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A320 Crossover Angle of attack

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A320 Crossover Angle of attack

Old 8th Apr 2021, 07:39
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A320 Crossover Angle of attack

I have recently watched a video about the 737 where at medium angle of attacks (flaps 5 speed Vref+40) a complete deflection of the rudder will generate a more powerful roll moment than the ailerons can. This comes from the 737-300 PCU issues you may have heard about. Note this is an all engine operating case.
I am wondering if this exists on the 320 family as well ( assume it is a factor, but the flight control computers will prevent this in normal law) and if this can become an issue in Alternate / Direct law, and roughly what angle of attack this is.

If anyone has some good documents of the test flight results of the A320 family, there are some more things I'd like to know, for example the Mach no where control services can reverse.
Thanks in advance
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Old 8th Apr 2021, 08:07
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Unlikely, I think. I've had full rudder in at low speeds in the simulator with engine failures and never felt that the roll authority was limited. I would also expect the spoilers to always be able to overcome the rolling moment of the rudder.

I can't conceive of a rudder hard-over event occurring on the A320 series - it would require, essentially, a fault in four independent computers (2 x 2 command and monitoring channels) and a failure of G+B or G+Y hydraulics.
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Old 8th Apr 2021, 08:35
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I'm not talking about an engine failure case, as the rudder counteracts the force on the engines. With engine failures, Vmca and Vmcg come into play and i've seen the effect of this, but this is the engine overcoming the rudder force. I've done this in the sim as well as I'm A320 rated.

I agree it's unlikely in normal conditions, as the yaw damper is limiting the amount of rudder authority you have. I'm just wondering if theoretically you can come in this situation when yaw damper is failed, no protections.
I will try it in the sim in 2 months, I am just wondering if anyone has asked / done / seen this before and in what conditions.
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Old 8th Apr 2021, 08:59
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Isn’t the RTLU that limits the rudder travel?. Yaw damper only provides inputs adding or subtracting to your mechanical inputs.
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Old 8th Apr 2021, 09:11
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My bad, how we get to the stage doesn't really matter, for the sake of the argument let's say both FACs failed, leading to a failure in RTLU and alternate law. If now for some reason the rudder gets jammed in a full deflection, when can I expect this to happen? Is it within the normal range of speeds, or maybe well beyond the stall range, so we should never get there anyway.
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Old 8th Apr 2021, 09:14
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SavedByTheTCAS: I am just wondering if anyone has asked / done / seen this before and in what conditions.

Yes. On TriStar (L1011), it sometimes felt like you would run out of aileron authority when aligning with the runway during a landing in strong crosswind (e.g. Bermuda). The learned technique was to land with up to 5 degrees of crab remaining and 5 degrees of into wind bank.

Have a read of the Lufthansa A320 at Hamburg 2008 AVHerald incident report . The Full BFU report.
Despite full side stick by both Captain & FO, the aircraft still rolled to 23 degrees whilst a wheel was on the ground. The BFU report mentions "
The limited effect of lateral control was unknown." the aircraft went in to "Ground Mode" (only half aileron deflection during on ground logic above 80 kts) - hence loss of aileron authority.

Last edited by Goldenrivett; 8th Apr 2021 at 11:24. Reason: BFU report link
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Old 8th Apr 2021, 09:21
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I would hazard a guess in practically all swept wing airliners that the use of large deflections of rudder at slow speeds (high alpha) would create rolling moments that could exceed the power of the ailerons to counter.

This would be because of the large slip angles that can be generated at low speed and the effect slip has on on the swept wings creates a large differential in wing lift that could not be corrected with full aileron.

The effect of any wing dihedral might exacerbate the rolling moment?

Hey, but I am only a pilot...we require an Aerodynamicist armed with theory and graphs to chip in!
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Old 8th Apr 2021, 11:03
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SavedByTheTCAS

I doubt it. All 3 actuators are simultaneously operational on the rudder. Unlike the other surfaces except elevator which can activate both in the event of saturation.

it’s a pretty cool system. Basically there is an input rod that goes from the bottom of the rudder at the quadrant to the top. If simultaneously actuated all 3 actuators via an individual spring rod to the servo part of the valve. You have an input to the rod, it moves the input lever to all servos and the servo moves nulling out the command. If one servo was to jam there is an individual spring rod at each servo input. Thereby the other 2 will overpower the failed actuator.

let’s say the input to the rtlu broke. The rtlu is at the top of the rudder. Only effect is no RTLU.

Or if the input at the bottom of the rudder broke. The RTLU has an output on the same arm that drives the rudder, it has a centering spring rod. Which keeps the rudder centred and you have no rudder input. But the rudder stays centred.

or if a rod broke between servos, the servo would be in zero and the others would override it. The middle and upper servo have an additional centering spring rod incase there is a disconnection between the artificial feel and the middle servo actuating lever.

the rudder has an RVDT monitoring the surface position via the SDAC’s so if in the event it moves and it’s not supposed to. I’m pretty sure you will know about it.

in summary. By design, I’d say virtually impossible.
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Old 8th Apr 2021, 12:36
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^^ brilliant post, thank you
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Old 8th Apr 2021, 13:08
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The description at #1 re the 737, is ‘Crossover Speed’ - not AoA. This is the speed at which full opposite roll-control cannot maintain wings level flight with full rudder application due to failure.
A quick search for ‘A320 crossover speed’ did not find anything of help.

Because crossover speed is not a certification requirement it is unlikely that there have been specific tests. Although some manufacturers / authorities will investigate the situation as a qualitative assessment of aerodynamic response and control effectiveness.
The 737 issues involved accident investigation after rudder failure (The 737 Rudder Story)

With much higher integrity control systems in modern aircraft it is unlikely that a failure situation will be encountered. However, there may be unlikely situations where the yaw input is generated by means; e.g. the un-noticed combination of having one engine at high thrust and the other at low thrust or failed, and the crew not using rudder.
A remote situation, and more interesting scenario as above, but with the AP engaged which reduces the available roll control.
Lateral control limiting with sideslip

Last edited by PEI_3721; 8th Apr 2021 at 16:38.
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Old 8th Apr 2021, 14:32
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I would also treat any data you obtain from the sim with a pinch of salt. A lot of them are far from perfect and some of it is "best guess" when you start operating at more extreme edges of the envelope.
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Old 9th Apr 2021, 05:31
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Originally Posted by PEI_3721 View Post
The description at #1 re the 737, is ‘Crossover Speed’ - not AoA. This is the speed at which full opposite roll-control cannot maintain wings level flight with full rudder application due to failure.
The more fundamental variance is with AOA, not speed. If it was just a matter of speed, then the relative effectiveness of the rudder vs. ailerons would not change, as they're always flying at the same speed.

But dihedral effect (pushing on the rudder causes a roll) for a swept wing, increases with AOA. So at high AOA (read: low speed at steady state) the rudder is more effective at rolling, and thus, may overpower the ailerons.
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Old 9th Apr 2021, 12:00
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Vessbot, thanks for that view.
Semantics aside, the speed based deffiniton is widely used thus might have helped with a search; apparently not so.
The AoA aspect, not debated, also reflects differences in viewpoint. Theory or practice, steady state vs the dynamics of real situations which crews have to manage.
Limit points are usually static - steady state; thus in flight tests the ‘crossover point’ can be defined with smooth coordinated cross-control inputs without changing bank angle or speed (AoA). However, in reality if an aircraft is suddenly yawed - rudder failure, the dynamics of yaw to roll, speed / AoA change, as you note, could overcome the roll control power. Thus although an aircraft is within a limit condition, flight control is lost because of the combined need to stop the roll and recover to wings level.
This supports the comment by The Rt Hon minister (#11), that simulators, particularly training devices are no more than best guess; even more so if the manufacturer has not established the limit condition or recorded the dynamics of off-limit conditions.
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Old 9th Apr 2021, 18:37
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Vessbot

Is that an opinion or a research based observation?

Rudder causes yaw,
Yaw redefines effective sweep and span.
Effective sweep creates lift imbalance L-R and thus a rolling moment.
One aileron works on a lengthened arm, the other on a shortened one, both opposing the rolling moment

The kinematics seems not overly complex, but far from "higher speed or slower, the lift increase on control surfaces evens out between the rudder and ailerons"

IDK, just a FBW driver.
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Old 9th Apr 2021, 19:57
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From the USNTPS Flight Test Manual (publicly available here):

Additionally, dihedral effect of the
airplane with swept wings is directly related to lift coefficient
[i.e., AOA - my comment] (Figure 5.11) Airplanes with
sweepback will exhibit increasing positive dihedral effect with increasing lift coefficient;
these airplanes may tend to have excessive positive dihedral at low airspeeds.


and the figure:


(A note on collision of terms... the C sub L on the horizontal axis is Coefficient of Lift, as it says. On the vertical axis, the C sub L sub beta (also, graphically misaligned) is rolling moment due to sideslip - aka dihedral effect)

Last edited by Vessbot; 9th Apr 2021 at 20:22.
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Old 10th Apr 2021, 03:39
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Very nice. In my book the dihedral effect is a stabilizing one, i.e. acting in opposition to a roll disturbance.

We agree easily that the dihedral effect on a swept wing is magnified with increasing AoA.

The discussion seemed to revolve correctly about a cross-over speed, where lift available at aileron centre of pressure creates moment insufficient to counter the lateral lift imbalance introduced by yawing a swept wing configuration, the airplane tries to capsize.

Your dihedral effect, the power of which increases with AoA, acts in opposition to that.

Lower speed when yawed takes you closer to that snap roll point.
Lower speed at constant path means higher AoA.
Higher AoA provides for stronger anti-roll effect as explained.
Forensic evidence shows even that is not sufficient and nothing saves once you go too slow.

Nothing wrong with discussing cross-over speeds.

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Old 18th Apr 2021, 11:08
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Aileron Displacement

Been a long time since flying a Boeing, but I’m thinking the amount of actual aileron displacement with a given control wheel movement is dependent upon flap setting. What I’m saying is, you don’t get full aileron displacement with full control wheel movement until you’ve selected Flaps 40. At Flaps 0, moving the control wheel to the stop gives little actual aileron displacement.

As I recall.....and I could very well be wrong...this is the case on both the 727 and 737. (I have no experience on the newer 737s.)
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