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.
Last edited by eckhard; 16th Jun 2017 at 10:30.