), below.

BOAC,

(1) We agree on one point: adherence to SOPs. That was the only post-posting misgiving I had yesterday evening. See (5), below.

(2) But you are still choosing to misinterpret the concept of avoiding an unsustainably low ground speed on the approach. Perhaps you should read my post again – more carefully. :ugh: And electricdeathjet's.
Given the chance, of course, Airbuses would stall in exactly the same way: at an IAS. The trick on the approach is to avoid a predictable, critical loss of IAS by anticipating the loss of headwind.
Let's look at the common example in my post above. Using your traditional technique: if the surface wind is known to be calm, you will have added nothing to your approach IAS. Assuming sea-level/ISA, an approach speed (VAPP) of 125kts, and a steady headwind of 25kts above (say) 100ft; you will be soldiering on in your B737 quite happily at a GS of 100kts. Fine so far... But what is the point, when you know that your GS is going to need to increase by 25kt in the last 100ft (10-12 seconds) of the approach; involving a big handful of thrust, and (not being an A320) a lot of pushing/re-trimming?
If the SOP is to fly a stabilised approach, why allow it to be predictably de-stabilised close to the ground?

(3) You imply that we are ignoring the all-importance of IAS. On the contrary: IAS is precisely what we are trying to conserve.

(4) The "GS-mini" concept protects IAS by constantly offering the pilot an IAS target ("managed speed") which results from:
the higher of VAPP and the IAS required to achieve the minimum GS.
If the headwind component on the approach is higher than reported on the ground (and entered into the PERF page of the FMGS), the IAS target will be above VAPP. It will also rise and fall with the current headwind component. Despite (in fact, because of) this changing IAS target, the thrust requirement remains roughly the same, because the aircraft's kinetic energy remains constant at the constant GS.

(5) This changing, managed IAS produces two issues that have to be addressed.
> (a) SOP stabilised-approach IAS criteria have to be relaxed slightly. In the above example, the managed IAS target at 500ft would be VAPP+25 (150kt).
> (b) If the difference between headwinds aloft and on the ground is very great – say, 45kt at 1500ft – the resulting IAS target of VAPP+45 may exceed the flap limit for the next flap extension. So a suitable selected IAS has to be maintained, which will often be dictated by ATC anyway. As the headwind declines, the GS rises, and managed IAS can be introduced, subject to ATC.

D.I.Y. GS-mini?
 

Just being a pedant, but inertia is not the same as kinetic energy.

The only thing its proportional to is mass.

Mass is a measure of inertia, not groundspeed, indicated airspeed or any other flavour of speed. To say a helicopter in the hover has no inertia is simply inaccurate, not pedantic as SR71 suggests. Momentum is a function of groundspeed (taking an Earth fixed frame of reference as absolute for our purposes) for a given mass, not to be confused with inertia. I don't require the Ladybird guide to kinetic energy either thanks.

On the subject of being accurate I was incorrect to suggest GS mini is controlled by autothrust, so let managed speed faff with the command speed until you're content then.

The postings this evening seem to be a sales pitch for the Airbus. It may be a clever machine but I'm not qualified to give backing nor counter arguement to your statements. I am qualified to say faffing with the command speed on the Boeing is not the way they nor the vast majority of operators propose to use the AFDS. To think you're smarter than them and the person three feet away by doing so is a shortcut to creating an incident in my own opinion. On the assumption we're talking about relatively modern passenger jets then the instant wind is going to be right under your nose so any drop in airspeed can be anticipated, it's basic situational awareness. Manipulating the thrust and flight controls is part of a pilot's job description so I don't object to having to do that. If the windshear is going to be really sudden then it calls into question whether or not you should be executing an approach in the first place, plus you'll probably get a predictive if not reactive windshear warning. Horses for courses, but transferring techniques between types simply doesn't seem sensible to me.

I'm just a dinosaur but, for what it might be worth -

(a) is not the Airbus technique similar in intent to the Boeing approach additives ?

(b) where we are looking at the potential for windshear, my observation has been that the majority of pilots will carry extra speed if the circuit wind is moderately different to that on the surface .. neither Boeing nor Airbus can read the real wind profile .. and the pilot retains the option of the miss if it turns out to be unmanageable.

The worry I see is the pilot who rigidly sticks to whatever protocol without thinking about what he/she is doing... as a wise checkie put it to me years ago .. "Lad, the Ops Manual should have a sentence on the preface sheet saying something like ..'to be read with an bit of commonsense ..' "

I absolutely 100% agree. What ought to be understood is the spirit of the rules, and what I'm getting at is creeping back the command speed is not an underlying principle Boeing wish to promulgate, so don't do it on a Boeing for common sense doesn't suggest you do so. I wouldn't rigidly stick to the manuals at the expense of operational advantage let alone safety, but I think I've said enough times my point is Airbus speed control techniques on the Boeing aren't the way forward when the minimum crew is two and half of them probably aren't educated in the technique.

SR71,

You're not being a pedant at all, and I'm beginning to regret ever using the word inertia; although my dictionary [Collins] defines it as: "the tendency of matter to remain at rest (or to keep moving in the same direction) unless affected by an outside force", which is precisely what I'm talking about.

However, my old Physics book [A.R.W.Hayes] says: "Inertia or mass... is the property by which it tends to resist changes in motion. Numerically it is the constant M..." So inertia is mass, as you say: independent of velocity.

By the way, it defines momentum as "...the product of its mass and its velocity", (i.e., Momentum=Mv, where M is mass and v is velocity). It also says: "Momentum should not be confused with kinetic energy. Momentum is indestructible. Unlike kinetic energy, it cannot be converted to some other form." Kinetic energy (they refer to it as "translational kinetic energy") is defined as the familiar ½ Mv².

So you are right: I should have been more careful about the use of the word "inertia"; and not suggested it was the same thing as kinetic energy.
Am tempted to edit my posts; but that would be cheating, and I don't think the error will have misled anyone in this empirical context. Serious readers will see this, and to them I offer my apologies.

Chris

Quote from Port Strobe:
The postings this evening seem to be a sales pitch for the Airbus.
[Unquote]
In my first post, I pointed out that the concept of a minimum acceptable ground speed was not invented by Airbus, as far as I know. If by explaining roughly how their GS-mini works – in the face of familiar anti-Airbus sentiment from one or two Boeing advocates – I have shown Airbuses in a favourable light, so be it.

Quote from Port Strobe:
...so let managed speed faff with the command speed until you're content then.
[Unquote]
See what I mean?

Quote from Port Strobe:
Horses for courses, but transferring techniques between types simply doesn't seem sensible to me.
[Unquote]
I agree as a general rule. Hence the several caveats to Olendirk in my post #17. But john tullamarine suggests: "where we are looking at the potential for windshear, my observation has been that the majority of pilots will carry extra speed if the circuit wind is moderately different to that on the surface."

Does Boeing issue specific advice on this? Olendirk would like to know.

Merely to say, as Port Strobe does, that "any drop in airspeed can be anticipated, it's basic situational awareness", does not do justice to the argument.

To Chris Scott's question:

I don't know if we invented "managed IAS", but the USAF uses something like this on its heavy transports. Originally, it was computed by the flight engineer, using approach TAS adjusted with current tower winds to calculate a "reference GS", the pilots then added enough knots to fly the plane at a groundspeed that was equal to the "reference GS". Later, it was automated thru the FMSs which did the calculated and displayed the "reference GS" and produced an aural warning, if the actual GS was less than reference GS.

It came out of windshear accidents and the ability of INS systems to show real time ground speeds. I thought it was useful for aircraft which have high momentums and low excess power-the C-5, for example. That said, we still had prohibitions about operations in windshear and TRW conditions.

GF

Chris,

Whilst an aircraft is in flight, it's inertia, momentum, kinetic energy or anything else can be measured with respect the the earth, the moon or alpha-centauri and it doesn't make the blindest bit of difference to the aircraft.

A Tiger Moth with a TAS of 70kt, flying into a 70Kt headwind has, according to you, no kinetic energy, yet will fly along (well, it's not actually GOING anywhere!) just fine. The aircraft flys due to the air flowing over it's surfaces. How fast that moves it WRT the earths surface has no bearing on it's performance.

The only relevance ground speed has is that we often have a read-out of it if we have INS or GPS aboard, and the difference between that and our TAS is the wind. Changes in that relationship indicate changes in the wind, which IS relevant as that is the air we are flying through. But GS in isolation (and therefore Momentum, Kv or anything else reference the earths surface) has no bearing on the aircraft at all.

Hi Wizofoz,

I agree with your first two paragraphs, but disagree with the implications contained in your final one.

You are making light of the fact that, if the 70-knot headwind suddenly dies away, the inertia of your Tiger Moth (aaah, De Havilland!) will have to be overcome in order to increase its GS from zero to whatever it needs to restore a flyable IAS. If to do this it needs to regain its original TAS of 70kt, it will need to accelerate to a GS of 70kt.

If GS has to be changed, so must be the kinetic energy, which is also a vector and therefore relative to a datum (we normally use the earth's surface but, as you say, it could be anything). Increasing the kinetic energy involves the application of power for a period of time. [I'm aware that, in purist terms, "kinetic energy" is more of a concept than a reality; but it's one that works quite well enough for the purpose of this discussion.]

If your Tiger Moth was achieving a TAS of 70kt into a 140-kt headwind, its GS would be minus-70kt. If the wind suddenly dropped to 70kt, the Tiger would need to increase its GS from minus-70kt to zero. That would require a similar amount of [power x time] as in the first case. Power x time = energy. So the Tiger needs to increase its "kinetic energy".

Now: are you a bit happier?

Just to note, I have seen a 50Kt drop off in about 50 Ft above touchdown, in the desert. (Desert Night time Jets). TIs a pretty hideous situation as you have to add a fair whack of power before you enter it. The clue is in the tower reporting winds calm whilst the air you are flying in is moving at 70 Kts on the nose!

can also get interesting if it is a tailwind component. Especially flying a jet without brakes.

No, its not.

This is great fun....

:ok:

Chris,

The amount which an aircraft needs to accelerate due to a change in the wind in order to regain it's original airspeed is equal to the change in the wind. All frames of reference relevant to the flight of the aircraft are to do with the air it's flying through. As a consequence, yes it's velocity relative to the ground will change, and as such all consequential values which have velocity as a factor will change relative to the surface, but that is consequential, not causational.

Lets suppose that while the aircraft is in flight, someone attaches a mega rocket to the earth and suddenly changes it's rotational speed. Let's, for argument, assume friction doesn't cause the atmosphere to change IT's velocity, so the aircraft is still flying in the same surrounding air. The aircraft now has a ground speed that might be greater, less or sideways!! Relative to an observer on the ground, it's ground speed, momentum and Kv will all have changed. Will it effect the way the aircraft flys? Not at all! The aircraft won't know, until such time as it tries to land on the earths surface.

Perhaps the concept you are not quite seeing is the amount of kinetic energy an object has is relative to the observer.

Higher IAS in headwinds / WS
 

I'm flying with Chris Scott, his approach is sensable.

BOAC, like the name, is old hat.

GS Mini works well and avoids unnessary THR changes, either automatixc or manual.

Plus you have energy available to overcome that last minute sinking feeling with rapid IAS decrease.

Good points from John_tullamarine
(a) is not the Airbus technique similar in intent to the Boeing approach additives ?

ATC like the higher GS, keeps the traffic flow optimal.

Anyway great discussion topic.:O

Hello cpt G - thank you for your deference to my age.:ok: I should, however, point out that the 'advice' to Olendirk is anything but 'old hat' but it right 'up to the minute' CORRECT a/c handling technique - for 737, that is, not Tiger Moth or Airbus, which is what was asked.

Regarding your last 2 points:

No they don't when they have asked for an airspeed, and it could completely scupper the plan causing a g/a for you or cancellation of a departure ahead:ugh:.

Yes it is

I'm not sure what a is, but is it slower than this:

YouTube - STOL - Short Take Off and Landing

If you substitute the word "momentum" for "kinetic energy" in the above, you'd be correct.

The problem arises when it changes relative to the aircraft. What it is doing relative to the earths surface has no bearing on the aircraft. One way of detecting the fact that it is changing is the reationship between airspeed and groundspeed, but, answer me this, could you safely do an approach based on groundspeed only, with no reference to airspeed and therefore no knowledge of the winds velocity?

Absolutely not.

A vector by definition is that which has both a magnitude and a direction.

A scalar by definition is that which has only a magnitude.

Kinetic energy is a scalar because the V^2 in the definition thereof is defined as the scalar product of the velocity with itself i.e., V.V, and is a scalar.

Kinetic energy is the energy an object possesses by virtue of its motion.

Or alternatively, the change in kinetic energy of an object is equal to the work done by a conservative force.

So kinetic energy and work must have the same units and type.

Work is the scalar product of force and displacement.

Force and displacement are vectors, and the scalar product of two vectors is a scalar.

But I'm confused...

The aircraft knows nothing about what the earth below it is doing. It doesn't need to, as Wizofoz suggests.

Whether it touches down at a GS of 0kts or 70kts, as long as the IAS/TAS is the appropriate one, what is the problem?

I've touched down in a light 737 at a GS of <90kts on a day when it was gusting 65kts. I didn't even think about GS. The only thing I seek to preserve on finals is IAS. If I fly through a shear, I need to regain IAS surely, not GS?

What am I missing?

I'm surprised that nobody has mentioned this. Qantas boeing crew use RGS or reference ground speed. It basically provides them with a ground speed they don't go below. Works well.
Has anyone corrected this yet?.... Come on Wiz....inertia has nout to do with velocity, you know that! It stays the same regardless of velocity.

I am extremely surprised and disappointed that there are so many who cannot accept the part that G/S mini has to play during windshear.

Been following the thread with interest, and I think this is the crux of it. Having a minimum GS serves to protect the IAS. BUT that's not the only way of doing it.

<disclaimer> I'm just an interested PPL, with aspirations to bigger tin..</disclaimer>

Starting assumption is that a large transport category aircraft has significant inertia (resistance to change), so if the 65kt headwind suddenly disappears, it's going to be a big issue accelerating the aeroplane. If it happens at just the wrong moment we might finish up with upset passengers, and possible landing gear protruding from places it shouldn't: We all know that IAS makes it fly, however, we have some need to protect the IAS.

If we implement a minimum GS <airbus>, we protect the IAS because even if the wind should spontaneously reduce to zero, we're still rocking along fast enough to fly - the IAS will take a dive, but it will take a dive from a higher point to a point at which the plane still flies.

In the event that there is no shear, and we fly through a continuously decreasing headwind, the IAS will slowly wind down to some target value, at which point the IAS will stop decreasing, and GS will start to increase (it's GS mini, not target GS). In the event the headwind remains, we will arrive at a higher IAS AND GS than might be necessary.. but it's still not an excessive GS (perhaps somewhere around a 0 headwind GS), so shouldn't be a problem.

If we don't reference GS <boeing> then given the reported conditions, and experience, then I presume we (the pilot) takes some action to protect the IAS by adding some knots for headwind/gust factor?

Which surely amounts to about the same thing (protecting the IAS), just by different routes?

Having read the thread being discussed by stalwarts, it finally required someone with less experience like Mark1234 to put thing simply so that there is no more confusing GS Mini with KE, inertia, momentum etc…. Energy level required to see you through a sudden loss of head wind component close to the ground is what the GS Mini provides. In other than the Airbus we come in with an additive factor (as John put it) which is a rough figure mentally calculated by the pilot (or the flt engineer if you have the luxury of having one). The GS Mini removes this mental maths and lets the pilot concentrate on the approach.