Position of Elevators at T/O - A-320
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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
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
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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.
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.
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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.
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.
Only half a speed-brake
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)
As far as the reasons, FCTM does not help very much either:
On a normal take-off, to counteract the pitch up moment during thrust application, the PF should apply half forward (full forward in cross wind case) sidestick at the start of the take-off roll until reaching 80 kts. At this point, the input should be gradually reduced to be zero by 100 kts.
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)
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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.
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.
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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.
Only half a speed-brake
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.
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?
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...
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.
but it really is FULL elev down for both cases at start of take-off roll.
Also I am pretty certain that half stick gives full elevator down 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?
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...
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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.
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.
Only half a speed-brake
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.
FD (the un-real)
FD (the un-real)
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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.
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.