Airbus approach phase activation

[Macarto]

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.

[The Banjo]

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.

[C_Star]

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 :{

[Uplinker]

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 !!!

[vodmor]

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).

[vilas]

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.

[Uplinker]

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.

[Stone_cold]

There is no reference Vilas..as I suspect you know...because it doesn't function as claimed.

[WhyByFlier]

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.

[WhyByFlier]

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.

[tubby linton]

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).

[Peter G-W]

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?

[Metro man]

* - 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.;)

[WhyByFlier]

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.:ok: 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.

[Amadis of Gaul]

I request approach phase activation, when I'm good and ready.

[MD83FO]

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.

[Peter G-W]

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?

[Cough]

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!

[qwsa]

here are another references:FCTM SI-090 P6/28 and Instructor support P194

[Uplinker]

MD83FO I always leave it automatic, so my friend doesn't have to push buttons, and use selected speed as necessary.

I prefer to do that too, but not always possible.


if Decel pseudo waypoint is sequenced in heading mode, it will activate at localizer capture or upon NAV capture in approach.

I didn't know it did that in heading mode. But 250 kts on LOC capture, That's a bit sporty isn't it? How do you avoid overshooting and get configured and stable with that intercept speed?


i would never push speed without knowing my managed target on PERF page.


A pilot who doesn't make mistakes - a rare thing indeed ! Good for you.

I've made this particular mistake 2 or 3 times in 10 years.

[Metro man]

I didn't know it did that in heading mode. But 250 kts on LOC capture, That's a bit sporty isn't it? How do you avoid overshooting and get configured and stable with that intercept speed?

There is a chart in the FCOM which shows distance, intercept angle and speed combinations within which LOC capture is possible. It could be a useful referance for making up your own rules of thumb.

[MD83FO]

you can use NAV to intercept the final approach course and lead the turn, it is legal to the final approach fix.

[Bubi352]

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.

I just want to make sure I understand this correctly. Slow guy here. What do you mean by the decel pseudo waypoint is sequenced in heading mode?

[WhyByFlier]

A lot of people saying things without reference - if you can't back it up, don't say it.

Cough, as for why that happened - well, there's a reason why the word 'coincidence' is in the dictionary. Are you SURE it's because of that? Or is it because your colleague changed the wind in the box? Is it because GS mini was having a play? Vapp changes all the time, Vls doesn't - Vls is determined by weight. Vapp is on the basis of Vls, 1/3 wind, athr, ice accretion etc.

Peter, not necessarily. Having an extra knot isn't unsafe. 1 tonne is 1 knot. Being a pilot and flying is all about being around about right. Pilot maths: 2+2 is about 5. There's 3% error in Vls calculation and we are instructed to increase the gap between Vls and Vapp to 5 knots as it often shows as less on the PFD.

[vilas]

MD83FO
From IAF to IF>FAF segment if the intercept angle is sharp then on the chart there is a lead in radial or bearing drawn from the nav aid only after crossing that radial it is permitted to change the intercept heading to more suitable one but when radar vectored if you are given a heading to intercept then you must intercept the LOC on that heading. Also when in HDG mode if you did not activate approach and intercepted LOC (obviously in select speed)unless the decel point is ahead of you it will not auto activate and when you go managed speed will increase to 250KTS.

[MD83FO]

correct villas, DECEL pseudo waypoint will sit at aprox 13 track miles, if you are inside this distance and intercept localizer or NAV, approach phase will activate automatically, in leu of flying over DECEL point.

[Cough]

WhyByFlier.

Exactly as written. We were checking Vls in FMGC was similar to Vls on speedtape (which it was) We then watched Vapp reduce to speedtape Vls +4 so I checked what was on the Perf page. The old Vapp was 124, the new one was 123 and it was written in small font, so definitely not overwritten.

We both commented on it at the time. I had seen it before, my colleague had not...

Edit to add.

Today, my colleague activated approach in a 321 quite early. Perf page Vls was 135 (Vapp 140) and then we entered unexpected holding. Exceedingly long approach followed, we landed with the numbers saying 133 and 138, no other modifications and no I'm not dreaming it.

Now there are FMGC's by different manufacturers. Maybe this is a type difference between the one we use (that does simply use GW after app activation) and yours which may not?

[Peter G-W]

Tried it today in a 333. Activated the approach over an hour before landing: approach page VLS jumped from 127 to 136 and steadily reduced back to 127 for landing. Uncanny: it was as though the aircraft knew how heavy it was and knew that we were getting lighter as the fuel burned down. (As expected, screwed all the predictions though...) As was said earlier, don't read too many nuances into FCOM English, it's a foreign/second language to most of the authors after all.

[767-300ER]

So if one "accidentally" activates the approach, is there anyway to "un-activate" it?

[vilas]

VLS is real time speed(shown on PFD). It is always correct even with flap/slat jam. As all airbus guys know just put present altitude on progress page to get back to cruise phase.

[737Jock]

So if one "accidentally" activates the approach, is there anyway to "un-activate" it?

Yes... Re-enter a cruise level.

The box does not memorise weight and freeze the speeds. Also the airbusses in the largest airbus operator of europe change speeds in the box as the weight reduces, for example due to extended downwind vector in.

Why is this even a discussion? You activate approach phase when you start final deceleration for the approach. In other words when you or the controller thinks that it is time for deceleration to minimum clean (in controller jargon 220kts).

The biggest problem with activating approach too early is that your fmgs descent prediction no longer works properly. In open des mode the blue arrow is no longer based on your actual selected speed.

[hikoushi]

Also, a quick shorthand method in the 330 (works on the McBoeing MD11 / B717 box, too) is to just type the number "1" in the scratchpad and drop it in LSK L1 on the PROG page (the altitude field). Makes the current FCU selected altitude your cruise altitude.

Incidentally, lots of similarities between the FMS's of the Airbus and the McDonnell Douglas boxes (LAT REV / VERT REV, "snowflake" instead of an execute key, McD's FMS speed basically the same as approach-phase managed speed etc). Anyone know why? Always wondered. Sure made the transition easier between the two.

[compressor stall]

So if one "accidentally" activates the approach, is there anyway to "un-activate" it?


Yes there is.

Reminds me of the time the MCDU temporarily seized up in CRZ at FL390 as I was making a position report. I was new and hadn't seen it do that before. I randomly pressed a few buttons and nothing. Reaching for the books, it suddenly sprang back to life - and remembered the sequence of each button I had pressed...

[346Heavy]

if you activate the app by accident you can go to the progress page and enter any crz altitude and you will un activate.

and i was reading on the post someone did mention that the speeds will be wrong because the a/c will capture the weight at that moment....that is wrong....the a/c will assume it will land soon so the speed representation on the approach page will assume that you will be landing at your current weight now....however its not frozen.....AIRBUS is smart

so long story short you can activate approach when ever you like brother.

[jdawg]

WBF
You couldn't be more wrong about the airbus using static data in a dynamic environment. An apology to Peter and recognition of your own arrogance will go a long way on this forum here. The best info will come without ego!


I'm sure your instructors at your large airline will love to know what you have discovered but perhaps they already know it and your misinterpreted them just as you mis interpreted the bulletin you referenced.


Slow down and keep reading/asking questions, you have a whole career to figure this thing out and it starts with understanding that you know very little and once you do figure something out, software changes may render that knowledge obsolete as well.


I'm new here, enjoyed this thread y'all.

[Amadis of Gaul]

Jdawg,

Welcome to the forum, my friend. I agree with you wholeheartedly on the whole lifelong learning thing, but, as you're new 'round these parts, let me just tell you not to put too much stock into things you read here. You'll thank me later. Or maybe you won't, I dunno, hell, I fully admit to not knowing much of anything.

[WhyByFlier]

Show me the reference. We are discussing the A320 here not the 330, 380 or Boeing 787. The reference is clear. I appreciate it probably does rectify the whole time but for the final time, seeing the wood and not the trees, on an A320 1 tonne is 1 knot. Get a grip.

[Stone_cold]

WBF. One tonne being one knot is not the issue. Please clarify that you are of the belief that if the approach phase is activated, say one hour into a five hour trip, the speeds calculated then will be frozen and unchanged at the end of the flight?

[vodmor]

WBF, you keep asking for reference that speeds are being recalculated continuously when APP phase is active, yet you didn`t provide any reference that the contrary is true. In FCOM it`s only written that when app phase is activated, speeds are recalculated using current GW, not that they are memorized/freezed/whatever. As anyone actually flying A320 can check him/herself, speeds are being continuously updated after app phase activation as the GW reduces.

[jdawg]

WBF
Its great that you demand documentation but all you really need to do, if you do in fact fly 320s, is watch the box. Clearly your instructors never provided a reference to support your current position because they're wrong and the bulletin you linked above clearly does not suggest what you believe to be the case.
I would suggest you consider the possibility that your bulletin means what it says, that "current" weight is used for the calculation and not the weight recorded at the time of approach phase activation. I have no idea what your instructors really told you but that's dead wrong.
Here in the states we usually hold 250kts until within 8 or 10 miles of FAF so we usually activate the app phase at transition altitude. Its becoming more and more common to fly RNAV arrivals with speed constraints and allowing the phase to self activate is a nice feature that I wish more US pilots used on such approaches but they are pretty rare.

[WhyByFlier]

Qwsa on page 1 provided the reference which I hadn't previously seen reading this thread - FCTM SI 90. I was taught it by an instructor early on in my career 5 years ago and the FMGC has had software updates since. I agree it updates based upon current gross weight and can hang my hat on it now I've seen it in writing rather than assuming or believing anonymitys on pprune. That and I have never seen a need or had a need to activate the approach that early that it updates. 1 tonne is 1 knot and it rounds up! How early are you lot seeing this on an A320 activating it?! Personally I do it when given an ATC speed on approach, when I need to select speed on approach because of my profile or at 20 miles, whichever the sooner - none of which burn a tonne or even half a tonne in the process.

[Peter G-W]

"Early on in my career 5 years ago..." QED.

[WhyByFlier]

Meaning what Peter? I understand QED but can't derive your meaning......

I'm very sure this site is full of miserable commuters, lonely people down route. I was one too. Make a change. I'm off.

[vilas]

I saw in the latest manual the bulletin is repeat of what WBF quoted from an earlier one. It is very clear that VLS(all speeds) are recomputed to the weight at activation but does it update further if approach is delayed is not clear.

[WhyByFlier]

Thank you Vilas, I'm glad you read it the same way.

The FCTM reference however , within the realms of a BA verbal reasoning test, is quite clear that VAPP prior to activation is on the basis of 'estimated landing weight' and after activation it's on the basis of 'current gross weight' - there is no 'at the time of application' there.

Reference is FCTM SI 90 'VAPP determination without failure' - last 6 sentences before the equation. It's all the point scorers needed to provide rather than providing assumed opinion, pearls of wisdom and vitriol.

[swh]

Activating the approach phase updates the GW with the actual fuel used, the weight is not being memorised. The reference should be under GW/CG error input after engines started.

[vilas]

WBF
That is what I also meant. This is from actual bulletin
GW and CG values are computed from entered ZFW and ZFWCG corrected for the predicted FOB and CG variation. When the Approach phase is activated, the characteristic speeds are recomputed using the actual weight and CG.
The performance model used to compute the characteristic speeds, is accurate enough to provide speed errors of less than ± 2 kt from the certified speeds.
Off course few knots practically makes no difference

[jdawg]

Guys if it didn't update it would not change. Don't need to think so hard about this its pretty obvious.
Revision 1A had no effect on this in our fleet.
Don't understand why you are so hung up on this but if you believe your fleet does not update I would suggest you set a cruise altitude in PROG and re activate after any significant holds or delays you encounter. Your doing it for nothing and will appear foolish because it won't change the data more than 1 kt but it might make you feel better.
Good Luck guys

[Amadis of Gaul]

Some of these guys would argue about the proper way to enter the line at Starbucks, right to left or left to right....

[Lantirn]

Also, a quick shorthand method in the 330 (works on the McBoeing MD11 / B717 box, too) is to just type the number "1" in the scratchpad and drop it in LSK L1 on the PROG page (the altitude field). Makes the current FCU selected altitude your cruise altitude.

True. Not only 1, but whatever altitude is below your crz altitude. Whatever number between 1 and current FL. Concept is, 1 is understood as FL001.

[vilas]

Any time after take off if you check the VLS on approach page it is calculated from estimated landing weight which is equal to ZFW+EFOB at destination. It remains constant as long as EFOB remains same.The actual aircraft gross weight is not taken in consideration, but if you activate approach now the VLS immediately changes to VLS for present aircraft weight. It is not frozen at that weight but will vary with the aircraft weight but is not noticeable as it changes only1KT/Ton.

[CMpilot1]

Vilas,

Regarding your query with respect to approach phase activation, you may refer to OEB 819 page 5/6 under the para 'CHARACTERISTICS SPEED COMPUTED BY FMGC'. There it is mentioned that when the approach phase is activated, the characteristic speeds are recomputed using the actual weight and speed.

[vilas]

CMpilot1
I know the OEB. That is what I am also saying. In my post 52 I explained it a bit further.