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.