Originally Posted by
Clinton McKenzie
If 43 was told s/he's closing at 10 to 20 knots, despite having an IAS that's supposedly 10 knots less than the 'lead' aircraft, how could that have been determined other than by ground speed data from the aircraft? Is it just calculated from the position of and changes to their radar returns?
I'm trying to get my head around why the IAS method of separation is used when ground speed data is available to both the controller and the pilots. If the 'lead' aircraft has a groundspeed that's e.g. 5 kts slower than the 'trail' aircraft on the same track, why does ATC 'care' what their respective IAS indications happen to be if ATC knows their respective positions and groundspeeds?
(And thanks for you answers, le P. I'm learning stuff.)
I am not a controller, but if an aircraft has been assigned a Mach/IAS and they are catching up the one in front - as noted on your radar display - then assign the trailing aircraft a slower Mach/IAS. You don't need to know their groundspeeds. (Sometimes pilots don't set the requested IAS/Mach - sometimes ATC can see what speed they have actually set !!)
Re your second point; Aircraft are not designed to fly by selected groundspeed. There is no selection window or data box in the FMGS to select a groundspeed, and there are no groundspeeds dictated on SID and STAR plates. There is nowhere to set a groundspeed bug. If you think about it, it could be incredibly dangerous to do so - it could cause an aircraft to stall or overspeed if the winds were particularly strong tail winds or head winds.
We can select or program a Mach number or an IAS, because that is what the lifting and control surfaces 'feel'and react to. We are not cars on a road; we need to move through the air molecules at precisely known speeds to provide the lifting performance we need to stay aloft. (And, no, we are not going to get out our CRP-5's to work out what IAS we need to fly a groundspeed, then work it all out again when we change our heading !)
To know your TAS or groundspeed, you need accurate OAT probes and air data computers and some form of very accurate inertial or GNSS position determining computer. Not all aircraft will have this - certainly not all of the same accuracy. But all aircraft - even a basic Cessna 152 - have an IAS readout.
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