737 Improved Climb with Tailwind
 

Mesdames et Messieurs

A recent performance lecture threw up an interesting conundrum. It was discussed that when using the Improved Climb pages of our Airport Analyses, by using the 10kt tailwind column, as opposed to the 0 wind column, this resulted in lower V-speeds (V1 and Vr. V2 seems to stay the same) for the same (or similar) weight. Removing all the arguements about reduced flex etc, the question I am asking is:

“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?”

Bearing in mind that the V-speeds are based on weight and airspeed over the wing, the only reason I have been able to come up with is that when there is a tailwind there may be a reduced IAS (for a given TAS), as the compression in the pitot tube may be slightly less even though the airspeed over the wing maybe the same, thereby giving an under-reading on our IAS (although not our TAS). So if the IAS is under-reading, the V-speeds may reduced accordingly (while the TAS actually remains the same). But all of this really depends on the linear relationship between tailwind and relative pitot compression and whether or not one fully cancels out the other. This is way above my level of understanding and I turn to your learned selves for guidance.

Thank you in advance

Might this not be more about ASDR? If you have a tailwind then for a given IAS you'll have more groundspeed and therefore be eating up your runway faster. So you have to make the V1 decision earlier and so live with lower speeds than you'd otherwise prefer.

That was my first thought as well. However, even if you are very light at a bonkersly long runway where ASDR or obstacle clearance is never an issue, the same seems to occur. Also, if it were due ASDR, it shouldn't affect Vr.

Good thinking though.

Improved Climb
 

Is this using OCTOPUS?

Flugprestanda and EAG (we have just changed over, but it's the same for both)

Improved Climb
 

This is a question for Old Smokey.... :> :> :>

I didn't read your quesiton carefully enough. You mentioned the 737. I assume OCTOPUS is for Airbus only, as my first experience with OCTOPUS is with an Asian carrier that operates the Bus.

I can quote you the FCOM with regard to this question, but I don't think that will answer your question.

IAS/TAS relationship has nothing to do with ground speed. During takeoff, the wings don't care how fast the wheels are turning. They're interested only in the airflow over them.

Boeing's improved climb philosophy considers a longer than needed runway...using up more runway than you'd normally need...to 'get a jump' on second-segment climb. It's normally used to the biggest advantage at high density altitude situations where you have a bunch of runway.

So, normally, you might consider a lesser flap setting for takeoff (although not necessarily), a longer takeoff roll, with a higher V1/Vr. This will give the aircraft 'a boost' if you lose one after V1.

In essence, you have much more runway than you need to get it stopped...but, you're getting tight on second-segment climb. So, you make a trade. Use more of that excessive runway, help yourself out on second-segment climb.

However, I'm always opposed to 'fudging' figures in the charts. Yes, Airbus says you can take the numbers for a five or ten knot tailwind, even though you may have a ten knot headwind. As long as you have the performance needed with the tailwind, it's really okay to actually have a headwind (more conservative). (Sort of like having numbers for a displaced-threshold departure...and then taking off using the whole runway.)

The crux of the issue is: how much runway do you want to sacrifice for second-segment climb? On some of the 737s, a FLAPS 1 takeoff is allowed. Doing so would use up more runway, but increase your second-segment climb gradient. Or, you could do a FLAPS 5...using an improved climb strategy, and come close to accomplishing the same thing.

It's interesting looking at OCTOPUS for the 320 series Airbus. Many times a FLAPS 2 gives a higher V1/Vr than FLAPS 1. Using FLAPS 2 would be an 'improved climb' situation. But, perhaps chosing FLAPS 1 would give lower speeds with a similar second-segment climb profile.

Again, I can quote you the FCOM with regard to all of this...what Airbus says. But, I don't think all of that is relevant to your question.

Tom

Tom

Thanks for that. Do I understand from all that, that basically there is an additional safety factor in normal V1 and Vr speeds that can be done without when in a tailwind situation?

This all came about after looking at AGP Rwy 32, but when I looked deeper for interests sake, it also seemed to the case when operating out of unrestricted, hugely long airfields where there was no apparent concern with second, third or fourth segment performance... hence the conundrum! Maybe it's just that they automatically remove the safety margin whenever there is a tailwind, regardless of where you are.

V1, Tailwind, Safety Margin, et. al
 

Well, to say that the safety margin is removed when operating in an existing tailwind is not quite accurate. However, would it not be true that your ability to stop after a take-off rejection at V1 minus one knot is better with ten knots of headwind, rather than with ten knots of tailwind? I suppose you could view this as a reduction in the safety margin if you take off with tailwind.

But, then you could view the increasing of takeoff weight as a reduction in the safety margin, too. Consider two scenarios...all things being equal... but one takeoff is at 53t, the other is at 55t. I suppose you could say the 53t takeoff is safer, and taking off at 55t is a reduction of the safety margin.

There are several factors that figure into the equation of takeoff performance...some that you, as captain, have control over, some factors that you as captain, have no control over.

Again, it all boils down to engineering/performance tradeoffs. If, under one set of circumstances, you have twice the runway you need for a takeoff roll to V1, then a reject...well that's great. Now, all things same-same, figure in a ten knot tail wind into the situation. You can still accelerate to V1, reject, and still get it stopped before the end of the runway. Your 'safety margin' is reduced, but things are still okay.

Now, consider the above situation with a slightly shorter runway. Takeoff with no wind or ten knots of headwind...at a given weight...no problem with the reject scenario. But, change the surface wind to a ten knot tailwind, and you no longer will be able to stop if you reject at V1.

So, you reduce your takeoff weight until the performance of the aircraft allows you to stop with the above circumstances.

Is the safety margin reduced? Well, I guess you could look at it that way... If we could always have 12,000 feet of runway, no obstacles in the takeoff path, all takeoffs at sea level and 15 degrees OAT...that would be great! But, in reality, we have less-than-optimal circumstances with most flights. We consider all the factors, and make tradeoffs.

Again, it's a tradeoff between accelerate/stop and accelerate/go. If you have 12,000 feet of runway, and you really need only 8000 feet (under a given set of factors) to accelerate to V1, then reject...well, you're a happy captain. Change this situation to a ten knot tailwind, and that 8000 feet requirement becomes 9000 feet. But, you're still 3000 feet below what you've really got to work with. You're still happy.

Now, let's throw in the same scenario, but add the problem of a second-segment climb limitation. Well, you've got ten knots of tailwind...so, if you accelerate to V1 and reject...you need 9000 feet. But, you have 12,000 to work with...3000 feet extra. Ah!!! You can increase your V1 slightly, increase your Vr slightly...in order to help out with the second-segment climb problem.

Okay, so you've increased your V1/Vr a bit...now, if you accelerate to your new V1 and reject, you will need 11,000 feet of runway (assume all else the same...ten knot tailwind and all). Now, you have only a 1000 foot safety margin. Captain is now not as happy...but, still okay. But, you've 'improved' your second-segment climb ability.

Okay, let's assume the above scenario...but, after the increase of V1/Vr, you still have the second-segment climb problem...So, you reduce your weight a bit. Now, problem solved.

So, quite possibly, it is perfectly fine to do an improved climb take off with a ten knot tailwind. Yes, it's true, you'd be a much happier captain if that surface wind were headwind, instead. But, if the performance capability of the aircraft allows, no problem.

Will we throw the margin of safety out the window? Not really. It's just a tradeoff. You've actually increased your safety margin if you lose an engine at V1 plus one knot...and that was the limiting factor...while decreased your safety factor in the event of a rejected takeoff. Since the margin of safety for the reject scenario was incredibly fat...you gave up some of that fat to make you a happier captain should you lose one at V1 plus one.


Tom

Gentlemen

V1 is a take off decision speed. It is based on runway length and conditions that affect your "point of no return" ability to lose an engine or other no go decision, and stop on the available runway. Tailwind, wet runway, snow or ice would all have the affect of lowering this take off decision point.

VR is based on weight and power setting. It is the speed at which you may safely defy the laws of gravity and get the beast over the tree at the end of the runway with out scaring the birds nesting in there too badly.

Yes...Ironwalt
 

Yes, you're 110% correct!!!! I hope I didn't imply otherwise. Yes, thank you for this... :> :> :> :>

Thank you Sir.

I had to brush the cobwebs off of my "AvBrain" as I have not flown anything in two years. Seriously want to come back.

Chaps

I'll be honest, the issue of the reduced V1 was never really a problem for me. What was bothering me was the reduction in Vr. We're only talking about 2kts or so and at those speeds and rates of acceleration, does it honestly make that much of a difference? Having gone past V1, you're going anyway and it seems strange to me to then reduce Vr. Surely it would make more sense to keep it as the 0 wind to give you that extra 'boost'.

(I realise this is probably just me being a bit thick - it's been a long night re-writing the MEL).

Again, it's a tradeoff
 

Of course, it would be better to keep Vr as it was...Or, better yet, it would be better to increase it...

Again, it's a tradeoff...You're sacrificing a small bit of performance (or safety margin) on one end, to give a bit more safety margin on the other.

So, the crux of your question is: So, we reduce V1 because of the tailwind...but why not keep Vr as it was?

Again, it's an issue of climb gradient...what is the minimum required...what you'll actually have under your specific set of circumstances...

Consider this:
Why not reduce the V1 by a bit due to the tailwind (as discussed)...but INCREASE the Vr to that above what would be for Zero wind? Okay, now you're accelerating from V1 thru Vr on one engine...going very fast down the runway...even faster due to the tailwind...so, all is fine...you rotate at Vr, but you're running out of runway because you've rotated later...while going very fast (due, in part, to the tailwind)...running out of runway...and, you've got a FIRST-SEGMENT requirement. Uh oh!!!!!

Or, what about tire speeds????

Again, it's all in the tradeoffs. You can do all kinds of adjusting...here and there... Ultimately, you can reduce your actual takeoff weight.

In the end, the charts are true and correct. The figures in these charts take into account (I hope) all these factors. :> :> :> :>

Tom

That's it exactly mate. I fully accept the charts are right (particularly at places like AGP) and I was just trying to get my head round the unrestricted airfields. V1s have a massive range min-to-max, and on those long runways you can pretty much decide what you want (so long as it's above Vmcg). The Vr's I'm talking about are still way below max tyre speeds (they are perfectly okay with the 0 winds). As I said, I presume that they do it as a standard thing with tailwinds, regardless of where you are as even on the hugely long runways with no segment restrictions, it's still there.

Having said all this, it's never been an issue on the line particularly - just exploring the principles behind it.

I really appreciate you taking the time.

B737 Improved climb with Tailwind - PART 2
 

The original thread disappeared about 5 minutes ago, I can only guess that the original poster deleted it. Anyway, I've spent ages writing this response, so i'm going to post it anyway! I have removed any reference to the original poster.


Are you sure? Whilst we all like to think that they are correct, this may not be the case.

Would you agree that if an aircraft required X distance at a certain weight to get to the screen height, if a headwind exists that distance would be shorter or the weight higher. Conversely, if a tailwind exists, that distance would be longer or the weight lower.

I set out to prove this by using the B777 AFM-DPI, this is the electronic version of the Airplane Flight Manual and is a certified program. I was surprised to see the following:

Zero Wind
CRITICAL: Field Length Limit 263549 KG
CRITICAL: Climb Limit 263549 KG

10 kt Head Wind
CRITICAL: Field Length Limit 265606 KG
CRITICAL: Climb Limit 265606 KG
(This makes sense, headwind gives more weight for same distance)

10kt Tail Wind
CRITICAL: Field Length Limit 265606 KG
CRITICAL: Climb Limit 265606 KG

The 10kt HW and 10kt TW weight/speed/distance data is identical up to the screen height. I can take a greater weight with a 10kt tailwind that with 0kts!

Guess that this goes back to Mr Boeing.



Ironwalt
I don’t totally agree with you, the runway gives the weight, the weight gives the speeds. Therefore for totally difference runway lengths with a fixed weight you will have the same speeds. Kinda like what comes first the chicken or the egg :)

Mutt

Interesting...
 

Mutt,

I don't know why the threat disappeared. Maybe my responses were too lengthy! :> :> :>

Yes, that's very interesting. So, I guess it's all a matter of the computer software.

Thanks for jumping in here with your expertise in aircraft performance. I was barely keeping my head above water with this. :> :> :>


Tom

thread...not threat... :> :> ::>

Sorry chaps

Don't know what came over me. I was worried I was coming across as a bit thick :}

The V1 part of it I get (tailwind is a bit like any other contaminant). What I am struggling with is the reduction of Vr..... unless anybody else can add fuel to the fire?

Tom
Your answers were great and I really appreciate you taking the time to help me get my head round it all (if I'm honest, I still haven't totally - brainache!). But good work fella :D

mutt
Now that just really confuses me! Just as I was starting to get some sort of grip on it :\ But at least it's not just me then!

.. then let me use my annointed God-like mod powers to ferret about at the bottom of the peat bog and do a Lazarus on the original .. which I will combine with this reincarnation ..

Please, all, refrain from deleting threads ... sometimes it is incredibly frustrating to lose an interesting thread .. at least, with the current implementation of PPRuNe .. we can find deleted threads and undelete them ..

.. and, if you think that you have made a "dumb" post, don't worry about it .. just dig back through some of mine and I am sure that you will find that I have made a few dumber posts than you ever possibly could ... point is, in this forum, no-one (but no-one) is ever going to (be permitted to) make light of someone else's learning process .. so please don't ever have any fear of asking a question or putting a view ... as we say from time to time .. the only dumb question is the one which didn't get asked ...

by using the 10kt tailwind column, as opposed to the 0 wind column, this resulted in lower V-speeds (V1 and Vr. V2 seems to stay the same) for the same (or similar) weight

.. this sounds rather strange .. Vr and V2 should be sensibly tied together. Keep in mind, though, that, for the overspeed case, the V2 schedule is increased above the minimum V2 so there is no necessary problem with having a different degree of overspeed across an RTOW page.

when there is a tailwind there may be a reduced IAS (for a given TAS),

Probably not the case as the ASI doesn't have any way of knowing what the groundspeed and wind is doing ...

Might this not be more about ASDR

.. but if you reduce V1 for ASDR reasons, you still have to get from that (reduced) V1 to Vr .. so, what you gain here .. you lose there.

Maybe it's just that they automatically remove the safety margin whenever there is a tailwind, regardless of where you are.

No .. the general rules are the same ... in fact, there is a bit more fat in the tailwind case ... recall that the AFM uses only half of the headwind component, but one and one half times the tailwind component in the performance charts .. this is why you see a sharp discontinuity in all the wind component carpets at the nil wind line.

What was bothering me was the reduction in Vr

This can get messy .. as a principle, Vr will be linked to V2 so that, at the prescribed rotation rate, V2 (or thereabouts) will be achieved passing the screen height. 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 .. the V1/Vr ratio might vary quite a bit, on the other hand. Keep in mind that a more detailed aerodynamic model might well split a few hairs along the way to determining the final numbers .. considerations such as vertical wind profile may be at play here ?

I fully accept the charts are right

.. usually .. but beware of presuming perfection on the part of the technocrats. One of the great values to be had in a pilot's general knowledge and appreciation of things is that it assists him/her in the role of being the last line of defence. Be aware that one sometimes sees significant errors in things like letdown plate obstacle data, RTOW charts and such like. All the QA processes in the world only result in a high system reliability ... but not perfection ... looking at Mutt's observation .. even Mr Boeing may not be immune to real world reality

Guess that this goes back to Mr Boeing

.. now, that might be a bit embarrassing ... you don't think someone along the way might have left a sign out of an expression, Mutt ?

Thanks John

I'll grab a set of figures tomorrow to show you what I mean. As far as I was aware, Vr is Vr is Vr, so to find it being reduced like this is just doing my head in!:ugh: I think I'm going to have to go with 'safety margins', but something just doesn't sit right.

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.

Think I might ask in the FT forum on this one ...

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

mutt

Nice. The mists are clearing a bit now (sorry if this has been a bit hard work! :O )

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

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

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

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?

Bumblebee

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?

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!

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.

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

Insomniac

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.

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.

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.

A possible suggestion; it's not the Vr which is being limited, it's V2? Which would be consistent with both V2 values being the same.

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?

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 ...

I would consider this unlikely, the input parameter form allows for multiple wind columns, so all of the other parameters should, remain the same.

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..?

Full Wings,

I have to ask, if you have concerns about the weather reporting, "Why are you using Improved Climb"?

Mutt

I'm not but I could be if I needed to get extra weight: I don't want to base the performance on an incorrect assumption, namely the wind. We might be talking at cross purposes here but I don't see how using tailwind figures when you may not actually have one is in any way unsafe...:confused: 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...