Flying faster because of decreasing winds [Archive]

Chris Scott

15th Nov 2008, 14:53

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

BOAC

15th Nov 2008, 17:02

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. - 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.

Wizofoz

15th Nov 2008, 17:22

Inertia is a function of GS,

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.

bookworm

15th Nov 2008, 18:02

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.

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.

Wizofoz

15th Nov 2008, 18:05

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.

bookworm

15th Nov 2008, 18:40

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.

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.

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

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.

Wizofoz

15th Nov 2008, 18:51

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.

bookworm

15th Nov 2008, 18:55

I concede both points.

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? ;)

Wizofoz

15th Nov 2008, 19:11

Hmmm,

OK, try this one:-

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

Loose rivets

15th Nov 2008, 19:23

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.

Port Strobe

15th Nov 2008, 19:35

Inertia is a function of GS

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.

Chris Scott

16th Nov 2008, 18:52

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.

Chris Scott

16th Nov 2008, 19:05

So what about you, Olendirk, trying to stay within SOPs in your Boeing 737; next to a captain like BOAC, wielding his rolled-up newspaper – ready to flagellate?

I presume you are flying an approach speed which has been calculated by yourself – or by the FMS, based on the reported surface wind; and that you are using an IAS knob manually to alter the target? If that surface wind is small, you will have added little to VREF, and are at the most vulnerable to the inevitable wind-sheer. With luck, the sheer will be gradual – but it may be sudden, as in my example (see my previous posts).

To deal with serious cases, my suggested technique starts with calculating an estimated threshold GS. This involves correcting threshold IAS to TAS, if necessary; then subtracting the headwind component. This is "GS-mini". Once established in landing config at the selected approach IAS, look at the indicated GS. If it is below GS-mini, wind up the selected IAS to try and correct it, but do not exceed the stabilised-approach IAS criterion for this approach. Do not exceed GS-mini. As the headwind falls, GS rises, so you must reduce the selected IAS. Once it has reached the original approach speed, leave it alone.

Ensure you NEVER select an IAS below the SOP approach speed. If the tower reports a big revision to the reported surface wind, revert to the normal SOP (and, just as normal, consider the possibility of a go-around). In any case, ensure that the calculated approach speed is selected by 100ft. [Once the AP has been disconnected, these selections have to be made by the PNF.]

If the above technique is unacceptable to your fleet managers and trainers, or to the captains you fly with, there may be very good reasons; the possibility of mis-selecting too low an IAS being one, depending on your FCU and FMS. BOAC's opposition may seem to be pure Luddism, but it could also be that he is in the honourable business of enforcing SOPs. In less-regulated times, it has been done on other aircraft. You, however, may have to wait for an Airbus...

SR71

16th Nov 2008, 20:38

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

The only thing its proportional to is mass.

Port Strobe

16th Nov 2008, 22:29

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

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.

john_tullamarine

16th Nov 2008, 22:39

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 ..' "

Port Strobe

16th Nov 2008, 22:49

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.

Chris Scott

16th Nov 2008, 23:51

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

Chris Scott

17th Nov 2008, 01:05

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.

galaxy flyer

17th Nov 2008, 03:10

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

Wizofoz

17th Nov 2008, 03:28

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.

Chris Scott

17th Nov 2008, 09:09

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?

VinRouge

17th Nov 2008, 09:27

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.

SR71

17th Nov 2008, 10:14

...kinetic energy, which is also a vector...

No, its not.

This is great fun....

:ok:

Wizofoz

17th Nov 2008, 10:36

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.

Capt Groper

17th Nov 2008, 11:50

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

BOAC

17th Nov 2008, 12:01

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

SR71

17th Nov 2008, 13:41

I'm not sure what a unsustainably low ground speed on the approach is, but is it slower than this:

YouTube - STOL - Short Take Off and Landing (http://uk.youtube.com/watch?v=9WPb5k4-HnY)

Chris Scott

17th Nov 2008, 15:53

Hi again, SR71,

Your minimalist and unhelpful interjections have certainly made me dust off my A-Level (1960s) Dynamics, which has been rewarding, and for which I'm most grateful. :} It has indeed been fun, but I suspect we are now at risk of getting a little more off topic than is really productive?

When I bravely stated that "kinetic energy" is a vector, I was thinking of the Tiger Moth with its TAS of 70kt into a headwind of 140 (see post #26). Its GS, in pilot terms, is minus 70, because aeroplane pilots think of GS in the direction of intended travel; i.e., "Track" (loosely speaking, forwards). The same must therefore apply, by definition, to its "kinetic energy"?

To illustrate my point, remember my example: when the headwind suddenly dropped to 70kt, the Tiger had to accelerate its GS from -70kt to 0kt to restore its TAS/IAS. This required an energy input to overcome its inertia (mass), i.e., its "kinetic energy" had to be increased from the point of view of the pilot.

But from the point of view of someone standing far below on the ground, who may not have been able to see which way the Tiger was trying to go, it actually appeared to decelerate from 70kt to 0kt (stationary). This implies a decrease in kinetic energy. So the extra power (or thrust, if you want to achieve the same result by a slightly different method) has, from this point of view, been negative.

Anyone confused yet? Kinetic energy, as I think Wizofoz may be already reminding me – like GS (velocity) – is purely relative to the observer. But, additionally, the direction in which it is acting is all-important. A speed is a speed (relative to a point); but a velocity is a speed and a direction/bearing (relative to a point). Similarly, the concept of kinetic energy can only be meaningful if its direction is specified. If that does not make it a vector, then please let me know what I can call it, and I'll oblige.

Quote from Wizofoz:
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.
[Unquote]

Yes, but let me remind you that the concept of wind is equally relative; it is universally defined as a velocity relative to the earth's surface. The problem for pilots arises when it changes, not when it is steady.

That's what the concept of a minimum acceptable GS on the approach is all about.
Can we get back to the subject, please?

bookworm

17th Nov 2008, 16:48

When I bravely stated that "kinetic energy" is a vector,

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

Wizofoz

17th Nov 2008, 17:23

The problem for pilots arises when it changes, not when it is steady

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?

SR71

17th Nov 2008, 18:48

Similarly, the concept of kinetic energy can only be meaningful if its direction is specified. If that does not make it a vector, then please let me know what I can call it, and I'll oblige.

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?

framer

17th Nov 2008, 19:54

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?....Inertia, also known as momentum, is Mass * Velocity. Come on Wiz....inertia has nout to do with velocity, you know that! It stays the same regardless of velocity.

autoflight

18th Nov 2008, 00:01

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.

Mark1234

18th Nov 2008, 00:30

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?

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?

Anp

18th Nov 2008, 06:19

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.

SR71

18th Nov 2008, 09:33

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

The OP wanted to fly the Airbus paradigm in his Boeing.

The Boeing proponents said, "Why?"

The Airbus proponent(s) said "Because its better. You protect IAS by flying a mini GS. You don't have de-stabilising thrust inputs late in approach."

(Not that I fly the Airbus, but are Airbus pilots and their machine so cool as to not bother to add thrust even if they did fly through a shear?)

The Boeing proponents said "We protect IAS by protecting IAS. Whats the big deal?" (Not many of us still here can say it hasn't worked for us...even BOAC, and by anecdotal accounts, he has been around a while.)

And then everybody got confused...

As for removing the mental maths, thats mental.

:\

Chris Scott

19th Nov 2008, 00:01

Excellent one, Mark1234! All correct. :ok:

Quote from galaxy flyer:
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.

Thanks for answering my question (in post #5), and I think you have covered about two-thirds of the whole GS-mini subject in just two paragraphs. :D It confirms my contention that the technique predated the A320. As they say, there is nothing new under the sun.

What you describe, once it had been automated, sounds pretty close to Airbus's "managed" IAS.

You hit the nail on the head when you talk of aircraft that have little surplus power/thrust, like your C-5. Without comparing typical thrust-to-mass ratios on various aircraft, I guess heavy B707s are also underpowered, and presumably B-52s.

B707s were retrofitted with INS eventually, but I don't remember Boeing offering guidance of this kind, which would have been useful. The only "additive" we used to apply to the "bug" (VREF) was a mental one (no objection to that) based on the [U]surface wind (half wind speed + gust, up to a maximum of 20kt). Can't comment on early 747s; but framer tells us that "Qantas boeing crew use RGS or reference ground speed. It basically provides them with a ground speed they don't go below. Works well."

It's only fair to admit that most twins, including BOAC's and SR71's B737, have the luxury of a good thrust-to-mass ratio in the all-engine case. Maybe that is why they can stick to the old version of energy management on the approach: simply chasing an IAS that is based only on VREF, plus an increment for the wind on the surface – which wind may be zero.

Chris Scott

19th Nov 2008, 00:20

Like autoflight, I am increasingly dismayed at the difficulty that so many otherwise intellectually-bright forumites appear to be having to grasp what is essentially a simple concept: the importance of protecting the all-important IAS by not allowing the GS to fall too low when a negative windshear is known to be ahead.

The fascinating – but essentially esoteric – discussions about the precise definitions of inertia, momentum, and kinetic energy are unnecessary in this context, although they might well be worthy of a separate thread. On this one, they amount to little more than fog-inducing semantics. I'm wondering if, in one or two cases, they are inspired by an element of perversity of the "not invented here" variety.

The sceptics constantly emphasise the importance of IAS, and seem to recognise that it can suddenly fall dangerously, due to a loss of headwind; as described by VinRouge (post #27). Their mantra is roughly this: IAS is the only thing that matters; so we must concentrate on it exclusively, and ignore the GS. Wizofoz (post #35) seems to be implying that we, on the other hand, are advocating that GS is more important than IAS.

This is a perverse misrepresentation of my previous posts, which clearly state that the target IAS is always the higher of: the conventional approach IAS (on the one hand); and the IAS which delivers the calculated minimum GS (on the other). So we are always at an equal or greater IAS than he is.

To save readers looking back 3 days, let me remind you that in my first post (#5), I wrote: "...lift requires IAS."
I then invited sceptics to look at IAS from a fresh perspective –
"IAS = GS plus headwind-component (sea-level/ISA). Shortage of (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."

Two days ago, in post #16 –
"You imply that we are ignoring the all-importance of IAS. On the contrary: IAS is precisely what we are trying to conserve."
and –
"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."

Quote from SR71,
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?

Does he want an honest answer? ;) Seriously, though, the secret is to anticipate the shear, if you can predict it is going to happen. [If you cannot, that's another topic.] We are not discussing the situation where the wind is strong and gusty all the way down to and including the threshold; it is the sudden [U]predicted suspension of a headwind that concerns us.

SR71 will continue concentrating manfully to regain his target IAS, which he need not have lost hold of in the first place. While doing so, he may have ample time to discuss whether his "scalar" kinetic energy, and the amount it needs to increase, is relative to an unsteady atmosphere, or to a stable platform like the earth's surface; I shall leave him to that. :ugh:

Mark1234 describes the advantage of (automated) GS-mini: as the headwind falls, taking the actual IAS with it, the target IAS normally falls by the same amount (but never below the threshold IAS). That is because the inertia of the aircraft protects its GS (unlike its airspeed). And remember, TAS = GS + HWC. The minimum GS is the threshold TAS minus the headwind component of the programmed surface wind. IAS and TAS are the same at sea-level/ISA, of course, but usually not elsewhere. The system corrects automatically for the conversion of target TAS to target IAS, which is one of the awkward things to do manually, and therefore invaluable.

Wizofoz

19th Nov 2008, 04:19

Chris, No, I think it's you that unintentionally misrepresented my position.

I've kept out of the minimum GS debate entirely. I understand the concept and have no doubt it works. I shall therefore, when on approach.....Follow my companies SOPs and do as I'm told!! If I'm ever put in a policy making position, I will revisit the whole matter and...Follow the manufacturers recommendations!!

All I have been doing is trying to do is point out several conceptual fallacies to which you seem to subscribe, encapsulated by one of your first statements:-

Inertia is a function of GS, not IAS.

Which was untrue at several levels, not least of which being you didn't know the difference between Inertia, Momentum and Kinetic energy, and that you constantly allude to an aircrafts ground speed somehow effecting it's aerodynamic performance.

The only relationship that effects the aircraft is between it and the air around it. Carrying extra airspeed to allow for changes in that relationship is valid. Basing the amount to carry on a fixed datum like the earths surface is also valid. Making pseudo-scientific statements about "Kinetic Energy" and how it should be based on the earths surface is not.

The fact that you still don't get it is shown by your mocking tone towards SR71 when you said-

SR71 will continue concentrating manfully to regain his target IAS, which he need not have lost hold of in the first place. While doing so, he may have ample time to discuss whether his "scalar" kinetic energy, and the amount it needs to increase, is relative to an unsteady atmosphere, or to a stable platform like the earth's surface; I shall leave him to that.

Dude, it's that unstable atmoshere we need to keep flowing over our wings, and it is are IAS that is a measure of that flow.The amount you need to change (or, in your method, the extra you need to hold in the first place) is equal to the amount the air is going to change it's velocity relative to your aircraft.

It was you who started (incorrectly) throwing scientific terms around, don't get snitty with someone for correcting you (BTW framer, thank you!).

I also note that you are a greater authority than Boeing on how to fly Boeings, and anyone who doesn't fly as you recommend is open to mockery and criticism

Meikleour

19th Nov 2008, 09:20

Chris,
I admire your perseverance in trying to open the minds of the `determindly closed`! Some people will just never get it. It`s a bit like the advocates of wing warping saying that ailerons `will never catch on`

Bon chance mate.

Wizofoz

19th Nov 2008, 09:56

Meik,

I'd be interested to know who you think is showing a closed mind here and how. I for one have stated that I understand the concept of GSmin, and agree it would work. I just don't go around ignoring my Boeing approved and company mandated procedures because someone on PPRUNE made a persuasive argument.

I'll happily fly a min GS approach- The day it appears in my ops manual!!

Chris Scott

19th Nov 2008, 11:34

Thanks, Miekleour.

Quote from Wizofoz:
I've kept out of the minimum GS debate entirely. I understand the concept and have no doubt it works.

Why? It was central to the topic. It was the main element of Olendirk's original post, which was a thoughtful request for an explanation of how the technique works. His question was effectively ruled out as heresy by BOAC (post #3).
Referring to anyone who advocated the technique, he wrote:
"ignore them - they are mad."
Just to ram the point home, he wrote:
"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."

This paternalist scolding seemed to me to be less than helpful. The concept needed to be explained, which I attempted to do in common cockpit parlance. In Physics terminology, of course, my use of the word "inertia" was wrong – as Wizofoz (and others) pointed out, and I quickly accepted. Since then, the "debate", as he calls it, has consisted mainly of nit-picking.

As Wizofoz understands the concept – and can presumably explain it better than my feeble efforts, would it not be more helpful to do so? There's still time...

Quote from Wizofoz:
...you constantly allude to an aircrafts ground speed somehow effecting it's aerodynamic performance.
[Unquote]
Rubbish. Any quote?

Quote from Wizofoz:
...don't get snitty with someone for correcting you
[Unquote]
Examples?

Quote from Wizofoz:
I also note that you are a greater authority than Boeing on how to fly Boeings, and anyone who doesn't fly as you recommend is open to mockery and criticism
[Unquote]
No. I recommend that pilots remain within the limits of their companies' SOPs. galaxy flyer and framer have provided evidence that manual versions of the concept have been in use elsewhere, since the introduction of INS. Although I attempted to explain a possible procedure to Olendirk ("D.I.Y. GS-mini?", post #17), I included several caveats, and ended it with a strongly-worded warning not to deviate from SOPs. Since then, I have added:
"It's only fair to admit that most twins, including BOAC's and SR71's B737, have the luxury of a good thrust-to-mass ratio in the all-engine case. Maybe that is why they can stick to the old version of energy management on the approach..."

The difference between me and many of the pilots on PPRuNe is that I spent 17 years flying approaches on a variety of jet transports, including the underpowered B707, using IAS with SOP-increments, and ignoring any GS read-out. I later spent 14 years flying GS-mini-equipped twins.

The B707-320, with a MLW of 112T and only about 30T (spare us a lecture about Newtons, etc.) of rated thrust, could have benefited from the technique more than most. There is no doubt that [U]automated GS-mini, used iaw SOPs, is a useful and protective system. It is particularly advantageous in the engine-out case.

But both pilots and the A/Thr need to avoid the mistake of using the "managed" IAS as a minimum speed, rather than a target.

BarbiesBoyfriend

19th Nov 2008, 12:00

I'd say thinking about your GS while on approach is a bit silly.

All that matters is your IAS and the wind.

On a windy day, if I ve got 40kt on the nose and the surface wind has been passed as 10kt, I know 30kt of HWC is going to disappear at some point. Therefore I fly a bit faster!

Got 10,000 hours now and never had a negative windshear- so it works!:ok:

Wizofoz

19th Nov 2008, 12:14

Rubbish. Any quote?

HERE:-
Inertia is a function of GS, not IAS

HERE:-
Shortage of inertia (GS) can only be corrected by applying extra thrust

and HERE:- 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.

You meant momentum, but were still wrong.

Examples?

Here:-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?

(Actually, that belongs in both columns)

And most notabley Here:-

While doing so, he may have ample time to discuss whether his "scalar" kinetic energy, and the amount it needs to increase, is relative to an unsteady atmosphere, or to a stable platform like the earth's surface; I shall leave him to that.

If you had simply given an explination of Gs Mini- A way of allowing for windshear by carrying sufficient IAS to allow for it, referenced to a ground speed, all would have been fine. Your delving into iffy physics was always going to be challenged- it is Tech Log after all.

BTW, what happens to an aircraft flying with a large headwind if it quickly turns 180Deg??

Port Strobe

19th Nov 2008, 12:16

So in summary Chris you've been right all along. You agree that we're all agreed we understand how GS mini works, some of us have learned a little about inertia, momentum, kinetic energy of the translational variety and their lack of interchangability, and in the future we'll all fly approaches based on a minimum reference grounspeed to save us from adding thrust to maintain the IAS as this is safer than having an increasing groundspeed throughout the approach. However the practical ramifications of this discussion are we'll still go to work and do as we're told by the manufactuer and any deviations from this approved by our respective employers which are authorised by their NAA all underpinned by airmanship questioning whether or not it is sensible to do so in the present and expected conditions, be they meteorological, proximate traffic its sequence or whatever. Which is a long way of saying what I said on the first page in response to the initial question, when it's managed speed faffing with the command speed then let it do so until you're content however until that day arrives then do as instructed. Which you said you're also in agreement with. Agreed we're all agreed?

Meikleour

19th Nov 2008, 12:18

Wizofoz,
What makes you think I was referring to you?!

If your main arguement is `comply with one`s own company SOP`s` then we are in complete agreement! HOWEVER that was not the reason for the original thread. On the three Boeing types I have flown (B707, B737, B747) the maximum Vref increment was +20kts. I also flew with one operator who used the minimum reference ground speed concept on these types. The problem arises when the expected change in headwind component decreases by more than that amount. To berate Chris about his use of scientific terms and to quibble about frames of reference misses to whole point of the arguement! An Airbus, using mini-GS arrives over the threshhold at a SUITABLE GROUND SPEED and a SAFE IAS!

QED

SR71

19th Nov 2008, 21:44

Chris,

I'm bowing out at this point...

Like the Wizard, I understand what the concept you allude to is seeking to guard against but I (also) objected to your use of terminology and the constant reference to GS.

To have a sensible discussion, one has to use the right terminology. I'm sure you agree.

I let you know early on I was a pedant.

john_tullamarine

20th Nov 2008, 00:41

might be a suitable time for some of the folks to have ten deep breaths .. the forum isn't about sheep stations .. only looking for reasonable and rational discussion on the topic. If this one or that has a different view, that's fine .. as always, the aim is to play the ball, not the player.

krujje

20th Nov 2008, 00:45

as always, the aim is to play the ball, not the player

but are we measuring the velocity of the ball relative to the players, the court, the air, the earth, or the sun...?
ah! forget it... :)

autoflight

22nd Nov 2008, 03:03

I flew military & airline for 38 years and only recognised a microburst one time. It seemed that the initial outward wind was about 80 knots. If that was on your final approach and you ignored all the evidence, expect a near instantaneous loss of 160 knots of IAS. Unrelated to the loss of speed would be an increase ROD due to the rapidly decending air. Please nobody tell me this is a survivable event! Avoidance is the only reasonable course.
I am also concerned that a few jet pilots out there still don't understand that an indication of impending loss of IAS is when G/S on final is lower than speed calculated using tower reported wind. A moderate shear will show as a G/S perhaps 10 - 15 knots less than the calculated touchdown G/S. This might not be an indication of a microburst. If you expected a G/S of 120 knots and at 500 ft, with a stabilised approach, it is 70 knots, I would call that very significant windshear, probably with worse to come. This could be a MB. All your worries about how your kids will turn out, will your marriage last, will you have a successful retirement, will probably disappear with you, your crew and pax in a smoking hole in the ground short of the runway.
You don't have to actually fly G/S. Just make sure you don't go much below it. If you're not able to force yourself to do this because you were trained to fly IAS, at least keep an eye on it to assist in identification of perilous conditions.

BOAC

22nd Nov 2008, 06:59

Well, I 've been doing JT's 'deep breathing' for a couple of days and returning now to read the thread I have to say that in my opinion 'Port Strobe' has said it all. As I said to Olendirk on Post #3, However the practical ramifications of this discussion are we'll still go to work and do as we're told by the manufactuer and any deviations from this approved by our respective employers which are authorised by their NAA all underpinned by airmanship questioning whether or not it is sensible to do so in the present and expected conditions, be they meteorological, proximate traffic its sequence or whatever.- i.e. when your 737 operating company TRAINS you to fly g/s on finals, learn how to do it and do it well. Until then, fly your company procedures - and conform to ATC speed request, of course, as always.

autoflight

24th Nov 2008, 08:19

It is quite rare for me to specifically identify those with a contrary view, but in the interest of flight safety I ask that less credibility be given to the entrenched opinion of those absolutely opposed to G/S considerations.

Capt Pit Bull

24th Nov 2008, 09:22

Looks like I'm a bit of a 'johny come lately' and missed most of the good bits!

One problem here is that its very common for people to build their understanding of principles of flight on a foundation of dodgy Physics. Most of the issues have been thrashed out above. I know it might sound pedantic, but things like mass, inertia, momentum and kinetic energy are often not just slightly different but are truly radically different quantities.

I think momentum and kinetic energy have been well dealt with above. A brief summary:

Momentum. mv. A vector quantity. Conservative (in a given system, total momentum is always conservative, this is intimately connected to Newton 2 and 3). Pitbull top tip: Use changes of momentum to figure out how long a TIME it takes for a force to cause a change.

Kinetic Energy. 1/2mv^2. A scalar quantity. NOT conservative (total energy is conservative, any given form isn't). Pitbull top tip: Use changes in Kinetic Energy to figure out how long a DISTANCE it takes for a force to cause a change.

So we can see, they are different in pretty much every way. Don't let the fact they both have an M and a V in them fool you into assuming they are very similar - they aren't!

What about Inertia and Mass then? Not sure this has been properly dealt with. There are two aspects to mechanics, translational and rotational. Everything above (I think) was about translation, i.e. an object moving around in its frame of reference. However, there are almost identical concepts relating to the rotation of objects.

e.g. We know (Newton 1) an object will maintain a given velocity unless acted upon by an unbalanced force. The rotational equivalent is that an object will maintain a given rotational speed and orientation in space unless acted upon by an unbalanced Torque. There are equivalent concepts for momentum and angular momentum, kinetic energy and rotational energy and so on.

Inertiais a term that applies to both parts of mechanics. It is, in general, the reluctance of a body to change what its doing. The reluctance of a body to be moved is its mass. However an objects mass does not tell us much about the reluctance of a body to change its rotation. For this we need a quantity that describes not just the mass but also the distribution of the mass (remember your basic gyro theory?). This is the moment of inertia.

So, Inertia is a general term that encompasses both mass and moment of inertia.

As such, mass and inertia are not strictly identical to one another, but rather mass is a subset of inertia.

However, if you are only talking about translational mechanics then you can petty much get away with using the terms interchangeably.

Clear as mud?

pb