A320 Flight Controls

[TheBusFlyer]

Good morning,

In normal law and in alternate law, side-stick commands g load for pitch.
You also have pitch angle protection.

So if you pull full AFT, you are basically asking maximum g load.
Am I right till here?

Let's say that we are in clear configuration.
So we can get +2.5g to -1.0g.

How do I reach +2.5G?

Will continue after some researches.....

Thanks a lot,
TBF

 

[TheBusFlyer]

The side-stick maintains 1g when neutral.

Can I say that 33° pitch angle = 2.5g?

 

[Henning]

How does pitch angle equate to G in this system?

 

[Rex Kramer]

Good morning,

In normal law and in alternate law, side-stick commands g load for pitch.
You also have pitch angle protection.

So if you pull full AFT, you are basically asking maximum g load.
Am I right till here?

Let's say that we are in clear configuration.
So we can get +2.5g to -1.0g.

How do I reach +2.5G?

Will continue after some researches.....

Thanks a lot,
TBF

Full aft stick will get you to max G or max AoA, whichever limit is respectively lower.

 

[Henning]

Full aft stick will get you to max G or max AoA, whichever limit is respectively lower.

Say I slap the stick back, will it pitch up to max G moment up to max pitch angle? If so, what is that G moment?

 

[RotorAndWing]

From the FCOM:

The normal-law flight mode is a load-factor-demand mode with automatic trim and protection throughout the flight envelope.
Following normal law, the sidestick controllers set the elevator and THS to maintain load factor proportional to stick deflection and independent of speed.

With the sidestick at neutral, wings level, the system maintains 1 g in pitch (corrected for pitch attitude), and there is no need for the pilot to trim by changing speed or configuration.
Pitch trim is automatic both in manual mode and when the autopilot is engaged. In normal turns (up to 33 ° of bank) the pilot does not have to make any pitch corrections once the turn is established.
The flight mode is active from takeoff to landing, and follows the logic shown schematically (Refer to DSC-27-20-10-10 General).

Automatic pitch trim freezes in the following situations:
‐ The pilot enters a manual trim order.
‐ The radio altitude is below 50 ft (100 ft with autopilot engaged).
‐ The load factor goes below 0.5 g.
When angle-of-attack protection is active, the THS setting is limited between the setting at the aircraft’s entry into this protection and 3.5 ° nose down. (Neither the pilot nor the system can apply additional nose-up trim).
Similarly, when the load factor is higher than 1.25 g or when the aircraft exceeds 33 ° of bank, the THS setting is limited to values between the actual setting and 3.5 ° nose down.
When High Speed or High Mach Protection is active, the THS Setting is limited between the setting at the aircraft’s entry into this protection and 11 ° nose-up.

 

[Henning]

So basically it limits pull to 1.25G and push to .5G?

Interesting that the auto trim disables at low altitude. Is this to guard against risk of low level runaway?

 

[RotorAndWing]

GENERAL
The normal law protects the aircraft throughout the flight envelope, as follows :
‐ load factor limitation
‐ pitch attitude protection
‐ high-angle-of-attack (AOA) protection
‐ high-speed protection.

LOAD FACTOR LIMITATION
The load factor is automatically limited to:
+2.5 g to -1 g for clean configuration.
+2 g to 0 for other configurations.

PITCH ATTITUDE PROTECTION
Pitch attitude is limited to :
‐ 30 ° nose up in conf 0 to 3 (progressively reduced to 25 ° at low speed).
‐ 25 ° nose up in conf FULL (progressively reduced to 20 ° at low speed).
‐ 15 ° nose down (indicated by green symbols “=” on the PFD’s pitch scale).
The flight director bars disappear from the PFD when the pitch attitude exceeds 25 ° up or 13 ° down. They return to the display when the pitch angle returns to the region between 22 ° up and 10 ° down.

HIGH ANGLE OF ATTACK PROTECTION
Under normal law, when the angle-of-attack becomes greater than αprot, the system switches elevator control from normal mode to a protection mode, in which the angle-of-attack is proportional to sidestick deflection. That is, in the αprot range, from α prot to αMAX, the sidestick commands α directly. However, the angle-of-attack will not exceed αMAX, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick, the angle-of-attack returns to αprot and stays there.
This protection against stall and windshear has priority over all other protections. The autopilot will disconnect if the α protection is active.

 

[RotorAndWing]

So basically it limits pull to 1.25G and push to .5G?

Interesting that the auto trim disables at low altitude. Is this to guard against risk of low level runaway?

From the FCOM

When the aircraft passes 50 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship(with some damping provided by the load factor and the pitch rate feedbacks).
The system memorizes the aircraft's attitude at 50 ft, and it becomes the initial reference for pitch attitude control.
As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude to -2 °nosedown over a period of 8 s. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.

 

[RotorAndWing]

Vα prot, Vα floor, Vα MAX vary according to weight and configuration.
To deactivate the angle of attack protection, the pilot must push the sidestick:
‐ Greater than 8 ° forward, or,
‐ Greater than 0.5 °for at least 0.5 s when α < α MAX.
In addition, below 200 ft, the angle of attack protection is also deactivated, when:
‐ Sidestick deflection is less than half nose-up, and
‐ Actual α is less than α prot – 2 °.
Note: 1. At takeoff α prot is equal to α MAX for 5 s.
2. αfloor is activated through the A/THR system, when:
‐ α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL) or
‐ Sidestick deflection is greater than 14 ° nose up, with either the pitch attitude or the angle-of-attack protection active.
The αfloor function is available from lift-off to 100 ft RA before landing.

HIGH SPEED PROTECTION
The aircraft automatically recovers, following a high speed upset. High Speed Protection is activated at/or above VMO/MMO.
When it is activated, the pitch trim is frozen. Positive spiral static stability is introduced to 0 ° bankangle (instead of 33 ° in normal law), so that with the sidestick released, the aircraft always returns to a bank angle of 0 °. The bank angle limit is reduced from 67 ° to 40 °.

As the speed increases above VMO/MMO, the sidestick nose-down authority is progressively reduced, and a permanent nose-up order is applied to aid recovery to normal flight conditions.
High Speed Protection is deactivated, when the aircraft speed decreases below VMO/MMO, where the usual normal control laws are recovered.
The autopilot disconnects, when High Speed Protection becomes active.

 

[Singlepilot]

So basically it limits pull to 1.25G and push to .5G?



Interesting that the auto trim disables at low altitude. Is this to guard against risk of low level runaway?

Henning,

No, that is not correct. When you apply full aft sidestick the system will give you an initial 2.5g pull to the climb then will limit the aircraft pitch to maintain V alpha Max if full aft sidestick is maintained. Think an EGPWS encounter here...

Auto trim is disabled at low altitude because you set trim for speed. I used to fly with some guys who would trim during the flare as a technique to soften the landing (different aircraft type) However in this case if you go around the trim will not be set appropriately for the go-around. Airbus fixes that issue by disabling it.




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[Henning]

Got it, thanks.

 

[flyingron]

I don't know about a plane that critiques the pilot's landing by calling him a retard.

 

[TheBusFlyer]

Good morning,

Thanks for these precesions R&W!

But what is the link of g limits and flight enveloppe protection?

Full AFT = 2.5g
At 1.25g, plane stops triming.

If we pull full aft, we hit Aprot where Positive trim stops.

So when we are at Aprot, is our g > 1,25g?

What is the relation between g and pitch?
What is the relation between g and AoA?

 

[Singlepilot]

So when we are at Aprot, is our g > 1,25g?



What is the relation between g and pitch?

What is the relation between g and AoA?

Not necessarily, Alpha prot is purely AOA based. It has nothing to do with G. It is based on the AOA for stall. You can stall an airplane at 1g. If you initially pull full aft and achieve 2.5g the aircraft will next stabilize at the max pitch limit protection and may then at some point achieve 1g but reach the high angle of attack protection (V alpha prot). At this point the sidestick commands alpha directly but will not exceed V alpha max. If full aft sidestick is maintained the aircraft will fly at V alpha max. If the sidestick is released while above V alpha prot the angle of attack will return to V Alpha prot and remain there until another sidestick command is ordered.

There is no direct relation between g and AOA or pitch. Any airplane can be at a negative pitch and still be pulling positive g's or be at a positive pitch but pushing negative g's. The same is also true for AOA. You could be at a high AOA but once established at that angle you will be at 1g. Think of g being in the transitions and the AOA after those transitions.

In simple terms the aircraft will first look at the g limitation, then the pitch attitude and then the angle of attack in determining any corrective action to the sidestick input.




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[TheBusFlyer]

Hmm....maybe I am not able to get the concept.

We pull side-stick full AFT.
We are asking 2.5g.
The F-CTLS's pitch the plane-up.
We reach max pitch limit (still at 2.5g).

So has our plane stopped to trim?

Now, we hit Vprot .
The plane pitches down.
Positive pitch trim stops.

But we are asking 2.5g! So basically, pitch trim has already stopped?

How can you reach alpha prot at 1g? You simply reduce thrust?

Thanks

 

[RotorAndWing]

You are way over analyzing this.

The reply in post 15 more than covers this.

 

[TheBusFlyer]

What do you mean R&W?

If we pull SS ful AFT, It means that if I will hit Alpha prot, trim will already be not available?

Thanks

 

[Henning]

Hmm....maybe I am not able to get the concept.

We pull side-stick full AFT.
We are asking 2.5g.
The F-CTLS's pitch the plane-up.
We reach max pitch limit (still at 2.5g).

So has our plane stopped to trim?

Now, we hit Vprot .
The plane pitches down.
Positive pitch trim stops.

But we are asking 2.5g! So basically, pitch trim has already stopped?

How can you reach alpha prot at 1g? You simply reduce thrust?

Thanks

You seem to not be clear on what a G is, a G is a measure of a rate of change in energy state or vector, it can't really be applied the way it sounds like you're trying to apply it. G is wholly independent of AOA except to relate to the speed at which the change happens.

 

[Singlepilot]

TBF, as said you are way over analyzing this.

When you pull full aft sidestick and command 2.5g it is only for the short period of time in transition from the previous flight path before the change was commanded. Again, think of a terrain avoidance maneuver where you want maximum performance from your aircraft. Without a FBW system most pilots in general have no idea what 2.5g feels like and will either pull too hard(not good-we want the the wings to stay on) or won't pull hard enough (the more common result since we don't want to over stress the wings). This wouldn't be so bad except you are trying to avoid the ground.

Once the 2.5g pull is completed the flight computers than transition to the pitch attitude protection since you are no longer pulling 2.5g. Once you are at the maximum pitch limit you are again at or about 1g. (Go out in your plane, pitch to 5degrees and stabilize, do you feel like you are at 1g?) The aircraft will stop trimming above 1.25g(or 33 degrees bank) and also when exceeding V alpha prot. Don't confuse what the flight computers are doing with the auto trim with what is happening with the flight controls.

When the aircraft reaches V alpha max it will stay there if full aft sidestick is maintained. It will not pitch down.

Refer to the diagram posted in post 10. I hesitate to mention it because you are having trouble understanding it but between V alpha prot and V alpha max you will encounter V alpha floor. When this happens the auto thrust commands TOGA thrust automatically and will remain there until you disconnect the auto thrust, select the thrust levers to the climb detent (dual engine, MCT single engine) and reactivate the auto thrust.

All of this is designed to give you the maximum performance from your aircraft when you need it most without having to worry about exceeding design limitations or stalling.

Hope that helps...


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[TheBusFlyer]

Yes. That helps!

But do you release the stick to go back to 1g?

So first, the plane will reach 2.5g (full AFT).
You release the stick.
Plane will go back towards 1.0g.
But it will remain at 2.5g if you keep pulling the stick.

It will then move to pitch angle protection.
If you are still below the limit, it will trim.
Or else, the protections will come.

But when does it transit between Gload and pitch angle protection?
You pull the stick = you demand G load = You get the g load --> You then release the stick = will maintain 1G.

Now, If you are at 2.3G, and you relase the stick. Willl it only start triming back when your G will be equal or below 1.3G?

I am getting it....but it's still confusing!
If you could please give me the steps of how the computers will react in these cases:

- Full AFT (2.5g). What about trim? Pitch? AoA?
- Full AFT (2.5g) but you don't release the stick. You keep demanding 2.5g. What about pitch?

Thanks....

 

[Singlepilot]

Read Hennings last post. Full aft sidestick only equates to
2.5g in the transition stage. As soon as full aft sidestick is no longer commanding 2.5g the system reverts to pitch protection. If you hold full aft sidestick and the pitch stabilizes you are at 1g without releasing the stick. There is absolutely no direct relationship between stick deflection and a specific g value. Action on the sidestick is commanding a rate of change and nothing else until you reach V alpha prot and then it is a proportional sidestick to flight control deflection response like in a non FBW aircraft.


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[TheBusFlyer]

Thanks a lot,

But you don't release the stick? It will maintain 1g even if we have full AFT????

So it is just the transition time...when we are no more moving the stick, it maintains 1g?

Getting it!

A BIG THANKS!

 

[Henning]

Yes. That helps!

But do you release the stick to go back to 1g?

So first, the plane will reach 2.5g (full AFT).
You release the stick.
Plane will go back towards 1.0g.
But it will remain at 2.5g if you keep pulling the stick.

It will then move to pitch angle protection.
If you are still below the limit, it will trim.
Or else, the protections will come.

But when does it transit between Gload and pitch angle protection?
You pull the stick = you demand G load = You get the g load --> You then release the stick = will maintain 1G.

Now, If you are at 2.3G, and you relase the stick. Willl it only start triming back when your G will be equal or below 1.3G?

I am getting it....but it's still confusing!
If you could please give me the steps of how the computers will react in these cases:

- Full AFT (2.5g). What about trim? Pitch? AoA?
- Full AFT (2.5g) but you don't release the stick. You keep demanding 2.5g. What about pitch?

Thanks....

You can release the stick, or let the plane hit the pitch limiter, either way will get you to 1G.

Full aft stick commands a rate of change of 2.5Gs that will not change until a conflicting value is chosen. Whether you chose to alter the information was th the stick, or the pitch limiter does it through the computer, the result will be the same, a reduction in the rate of change as you stabilize on an AoA.

Two different factors, "Where are we going?" (AoA) and "How fast are we going to get there?" (G)

 

[TheBusFlyer]

Thanks a lot!

I am getting it...

PITCH ATTITUDE PROTECTION
Pitch attitude is limited to :
‐ 30 ° nose up in conf 0 to 3 (progressively reduced to 25 ° at low speed).
‐ 25 ° nose up in conf FULL (progressively reduced to 20 ° at low speed).
‐ 15 ° nose down (indicated by green symbols “=” on the PFD’s pitch scale).
The flight director bars disappear from the PFD when the pitch attitude exceeds 25 ° up or 13 ° down. They return to the display when the pitch angle returns to the region between 22 ° up and 10 ° down.

LOAD FACTOR LIMITATION
The load factor is automatically limited to:
+2.5 g to -1 g for clean configuration.
+2 g to 0 for other configurations.

Let's say that I am in configuration 0.
My pitch is limited to 30° nose-up.

I pull the side-stick FULL AFT. I am basically demanding 2.5g.
It will keep giving me 2.5g till I maintain the stick full AFT.

If I release the stick or If I hit 30° Nose-up, I will go to 1g.

But what about trim in all this? It stops trimming at 1.3g!

So If we are at 2.5g, it is basically no more trimming.

"Once the 2.5g pull is completed the flight computers than transition to the pitch attitude protection since you are no longer pulling 2.5g."

What do you mean by "transition" and " you are no longer pulling 2.5g"???
Basically, you will never hit 2.5 g because Pitch angle limiter will come first and ask you to maintain 1g, isn't it?

Thanks a lot,
TBF

 

[TheBusFlyer]

What do they mean by this:

Can pitch angle protection override g load demand?

 

[Henning]

Because you don't apply trim to make changes, you apply trim to stabilize the changes you made. Most likely it quits trimming at high G values until the the rates of change head into stabilizing territory.

 

[TheBusFlyer]

Yes, indeed.

Could you please explain the last sentence (post 25)?

What is pitch pitch protection in G? When you hit 30° pitch up, you automatically go back to 1g in any case?

So what comes before? 30° pitch up or 2.5G?

 

[Singlepilot]

The 2.5g limit is first and almost instantaneous.


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[TheBusFlyer]

Ok...let me do it in the right order....

So trim has already stopped?

 

[TheBusFlyer]

The 2.5g limit is first and almost instantaneous.


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So it means that we are in a direct side stick to elevator "mode".
Trim stopped at 1.3g
Now it is the turn of pitch protection.

Is it possible to be at 2.5g but between the pitch limits?

What next?

 

[Singlepilot]

Absolutely. The aircraft will be pitching up at 2.5g until the pitch limit protection.

Further, there isn't a separate pitch protection in g. All modes work together to monitor the pilot input and apply corrective action if necessary.

The load factor protection is an enhancement of the angle of attack protection which has priority over all other modes.

Perhaps I haven't been clear. Let's say you are flying along straight and level and you get a terrain warning. You pull full aft sidestick to command a climb. The computers will give you an initial 2.5g climb. This happens quickly. I can't remember the exact pitch rate per second but it's quick. It will continue to give you 2.5g until you reach the pitch limit protection for your speed and configuration. This occurs quickly as I said. This is the transition I speak of. That is, the period from what you were doing (straight and level) to what you are doing (climbing to the pitch limit). Once you hit the pitch limit the computers will adjust the flight control response to maintain the maximum pitch allowed (even with the sidestick fully aft). Once that response stabilizes you are effectively at 1g. If you continue to hold full aft sidestick the computers interpret that as a max rate of climb command. As your doing this the speed is decreasing. Alpha floor activates after V alpha prot and commands TOGA thrust to maintain a minimum energy level. You will then reach V alpha max and maintain that if full aft sidestick is maintained.


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[Henning]

So it means that we are in a direct side stick to elevator "mode".
Trim stopped at 1.3g
Now it is the turn of pitch protection.

Is it possible to be at 2.5g but between the pitch limits?

What next?

Pull the stick back, the elevator pitch changes until 2.5G of aircraft pitch change are hit, then maintains that rate of aircraft pitch change until the pitch limiter is hit.

G is the measurement of how fast you are asking it to change pitch attitude, it is a measurement of nothing else but acceleration.

 

[Singlepilot]

Pull the stick back, the elevator pitch changes until 2.5G of aircraft pitch change are hit, then maintains that rate of aircraft pitch change until the pitch limiter is hit.

G is the measurement of how fast you are asking it to change pitch attitude, it is a measurement of nothing else but acceleration.

That's entirely correct, iHenning...hopefully TBF sees the light now...

 

[TheBusFlyer]

Yes my friend! I can see the light! Thanks a lot!

You say: Once you hit the pitch limit the computers will adjust the flight control response to maintain the maximum pitch allowed (even with the sidestick fully aft). Once that response stabilizes you are effectively at 1g.*

It will basically trim at max pitch (30° - conf 0)
But we don't have pitch trim. So could you please explain that part?

When we hit 30° pitch, we are at 2.5g
So it reduces g to 1g when we hit 30° pitch? So we are pulling the stick but g is reducing....when g < 1.3g, trim starts back and it maintains 30° pitch?

What happens if I release the stick when we hit 30° pitch??
There is no trim till we reach 1.3g so it will basically start triming back when our g will go towards 1 (when it goes below 1.3)??

So are we in direct link between 1.3g and 2.5g?

Thanks a lot!

 

[Henning]

Yes my friend! I can see the light! Thanks a lot!

You say: Once you hit the pitch limit the computers will adjust the flight control response to maintain the maximum pitch allowed (even with the sidestick fully aft). Once that response stabilizes you are effectively at 1g.*

It will basically trim at max pitch (30° - conf 0)
But we don't have pitch trim. So could you please explain that part?

When we hit 30° pitch, we are at 2.5g
So it reduces g to 1g when we hit 30° pitch? So we are pulling the stick but g is reducing....when g < 1.3g, trim starts back and it maintains 30° pitch?

What happens if I release the stick when we hit 30° pitch??
There is no trim till we reach 1.3g so it will basically start triming back when our g will go towards 1 (when it goes below 1.3)??

So are we in direct link between 1.3g and 2.5g?

Thanks a lot!

No, when you hit 30° you are back to 1G because there is no longer a change happening. The only relevance G has to trim is that when it senses high rates of change, it quits trying to catch up to the requests, and just lets the servos do the work of moving the control surfaces. Once the maneuvering is completed and the control inputs have it stabilizing on a new attitude, the trim comes back in and takes the load off the servos. Trim doesn't do anything except use aerodynamic force to stabilize the control surface into a balanced condition to take the load off the electro-mechanical controls.

 

[TheBusFlyer]

Thanks a lot,

You say that g is acceleration: how fast we want to increase/decrease pitch altitude
So basically you need to keep the pitch increasing to maintain 2.5g but as there is a limitation (30°), it stops pitching up and maintains 1 g.

Am I right?

 

[Henning]

Thanks a lot,

You say that g is acceleration: how fast we want to increase/decrease pitch altitude
So basically you need to keep the pitch increasing to maintain 2.5g but as there is a limitation (30°), it stops pitching up and maintains 1 g.

Am I right?

Exactly. Think of it like this, you have a graph with a bell curve on it. The horizontal axis is pitch referenced to level datum. On the right is 0° On the left is 30°. The vertical axis represents G, or rate of change. Starting at the right, you yank back on the stick, the event to follow will progress in the manner represented by the graph of the bell.

 

[jesse]

Thanks a lot,

You say that g is acceleration: how fast we want to increase/decrease pitch altitude
So basically you need to keep the pitch increasing to maintain 2.5g but as there is a limitation (30°), it stops pitching up and maintains 1 g.

Am I right?

Assuming wings level. Yes.

Add a bank, and to maintain altitude, you'll have a constant G force higher than 1. Maintain 60 degrees of bank, constant pitch to maintain altitude, will give you continuous 2G.

 

[TheBusFlyer]

Thanks a lot!

Will come back to back angle later!
Was thinking about this: Side sticks commands G load.

1 --> G load is the acceleration: how fast we want to increase /decrease pitch.
2 --> 2G :Twice our weight.

So let's say that full SS AFT = 2G.
How can you relate 1 and 2? Weight and speed?

In 1 sentence: Pulling Side-stick AFT = 1 + 2
How would you say it?