ATC are also very aware that 99.9% of aircraft are perfectly capable of transitioning at a speed rather than an FMS chosen height. I've used many different ways of trying to say "slow down" and still am baffled by aircraft doing 300+kts because they "haven't transitioned to indicated" yet, but were at FL220......

Have to say that from my point of view on transition/conversion means exactly that - an IAS or Mach to fly when transitioning from one speed schedule to the next. If I was given 250 knots on transition in the descent then I would never have assumed a problem with maintaining a higher speed until the transition, and likewise if I was give 320 knots on transition in the descent I would not see a problem with keeping a slower equivalent mach until transitioning.

Surely if you folk in ATC say "speed 250 knots on transition" and what you actually want/require is 250 knots know then you should say "reduce speed 250 knots"?

eckhard, can you give a reference for on conversion implying an acceleration? I am not assuming you are incorrect but without a reference that isn't what the instruction implies to me nor any of my colleagues I have recently asked.

That's certainly how I understand it and it's worked for me worldwide for a few years. In my experience going into the likes of London if the ATCers initially give you an "on transition" and then see it isn't working they'll simply put you on a hard IAS.

OhNoCB, my issue is that I don't use "on transition" but I use "when able" instead and nearly always pass a reason for needing a slow speed to highlight I need a slow speed. London ATC on the sectors being discussed in the first post have IAS displayed on radar via Mode S so we can see exactly what the equivalent IAS is knowing that the aircraft is flying on a Mach Number. The majority of the traffic these sectors work cruise around FL360/380 at Mach .78, and show an equivalent IAS of roughly 240kts. It surprises us how many times in the descent aircraft accelerate to 300+kts before reducing, however some others will just accelerate to 250kts and then maintain all the way down. We are also aware that the bigger jets normally show 260/270kts in the cruise at FL380/400 and either cannot reduce immediately to 250kts or are uneasy due to being close to stall, we accept that but again, cannot understand why the current 270/280 kts equivalent isn't maintained until you are able/happier to reduce to 250 kts but instead you still increase to 300+kts before pulling it back. I'm aware that the level restrictions can sometimes complicate it and that Airbus FMSs are awful at planning speed/level restricted descents, but we work and live with that all the time and can get frustrated when in our eyes, someone hasn't quite helped "the game". I was moaned at for getting a minibus a little tighter than they liked behind a very very large airbus, they came to me 12 miles in trail with the largest 4000ft higher and in front, they were both given exactly the same descent level restriction/speed/delay transmission one straight after the other and both began descending at the same time (so the minibus started down from 4000ft lower with 12 nm more mileage) yet the minibus said that they had to increase to 324kts to comply with the level restriction, however the maxibus was showing 260kts as it left FL400 and was at 250kts for the entirety of the descent from FL385 and didn't struggle to achieve the restriction at all.

Zonoma,

Thanks for the insight, always good to hear a little bit more detail about what you folk are doing. One thing however is that you said you say 'when able'. I assumed from the thread title that you were using 'on transition'. This makes all the difference and I would certainly fly the requested speed as soon as possible if told to 'when able'. I do not understand the logic if others are speeding up on a mach number and claiming that they are not 'able' an IAS until transition!

ATC college days...
 

Gosh this thread takes me back to what used to be taught at the ATC college...

Didn't the college used to say something like you can't go limiting an aircraft's IAS to less than it's Flight Level around the transition? E.g. For an aircraft at FL 290 then IAS 290kt, FL280 then IAS 280kt, FL270 then IAS 270kt, etc., etc.

(Maybe I wasn't listening too hard but it seemed an easy concept?)

Whoa, imagine finding you here, good egg.

I like that simple explanation though.

OhNoCB - the thread is about the use of "on transition" and my "when able" isn't official phraseology so technically shouldn't be used. I find it the most effective to get what I need but as I said above, it still can be misinterpreted or abused. I opened my post with slightly bad English as my issue isn't the use of "when able" or "on transition" as it alludes to, speed control is used so much more these days but the accompanying phraseology is still that of old which is my issue, it can be openly interpreted and things get a little tighter than we'd like. How about "convert to indicated speed intercepting 250kts, if able", would that be understood internationally? With the bigger jets going quicker than 250kts in the cruise I normally say "when able slow speed 250kts" but it is still surprising when each of them actually do it!

Interesting to hear from ATCs and pilots about their different interpretations.

Juggler25 makes the point that mode S enables ATC to see your current IAS but not Mach no.

zonoma is baffled by a/c doing 300+kts before converting. He/she also sees a/c at FL360/380 doing M0.78/240kts who then surprise him/her by accelerating to 300kts before reducing to 250kts, whereas others accelerate to 250kts and then maintain that speed.

He/she also knows that bigger jets cruise at 270/280kts who may be unhappy to reduce immediately to 250kts but cannot understand why they don't maintain their current IAS in the descent until they are happy to reduce. Instead, he/she sees them increase to 300+kts before pulling it back.

I understand and recognise the scenarios in zonoma's posts and share the bafflement.

I think that the problem stems from letting the FMS decide on the conversion altitude and leaving the jet to follow the programmed Mach/speed. Neither ATC nor the pilot know exactly at which level this conversion will occur. Yes, it's generally in the high 20s but that may be a bit late for some tactical speed controls. Would it not be better to take a proactive approach and define the speed manually?

OhNoCB asked me for a reference for 'on conversion implying an acceleration'. I can't give one but I think zonoma illustrates it well when he/she says, "convert to indicated speed intercepting 250kts, if able" in post #28. The clue is in the word 'intercepting'.

Descending at a fixed Mach no. will mean that the IAS increases as altitude reduces. This is the 'acceleration' to which I refer. When the desired IAS is intercepted, forget the Mach no. and hold the IAS.

Descending at a fixed Mach no. with the target IAS already slower than the current IAS will only increase the discrepancy. You then have to decelerate to the target IAS but when? Do you leave it to the FMS to decide?

To sum up my understanding:

1.You are in the cruise at a given Mach no. and IAS. (Speed 'x')
2.ATC clear you to descend and ask you to fly at a given IAS 'after conversion'. (Speed 'y')
3.If 'x' is slower than 'y', let the descent commence at a Mach no. and then maintain 'y' when it is intercepted. In other words, let the IAS accelerate towards the requested value, then hold it.
4.If 'x' is faster than 'y', you have two options:

a) Tell ATC that you cannot reduce to 'y' immediately but start the descent at your current IAS (not Mach no.) and then reduce to 'y' when able to at a lower altitude; or
b) If able, reduce to 'y' immediately and maintain this IAS during the descent.

In both cases a) and b), Mach no. has no relevance (unless severe turbulence is encountered) and the FMS descent programming may have to be overwritten with the new IAS. The VNAV/DES profile may well change and the top of descent point may move towards you (or indeed now be behind you!). Down-path altitude restrictions may now require speed brake to achieve. Considering all of these factors makes flying airliners so much fun!

I would welcome any insight into my summary above from ATCs or pilots.

I use "in descent 250 kts". It occasionally means a high level inbound accelerates but does seem to keep all of them at similar groundspeed (which we are all aiming for when using speed control). "On transition" and "when able" are open to too many interpretations

Most of these issues can be solved by issuing a mach number and an IAS to convert too which I quite often do. With a stream of aircraft ask the first what their mach number is, then either increase/decrease it a couple of points and lock the aircraft behind to the same or slower (within reason of course).

It's just more R/T intensive which when you're busy you want to reduce as much as possible.

Like 1985 says we separate based on ground speed so it's always fun and games trying to get everybody doing the same with all the variables involved.

Good to see the discussion though!

Speed 'on conversion' means what it says to me. Maintain your Mach Number of choice until you or the FMC 'converts' to IAS and then fly at the IAS I have requested. If you are tight against someone else before reaching the 'conversion' level, then I will give you the IAS there and then with a 'when able' instruction.

Incidentally, the Mode S in our Centre gives aircraft Mach No. for the sectors operating generally above FL255 and IAS for those generally operating below. The sector groups operate on 2 different pieces of equipment.

I am confused, t-bag. Addmitedly not knowing how 787 FMC speed schedule works, but at at the moment I am scratching my head around your description.

1) At FL370, a reasonable actual level for your scenario, M84 = 277 kt IAS.

2) ATC issued a speed reduction, and the FMS / crew (*) carried 0.84 in the descent, accelerating until reaching the crossover altitude.

3) At some point, your non-compliance become a concern and questions were raised as ATC can see the actual IAS. At that point, your IAS was probably being observed for quite some time already, before they spoke up.

a) In a perfect world, the ATC would have said just "reduce speed 250 kts", as your IAS was already higher - opposite from the typical crossover sceanrio.

b) A perfect pilot (quarterback's), in a situation similar to the originally described, could have said:
- reducing 250 kt now, or
- unable, minimum clean 2xx kts [where it is more than 250]
- roger, 250 kts as soon as we can. [min Mach restricted]

I wonder if 787 falls into the 3rd option?

@zonoma: Strange. The minibus achieves around 1000ft/3NM descent easily, if the speed (whatever speed!) is held constant.

The extra 12 miles, to make a same level restriction, should have given the trailing aircraft another 4000 ft of geometrical room. Thus with 8000 feet of "energy advantage" it is hard to imagine one would need 300+ kts to match a profile of other jet (any) descending at 250 IAS.

Lots of great replies here. ATC are well aware that very few jets can go down and slow down so aren't always that surprised when they get a "cannot do that" and should always have an alternative plan anyway. However when they have sat in a particular seat long enough they will know from other aircraft what is achievable, the benefits of having steady streams following similar profiles, so the "sore thumb" really sticks out.

Spade. Today's ATC are more rigorously selected, trained, and held to higher standards of performance than pilots.

What are the reasons, not to use "speed 250 kt (as soon as possible)" or similar as a standard? Removing the Mach / crossover element altogehter. Genuine question, there must be disadvantages otherwise it would have been adopted long ago ...

See this is where I obviously have a different opinion - because I do not see this as non compliance. ATC did not issue a speed reduction, they issued a fixed speed to fly at whenever converting to IAS - which it sounds like t-bag did.

The problem here seems obvious in that with FMC derived speed scheduling, ATC doesn't know when "transition" is and flight crew (including myself and colleagues) do not see "250kts on conversion" as an immediate request to slow towards 250 when able, we understand that to me literally on conversion so will maintain a fixed mach and therefore potentially higher IAS until converting.

Having one party believe there is non compliance with an instruction and having the other believe they are complying perfectly is obviously not a good thing, and I can tell you that myself and my colleagues would act as t-bag did.

250 knots as soon as possible would make me reduce to 250 knots as soon as possible so that one works with me! :D

I disagree. If ATC say "250kts on conversion" I stick 250 in the FMS as the descent speed and the aircraft will convert to that when it can which, co-incidentally, will be as soon as possible and on transition. It ain't rocket science but I agree that "250kts when able" would be better.

This sums up the problem. Over reliance on the 'box'...

It's quite easy really. If descending at say 0.84 and am asked for 250 knots on conversion I take a look at my IAS. If say I'm showing 270 kts I just select speed and pull for Open Descent. Voila. Very easy and never had a complaint from London ATC.

Conversely if I'm below the requested speed then I select the Mach and when it hits the required number just hit the mach/speed button. It's not difficult.

And to answer you FZRA I can tell you that the Maxibus can come down like a sack of spuds:D:ok: