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Airbus approach phase activation

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Airbus approach phase activation

Old 2nd Feb 2015, 04:08
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Airbus approach phase activation

Good Day Gentlemen,

There has been some debate over when the activation of the approach phase should be activated during the course of descent down to the destination.

I have list down a few factors of considerations and techniques to determine when the approach phase should be activated and I hope you can provide your individual insight and opinion over when and how one should determine the period for the activation of the approach phase.

Scenario 1
High Speed descent, ATC Speed restrictions or A/C not abiding by speed constraints in managed mode, PF sets selected speed.

Activates approach phase early in the descent after PF approach brief to get it out of the way and continues on select speed. Sets speed to managed when he/she plans to decelerate to greendot speed. Eg 300kts to Greendot, an approximate distance of 10nm assuming nil wind conditions. Plans to start configuration of A/C at 20nm thus activates approach phase at 30nm to touchdown.


Scenario 2
Delayed activation of Approach phase.

Activate approach phase only when you want to managed speed. Practice is to set managed speed right after the activation of the approach phase. Reason for this is because the sole purpose of activation of approach phase is for the A/THR system to provide auto-speed. Thus it wouldn't make sense to activate it when you want to keep selected speed and not allow the A/THR system manage speed or when you're not ready to use it.

Also, you don't want to be dragging the aircraft at greendot speed if you activate it early on the descent all the way to the point at which you plan to configure the aircraft for the approach. If you did, PF will have to have to pull speed and select above greendot to ensure you are not descending to slowly admist all the traffic behind you.

On that note, why activate the approach phase early on to get it out of the way when you're not ready to use it and pull speed after, it is additional workload.

Worse case scenario, PF forgets to manage speed on approach as sometimes he manage speed after activation of approach phase and sometimes he doesn't, lack of inconsistency can bring about such errors during high workload situations.


I hope what I've mention makes sense and do provide your 2 cents worth, your insights is very much appreciated.

Last edited by Macarto; 2nd Feb 2015 at 05:21.
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Old 2nd Feb 2015, 07:49
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Not current on the 320 but from memory, when you activate approach phase the box memorises the weight at that point, so if you do it too early the speeds will be in error as the weight reduces further.
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Old 2nd Feb 2015, 08:16
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I normally leave it until "decel" point and let FMGC activate it automatically, unless I have reasons to decelerate earlier (tailwind, interceping G/S from above platform ALT, etc.)

The only time I activate it early is when I need to fly selected speed below managed target (eg. ATC tells you to fly 220kt) to avoid unwanted acceleration when going back to managed speed.

Seing guys going selected speed at 50 miles out for no apparent reason is one of my pet peeves

Last edited by C_Star; 2nd Feb 2015 at 08:33.
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Old 3rd Feb 2015, 10:32
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Not current on the 320 but from memory, when you activate approach phase the box memorises the weight at that point, so if you do it too early the speeds will be in error as the weight reduces further.
I had either forgotten this, or never knew it. Our lot tell us to do it during the descent "in case we forget"* but you actually only need to do it in order to slow down and start getting flap out, so that could be at around 20-25 miles. If you are flying in Managed NAV, the system will automatically activate approach itself.

Cheers !

* - and it is VERY embarrassing to be flying in selected speed on the ILS and having forgotten to activate the approach, you then push for managed speed at about 5 miles, only to have the A/THR put in a shed-load of power to get you back to 250kts !!!

Last edited by Uplinker; 3rd Feb 2015 at 13:21.
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Old 3rd Feb 2015, 11:12
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Before Approach Phase activation speeds are computed according to predicted landing weight, after activation they are computed according to the current weight. So if you activate the approach phase really early, your speeds might me 1kt higher than they will be at landing, but the error should not be bigger than that, unless you activate APPR phase 400NM out, and the speeds will be calculated correctly at landing using actual weight anyway, as long as weights in FMGS are correct (ZFW).
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Old 3rd Feb 2015, 11:40
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Can anyone provide a reference that the weight is memorised at approach activation? I have not seen anything in the manuals and Uplinker I think you are mixing up arming approach with activating approach. Normally you should activate approach when you have approximately 5miles cross track to final approach course so that the speed decreases from 250 to GD and when cleared to intercept arm approach.
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Old 3rd Feb 2015, 13:04
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Hi vilas. Thanks, that was a typo, now corrected.

My company recommends activating the approach once in selected speed, which can be as far back as during initial descent if a specific speed is required by ATC in the descent.

My point about pushing for managed speed on the approach was that if you have forgotten to ACTIVATE the approach, (because you are using selected speed at ATC request); then when you push for managed speed when on the ILS, your speed target will become 250kts instead of F speed or VLS+ mini ground speed.

Last edited by Uplinker; 3rd Feb 2015 at 13:25.
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Old 3rd Feb 2015, 16:04
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There is no reference Vilas..as I suspect you know...because it doesn't function as claimed.
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Old 3rd Feb 2015, 17:10
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Not current on the 320 but from memory, when you activate approach phase the box memorises the weight at that point, so if you do it too early the speeds will be in error as the weight reduces further.
1 tonne makes 1 knot difference to Vapp (which rounds up) - see Vapp computation in QRH. How many tonnes do you think an A320 burns from 80 miles before TOD? Who and what flies that accurately?! It's irrelevant and people being peeved by it need to heed Einstein's advice:

Any fool can know. The point is to understand.
It is in FCOM, I'll provide the reference later as I'm away from the iPad. It's in the characteristic speeds bulletin from the old FCOM.
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Old 3rd Feb 2015, 22:11
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http://www.blackholes.org.uk/PP/oeb-819-1.pdf

There you go, that's the bulletin. Page 5, 3rd sentence down.

When the approach phase is activated, the characteristic speeds are recomputed using the actual weight and C of G
So the later you leave it the closer the speeds are calculated to your actual landing weight. But as I've said, 1 tonne makes 1 knot difference so don't get your knickers in a twist.
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Old 3rd Feb 2015, 22:28
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The latest version of the fcom states-
The FMGC uses the performance model and either the predicted landing weight or the current gross weight at transition to the approach phase to compute approach speeds (VLS, VAPP, F, S, Green Dot).
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Old 3rd Feb 2015, 23:21
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Once you activate approach phase, it will effect
- the altitude predictions/VDEV profile shown in the MCDU
- the position of the bottom of descent arrow (blue arrow)
even if you are in selected speed.

If you are using either of this information to monitor your descent (as I do), then I say don't activate approach phase until you are actually ready to use the approach phase (ie slow down).

If you don't believe me, verify yourself. I am happy to be corrected.

I once observed someone do a high speed descent (320+ knots) and activate approach phase very early, whilst in selected speed. The FMGS misleadingly miscalculated all remaining altitude predictions, as the FMGS now assumed the pilot would be slowing down and not continue the high speed descent. All off a sudden this pilot thought he was well above profile, as per the misleading VDEV profile/altitude predictions, when he actually was not. Next thing he had to disconnect the AP and use full speed brake to get back on profile/be below the next altitude constraint when it wasn't necessary. He was not aware why there was a large change in the VDEV profile once approach phase was activated. He was on profile before but once approach phase was activated he was about 5,000' too high. (according to the MCDU)

Last edited by a_pilot; 4th Feb 2015 at 04:28.
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Old 4th Feb 2015, 23:30
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Nowhere does it say that the characteristic speeds are frozen at approach phase activation. All it says is that, after activation, the characteristic speeds are based on calculated, ie actual, GW rather than the predicted weight. This is dynamic and is continuously updated. Stop trying to be too clever, folks. If the speeds were frozen, it would make base training a bit tricky by the time of the twentieth approach using speeds from 2 hours previously?
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Old 5th Feb 2015, 04:36
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* - and it is VERY embarrassing to be flying in selected speed on the ILS and having forgotten to activate the approach, you then push for managed speed at about 5 miles, only to have the A/THR put in a shed-load of power to get you back to 250kts !!!
I know exactly what you mean, I did it during my line training with the Head of the Training Department in the left seat. I looked at him expecting him to tell me what to do, he just looked back with a "What are you looking at me for ?" expression on his face.

I was rewarded with some extra sectors.

With over 8000hrs on the bus since then I've never done it again.
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Old 5th Feb 2015, 08:01
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Peter, it says quite clearly it's calculated based upon the GW AT TRANSITION to the Approach phase. I've had several trainers in the biggest Airbus operator in Europe, second biggest in the world also tell me it. When you go around or touch and go everyone knows, or will find out, that you need to RE-ACTIVATE the approach phase. Clearly you're such an ace you've never gone around. I however have done my fair share and know you need the 'As': activate approach phase, autobrake, approach checklist. With an activate secondary thrown in at the start of that for an immediate return. Trust me.


Worth also noting that if you go to headings I.e. are RVd for an approach prior to approach phase activation (be it manual or automatic at the D point) as is the case on 90% of approaches then you'll need to activate it yourself or you'll get the pain in the ar$e situation described above.

Last edited by WhyByFlier; 5th Feb 2015 at 08:29.
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Old 5th Feb 2015, 10:47
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I request approach phase activation, when I'm good and ready.
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Old 5th Feb 2015, 14:08
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I always leave it automatic, so my friend doesn't have to push buttons, and use selected speed as necessary.
if Decel pseudo waypoint is sequenced in heading mode, it will activate at localizer capture or upon NAV capture in approach.
i would never push speed without knowing my managed target on PERF page.
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Old 5th Feb 2015, 14:25
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WBF. I respect what you are saying but why would Airbus take the conscious design decision to actively inhibit the FCMC and MCDU from talking to each other so as to not update the characteristic speeds? Surely, is it not more likely that the translation process from French to English has allowed some nuanced misinterpretation of the mechanics of the system (even by people in large organisations)? I (mis?)read it as saying that at or after approach phase activation, the speeds become accurate based on current information, not predicted information. That's how you would design it yes?
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Old 5th Feb 2015, 15:39
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During today's approach in an ikle A319, after the approach phase was activated, Vapp reduced by 1 knot as the aircraft weight reduced...

It's not the first time I have seen it either!
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Old 5th Feb 2015, 18:34
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here are another references:FCTM SI-090 P6/28 and Instructor support P194
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