ALPHA PROT/FLOOR/MAX

[Dupre]

1. If everything is normal, the stick commands a g loading... 1g is normal gravity, 0g is weightless, 2g is pulling back real hard. with the stick neutral, the plane maintains 1g.

2. If everything is normal, elac2 gives the orders to the elevators to maintain vprot if the thrust is insufficient. Fac has nothing to do with it. elac knows v.prot.

3. Actually is A.floor... not a speed, but an angle of attack. Calculated by the Facs, yes.

4. The autothrust is not on. in fact it will even disengage at A.floor if it was on. the TOGA command is sent straight to the engine . Autothrust is off throughout the whole thing... just the fac tells the engines to hit toga power and stay there.

helping?

[AF330]

Yes! Thanks a lot!

So in normal law:

Vprot calculated and mainatained with ELAC.
Alpha floor range calculated and managed with FAC's till....
we exit the range: TOGA LCK.

1) Right?

2) But do at least FAC's send a signal to ELAC to tell him that it has to do something because we are at Vprot? Or do FAC's only come when we hit Alpha Floor?

Thanks a lot!

Good night...

[Dupre]

haha i am really having to read the book in depth to get clear on all this now!

Firstly I must apologise for an error on point 4... you were correct and I was completely wrong, the thrust going to TOGA at A.floor is in fact an autothrust function!

Quote from FCOM DSC-22-30-90-A/THR ACTIVATION
"When ALPHA FLOOR is activated, regardless of the initial status of A/THR and the position of the thrust levers, the A/THR activates."


Next, the FACs do talk to the ELACs.
Quoting FCOM DSC-22-40-10-GENERAL
"Each FAC interfaces with the elevator aileron computers (ELACs) when the APs are disengaged, or with the FMGS when at least one AP is engaged."

Next, the book is slightly unclear whether these FAC calculated speeds (V.a.prot and V.a.max) are sent to the ELAC for use... it seems to suggest not, but i am certainly unaware of any capability of the ELACs to calculate these speeds themselves.
Quote from FCOM PRO-SUP-10-PROTECTION SPEEDS.
"Vα PROT, Vα MAX and VSW are computed by the FAC, based on aerodynamic data. They are only used for display on the PFD, and not for flight control protection (the activation of the protections is computed by the ELAC)."

More on A.floor, Quote from FCOM DSC-22-40-30 ALPHA FLOOR PROTECTION
"The FAC sends this signal (for A.floor) when :‐The angle of attack is above a predetermined threshold, that is a function of the configuration.‐In CONF3 and CONF FULL, this threshold decreases as a function of the aircraft deceleration rate (down to – 3 °)."

does that go some way to answering your questions?

[Dupre]

Also I had a look today at the pitch attitude on the PFD in a moderately loaded A320. Cruise about 2deg up. Steep descent (High altitude, idle thrust, no speedbrake) 1deg nose down. Approach with flaps full, 3deg up.

[AF330]

Ok...so more questions now!

1) If we hit Vfloor, A/THR gives TOGA. Can we exit leave A/THR at Vfloor (by pressing red button on levers e.g)?

2) So how does it happen? FAC knows Vfloor. FAC sends a signal to A/THR to give TOGA?

3) Well...confused for Vprot....

What maintains Vprot???? Is it FAC or ELAC? Your F-COM says:
" and not for flight control protection (the activation of the protections is computed by the ELAC)."

It's pretty confusing. It says that FAC only displays Vprot...etc. So just 1 question here:

Option 1: ELAC knows Vprot. It knows it's IAS, it knows it's attitude --> No need of FAC's. ELAC's are managing Vprot.
Option 2: ELAC knows nothing (of course, it does but does not take any decision itself), FAC knows Vprot, our IAS and the attitude --> It sends to ELAC pitch command to maintain Vprot.

Your F-COM tends to say that ELAC do it alone. Of course, FAC's do other things such as stoping trim at Vprot. (Normal Law)
What do you think? (Option 1 or 2?)

Thanks a lot!

[Dupre]

1) Don't understand your question. What do you mean by "Can we exit leave A/THR at Vfloor"?
(Also it is A.floor, not V.floor - is an angle of attack, not a speed. It is possible to get A.floor protection activate at a speed well above Vls, by pulling back hard enough to increase angle of attack to above the A.floor value)

2) Yes. FAC knows current angle of attack from the probes. FAC also calculates the value of A.floor from the config of the aircraft. FAC makes a simple comparison of the two, and any time AoA > A.floor, activates the protection by signalling the Autothrust to give TOGA.

3) It is confusing and I am not entirely sure. My best guess is option (1). What remains unknown to me is whether the FAC tells the ELAC the value of V.a.prot or whether the ELAC calculates it itself.

[AF330]

Ok...confused.

Are you a real airline pilot?
Well, I really appreciate your help! Thanks a lot! Here are my doubts:

How can you get alpha floor above VLS?
VLS is the lowest speed you can select. Vprot is the speed where the maximum AoA at which Alpha protection becomes active (the plane pitches down and maintains Vprot with ELAC).

You can only reach Vprot if your thrust levers are at IDLE.

At Vmax, your plane is stalling so your speed is below VLS.
I haven't seen any stall with TOGA thrust... (so why is Vprot and Vmax BELOW VLS????)

You say that Aloha floor is a special AoA where you basically start stalling (between Vprot and Vmax - VSW). I agree with you.
But then you say that you can have Afloor above VLS.
If you are stalling, you don't have enough lift ---> Your speed is low.
So at a special AoA, you get TOGA from A/THR.

Do you have a video where you hit Afloor and you get TOGA LCK above VLS???

My question: Can we stop A/THR to give us TOGA when we are at Alpha Floor? Or can we only stop A/THR when we have left the critical AoA (A.floor)?

Thanks a lot!

[Dupre]

haha yes I am... well at least i try to be one. What is your interest in these questions? they are very detailed!

Did a youtube search and came up with this.

You Tube

It is only a microsoft flight sim vid (and is not mine) but illustrates the point. Thrust does not need to be at idle to activate A.floor. I have done it myself in full flight sims a few times (normally associated with a gpws manoevre which requires full backstick and manually selecting toga, which is quickly replaced by A.floor (also toga)) but obviously never in the real aircraft.

The actual stall speed is always below V.a.max. At A.floor you still have a reasonable margin above the stalling AoA.

Your question - answer is the second one. You cannot stop it giving you TOGA and can only get out of TOGA when AoA is above A.floor. That is if you have protections... if you have some failures that put you in alternate law without protections then you will not see A.floor at all.

[AF330]

Thanks a lot,

Yes....

1) Basically, Alpha prot is also an AoA. The plane just shows where your Alpha prot/floor/max AoA is if you keep this attitude. Do you agree? But he/she pulled the stick, the attitude changed ---> A new Alpha protection AoA. Right?

2) Something very intresting in your video:
A/THR doesn't go below VLS. So it remains ON.
So what happens? Ok...here he reaches Alpha Floor AoA.
--> As A/P is OFF, the default mode is SPEED. When you pull the stick, as you are still above VLS, A/THR tries to maintain that speed. But as you pull the stick, you reach Vprot (A/THR still ON). So I had said that as A/THR is OFF, Vprot managed by ELAC (completely wrong for the first part).

---> As, A/THR is ON, can Vprot be managed by A/THR? Or is it ALWAYS ELAC?
Ok, so you can NOT disconnect A/THR at AoA Alpha floor.

Question 2:

---> In that case, Afloor was over VLS.
We can also notice that 2 things come on the FMA:
- A.FLOOR (I think that at that moment you can NOT disconnect A/THR)
- TOGA LCK (You can disconnect TOGA).
But I imagine that Alpha max is BELOW VLS.
But I can not see the VLS bar. (maybe it's hidden in the red part!)
What do you think?

Question 3:
What are the ways to leave A/THR.? In his case, we didn't even see what happens to the thrust levers! We just see that he goes back to SPEED mode!!!!
Do the levers move to TOGA? How does he come back to A/THR range??
Well, he disconnects A/THR ~ 140kts. So it's enough (and I think that his levers are already at CLB - A/THR range)

Thanks a lot,
AF330

[Dupre]

1. Almost. V.a.prot and V.a.max take account of current load factor (LF).

To correct your quote "But he/she pulled the stick, the LOAD FACTOR changed ---> V.a.prot moved up the scale to a speed that would now (with the big LF) result in.... Alpha protection AoA"

I am not aware of athr disengaging below Vls... do you have a reference for that? What does disengage is the autopilot, at alpha.prot +1°

The high AoA protection works by reducing pitch attitude only. This is ELAC control, Athr has nothing to do with it. However Athr, when engaged will always try to maintain target speed. Min target speed is Vls, so when engaged it is always trying to maintain a higher speed than v.a.prot anyway.

Also, the athr is limited to how far advanced the levers are. so if you have them near idle, with athr on, maybe it cannot even maintain Vls. Normally they are in CLB detent, giving the athr authority to command anything from idle to climb thrust.

2. Because Vls doesnt change with load factor, it stays constant, while V.a.prot and V.a.max shoot up the speed scale. For a brief moment, V.a.max is above Vls.

3.The levers are not back driven, so they stay where the pilot put them.

The whole sequence breaks down...

Athr on, speed mode, target 165kt selected.
Vls 135kt, constant.
Pulls stick back, big LF generated.
Airspeed decreases and V.a.max shoots up. Athr still targetting 165.
Airspeed and V.a.max meet at 158kt, and the ELAC forces nose down to keep AoA at V.a.max
A.floor activates simultaneously. Thrust goes TOGA.
Pilot relaxes the stick, allowing LF to decrease, V.a.prot/max go back down.
No longer in A.floor, thrust still in TOGA, FMA changes to TOGA LK
Pilot pushes instinctive disconnect, Athr disengages. Thrust now manually controlled.
Pilot pushes the Athr button to re engage. It engages, in THR LVR mode which indicates thr levers are below the CLB detent. Target speed now 165kt.
Pilot puts levers in CLB detent, FMA changes to SPEED

[AF330]

Thanks a lot,

1) What is load factor?

2) But at AoA alpha protection, ELAC maintains Vprot. But you say that A/THR is still trying to manage the selected speed. That is impossible. One is trying to manage Vprot, the other one is trying to maintain 165kts!!!!! Does FAC's send a signal to A/THR to stop sending orders to FADEC/FMGS at Vprot?

3) How do A/THR signals work? FMGS sends the next speed/altitude (constraint). A/THR knows the plane's IAS. A/THR calculates the fuel required and sends it to FADEC (ECU). It then sends a signal to FMGS to tell him the pitch required if in CLB/DES mode.
What do you think?

I also have some questions about Airbus laws, would you mind If I ask them?

I am just waiting for your answer and normally we will be good for AoA protections!

[Dupre]

1. Load Factor is the vertical g force the aircraft experiences. (Lift/Weight).

2. ELAC controls pitch. Athr controls thrust. The ELAC will only need to pitch down to avoid stall if the autothrust is insufficient to accelerate the aircraft. The autothrusts target is higher than V.a.prot or V.a.max... they are both moving to accelerate the aircraft.

It would only be a problem if their targets were in opposite directions.

Practically, the only was to activate this protection is to 1. disengage autothrust and use low thrust, or a steep climb to reduce speed. 2. Keep autothrust on, but retard the thrust levers far enough back that you restrict the autothrusts authority so much that the aircraft reduces speed even though autothrust is giving its max thrust. 3. Pull back on the stick so hard that the V.a.prot/max go above your airspeed.

3. Quoting FCOM-DSC-22-30-90-GENERAL
"The autothrust (A/THR) is a function of the FMGS, it includes two independent A/THR commands, one per FMGC. Each one is able to control the thrust of both engines simultaneously through two Engine Interface Units and two Electronic Engine Controls (IAE engines) or two Engine Control Units (CFM engines). Only one FMGC controls the active A/THR, it is called the master FMGC."

So the Athr is a part of the FMGS. FMGS knows the target speed (Managed or selected), and current speed. The ATHR part then works out how much thrust it wants, and sends this to the EIUs, which in turn tell the FADECs, which work out how much fuel to put in the nozzles.

Separately, the FMGS calculates the pitch required and sends this to the FD

Another quote, this time about how the FD pitch mode and Athr mode relate.
FCOM-DSC-22-30-10-INTERACTION BETWEEN AP/FD AND ATHR MODES.
"The AP and FD pitch modes can control a target SPD/MACH or a vertical trajectory, and the A/THR mode can control a fixed thrust or a target SPD/MACH. However, the AP/FD and the A/THR cannot both control a target SPD/MACH simultaneously. Therefore the AP/FD pitch modes and A/THR mode are coordinated as follows:‐If an AP/FD pitch mode controls a vertical trajectory, the A/THR mode controls the target SPD/MACH.‐If an AP/FD pitch mode controls a target SPD or MACH, the A/THR mode controls the thrust.‐If no AP/FD pitch mode is engaged, the A/THR mode reverts to controlling the SPD/MACH mode. In other words, the selection of an AP/FD pitch mode determines which mode the A/THR controls.


Go ahead Ill see if i can help.

What makes you so interested in these things?

[AF330]

Thanks for your answer.

I just love Airbus!

1) Ok, but I still don't get it.

If ELAC is controling pitch and A/THR is controlling thrust, do you mean that if A/THR is ON and if we reach Vprot, ELAC will just pitch down at a certain programmed value and A/THR will keep trying to maintain the selected speed but if A/THR is OFF, when we reach Vprot, ELAC will pitch down and manage itself the Vprot? So:
A/THR ON = ELAC pitches down at X° (programmed) - target speed managed by A/THR.
A/THR OFF = ELAC pitches down and maintains Vprot.
Am I right??

2) What is EIU's use?

3) Ok, A/THR will send X fuel amount to FADEC.
A/THR is part of the FMGS. FMGS knows the target speed.

Ok so you are saying this:

If A/THR is in SPEED/MACH mode, AP/FD control pitch.
I really don't understand then...what do you mean by vertical trajectory??
Do you mean to say what I have written before?

Well, for me, If A/THR is in SPD mode, pitch is then controlled by FMGS.
If A/THR is in CLB/DES mode, pitch and thrust are controlled by A/THR. What is FMGS doing?? He always controls the trajectory...A/THR never controls HDG/CRS.
How can AP/FD control a target SPD? Or do you mean, FD/AP controls pitch?
Don't get your last F-COM quote.
Please explain,

EDIT:

When AP/FD manages vertical trajectory: Does that mean that AP/FD are managing pitch? Or is it simply the route?

When A/THR manages a target SPD/MACH: Does that mean that A/THR is in CLB/DES mode and is pitching up/down to maintain target speed? So - to make it shorter - A/THR controling pitch?

When A/THR is controlling thrust: Well, that's speed mode, isn't it?

When AP/FD is controlling a target SPD/MACH: ???????? A/THR will always control speed (by pitch or by thrust). So what is this???

Thanks a lot

[AF330]

I really want to finish Afloor and stuff before talking about the laws..

Thanks

[Dupre]

1. Think of it like this:

ELAC makes sure that the aircraft will not go BELOW V.a.prot/max. It is a minimum speed, not really a target speed as such. It will pitch down just enough to maintain the minimum airspeed. No more, no less. It is not a fixed value, and depends on lots of factors.

Say you are climbing really steeply, and the autothrust is already at Climb power, but you are still decelerating. When you reach V.a.prot/max the ELAC will not allow you to climb that steeply, so will lower the nose so you don't go below and stall.

2. EIU (Engine Interface Unit) is the bit that interfaces between the FMGS and the FADEC. Not sure exactly what that means... I have a picture of the architecture but can't figure out how to post it on here. If you can help, I will post it!

3. By vertical trajectory it means a certain climb rate or flight path angle.

Just briefly (I have to go to work, will be back on Jan 1st)

Set yourself up in the simulator, normal flight, Athr On, FL300. Select speed.
Select FL100, pull for open descent. You will see Athr commands THR IDLE. Select a high speed, watch as the pitch increases, descent rate increases, then reduces as the speed is reached. Then decrease selected speed (say by 20kt) and watch as it increases pitch, reduces descent rate, to decelerate, then steepens up the pitch once the speed is reached.

Now turn off autothrust, set a low thrust value (say 700kg.hr per engine). Do the same. It will pitch to maintain the speed. Thrust is fixed, so it will give whatever pitch is required to maintain the speed.

Now re engage autothrust, and select vertical speed -1000fpm. Again increase and decrease the selected airspeed, and watch how the autothrust compensates to maintain it this time, not the pitch. This is because the FD is controlling the VERTICAL TRAJECTORY.

When you are done, set up a climb and do the same exercises (using climb thrust if Athr is not on) and see how it works there - pretty much the same.

That should give you a good understanding.

Sorry I won't be around for next couple of days!

[AF330]

Thanks,

Well, will come to A/THR when you will be back!

So you said: " ELAC makes sure that the aircraft will not go BELOW V.a.prot/max. It is a minimum speed, not really a target speed as such. It will pitch down just enough to maintain the minimum airspeed. No more, no less. It is not a fixed value, and depends on lots of factors.

Say you are climbing really steeply, and the autothrust is already at Climb power, but you are still decelerating. When you reach V.a.prot/max the ELAC will not allow you to climb that steeply, so will lower the nose so you don't go below and stall."

I agree with you - but - I am 99,9% sure that airbus will maintain Vprot after piching down (Flight Detent said it in his previous posts).
---> So I agree with you if A/THR is ON. ELAC will pitch down the aircraft. A/THR (over VLS!), will still try to maintain the target speed. ELAC has been programmed to pitch down at X° if A/THR is ON.
But what if A/THR OFF? We had said that ELAC will MAINTAIN Vprot by pitching down. I don't think it's more complicated. What do you think? Hope it's right...

Kind regards,
A.Roy

[Dupre]

it really doesnt matter iv athr is o or off. The protection works the same way i.e. if speed decelerating to v.a.prot, elac orders pitch down so as yo avoid speed dropping any lower.

As for elac maintaining v.a.prot? Once the plane is capable of flying faster, it will do so, and the high aoa protection is no longer active.

You say "airbus will maintain Vprot after piching down". Are you saying that the elac will maintain v.a.prot even after the plane accelerates, by pitching up??

[AF330]

Hmmm...good point.

Are you 100% sure that ELAC simply pitches down the aircraft? So what happens if ELAC itself hits VMO/MMO?? That will happen if you don't do anything.
If A/THR is ON, then it's fine.
MMO/VMO protection will come if you don't do anything...

So yes, i thought that ELAC could pitch up to maintain Vprot but as the speed is already low, so if it keeps pitching down, it will maintain Vprot. No need to pitch up.

Are you sure?

[Dupre]

"Are you 100% sure that ELAC simply pitches down the aircraft?" What else do you think it could do?

When high speed protections activate, the elac commands pitch up to prevent excessive overspeed.

The protections are just maximum/minimums that the elac will not allow the plane to exceed, regardless of thrust setting, stick input etc. Once the speed is within the extremes again, the protections are not active and the plane returns to normal law.

[Dupre]

Once again I learn something new.

Quote FCOM-27-20-10-20 HIGH AOA PROTECTION

"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.
Note 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."