Flying faster because of decreasing winds
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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
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
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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
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
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Olendirk - you certainly keep on coming up with strange questions.
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.
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.
so many guys say you can fly a constant ground speed. but how?
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.
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
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
Last edited by Chris Scott; 17th Nov 2008 at 00:45. Reason: No changes since John Tullamarine's. In the penultimate paragraph, readers are warned that I loosely used the word "inertia" to mean "kinetic energy". See post #22.
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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 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.
Inertia is a function of GS,
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.
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.
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.
The misconception, however, that velocity is mesurable only reference the earths surface is what leads to faulty concepts like the infamous "downwind turn" myth.
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.
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.
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.
Psychophysiological entity
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.
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.
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Inertia is a function of GS
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.
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.
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.
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?
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...
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...
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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
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
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.
Last edited by Port Strobe; 16th Nov 2008 at 22:39.
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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 ..' "
(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 ..' "