Blip

There is opinion out there that says we should minimise control wheel displacement beyond a couple of units during a crosswind take-off as this will cause the flight spoilers on the up-wind wing to raise, causing additional drag. It has been said that this extra drag will reduce Vmcg and adversly affect take-off performance.

I can understand these concerns to some extent however with regards to Vmcg, unless V1 is equal to minimum V1 (due to Vmcg considerations), it doesn't really matter that Vmcg increases does it? For example if V1 is 130 kt, the Vmcg quoted in the manual for a particular take-off is 120 kt and due to spoiler deployment the actual Vmcg increases to 125 kts, if I suffer an engine failure at 120-125 kt I am going to be closing the thrust levers and stop anyway. Closing the thrust levers immediately removes the assymetric thrust

How often is V1 any where near Vmcg? For the vast majority of the time I would say it is not. And during my search on the subject before typing this post I read on an archived thread that Vmcg assumes nil crosswind in Europe and just 7 kts in the USA anyway. It seems the simple fact that a crosswind exists makes all the Vmcg data invalid even before I consider puting aileron into wind.

Now my question is, if I put in the required control wheel displacement such that it is roughly in proportion to the amount of rudder input and the wings stay level during the take-off roll and rotation, how much extra drag will there be? (I'm talking B737, B767, B747, A320, A330, A340.)

How would you quantify it in terms of head wind component or assumed temperature? That is to say if there is a 10 kt headwind and we make the take-off calculations assuming nil wind, would that be enough compensation to make the extra spoiler drag a non-issue? Or if the actual temperature is 30 degrees C and the assummed temperature is 45 degrees C, would using an assumed temperature of 40 degrees C be enough to compensate for the extra drag.

The reason I ask is because my gut feeling is there's more risk to the operation by not puting in enough aileron during a crosswind take-off for fear of spoiler deployment than there is by puting in aileron in proportion to rudder displacement and accepting the extra drag. This risk by under doing the ailerons comes during rotation.

I have seen first hand the 1-2 degrees roll during the take-off run (yes you can see it on the ADI) suddenly become roll of 5 to 10 degrees during rotation because there was insufficient aileron into wind. How we didn't get a pod scrape I do not know.

This sudden requirement to adjust the ailerons to where they should be to prevent the roll also causes a subsequent tendancy to rush the rotation. Is it any wonder many tail scrapes happen during crosswind conditions!

May I say having to REACT to a sudden roll at such a critical moment in the flight to prevent aircraft structural damage rather than having the correct aileron input to begin with seems like bad operational policy to me!

john_tullamarine

I shall watch with interest as the thread develops ..

A couple of points to note re Vmcg ..

(a) Vmcg is a bit of a vicious beast .. it sneaks up very rapidly and the aircraft can change from pussycat to tiger in the space of a few knots .. unless you have seen the situation, there is the risk that the pilot might be caught out ... unless you have your wits about you, one could find oneself in the grass without much delay or ceremony

I, for one, wouldn't be getting too close to the real Vmcg in a cavalier manner ..

(b) while the certification Vmcg is based on nil wind (the older BCARs used 7kt) - keep in mind, like many certification things, Vmcg is there to put a line in the sand - the realworld Vmcg is VERY dependent on crosswind .. increases in the range half kt/kt for twins to in excess of one kt/kt for quads is not atypical. Given a decent crosswind and a low weight ferry-type takeoff .. a failure on the "wrong" side can get super interesting.

Jonty

The way I do cross wind take-off's in the 757 is to put up to 2 units of aileron in, due to spoiler drag. But then as rotation is called apply more into wind aileron as the aircraft is rotated. This means that the spoilers have not compromised your position on the runway for V1 or rotate, but you wont hit a pod during the actual rotation.
The change in Vmcg is not quantified, that I have seen, for the 757. So its best not to try and second guess it.
Of course I stand to get shot down in flames for all this!

MrBernoulli

If the spoilers on one side deploy/partially deploy as part of the roll control system (as opposed to ALL spoilers being deployed for landing or in-flight speed reduction) then that is part of the design! Having roll control spoilers deployed during a cross-wind take-off will have been accounted for in the performance certification.

You may wish to look at these threads:

http://www.pprune.org/forums/showthr...sswind+takeoff

http://www.pprune.org/forums/showthr...sswind+takeoff

Hudson Bay

Correct!

You don't think the designers would have allowed a Pilot to work out how much aileron to put into wind after considering how much spoiler pokes up into the airflow! Do as your company SOP's state. Don't think you know any better.

Blip

Mr Bernoulli thank you for those links. I am sorry I missed such a recent discussion of the issue. I certainly do not wish to start another thread that covers the same ground.

Hudson Bay. Sorry. What are you saying "Correct!" to? Are you saying that you should put in whatever control wheel imput you require to keep the wings level during the ground roll prior to rotation?

" Do as your company SOP's state. Don't think you know any better."

The problem is, our company procedures do say to limit the aileron to 1 or 2 units and yes we have had a recent spate of pod scrapes and tail scrapes!

I have seen a post on another thead that quotes the B777 manual stating to put in as much aileron as you need, or perhaps more precisly it doen't mention limiting the aileron input at all. Yet on other Boeing type manuals it does say to limit aileron input.

Anyway it is hard to argue with "Do as the manual says". I would appreciate if someone could QUANTIFY the performance penalty. Thank you.

ruddman

Sorry to interrupt the thread, but what do you guys mean by 'units' when describing aileron use?

Rainboe

Divisions or units are marked on the control wheel boss. I have always worked on 1 unit/5kts crosswind.....it works.
BlipThere are 2 quite conflicting camps of opinion here. I believe you said it above- the drag from a few spoilers on one side slightly uplifted, at the low speeds before rotation, is immaterial. You only have to see how relatively ineffective full spoilers are below 250kts to appreciate that- at far lower take-off roll speeds, it's irrelevant. What is startlingly noticeable is rotating in a strong crosswind with not enough aileron held on! On a 747 7 hour flight, a copilot didn't hold any aileron on in a crosswind- I thought 'I'll let him feel what happens and talk about it later'. Even I wasn't ready for what happened- it was startling. When you do long heavyweight take-offs in a 747, I still think it better to have all the aileron you need until lift-off. I really think the drag is negligible.

Old Smokey

The drag attributable to sufficient roll spoiler to off-set crosswind effect during takeoff is negligible, and it is accounted for. The primary purpose of the use of all roll control during a cross-wind takeoff is to negate the assymetric lift effects caused by the crosswind, a reasonable lift differential, but only a small increase in drag.

The actual drag will only be of the order of a few hundred pounds, balanced against tens of thousands of pounds of thrust, it truly becomes negligible. From a performance aspect, it is a "nothing", or close to it.

What is of importance, as illustrated in Boeing manuals, is that these few hundred pounds of drag are assymetric, i.e. they are on one side only, and this CAN have a significant effect upon VMCG and VMCA. V1 often equals VMCG, and a significant directional control problem can be in the making if EXCESSIVE spoiler application has been applied at this point, and engine failure occurs.

Boeing's technique is to apply roll control (including spoiler) as required during the Takeoff roll, but with decreasing wheel angle as V1 is approached. Ideally, the wheel angle should not be beyond the point where spoiler activation commences - Nor should it be, at that higher speed, aileron alone should suffice.

Regards,

Old Smokey

RYR-738-JOCKEY

In the last few days there's been quite interesting x-wind conditions several of the places I've been to. At LBA yesterday 32 in use, wind was 240/26G40 Using 2 units of aileron input would not have kept the wing down. I try to adhere to SOP's as much as I can, but at the end of the day all I want is to get home in one piece. So I am using whatever input I feel is needed to keep us level.

Rainboe

I think that is the right answer. It is only when you experience the effects of a strong crosswind with not enough aileron that you weigh up properly the strong roll element on lift off, at minimal speed, against the unnoticably marginal drag element through having your spoilers cracked open on one side. It's not a time for take-off performance theory- the only priority is to keep wings level and get the aeroplane settling down as it yaws into wind and commences climb. With about a 10 degree margin before something strikes the ground at that moment, you can't afford an inadvertent roll just so that you can save a very small bit of drag from cracked open spoilers!

Jonty

In the B757 FCTM it states that "large control wheel inputs can have an adverse effect on directional control near V1(MCG) due to the additional drag of the extended spoilers."
So in the B757, no spoilers on the take off roll please! only after the rotation is started.

At the end of the day, you should ask your airlines training department what the recommended technique is for your aircraft and airline.

Jonty

Blip,
Nothing quantifiable for performance, as its more to do with controllability near Vmcg, than take off distance performance.

For the 757 the manual states that at normal UK temperatures and sea level Vmcg is 111kts

Now at light weights and short, wet runways the V1 can be below that:
80,000kgs, flap 15, 6000ft wet runway V1 is 109kts and at 70,000kgs its 101kts

Now without getting into a full blown disscution about take-off performance, which I know very little about, you can see that V1 and Vmcg can be that same and that can create controllability issues with spoilers up for the B757. Other aircraft my not have this issue.

Looker

An interesting topic and I have to admit switching from the 'limit control wheel input' camp to the 'use as much control wheel as you need' camp.

The reason for the switch was a recent B737 sim session in which exactly this problem was demonstrated and the wing lift with limited control wheel input on a 35kt crosswind was impressive and startling at the same time.

I fully respect Old Smokey's input regarding problems with vmcg and vmca with a low V1 but would I be correct to say that V1 / vmcg problems occur mostly on contaminated runways, short runways and low take off weights? Strong crosswinds and contaminated runways don't mix so this is an unlikely scenario, at low take off weights you can usually increase V1 = Vmcg without without compromising take off performance and for short fields there is no easy answer!

Statistical probability of engine failure at V1 is miniscule and on that basis I prefer to use the correct amount of control wheel input to prevent the rapid wing lift that will occur with a strong crosswind rather than fret about the possibility of an engine failure with a higher vmgc/vmca caused by spoiler deployment.

Happy to be corrected if I'm barking up the wrong tree.

BOAC

- this is the 737 guide - and it also warns of potential problems around Vmcg.

Rainboe

The recommendation on my last type, and as far as I am aware on the current, was 1 division or unit per 5 kt crosswind component. My experience supports this. I don't believe there is any harm in putting what you need on at the start of roll, and you can forget it for a while because up to 80 kts or so, ailerons are ineffective. Meanwhile, you can confirm the crosswind by feeling what rudder you need to keep straight, and then decide if that amount of aileron is actually needed. How do you know how much aileron is needed to hold level unless you actually start to experience wing lift? I think the aim to to avoid that situation. It's vital to have that aileron on for you when the wheels lift, because that is when startling things begin to happen if it's not there. From what I've seen, it happens very quickly indeed. So you have to decide are you going to stop it happening in the first place, or deal with it when you suddenly end up with a rapid roll away from the wind as soon as the wheels lift, and IMO opinion, belatedly correct a situation you have allowed to develop? As always, there are many ways to skin a cat.

ray cosmic

Recently converted from the 73 to the 74, and both operators had opposite ideas. On the 73 I was not allowed to roll into the wind, on the 74 I have to. I noticed that on teh 74, after about 2.5 units the spoilers come out. And that is actually already quite a deflection.
Funny technique: During rotation, roll to twice the amount of units you had before and you nicely fly wings level!

742

Life is full of compromises. And aircraft performance, as taught in typical ground schools, is full of hypocrisy and gapping holes. It is, IMO, bordering on delusional to think that the sharp pencil derived performance data is worth anything when the winds are gusting to 40 Knots. In high wind conditions I suspect that even crayon derived data would be an exercise in wishful thinking.

So I sit and ground school and behave while screen heights and “guaranteed performance” is talked about. And then when I go to work and arrive at PIK I fly the airplane. A straight track, no side loads on the gear and a few degrees of bank into the wind are all good. Drift, side loads and upwind wing lift are all bad. My view is to do what you have too in order to make the former happen and avoid the latter.

This increasing practice of sacrificing aircraft control for hypothetical performance issues is a very poor trade.

Blip

Here's an idea that might pacify the "no spoiler" camp.

Why not allow the pilot to put in the required aileron to keep the wings level, (basically in proportion to the rudder inputs) as a number of us have argued so far.

If we should suffer an engine failure approaching V1 and decide to continue, we can simply reduce the contol wheel input back to a couple of units while putting in the required rudder. Assess how the centreline tracking is going and if all is good and you feel there is excess rudder authority, start feeding in the aileron (and spoilers) back to where you want them. By that time you should be somewhere near Vr and all will be fine and dandy.

Even then this is only required if you estimate the REAL Vmcg (due to the existance of the crosswind in the first place) is between V1 and Vr.

For example the B737-800 with 26k engines. The highest Vmcg quoted is 105 kt at Max Thrust, 0 ft pressure altitude, and -60C. (At 20C it is 104 kt)

If I assume the ratio of crosswind to increase Vmcg is 1:1 then:

10 kt crosswind. Vmcg = 115 kt
20 kt crosswind. Vmcg = 125 kt
30 kt crosswind. Vmcg = 135 kt
35 kt crosswind. Vmcg = 140 kt


If V1 is less than this REAL Vmcg value, there is no issue, as an engine failure will result in an aborted take-off and the asymmetry disappears. Once you reach Vr well then again it doesn't really matter any more as you are now becoming airbourne and you are now concerning yourself with Vmca which is even lower than Vmcg (oh no is this another can of worms I see?)

It is only a concern if the REAL Vmcg fits in between V1 and Vr.

Often the difference between V1 and Vr is very small, just 5 kts or so. In many instances if your company utilises an unbalanced V1, it can be equal to Vr. In this last case have I just successfully argued that Vmcg is not a concern as there is no space between V1 and Vr for Vmcg to fit??

In the vast majority of take-offs, what is the difference between V1 and Vr? How long would it take to accelerate from V1 to Vr on one engine? Is this period long enough and is the shortfall in rudder authority great enough to really cause the whole event to end in tears on the side of the runway?

I don't know, I'm just thinking aloud.

Blip

Or...

Use aileron/spoiler as you require to keep the wings level.

If you suffer an engine failure between V1 and Vr, put in as much rudder as you require to keep the aircraft tracking the centreline. If you find the nose is still turning in to wind, THEN reduce the control wheel/sidestick input to a couple of units. The nose should stop turning in to wind and come back towards the runway centreline. By that time it should be time for rotation and the control wheel imput can be increased again.