A320 OP CLB behaviour

[Check Airman]

Gents, yesterday as we were flying along at Vmax-5kt, I selected OP climb, and then rolled the speed back to something more reasonable for a climb, and Fifi reduced power to slow, then started climbing and added power.

Only ever seen that happen once. Is there a system explanation that I’m missing? Did it do that simply because we were close to Vmax?

[Inverted Flat Spin]

Was it just a small climb? If the change is less that 1200ft it will do 1000ft/min

[aerobatic_dude]

Was it just a small climb? If the change is less that 1200ft it will do 1000ft/min

Have you an FCOM reference for this ?

Recent convert to Airbus from Boeing here. It has it's oddities.

[Check Airman]

It was a 5000ft climb.

[krismiler]

The autothrust is quite slow to react to changes, it probably targeted the lower speed first by reducing power and then realised you wanted to climb so it increased power. It's a 1980s design and can't keep up with even a modern smartphone. You would have been better off waiting until the aircraft was properly established in the climb before winding the speed back or managing it.

[Inverted Flat Spin]

Have you an FCOM reference for this ?

Recent convert to Airbus from Boeing here. It has it's oddities.

DSC-22_30-70-30 P 3/4

[FlightDetent]

Is there a system explanation that I’m missing? Nothing, it's the 5-year barrier. It happens, fuzzy logic.

[Check Airman]

Nothing, it's the 5-year barrier. It happens, fuzzy logic.

Ha. Thanks FlightDetent

I hope that doesn’t assume 900hrs a year. It may be more like a 10 year barrier for me!

[neilki]

Gents, yesterday as we were flying along at Vmax-5kt, I selected OP climb, and then rolled the speed back to something more reasonable for a climb, and Fifi reduced power to slow, then started climbing and added power.

Only ever seen that happen once. Is there a system explanation that I’m missing? Did it do that simply because we were close to Vmax?

There's no communication between the Ovespeed Protections and Autothrust. The system architecture is discussed in Safety First (https://safetyfirst.airbus.com/management-of-overspeed-events-in-cruise/)
Yes. Fifi can be very exciting sometimes!

[Roj approved]

The autothrust is quite slow to react to changes, it probably targeted the lower speed first by reducing power and then realised you wanted to climb so it increased power. It's a 1980s design and can't keep up with even a modern smartphone. You would have been better off waiting until the aircraft was properly established in the climb before winding the speed back or managing it.

Krismiler is correct, in this scenario it will target the speed first, even though the book says it will always give CLB Thrust in OP CLB.

If you just leave the speed were it is, the a/c will raise the nose at max 1G, and the thrust will come on as required to maintain the target speed. Once established in the climb, with CLB thrust set, the desired climb speed can be set.

Alternatively, if you need to start climbing asap, you can EXPEDITE (if fitted), and this will pitch to maintain Green Dot, therefore the thrust will increase sooner. I don't like this for a number of reasons:

The pitch is aggressive for Cabin Crew and pax, and the speed decays very quickly. If you go below your desired Climb speed, it can be a struggle to get the aircraft to pitch down to regain it while continuing the climb.

I have seen folks set a V/S initially too, and increase it as the thrust increases, before OP CLB, it works, but requires a lot more fiddling.

My preferred method, if down low, i.e. 5000ft to FL100 ish, i.e.: On a SID with speed canceled, and expecting a climb soon, keep a bit of a buffer to Vmax, say 15-20kts. (290 works well, and conforms with SID design parameters)

Then:
Set the cleared Altitude,
OP CLB
INCREASE the target speed by 5-10kts. (This will initiate a thrust increase, as the a/c has already pitched for the climb, and you will have CLB THR and OP CLB)
Set desired CLB speed target (I.e: Managed speed or en route climb speed)

It gives a smoother transition to the CLB Thrust, and CLB attitude, also a larger buffer to Vmax, if you hit some bumps.

(NOT TO BE USED IN THE HIGHER FLIGHT LEVELS)

There is many ways to achieve the same thing, but the Airbus works in mysterious ways, this is the way i have found to manage this problem.

Feel free to offer up other solutions.

[Vessbot]

Jeez man, all this hoopla about advanced avionics and elegant user-friendliness, and reading this it looks like I have to hack into the Pentagon to start a climb!

[Goldenrivett]

Hi Check Airman,
Did it do that simply because we were close to Vmax?
No. Unlike ALT* which is annunciated, the transition from ALT to CLB is annunciated immediately - but it can't be so.
There has to be a blend from AT controlling speed in ALT to Elevators controlling speed in CLM.
Your speed intervention was done during the transition phase whilst AT was still controlling speed.

[Farmer106]

Fun fact: at the same time the FMA will already tell you "climb thrust".
So in this case the FMA is obviously lying to you.
But only for a few seconds..

[Roj approved]

Fun fact: at the same time the FMA will already tell you "climb thrust".
So in this case the FMA is obviously lying to you.
But only for a few seconds..

welcome to the Airbus :-)

[Check Airman]

Thanks all. I’ll try pay attention to exactly when I change the speed next time, to see what effect it has. One is always learning.

[vilas]

The following information was released by Airbus in Sochi crash investigation:
The logic of integration of the autopilot/flight director (AP/FD) pitch control and the
autothrust control
¬If AP/FD pitch mode controls a vertical trajectory (e.g. V/S, ALT), then AT controls speed.
¬If AP/FD pitch mode controls a speed (e.g. OP CLB), then AT controls thrust.
¬If no AP/FD pitch mode is engaged, then АТ controls speed.
Logic sequence of the OPEN CLB mode
¬for level change more than 1200 ft:
¬at OPEN CLB mode engagement by the pilot, V/S control with V/S target = +8000 ft/min (40 m/s) is applied for AP/FD, and SPEED/MACH mode is engaged for AT
¬when engine N1 reaches 95% N1CLB mode, AP/FD switches to SPEED/MACH control law, whereas AT switches to the THRUST mode
¬Throughout this time the FMA displays THR CLB for AT and OP CLB for AP/FD
The given scheme of engagement for the OPEN CLB mode ensures the uniformity of the aircraft response in all configurations and within the whole range of the flight altitudes and speeds.
¬For level change less than 1200 ft:
¬at OPEN CLB mode engagement by the pilot, V/S control with V/S target = + 1000 ft/min (5 m/s) is applied for AP/FD, and SPEED/MACH mode is engaged for AT
¬ Throughout this time FMA displays THR CLB for AT and OP CLB for AP/FD
In this case the climb is in fact performed in the vertical speed control mode.
***
It should also be noted that if the OPEN CLB mode is engaged less than 30 seconds after the aircraft level off function is activated, the autopilot is authorized to use the vertical acceleration at the maximum value of 0.3g, whereas usually it is only 0.15g.

[Uplinker]

Hi Check Airman, Just out of interest and background, why were you climbing the way you were, and why were you not using Managed Climb?

As some have said, Airbus FBW tends to try to operate smoothly, (unless you select TOGA).

[ironbutt57]

Jeez man, all this hoopla about advanced avionics and elegant user-friendliness, and reading this it looks like I have to hack into the Pentagon to start a climb!

how to complicate something so simple...select the altitude, pull the damn knob..after it starts to climb, select any speed change you desire...

[Goldenrivett]

..after it starts to climb, select any speed change you desire...

Or more technically correct - wait until N1 reaches 95% as vllas posted #16

"when engine N1 reaches 95% N1CLB mode, AP/FD switches to SPEED/MACH
control law, whereas AT switches to the THRUST mode
¬Throughout this time the FMA displays THR CLB for AT and OP CLB for
AP/FD"

[Check Airman]

Hi Check Airman, Just out of interest and background, why were you climbing the way you were, and why were you not using Managed Climb?

As some have said, Airbus FBW tends to try to operate smoothly, (unless you select TOGA).



Not sure I understand your first question, but we were in a hurry, and ATC had us level off for some traffic above 10,000, so we made the best use of the low altitude. The thought was to convert the kinetic energy to a fairly rapid climb once the clearance was given, but that didn’t quite happen as expected.

To the question of managed vs open climb, I’m 50/50 on that. Most SIDs in the US don’t have altitude restrictions other than “at or above” (LAS and IAH are the only exceptions that come to mind) and if I recall, we may have been in HDG mode anyway.

All else being equal, do people generally have a preference for managed vs selected?

[Check Airman]

The following information was released by Airbus in Sochi crash investigation:
The logic of integration of the autopilot/flight director (AP/FD) pitch control and the
autothrust control
¬If AP/FD pitch mode controls a vertical trajectory (e.g. V/S, ALT), then AT
controls speed.
¬If AP/FD pitch mode controls a speed (e.g. OP CLB), then AT controls thrust.
¬If no AP/FD pitch mode is engaged, then АТ controls speed.
Logic sequence of the OPEN CLB mode
¬for level change more than 1200 ft:
¬at OPEN CLB mode engagement by the pilot, V/S control with V/S target = +
8000 ft/min (40 m/s) is applied for AP/FD, and SPEED/MACH mode is engaged
for AT
¬when engine N1 reaches 95% N1CLB mode, AP/FD switches to SPEED/MACH
control law, whereas AT switches to the THRUST mode
¬Throughout this time the FMA displays THR CLB for AT and OP CLB for
AP/FD
The given scheme of engagement for the OPEN CLB mode ensures the uniformity
of the aircraft response in all configurations and within the whole range of the flight altitudes and
speeds.
¬For level change less than 1200 ft:
¬at OPEN CLB mode engagement by the pilot, V/S control with V/S target = +
1000 ft/min (5 m/s) is applied for AP/FD, and SPEED/MACH mode is engaged
for AT
¬ Throughout this time FMA displays THR CLB for AT and OP CLB for AP/FD
In this case the climb is in fact performed in the vertical speed control mode.
***
It should also be noted that if the OPEN CLB mode is engaged less than 30 seconds after the
aircraft level off function is activated, the autopilot is authorized to use the vertical acceleration at the maximum value of 0.3g, whereas usually it is only 0.15g.
In the automatic flight, when the OPEN CLIMB mode was engaged, the aircraft started climbing
rapidly, at a vertical speed up to 12 m/s, with pitch angle increased to 21º by 22:12:06, maximum
vertical acceleration 1.27g, maximum angle of attack 10.7º, and the indicated airspeed reduced to
129 kt (240 km/h), which is 8 kt lower than the target speed. Engine r.p.m. were increased to the
maximum possible value for the given position of the thrust control levers (Attachment 2, Fig.4).

Thanks vilas. Can always count on you to teach me something I didn’t know. Will pay attention to this in a few hours.

[Uplinker]

From Airbus FCOM:

The managed climb speed, computed by the FMGS, provides the most economical climb profile as it takes into account weight, actual and predicted winds, ISA deviation and Cost Index (CI). The managed climb speed also takes into account any speed constraints, e.g. the default speed limit which is 250 kt up to 10 000 ft..

A long time Airbus FBW TRE told us it was better to climb in managed if you can, and I think it was viewed as good practice. I always use managed climb/descent unless there is a good reason not to, (and just in case there is a constraint that I’ve forgotten).

The only statement I can find for Open Climb:
The OPEN CLB mode is a selected mode. It uses the AP/FD pitch mode to maintain a SPD/MACH​ (selected or managed) while the autothrust (if active) maintains maximum climb thrust.Nothing about economy, but that’s not to say it doesn’t.

[AerocatS2A]

Uplinker, your first paragraph is referring to managed speed during climb. If you use open climb and managed speed you will get exactly the same economic climb profile as if you’d used managed climb and managed speed.

[giggitygiggity]

From Airbus FCOM:

.

A long time Airbus FBW TRE told us it was better to climb in managed if you can, and I think it was viewed as good practice. I always use managed climb/descent unless there is a good reason not to, (and just in case there is a constraint that I’ve forgotten).

The only statement I can find for Open Climb:
Nothing about economy, but that’s not to say it doesn’t.



THR CLB/OP CLB uses a managed speed target which is whatever the CI asks for. As far as I know, THR CLB/CLB does exactly the same as OP CLB but uses the fmgc climb altitude constraints. The managed speed target remains the same regardless of whether it is open or managed climb. It will follow speed constraints in the flight plan as long as you’ve got the speed in managed.

The FCTM suggests not using managed climb unless you are flying constraints (PR-NP-SOP-150: FMS Use).
OP CLB is to be used if ATC gives radar vectors or clears the aircraft to a give FL without any climb constraints.

[vilas]

THR CLB/OP CLB uses a managed speed target which is whatever the CI asks for. As far as I know, THR CLB/CLB does exactly the same as OP CLB but uses the fmgc climb altitude constraints. The managed speed target remains the same regardless of whether it is open or managed climb. It will follow speed constraints in the flight plan as long as you’ve got the speed in managed.
The FCTM suggests not using managed climb unless you are flying constraints (PR-NP-SOP-150: FMS Use). What you said is correct. But the reason for using OP CLB in unrestricted climb is rather obvious because in CLB it will level of at constraints and ATC is not going to like that.

[giggitygiggity]

What you said is correct. But the reason for using OP CLB in unrestricted climb is rather obvious because in CLB it will level of at constraints and ATC is not going to like that.

Like I said, the FCTM says you shouldn’t do it for the for those obvious reasons. I see lots of guys putting it back into managed climb if it reverts to open climb after finishing the SID.

As for the original topic, it seems to happen pretty consistently. If you’re in ALT with a high speed and pull for OP CLB, then wind the speed back, thrust rather inappropriately comes back. Not sure why this would be a ‘feature’ of the airbus though? I can’t find anything in the FCOM. If you select a higher speed then pull the speed back I think it keeps the thrust on like it should.

[safelife]

Thank you vilas, that's epic!

[pineteam]

It should also be noted that if the OPEN CLB mode is engaged less than 30 seconds after the aircraft level off function is activated, the autopilot is authorized to use the vertical acceleration at the maximum value of 0.3g, whereas usually it is only 0.15g..

Hi Vilas, thank you for the infos. I just have a doubt about this bit. I never noticed that the aircraft will do a more agressive climb in OPEN CLB if less than 30 seconds of level off. By level off, if I understand correctly it could be vertical speed zero or ALT* engagement. I will try to pay attention today.
Also I have read on another post on Pprune that in CLB/OPEN CLB the G load will be max 1.1G and with expedite it will be 1.15G. Which differs quite a bit from your figures.






Like I said, the FCTM says you shouldn’t do it for the for those obvious reasons. I see lots of guys putting it back into managed climb if it reverts to open climb after finishing the SID.



I see some skippers doing that and it’s annoying. Why do you want to put in CLB detent again unless you have another altitude constraint? Just leave it in OP CLB. As some guys sometimes revert from OP CLB to CLB mode exactly when passing the constraint altitude and then the aircraft inadvertently level off... -___-
That and the guys using managed CLB when only 1000 feet to climb.. smh

[AerocatS2A]

Our operator seems to love managed climb. If given unrestricted climb we will delete the restriction in the McDu in preference to using open climb. Our FCTM recommends CLB whenever we are in NAV.

[Check Airman]

Our operator seems to love managed climb. If given unrestricted climb we will delete the restriction in the McDu in preference to using open climb. Our FCTM recommends CLB whenever we are in NAV.

Well that’s certainly one way to do it...

[sonicbum]

PR-NP-SOP-140

OP CLB is to be used if ATC gives radar vector or clears the aircraft direct to a given FL without any
climb constraints.

BUT

The managed AP/FD mode in climb is CLB. Its use is recommended as long as the aircraft is
cleared along the F-PLN.

[pineteam]

FCTM also states that below 10 000 feet, MCDU inputs should be restricted. I don’t think it is wise to go heads down at low level just to clear a constraint when you just have to select a knob on the FCU.

[FlightDetent]

My overall experience is that (managed) CLB is the standard and default mode. OP CLB only used when no choice (HDG) or actively wishing to ignore FMS altitude restrictions.

As if ATC gives radar vectors or clears the aircraft to a give FL without any climb constraints. meant when ATC tells you to ignore those constraints.

I see the point that ironbutt and pinteam make, just in my area it is considered appropriate to have CLB instead of OPCLB when possible. Basic design philosophy:"all managed, all pushed".

Check Airman I usually wait for "almost" full N1 before activating the climb to avoid that. BTW in your situation, I would had flown exactly the same modes, most likely forgetting and ending with the same result ("oh nintendo, was that really necessary?!"). Being a rather fully managed man myself, I do not consider operational choice of sel spd or V/S in busy airspace a sin.

vilas I ran out of vocabulary praising you a long ago, but this one is a real diamond.

[vilas]

FD, CA most welcome to share anytime anything

[vilas]

Pineteam what I have posted is copy paste from Sochi investigation report and it's from Airbus. I had read it long time ago and only vaguely remembered luckily it's available on Google.

[Vessbot]

I wonder what the plan is if they later give you the restrictions again.

[neilki]

I wonder what the plan is if they later give you the restrictions again.
Mutter expletive. Throw hands in air. Look confused. Look confused towards FCP. Transfer 'look of confusion' to FMC.
-oh. wait. I remember. Fly the Airplane....

[Check Airman]

Mutter expletive. Throw hands in air. Look confused. Look confused towards FCP. Transfer 'look of confusion' to FMC.
-oh. wait. I remember. Fly the Airplane....

And then say “next time, we’re using OP CLB”.

[Uplinker]

I wonder what the plan is if they later give you the restrictions again.

Simply press the Alt knob and you will go back into Managed Climb.

Pulling the knob towards you tells the system, that you will decide how to fly the climb/descent, by adjusting the FCU.

Pushing the knob towards the aircraft tells the aircraft to decide how to climb/descend, according to the data programmed into the FMGS.

It’s really simple and an excellent arrangement.:ok:

.

[Vessbot]

Simply press the Alt knob and you will go back into Managed Climb.

Sorry, I should have used a quote. I was talking about the scenario where they were already in managed, with all the restrictions deleted.

Seems to me like an absurd piece of makework.

[hans brinker]

Deleting restrictions to be able to use managed climb over open climb is the gold medal of putting SOP over common sense.

[pineteam]

Deleting restrictions to be able to use managed climb over open climb is the gold medal of putting SOP over common sense.

Hahaha! Could not agree more! A bit like guys going heads down in the MCDU to delete the 250kt speed constraint below FL100 instead of selecting the speed on the FCU... smh

[AerocatS2A]

Deleting restrictions to be able to use managed climb over open climb is the gold medal of putting SOP over common sense.
Absolutely. I'm not entirely sure why we do it that way, I haven't been here long and this is just what they're doing. As far as I know there is no prohibition on using open climb instead.

[Check Airman]

Deleting restrictions to be able to use managed climb over open climb is the gold medal of putting SOP over common sense.

HAHAHAHA

This made my day! Well said, sir.

We need a "like" button on this forum.

[vilas]

NO doubt when possible managed modes should be used but when unrestricted climb has been given use of OP CLB is what it is for. To stay in CLB by deleting waypoint is creating laborious head down work because you have to ensure you are not deleting a wrong constraint. Climb and descent have a difference. CLB and OP CLB are same without constraints they don't track any vertical path but in descent even without constraints DES follows a path calculated all the way to 1000ft on approach. OP DES is used only when you want max rate of descent with idle thrust. If you deleted constraints in descent FMS will rework the path but nothing of that sort happens in climb.

[Bus Driver Man]

The autothrust is quite slow to react to changes, it probably targeted the lower speed first by reducing power and then realised you wanted to climb so it increased power. It's a 1980s design and can't keep up with even a modern smartphone. You would have been better off waiting until the aircraft was properly established in the climb before winding the speed back or managing it.
I have the impression that the older A320s had a better A/THR response and spool up time. Did something change in the FADEC software? Both CFM and IAE engines at the previous companies that I’ve worked for spooled up faster and had better A/THR response than the current (new but still CEO) IAE engines that I’m flying now.

[tubby linton]

A previous operator had a number of aircraft that would exhibit this behaviour. Traditionally in open climb reducing speed would produce a pitch response to meet the target but one software fit would trigger the auto thrust to reduce thrust instead. I remember the first time I saw it there were a few expletives and why is it doing that. The software changed a few months later back to what I had been used to.
Open climb should be a speed based pitch mode with auto thrust commanding climb thrust, not the autothrust commanding speed

[vilas]

Open climb should be a speed based pitch mode with auto thrust commanding climb thrust, not the autothrust commanding speed
Off course it is but the way it is initiated is through VS, speed in the background. The pitch up occurs to give you a pre set VS and thrust comes up to maintain speed till 95% N1 and then continues to THR CLB and speed maintanence is switched to pitch. Had it not been so and thrust would have moved to climb then to control speed the pitch up may have been too harsh. It's Airbus design I don't how Boeing does it.