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Flying faster because of decreasing winds

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Flying faster because of decreasing winds

Old 16th Nov 2008, 22:49
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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.
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Old 16th Nov 2008, 23:51
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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
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Old 17th Nov 2008, 01:05
  #23 (permalink)  
 
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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.
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Old 17th Nov 2008, 03:10
  #24 (permalink)  
 
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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
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Old 17th Nov 2008, 03:28
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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.
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Old 17th Nov 2008, 09:09
  #26 (permalink)  
 
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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?
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Old 17th Nov 2008, 09:27
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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.
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Old 17th Nov 2008, 10:14
  #28 (permalink)  

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...kinetic energy, which is also a vector...
No, its not.

This is great fun....

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Old 17th Nov 2008, 10:36
  #29 (permalink)  
 
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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.
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Old 17th Nov 2008, 11:50
  #30 (permalink)  
 
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Smile 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.
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Old 17th Nov 2008, 12:01
  #31 (permalink)  
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Hello cpt G - thank you for your deference to my age. 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.

Yes it is
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Old 17th Nov 2008, 13:41
  #32 (permalink)  

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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
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Old 17th Nov 2008, 15:53
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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?
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Old 17th Nov 2008, 16:48
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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.
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Old 17th Nov 2008, 17:23
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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?
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Old 17th Nov 2008, 18:48
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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?
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Old 17th Nov 2008, 19:54
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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.
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Old 18th Nov 2008, 00:01
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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.
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Old 18th Nov 2008, 00:30
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Originally Posted by 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?
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?
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Old 18th Nov 2008, 06:19
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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.
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