Position of Elevators at T/O - A-320
 

Hi all,

To offset the effect of thrust application at T/off, FCOM recommends moving elevators halfway down till reaching 80kts and full down in case of tailwind or crosswind in excess of 20 kts. Why the difference and does it really prevent the nose from coming up when T/Off power is applied???

What is your tailwind limitation?

Mutt

To clarify: the recommendation is for the event of any tailwind (up to limit (A320) of 10kts) or crosswind in excess of 20kts.

I thought it was only up to 10 kts? I wouldn't be too happy departing with a tailwind in excess of 20kts.

I think you'll find the FCOM might say "in a tailwind or in a crosswind in excess of 20kts" - they just forgot the comma after tailwind. Meaning you do this in ANY tailwind, AND in crosswinds exceeding 20kts. I don't think you can takeoff in tailwinds of greater than 10 in the bus. Have just started though, so could be wrong.

Why don't companies understand the importance of punctuation?:mad:

The answer to your question, I personally believe it does make a difference, and does stop the nose coming up excessively on the t/o roll. It also allows for better directional control as you roll down the rwy. So though it sounds a little stupid, I think it's quite effective.

1234

Haven't seen it myself, but TP mates have told tales of T/W increase trials with aircraft sitting on their tails when the brakes are released ...

Can't see any reason to increase separation as same dynamic would apply to the following departure,,,therefore same separation standard.

To clarify the topic further Airbus ask you to

- input half forward stick for normal take offs which results in full elevator down at roll start and progressive decrease of this deflection until the sitck is gradually released to reach neutral by 100 kt
- input full forward stick for all tailwind take offs and also if x-wind is above 20 kt. This results in full down elevator all the way until the stick is gradually released to reach neutral by 100 kt

The reasoning behind I was told is to increase load on NLG for better control.

Yours,
FD (the un-real).

My apologies for the confusion which seem to have
resulted because of my missing a miserly comma somewhere. Hopefully any 320 driver after reading my post would understand that it is half elevator down in normal case and full elevator down in case of any amount of tailwind( I do know that the tailwind limit is 10 kts) or crosswind in excess of 20 kts.

With this clarification, I am still hoping someone experienced on the 320 will be able to clarify as to why this difference? Really interested in the aerodynamic effect.

Grateful to all those who have contributed so far

FD
Surely half stick gives half elevator deflection?

In theory, but have a look at the FLT CTRL page next time you do it!

You don't need a comma here. The phrase is written correctly.

Not on an A320 (or other Airbuse FBW aircraft) stick position has no bearing on surface position. When the stick is moved the computers position the surface where it should be for the current conditions.

Or maybe it is to over come the effects of the horiziontal stabilizer which is trimmed for takeoff, preventing premature rotation.

But on the ground, during the take-off roll the flight control system is in direct law so stick position is directly related to control surface position.

Correct "Max Angle" (You beat me to it)
Plus, just to be a pedant. Some of the A320/1 series have a 15kt tailwind limit for takeoff! (MSN [for 321]around 2000 & below, if memory still works)

You Airbus guys can't take a joke?

Sidestick command is mapped similarly for nose-up and nose-down inputs, so that you don't get twice the sensitivity when applying a nose-down input (since nose-down travel is 1/2 that of nose-up: 15 degrees vs. 30). Similarly, when you apply half nose down stick, you are commanding half of 30 degrees, or 15, which is full nose down elevator. Simple, really :)

P.S. Nothing is ever what it seems on the Bus! :p

Thanks guys, but still hoping for a definite answer to this one. I am sure there is a experienced A-320 hand out there, ready with the answer.

First why the difference in sidestick position and secondly what does it actually achieve. Does the jet exhaust go and hit the elevator and as a result push the nose down.

I am sure the THS position has nothing to do with it.

it counters the nose up effect of setting engine takeoff thrust

one point of note, if you look at the FCTL ECAM pg as you do the flt control checks, half stick down already gives full elevator deflection; hence full stick is not actually achieving anything.
changing from half to full stick down in the conditions on topic, I am told, is a procedure that all Airbus perform, and on the 330, there is a difference, but not on the 320. A 330 jock may be able to shed some light on this information.

...but to play devil's advocate, at the start of the roll there's no airflow over the elevator, so it's not doing anything. By the time there is some useful airflow over the aeroplane, (e.g. 80kts) you release the stick to neutral! Your point about the F/CTL page is what I was getting at.

So, what function does the elevator do at low speed? Is the jet exhaust enough to make a difference?

that's simply what it says in the SOPs
as to the reasoning, obviously there is no aerodynamic influence of the elevator position from a standing start, I'll give you that
it really doesn't appear to be entirely logical, and I can only surmise it is more logical for a rolling take-off where the power is set at a higher speed where there will then be some tailplane effectiveness.
as to what the speed is where it has an influence, Airbus do not publish that with the manual set.

well, there *is* already airflow over the elevator as soon as your speed is above zero, whether useful or not is depending on the situation. From experience on traditional and Airbus aircraft I can tell that there is already significantly more pressure on the nose wheel after a few meters during TO.

Airbus could have written "push the stick half full forward at 20kts", but that also makes not much sense, so it's better to start with the stick position at the start of the TO procedure.

Above 80kts the pressure on the nose wheel is so strong that it is a good idea to release the stick. I'm pretty happy that AI had the cleverness to write this down, because I see so many people pushing that yoke so hard (on other types), I sometimes fear that there will be a hole left on the runway...

HB

To increase tyre cornering effect and thus directional control.

(Please, no grammar classes , since english is not my native language)

A-3TWENTY

Gary Lager & DNR

My thoughts exactly. So, still no final word on why half elevator down and full elevator down. It has to have a reason. I don't think Airbus will put in such a thing without any proper justification.

I expect Harry and the others have it right; that there is some aerodynamic effect in favour of improving control, even at low speeds, and it is more sensible from an SOP point of view to select the elevator reqd before takeoff than trying to do it on the roll.

Perhaps there might be an answer on the flight testing forum - it sounds like the kind of procedure which might evolve from operational testing, as opposed to the drawing-board theory.

If there is a tail wind of 10 knots wouldn't lowering the elevator make the initial situation worse (at least until the aircraft reaches 10 knots ground speed)?

ss41: Sorry to be picky, but it really is FULL elev down for both cases at start of take-off roll. True, it is achieved by different sidestick deflection.

As far as the reasons, FCTM does not help very much either:
[Speculation ON:] If it were for better NWS grip the situation when more is required is not a normal TO but engine fail scenario at low speed, when aerodynamic surfaces are not yet effective. The different thrust application technique is seeimgly developed to lower the risk of surge under tailwind/crosswind. Should that happen perhaps there is benefit of having full stick down. [Speculation: OFF]

I am pretty certain that full stick down gives full elevator down all the way until the stick is released by 80 kt. Also I am pretty certain that half stick gives full elevator down with IAS=0. What goes on with elevator between 0 and 80 kts when only half stick is applied I am not sure.

Yours,
FD (the un-real)

The nose-up couple at T/O thrust is significant due to the low-slung engines, and will unweight the nosewheel to some degree, reducing steering effectiveness. If you look at FCOM 3.03.08 P3 GROUND RUNUP DANGER AREAS you will see that the exhaust gases pass the outer half of the tailplane and elevator at over 65mph (CFM) and 68mph(IAE). This is where the upforce on the tail comes from at low speed.

FD
You seem to have gone from "it really is FULL elev down" to "I am pretty certain that half stick gives full elevator down with IAS=0" in one post!

From FCOM 1 : "Ground mode is a direct relationship between sidestick deflection and elevator deflection, without auto trim." Why would half sidestick deflection give full elevator travel? Please explain because if what you say is true
1. I have been labouring under a misapprehension during all these years of watching flight control checks
2. The different procedure in a tail or crosswind is pointless.

As suggested before TP, when looking at the F/CTL page during your checks have a look how much down elevator you get with half-stick fwd. Looks pretty close to full down to me.

How are you estimating half sidestick deflection?

use the maltese cross on PFD... TP.

Yes I know that I wondered how Gary and FD do it!

No different than you, I am certain - eyeball Mk.1 and the instrument provided. For instance here I would say here we have about 40% up input.
http://img89.imageshack.us/img89/4149/maltese.th.gif
My observation shows that when you input half forward stick as observed on PFD you get full down deflection of the elevator on F/CTL page. I had tried to locate a reference in the manuals but did not find it.

The ground mode description you quote is indeed correct but please note that it does not say that "amount of side stick deflection commands the (elev) deflection in full range of travel". Rather it says "direct relationship". What kind of proportion or conversion factor is there in the relationship, if any, is not explained. There are other cases, where control to surface ratio is not 1:1 and/or varies with speed: RUD TRVL LIM, NWS spring to mind. Even the ground mode description of elev UP says that max. deflection is reduced by 75 knots to 66 % of travel.

As I said I cannot find written explanation of what I saw, ... but people are seeing things.:) I will try to get a snapshot.

Back to the original question ss41 asked: Why does Airbus call for half or full elev DN for tkof? I comment that Airbus does nothing like that they ask us for half/full sidestick input - and no, that is not the same thing.

The reason for DN elev input (however large and achieved by whichever means) had been established. As the underslung engines introduce significant nose up moment the NW is unloaded. To re-introduce more weight onto NW, down elevator is applied. Even with aircraft stationary the engine exhaust provide sufficient airflow to achieve the desired effect.

I only substituted "start of take of roll" with "IAS=0" ?.

It is not necessary to dig any deeper until we can establish the facts. I shall get the photo and you will stop your next flight control check halfway stick down and observe elev pos on F/CTL page before proceeding as required. Deal?:ok: Of course, all parties are sincerely welcome, AIDS Alpha callout would be nice or, IFixPlanes where are you?

Oh, speaking A320 and not the other buses, are we?

Yours,
FD (the un-real)

Lord, I just hope so much I hadn't been seeing things, please, please...

Thank you FD ( hope the real one)

I will also check, what you say, on my next flight. Till then, I am also keeping my fingers crossed.

also done to avoid a Tailstrike

Definitely some blast back there.
B737 at TO thrust.. .

http://www.xplanefreeware.net/morten/DOCS/737ex.jpg

That's a very illuminating diagram, XPMorten, thanks. I think that probably puts the original question to bed!

http://img30.imageshack.us/img30/4043/100pcdn.th.jpg http://img136.imageshack.us/img136/8421/48pcdn.th.jpg

Looking at a profile shot of the Airbus, as opposed to the 737, I cannot see how there would be any effect of thrust over the tailplane; it's simply too high above the engine outlets.

Had a look on a take-off at the FLT CTL page as I rigidly held the half stick down position, and witnessed the full down elevator begin to rise at around 60 knots. Have not had a chance to look at the full stick down as yet; I now suspect it would retain the full elevator position all the way to 80kts.

Hence the fly-by-wire changes the relationship between the stick position and elevator authority as the airspeed changes even at these slow speeds.

The elev position is not identical. For half stick the indication stops atop the end-band while with even little more (and also full) forward stick is jumps (non linear movement) to the middle of the end band.
http://img20.imageshack.us/img20/2912/halfb.th.jpg http://img16.imageshack.us/img16/2706/fulllkk.th.jpg

FD (the un-real)

I had a look at the Flight controls and am quite confident that you can hold the elevators half down and full down; just as the FCOM recommends.

Now the original question once again. Why this difference?? What is the benefit of holding elevators full down in case of tailwind or cross wind in excess of 20 kts.