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-   -   "On transition" (https://www.pprune.org/atc-issues/595658-transition.html)

t-bag 8th Jun 2017 14:14

"On transition"
 
Just a question as to what ATC ( UK south ) expect?
We were descending at M0.84 with a planned 270 KT transition and were told 250 or below on transition, FMC reprogrammed and allowed the A/C to slow on the transition.
A rather curt transmission to the effect of I told you 250 or less followed after a couple of minutes.
So if you ask us for a speed on transition when do you expect it- my assumption was at the Mach/Airspeed transition, which for us is driven fron the FMC , but clearly in this case I was wrong?
Thanks.

eckhard 8th Jun 2017 16:43

My guess is that you were already above 250kts when you were at M0.84?
In this case, ATC would expect you to forget about Mach no. and slow to 250 kts immediately. When ATC give an IAS 'on conversion', they assume that you will accelerate to that speed as you descend at fixed Mach and then maintain it.
If you had said something like, "we currently have 270kts indicated; would you like us to reduce to 250 now?" then ATC would probably have confirmed that. If 250 is too slow for you at that altitude, just tell them.

BTW, the phrase, "on transition" in this context was replaced some years ago with, "on conversion", to avoid confusion with transition altitude.

22/04 8th Jun 2017 17:11

Might be wrong but think UK ATC still use transition.

Juggler25 8th Jun 2017 18:38

As eckhard says, 'on transition' was replaced with 'on conversion' a while back, however a lot of UK controllers will still use 'on transition' (old habits die hard...like some will still use millibars instead of hectopascals).

Anyhow. There is a lot of misinformation amongst controllers about when a/c are flying on what speed. I imagine what happened here is the controller wanted you to fly 250kts when you converted from flying on a Mach Number. We are taught that is usually around FL290 when descending however understand that it can vary greatly depending on a/c type/FMC and profiles etc. Therefore impossible for us to know when you will convert. In the UK we can see what IAS you are flying from the Mode S, however it does not show what that is as a Mach Number and which you are currently flying. So it's very difficult for us to know if you have converted or not.

In the future just say to the controller that you haven't converted to IAS yet and will do when you convert. Or if you're able to fly the IAS straight away then do so as this will show on the Mode S.

Hope that makes sense.

zonoma 8th Jun 2017 19:36

If you have been given speed 250kts or less, then I would imagine that a delay would have also been passed so why would you want to accelerate to 300kts+ before coming back on the speed? There is a debate about whether ATC can give aircraft an indicated speed to transition at rather than allowing the aircraft to automatically convert at whatever altitude it desires, however there are obvious arguments against (brick walls). I don't mind aircraft accelerating beyond the speed I have issued, I just hope that the extended mileage it then accrues from a couple of rather harsh turns isn't minded either.

t-bag 9th Jun 2017 18:23

Thanks Guys
Maybe 250 "as soon as you can" is a better way of putting it - the mach/speed transition is very variable dependent on type.
With regards to delay ... theres another conundrum - this was due LGW delays - the next controller gave us direct MAY and LGW a base leg for 26L - hey I know its dynamic, but there you go!
I was surprised, I have been operating into LGW for nearly 30 years and it was the first time I have be "rebuked". The 787 is a very different beast and Im not sure that ATC have got their collective head(s) around the way it works.
Bring back the jumpseat rides .....:ugh:

Juggler25 9th Jun 2017 18:39

If it's a speed for delay then personally I often just say 'when able to make the level restrictions, bring it back to 250kts'. Bit trickier if you're in the middle of a stream of inbounds however.

Expect to be given 250kts due to Gatwick delays an awful lot more in the future. I believe there are plans to bring in the same system as is currently used at Heathrow where aircraft are slowed down in the descent in order to minimise holding. Whether the adjacent ANSP's will be slowing Gatwick traffic down in the cruise for delay as they currently do for Heathrow I'm not sure, however definitely expect it on first contact with London soon.

P.S. I'd love a jumpseat ride in a 787! :8

Conversely, have you ever been down to Swanwick to see how it works from the other end? Definitely worth a visit...

Cirrussy 10th Jun 2017 07:22

Jumpseat rides can and do still happen. I had a controller in the FD last year.

GlobalJourney 11th Jun 2017 04:06


Originally Posted by Juggler25 (Post 9797840)
I believe there are plans to bring in the same system as is currently used at Heathrow where aircraft are slowed down in the descent in order to minimise holding. Whether the adjacent ANSP's will be slowing Gatwick traffic down in the cruise for delay as they currently do for Heathrow I'm not sure, however definitely expect it on first contact with London soon.

Yes it is in the pipeline, in theory in exactly the same implementation as XMAN Heathrow. In practice it probably won't actually be done because NATS are refusing to co-operate with any of the arrival managment projects for the other surrounding ANSPs.


Originally Posted by t-bag (Post 9797827)
Maybe 250 "as soon as you can" is a better way of putting it

For many operators "as soon as you can" equates to "whenever you feel like it".

Doug E Style 11th Jun 2017 08:16


Originally Posted by t-bag (Post 9797827)
The 787 is a very different beast and Im not sure that ATC have got their collective head(s) around the way it works.

For the benefit of controllers everywhere and those of us who fly normal aeroplanes, can you please elaborate on what you mean by that statement? I'd certainly like to know how "very different" it is.

eckhard 11th Jun 2017 11:00

t-bag: Not wishing to be controversial but in my experience (737, 747, A320, 787) they all behave pretty much the same in the descent. Sure, the FMC/FMGC and autopilot behaviour may vary but aerodynamically they are all heavy, swept-wing jets; they will go down or slow down, but not at the same time.

Whether 250KIAS in the descent is achievable at e.g.FL350 is another question. IIRC, all except the 747 should be capable of it.

If the FMC/FMGC speed schedule is preventing or delaying compliance with an ATC request, you can always use 'FLCH' or 'Open Descent'. That way you can control the speed manually.

t-bag 11th Jun 2017 14:21

Eckhard - completley agree, 250 KTS is achievable, but from where we were it wasnt "instant" as we allowed the FMC to manage the transition in VNAV, hence my question above - if the controller wanted a more immediate response there are, as you know, other options that we could have used.
Re the 787 on short haul european work we are normally at higher levels (410+) and cruising at M84/85, it also descends on a much shallower profile than other Boeings I have flown ,so yes it is different- fitting us into the lower traffic in the descent seems to be a bit of a headache.

The Many Tentacles 11th Jun 2017 16:30


Originally Posted by t-bag (Post 9799239)
fitting us into the lower traffic in the descent seems to be a bit of a headache.

That's what headings are for :}

eckhard 11th Jun 2017 18:50


Re the 787 on short haul european work we are normally at higher levels (410+) and cruising at M84/85, it also descends on a much shallower profile than other Boeings I have flown ,so yes it is different
Fair comment as I am on long-haul. Do you have the 'thrust levers slightly open' feature during VNAV descents? I think we used to but after some Blockpoint upgrade or other they now appear to close completely.

BBK 12th Jun 2017 17:40

Had something similar recently. Still in the cruise and informed delays into LGW. Asked to slow down to 250 and to expect some holding. Replied we would slow down immediately to min speed and then 250 when able. Controller was ok with that.

Saved about 5 minutes with the speed reduction but still given the opportunity to admire the Sussex countryside for about ten minutes! Pretty rare to hold but from comments above perhaps it'll become the norm. As an aside always a pleasure to work U.K. ATC. Thanks guys.

Doug E Style 13th Jun 2017 06:02


Originally Posted by BBK (Post 9800403)
Had something similar recently. Still in the cruise and informed delays into LGW. Asked to slow down to 250 and to expect some holding. Replied we would slow down immediately to min speed and then 250 when able. Controller was ok with that.

Saved about 5 minutes with the speed reduction but still given the opportunity to admire the Sussex countryside for about ten minutes! Pretty rare to hold but from comments above perhaps it'll become the norm. As an aside always a pleasure to work U.K. ATC. Thanks guys.

It is very common for traffic inbound to LHR to be given a speed reduction some way out if there are holding delays. It is co-ordinated with neighbouring ATC units so you could be asked to slow down by Scottish, Maastricht, Brest etc. A Mach number reduction of .04 in the cruise and 250 kts in the descent seems to be the norm.

Cough 13th Jun 2017 11:08

Tbag...

If they want 250kt, then FLCH is your friend. VNAV Follows the path at the expense of speed control, so if it rides fast, you end up not following the clearance...

t-bag 13th Jun 2017 13:20

Cough
Doh, I give up, yes I know how the aeroplane works.:ugh::ugh:
What is the title of the thread -250 "on transition" not 250 now?
Thanks to those of you that made sensible contributions.

middles 13th Jun 2017 15:02

From an old git-
On transition was used by AC CLN as a means of applying, but not actually applying in some cases, separation to streamed aircraft. They could then transfer the aircraft ASAP to TC East but not entirely confirming that they would both be flying at the same speed. I personally gave up ringing CLN to inform them that the following aircraft was actually flying faster and had they ever read the section about separation being 'constant or increasing'.
Parallel headings-Always fails safe.

eckhard 13th Jun 2017 19:03


What is the title of the thread -250 "on transition" not 250 now?
But I think that is why some of us are confused. It seems from your posts that you think that it's OK to be flying faster than 250kts while in 'Mach mode', then let the FMC decide when it is going to change over to 'IAS mode' and only then wind it back to 250kts. As I said, the phrase 'on conversion' implies an acceleration to the requested IAS as altitude reduces.

If you like, It's the reverse of the climb schedule, e.g. 320/M0.85, where after conversion to mach, the IAS reduces from 320 as you climb.

Going back to descent, if ATC asked you to "maintain 320kts after conversion" there would be no real issue as your current IAS would in all likelihood be less than 320. The problem with the FMC arises when 250kts is slower than the current IAS at top of descent.

So, it's not that we think 'you don't know how the aeroplane works', but rather that you might not know what ATC want?

All offered with the highest respect and in the spirit of constructive debriefing!


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