737 Improved Climb with Tailwind
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Probably not the case as the ASI doesn't have any way of knowing what the groundspeed and wind is doing ...
True. I was just exploring the idea of whether or not there was a known ASI/compresssion error that was being taken into account when backs were up against the wall!
If we are only talking a knot or two, that's fine and came out of the flight test confirmation of the aerodynamic model .. but a significant playing about with the Vr V2 split would raise the eyebrows ..
Most times it is just a knot or two, but occasionally (only because I looked for it) it could be 5 or 6 kts. Eyebrows raised. Hence my brainache.
True. I was just exploring the idea of whether or not there was a known ASI/compresssion error that was being taken into account when backs were up against the wall!
If we are only talking a knot or two, that's fine and came out of the flight test confirmation of the aerodynamic model .. but a significant playing about with the Vr V2 split would raise the eyebrows ..
Most times it is just a knot or two, but occasionally (only because I looked for it) it could be 5 or 6 kts. Eyebrows raised. Hence my brainache.
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I believe that you will find that the answer lies in the takeoff analysis program logic. I’m presuming that you have a chart that states it’s based upon improved climb, but it doesn’t actually tell you the percentage of the speed increase.
One of the program inputs is optimum improved climb & V-speed, for the examples that I have examined, the improved climb index changes between 0-8 units depending upon the weight/wind. The tailwind will have an impact on the distance required from Vlof to screen height, therefore to achieve the same screen height at a specific weight in a given distance, the VR has to change, as will the V1. So for a zero wind calculation, it may use 5 units but 0 units for the tailwind. The zero wind takeoff distance will be less than TODA, the tailwind distance will be limited by the TODA. You cannot use the tailwind v-speeds for zero wind…. Unless of course you have a good lawyer
Its hard to examine these things when they come from an outside vendor, but I suggest that you look at the Improved Climb tables in the Operations Manual, and try to replicate the takeoff weights/speeds manually.
Safety Margins? Apart from the 50% headwind/150% tailwind, I don’t know of any additional safety margins when operating with a tailwind.
now, that might be a bit embarrassing It wouldn’t be the first time, but before I make a fool outta myself, I gotta check that my home version of the program is the latest version.
Mutt
One of the program inputs is optimum improved climb & V-speed, for the examples that I have examined, the improved climb index changes between 0-8 units depending upon the weight/wind. The tailwind will have an impact on the distance required from Vlof to screen height, therefore to achieve the same screen height at a specific weight in a given distance, the VR has to change, as will the V1. So for a zero wind calculation, it may use 5 units but 0 units for the tailwind. The zero wind takeoff distance will be less than TODA, the tailwind distance will be limited by the TODA. You cannot use the tailwind v-speeds for zero wind…. Unless of course you have a good lawyer
Its hard to examine these things when they come from an outside vendor, but I suggest that you look at the Improved Climb tables in the Operations Manual, and try to replicate the takeoff weights/speeds manually.
Safety Margins? Apart from the 50% headwind/150% tailwind, I don’t know of any additional safety margins when operating with a tailwind.
now, that might be a bit embarrassing It wouldn’t be the first time, but before I make a fool outta myself, I gotta check that my home version of the program is the latest version.
Mutt
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Originally Posted by mutt
One of the program inputs is optimum improved climb & V-speed......
Nice. The mists are clearing a bit now (sorry if this has been a bit hard work!
)Now, suppose that the weight you are rolling with is still well below the Corrected Allowable Take-Off Weight for 10kt tailwind, even if you are actually 0 wind, you still cannot use those speeds? (.... in human speak
I'm trying to follow this, but I'm getting a bit out of my depth.)My stance is that we should use the speeds appropriate to the wind (0 or 10 tail), however, I need to be able to give a definitive answer as to why using the lower tailwind speeds shouldn't be done.
Thanks
Last edited by metabolix; 22nd May 2006 at 09:04.
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Let look at it from two sides, the first is the aircrafts actual performance, if you have a 10 kt tailwind the calculation is actually based upon 15 kts by regulation. So the weights/V-speeds in the 10kt column are actually for 15kts, this is a conservative method of protecting you against a higher tailwind. It’s therefore fair to say that if the wind is less than 10 kts, you are still perfectly safe.
The second side is the “what if” scenario, for some obscure reason you find yourself sitting in front of an accident investigation board, they have had the time to review your FDR, your takeoff calculation and the weather, they also can review your companies and manufacturers operational procedures. How are you going to justify using the tailwind data for zero wind, what written procedure did you follow? Would you only apply your personal procedure to zero wind conditions or would you apply it with a 5kt, 10kt, 20kt HEADWIND? Its an extremely gray area and I believe that you will find that both your company and the manufacturer will hang you out to dry…….
So I suggest that as there is NO procedure that allows you to use the 10 kts tailwind data for zero wind conditions, you don’t do it!
Mutt
The second side is the “what if” scenario, for some obscure reason you find yourself sitting in front of an accident investigation board, they have had the time to review your FDR, your takeoff calculation and the weather, they also can review your companies and manufacturers operational procedures. How are you going to justify using the tailwind data for zero wind, what written procedure did you follow? Would you only apply your personal procedure to zero wind conditions or would you apply it with a 5kt, 10kt, 20kt HEADWIND? Its an extremely gray area and I believe that you will find that both your company and the manufacturer will hang you out to dry…….
So I suggest that as there is NO procedure that allows you to use the 10 kts tailwind data for zero wind conditions, you don’t do it!
Mutt
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mutt
My personal feelings exactly. As I said, I just needed to know if there was technically a reason not to do it. Appreciate your input - it's what I wanted to hear.
metabolix
My personal feelings exactly. As I said, I just needed to know if there was technically a reason not to do it. Appreciate your input - it's what I wanted to hear.
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V interesting thread..may I throw in a thought or two about the reduced Vr?
Using improved climb procedures, perhaps the 0-wind Vr may occur at the furthest point down the runway at which the climb gradient permits you to achieve the screen height, or meet other over-riding obstacle-clearance requirements?
Does it follow that with a tailwind, you might therefore need Vr to occur earlier along the runway for the same weight (a few knots less) to achieve the screen/obstacle clearance gradient required?
Or am I oversimplifyng things?
Using improved climb procedures, perhaps the 0-wind Vr may occur at the furthest point down the runway at which the climb gradient permits you to achieve the screen height, or meet other over-riding obstacle-clearance requirements?
Does it follow that with a tailwind, you might therefore need Vr to occur earlier along the runway for the same weight (a few knots less) to achieve the screen/obstacle clearance gradient required?
Or am I oversimplifyng things?
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Originally Posted by Bumblebee
Using improved climb procedures, perhaps the 0-wind Vr may occur at the furthest point down the runway at which the climb gradient permits you to achieve the screen height, or meet other over-riding obstacle-clearance requirements?
Does it follow that with a tailwind, you might therefore need Vr to occur earlier along the runway for the same weight (a few knots less) to achieve the screen/obstacle clearance gradient required?
Does it follow that with a tailwind, you might therefore need Vr to occur earlier along the runway for the same weight (a few knots less) to achieve the screen/obstacle clearance gradient required?
Thanks for taking the time to help out here. I'm on paternity leave at the mo and don't have the AAs to hand, but certainly I think you're right when it comes to the more 'restrictive' airfields (the ones with a 5 or 6kt drop in Vr). And the bonkersly long unrestricted airfields which also have a drop in Vr (albeit only a couple of kts at these places) I guess I'm going to have to put down to the fine tuning of calculations
And a quick question for non 737 drivers - does this phenomenon happen with your aircraft Improved Climb performance tables as well?
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Originally Posted by metabolix
And the bonkersly long unrestricted airfields which also have a drop in Vr (albeit only a couple of kts at these places) I guess I'm going to have to put down to the fine tuning of calculations
And a quick question for non 737 drivers - does this phenomenon happen with your aircraft Improved Climb performance tables as well?
I wonder whether the length of the actual strip of tarmac is really the focal point of the issue. The relative improved performance gains are not primarily for lifting extra weight off the runway as such, but translate into maintaining obstacle clearance ability at higher weights than a normal take-off would allow. Achieving engine-out screen-height and avoiding fixed obstacles within the take-off 'cone' might not necessarily be coincident - ie it might be the obstacles that produce the limitation, even though the length of the runway is miles and miles and miles!
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Sorry, I should have made myself clearer. When I said "bonkersly long unrestricted airfields", what I meant was bonkersly long, and seemingly unrestricted (obstacle clearance in the climb-out) airfields. I say 'seemingly', because I can't be sure without the tables in front of me, but I think so.
Edited to add: But I suppose there must be some sort of restriction there somewhere, otherwise you'd be able to lift max weight at whatever temp, provided you can reach engine out screen height and can achieve basic climb performance.
Edited to add: But I suppose there must be some sort of restriction there somewhere, otherwise you'd be able to lift max weight at whatever temp, provided you can reach engine out screen height and can achieve basic climb performance.
Last edited by metabolix; 23rd May 2006 at 16:32.
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metabolix; “If lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?”
Improved Climb is used when we are climb weight limited and we have a long runway. this allows us to increase the Climb weight limit by a certain amount.
This is done by adding an increment to the normal takeoff V speeds.
we basically use up more runway than we otherwise would if we used the normal speeds this gives us better climb capabilities.
in Zero wind conditions we would need less runway to get to v1 than we would in a tailwind. so it is for that reason that we reduce the Climb weight Limit increment..
eg Normal Climb Limit = 54.6t v1:137 vr:138 v2:145 (Zero wind)
if we use imporved climb we get a new climb limit of 55.8t v1:150 vr:154 v2:159 (Zero wind) (Climb weight limit is now 1.2t more than with normal speeds)
if we had 10kt tailwind we would have
a normal Climb Limit of 52.5t v1:132 vr:135 v2:142
if we used improved climb we would have
a new climb limit of 53.3t v1:141 vr:146 v2:153 (we can take 800kg more)
So therefore we can see that because of the tailwind we would need more runway to acheive the same speed as in nil wind. So they reduce the improved climb limit from 55.8t in nil wind to 53.3t in -10kts and hence use the lower speeds.
as for the V2 not changing. i disagree. if u look at the boeing improved climb performance charts u can see that there is an increment for v1 and another increment for Vr & V2 (ie the same increment for Vr as for v2) so i dont see how mr boeing could have gotten it wrong.
I hope that helps a bit
Improved Climb is used when we are climb weight limited and we have a long runway. this allows us to increase the Climb weight limit by a certain amount.
This is done by adding an increment to the normal takeoff V speeds.
we basically use up more runway than we otherwise would if we used the normal speeds this gives us better climb capabilities.
in Zero wind conditions we would need less runway to get to v1 than we would in a tailwind. so it is for that reason that we reduce the Climb weight Limit increment..
eg Normal Climb Limit = 54.6t v1:137 vr:138 v2:145 (Zero wind)
if we use imporved climb we get a new climb limit of 55.8t v1:150 vr:154 v2:159 (Zero wind) (Climb weight limit is now 1.2t more than with normal speeds)
if we had 10kt tailwind we would have
a normal Climb Limit of 52.5t v1:132 vr:135 v2:142
if we used improved climb we would have
a new climb limit of 53.3t v1:141 vr:146 v2:153 (we can take 800kg more)
So therefore we can see that because of the tailwind we would need more runway to acheive the same speed as in nil wind. So they reduce the improved climb limit from 55.8t in nil wind to 53.3t in -10kts and hence use the lower speeds.
as for the V2 not changing. i disagree. if u look at the boeing improved climb performance charts u can see that there is an increment for v1 and another increment for Vr & V2 (ie the same increment for Vr as for v2) so i dont see how mr boeing could have gotten it wrong.
I hope that helps a bit
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I hear what you're saying, but the bit I had a problem with is as follows:
(The numbers are not accurate, I'm just using them to show what I mean)
If we use Improved Climb (Airport Analyses) tables with a given weight of lets say 55.8t under the 0 wind column, it gives us speeds of v1:150 vr:154 v2:159
Now, although you are more temperature limited for a given weight under the 10kt tailwind column (although not prohibitively in most cases) the speeds for 55.8t are v1:144 vr:150 v2:159
As I said, these are not actual figures, just an example.
So the question is, for a given weight if lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?
The V1 is not a problem - I got that. But why Vr?
I am starting to see though that it has probably got a lot to do with engine out performance and achieving screen heights, which with a tailwind, might not be so easy if you spend longer on the tarmac with one engine inop. And the reason for not putting those same speeds in the 0 wind column seems to be that the 0 wind speeds are worked out for optimum performance whereas the 10kt tailwind speeds are calculated for minimum required performance. (Did I get that right? Or even close?) It should be borne in mind that the figures I am talking about come from the Airport Analyses, and the resultant speeds really depend on the criteria that they have used for calculation and the factors they use for different wind conditions.
I'm sorry if I haven't explained it very well, but it's the best I can do at this time of night! I do, of course, appreciate any other help you can give me.
I hear what you're saying, but the bit I had a problem with is as follows:
(The numbers are not accurate, I'm just using them to show what I mean)
If we use Improved Climb (Airport Analyses) tables with a given weight of lets say 55.8t under the 0 wind column, it gives us speeds of v1:150 vr:154 v2:159
Now, although you are more temperature limited for a given weight under the 10kt tailwind column (although not prohibitively in most cases) the speeds for 55.8t are v1:144 vr:150 v2:159
As I said, these are not actual figures, just an example.
So the question is, for a given weight if lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?
The V1 is not a problem - I got that. But why Vr?
I am starting to see though that it has probably got a lot to do with engine out performance and achieving screen heights, which with a tailwind, might not be so easy if you spend longer on the tarmac with one engine inop. And the reason for not putting those same speeds in the 0 wind column seems to be that the 0 wind speeds are worked out for optimum performance whereas the 10kt tailwind speeds are calculated for minimum required performance. (Did I get that right? Or even close?) It should be borne in mind that the figures I am talking about come from the Airport Analyses, and the resultant speeds really depend on the criteria that they have used for calculation and the factors they use for different wind conditions.
I'm sorry if I haven't explained it very well, but it's the best I can do at this time of night!
I do, of course, appreciate any other help you can give me.
Last edited by metabolix; 24th May 2006 at 00:52.
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Metabolix (I presume you are a fan of Asterix the Gaul ?),
I think that we need to have a look at the specific RTOW chart from which you are plucking figures as the story doesn't seem to be quite kosher .. is it possible for you to scan the page and email it to one of me, Mutt, or Old Smokey ? Likewise a copy of the WAT limit page for the flap setting would be helpful.
I think that we need to have a look at the specific RTOW chart from which you are plucking figures as the story doesn't seem to be quite kosher .. is it possible for you to scan the page and email it to one of me, Mutt, or Old Smokey ? Likewise a copy of the WAT limit page for the flap setting would be helpful.
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John
Will do.... (paternity leave at the mo, so may take me a day or two). The figures used in the example above were just for illustration purposes only and are not actual figures. However, it is a fair representation of what I'm talking about. I swear to God I'm not making this up!
As I said, I presume that when Flugprestanda enter the performance criteria (factor) for each column, the 0 wind speeds are worked out for optimum performance whereas the 10kt tailwind speeds are calculated for minimum required performance.
Will do.... (paternity leave at the mo, so may take me a day or two). The figures used in the example above were just for illustration purposes only and are not actual figures. However, it is a fair representation of what I'm talking about. I swear to God I'm not making this up!
As I said, I presume that when Flugprestanda enter the performance criteria (factor) for each column, the 0 wind speeds are worked out for optimum performance whereas the 10kt tailwind speeds are calculated for minimum required performance.
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Originally Posted by metabolix
If we use Improved Climb (Airport Analyses) tables with a given weight of lets say 55.8t under the 0 wind column, it gives us speeds of v1:150 vr:154 v2:159
Now, although you are more temperature limited for a given weight under the 10kt tailwind column (although not prohibitively in most cases) the speeds for 55.8t are v1:144 vr:150 v2:159
As I said, these are not actual figures, just an example.
So the question is, for a given weight if lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?
The V1 is not a problem - I got that. But why Vr?
Now, although you are more temperature limited for a given weight under the 10kt tailwind column (although not prohibitively in most cases) the speeds for 55.8t are v1:144 vr:150 v2:159
As I said, these are not actual figures, just an example.
So the question is, for a given weight if lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?
The V1 is not a problem - I got that. But why Vr?
Then we need to wonder whether there is something about the speed-spread model which is forcing a higher Vr-V2 split with a tailwind.
I haven't thought it through, but if the asumed takeoff technique is the same - rotation speed, target pitch, etc - and the 'tailwind' aircraft is actually at a faster groundspeed, could there be something in the dynamics allowing the aircraft to accelerate more during the rotation phase. Perhaps it's harder to get an accurate pitch target with tailwind takeoffs, so the FT data is worse for speed spread?
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Metabolix,
.. not doubting your reporting .. but the whole table might give us a clue as to what is going on .. the present tale sounds a tad strange to me.
MFS,
I can't recall any difference on the 733 for wind and nailing the target pitch was always a doddle .. especially if one did a raw data takeoff and didn't have that pesky FD getting in the way .. and I would be a bit concerned with the longer fuselages regarding the tailscrape problem using a lower Vr .. the story, though, just doesn't seem to add up ...
.. not doubting your reporting .. but the whole table might give us a clue as to what is going on .. the present tale sounds a tad strange to me.
MFS,
I can't recall any difference on the 733 for wind and nailing the target pitch was always a doddle .. especially if one did a raw data takeoff and didn't have that pesky FD getting in the way .. and I would be a bit concerned with the longer fuselages regarding the tailscrape problem using a lower Vr .. the story, though, just doesn't seem to add up ...
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As I said, I presume that when Flugprestanda enter the performance criteria (factor) for each column, the 0 wind speeds are worked out for optimum performance whereas the 10kt tailwind speeds are calculated for minimum required performance.
Note to anyone from Flugprestanda who is reading this, its time for you to have a Q&A section on your website!
Mutt.
mutt, you say:
Practically, I would beg to differ. Many places I fly to (Africa & Caribbean for starters) often give quite misleading weather data. I can cope with temperature because I have an accurate reading on the screen in front of me but they haven't yet fitted the 777 with an ground-reading anemometer.
If Abuja tower are giving 'light and variable', I use 10kts tail in the calculations as a 'worst case' (probably not enough, actually!), based on the reasoning that that I will get a better safety margin. I have thought long and hard and fail to see any case where you will get worse performance out of the aircraft if you don't get as much tailwind as you planned. A lack of accredited headwind, yes but tailwind, surely not..?
So I suggest that as there is NO procedure that allows you to use the 10 kts tailwind data for zero wind conditions, you don’t do it!
If Abuja tower are giving 'light and variable', I use 10kts tail in the calculations as a 'worst case' (probably not enough, actually!), based on the reasoning that that I will get a better safety margin. I have thought long and hard and fail to see any case where you will get worse performance out of the aircraft if you don't get as much tailwind as you planned. A lack of accredited headwind, yes but tailwind, surely not..?
I have to ask, if you have concerns about the weather reporting, "Why are you using Improved Climb"?
I can see the opposite, of course. It's a bit (only a bit, OK 
) like using assumed temperature - fine to assume 30 when the OAT is 15 but not 15 when the OAT is 30...