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A320 Managed speed after touchdown

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A320 Managed speed after touchdown

Old 24th Dec 2016, 20:14
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A320 Managed speed after touchdown

Good evening,

Please have a look at the following video (9:16): https://m.youtube.com/watch?v=9VfI4Pyam1E

Why does the managed speed increase after landing? What's happening?

Thanks!
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Old 24th Dec 2016, 20:28
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My guess is that the flaps were being retracted on the runway before the FDs were switched off.
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Old 24th Dec 2016, 20:32
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Thanks for your answer Chris! That would of course increase Vapp...
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Old 24th Dec 2016, 20:52
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Sorry Chris, that does not look to be the reason:

https://m.youtube.com/watch?v=bjGT9aJU6jo

They did not retract flaps but managed speed still increased...
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Old 24th Dec 2016, 21:16
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My guess, and I stress guess, is that it is connected to GS Mini. In flap full that will command an IAS target up to VFE-5. In flap full this is 172 knots which is what was shown on the second video.

Where's Vilas when you need him?
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Old 24th Dec 2016, 21:59
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Dommage! Yes, my first guess is evidently wrong...

Like lurkio, I notice that the speed target runs up to 172 kt and then remains at that speed, which as he says is VFE(full)-5. But I do not think this can be a GS-MINI speed target, because that would imply that the IRS has detected a sudden headwind-component of 35 knots or more.

So for my second guess! I think this may be a SRS final target speed, catering for a go-around. But I've never seen such a low SRS target displayed. Normally it is 250, IIRC. But of course one is not looking at that kind of minor, transient detail when preparing to turn off the runway.

I realise it should not make any difference, but - just for interest - is the video of a simulator or an aircraft?
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Old 24th Dec 2016, 22:45
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Thanks to both of you!

Both videos are from the real A320 I believe.
So what do you mean to say Chris? SRS is the speed reference system which is simply equal to V2+10kts. What is a SRS Final Target display?
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Old 24th Dec 2016, 23:24
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Quote from me:
"I think this may be a SRS final target speed.."

Sorry AF330, I was using the wrong terminology, and in any case the FMGC is still in ROLL OUT mode: so logically SRS could not be displayed, nor I think any speed related to a go-around. The speed I was thinking of is the speed limit for the CLB phase. (The default is 250, but as you probably know can be pre-selected by the crew before departure.) So apologies for any confusion.

For the time being, and in the absence of the FCOM, I am not going to try a third "guess"! So maybe you will have to wait for a proper expert to explain this one.
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Old 24th Dec 2016, 23:51
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Hi Chris, one thing that made me think of GS mini was that on both occasions the IAS target starts to increase as groundspeed decays through 45 knots. The rate of increase seems to be proportional to the rate of deceleration of the aircraft which in the second is much more rapid and the IAS target increases faster.
It seems logical but how many times do we say that about the bus only to be proven completely wrong by the experts.
Normally at this time on the landing roll I have other things to look at but will check it next time I'm in.
Have a good day everyone.
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Old 25th Dec 2016, 09:28
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Hi guys
It seems to be just computer glitch. Technically aircraft remains in approach phase for 30 seconds after touch down then it's in done phase. There is no target in SRS it can be max of Vref+25 and you cannot preselect GA CLB speed.

Last edited by vilas; 26th Dec 2016 at 07:51.
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Old 6th Jan 2017, 06:57
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Saw this yesterday on landing. Again on decel from 45kts groundspeed the managed speed started to increase before the FMGC reached the done phase.
My guess is that it's connected with groundspeed mini somehow but I'm not sure how!!
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Old 2nd Feb 2018, 10:59
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If anyone wanted to know the actual reason for this increase of the managed speed on landing on an Airbus aircraft.
Approach speed = maximum of VAPP or VAPP + GROUNDSPEED MINI
Groundspeed mini = (Tower wind + (TAS-Vground))
TAS is frozen at 60knots on landing and becomes invalid (on the ND dashes are shown) but groundspeed continues to be displayed down to 0 knots.
Therefore with the TAS frozen at 60knots and the groundspeed continuing to decrease the groundspeed mini is forced to increase up to the maximum allowed which in CONF FULL A320 is 172 knots. VFE FULL -5knots.
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Old 2nd Feb 2018, 20:57
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I think lurkio and applecrumble may be onto something.

Couple of observations towards the last post:

The TAS does not enter the GS mini equation.
In fact, no measured airspeed enters the GS equation.
In the video, the magenta speed starts to increase not at 60 kts TAS, but 35 IAS.
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Old 3rd Feb 2018, 10:57
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That was from Airbus.
The TAS is used in the calculation as given above. It's another way of writing the equation GROUNDSPEED MINI = TOWER WIND + TAS - VGROUND
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Old 3rd Feb 2018, 11:48
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TAS minus GS gives you momentary wind component which is needed for computation of the corrected Vapp (and in this respect, I was plain wrong about measured speeds in my post).

But your equation? Not so, imagine
Vias = 133
Vtas = 130
GS = 120
HW[a/c] = 10
HW[twr] = 10
----------------
GS mini - according to your equation - is 0. That's not how it works.
A32S.FCOM_3.bulletins.820-1A.pdf

Yes, the Vapp increment due to observing GS mini IS 0, but not GS mini itself.

Sorry for being pedantic, but later on you will too realize that proper terminology is essential to understand what are the manuals trying to say. They are complex and easily confusing without us making a mess of it.


Not just academically, but real lifer too. Such as "recycle gear lever" and "recycle gear". https://aviation-safety.net/database...?id=20130608-0
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Old 3rd Feb 2018, 13:23
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The equation quoted above has come straight from the design office at Airbus. I asked this question on tech request and this is the flight operations engineer answer. I know it is written slightly differently in the FCOM but this is outside of the FCOM but that is how Airbus stated it themselves.
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Old 3rd Feb 2018, 13:37
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Here is the answer from Airbus customer services:

QUESTION:
Planned Answer :
I have been asked why on landing rollout the managed speed starts to increase passing 45knots ground speed. Why does the manage speed start to increase passing this speed when decelerating on landing?
ANSWER:
The managed speed on landing rollout is calculated by the FG part of the FMGC and is called "Approach speed target" (APP SPD TGT).
The formula is:
APP SPD TGT = max[VAPP ; VAPP+GROUND SPEED MINI]
* The VAPP comes from the FM part of FMGC and is memorized by the FG in G/S mode below 700ft. So the increase on the approach speed target must be due to the ground speed mini.
* GROUND SPEED MINI = TOWER WIND + TAS - VGROUND
With:
- TOWER WIND memorized in approach below 700ft, limited to [-10kt ; -100kt]. - TAS frozen below 60kt.

So, if VGROUND is still valid below 60kt, GROUND SPEED MINI will mechanically increase below this speed (TAS being frozen and VGROUND decreasing).
When VAPP+GROUND SPEED MINI becomes greater than VAPP (at 45kt in your case), the APP SPD TGT starts to increase.
So, the APP SPD TGT increase at the end of rollout should be due to the air speeds (TAS) becoming invalid.
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