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A320 OP CLB behaviour

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A320 OP CLB behaviour

Old 29th Sep 2019, 21:44
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A320 OP CLB behaviour

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 Iím missing? Did it do that simply because we were close to Vmax?
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Old 29th Sep 2019, 22:25
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Was it just a small climb? If the change is less that 1200ft it will do 1000ft/min
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Old 30th Sep 2019, 00:03
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Originally Posted by Inverted Flat Spin View Post
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.
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Old 30th Sep 2019, 00:05
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It was a 5000ft climb.
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Old 30th Sep 2019, 00:15
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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.
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Old 30th Sep 2019, 00:38
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Originally Posted by aerobatic_dude View Post
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
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Old 30th Sep 2019, 00:47
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Originally Posted by Check Airman View Post
Is there a system explanation that Iím missing?
Nothing, it's the 5-year barrier. It happens, fuzzy logic.
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Old 30th Sep 2019, 01:23
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Originally Posted by FlightDetent View Post
Nothing, it's the 5-year barrier. It happens, fuzzy logic.
Ha. Thanks FlightDetent

I hope that doesnít assume 900hrs a year. It may be more like a 10 year barrier for me!
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Old 30th Sep 2019, 01:32
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Originally Posted by Check Airman View Post
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 Iím 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
Yes. Fifi can be very exciting sometimes!
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Old 30th Sep 2019, 03:33
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Check Airman see below

Originally Posted by krismiler View Post
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.
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Old 30th Sep 2019, 06:35
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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!
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Old 30th Sep 2019, 10:11
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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.
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Old 30th Sep 2019, 10:36
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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..
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Old 30th Sep 2019, 11:45
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Originally Posted by Farmer106 View Post
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 :-)
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Old 30th Sep 2019, 12:47
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Thanks all. Iíll try pay attention to exactly when I change the speed next time, to see what effect it has. One is always learning.
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Old 30th Sep 2019, 13:34
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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.

Last edited by vilas; 1st Oct 2019 at 16:43.
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Old 30th Sep 2019, 14:48
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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).

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Old 30th Sep 2019, 14:50
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Originally Posted by Vessbot View Post
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...
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Old 30th Sep 2019, 16:16
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Originally Posted by ironbutt57 View Post
..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"
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Old 30th Sep 2019, 16:59
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Originally Posted by Uplinker View Post
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 didnít quite happen as expected.

To the question of managed vs open climb, Iím 50/50 on that. Most SIDs in the US donít have altitude restrictions other than ďat or aboveĒ (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?
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