Question from dumb pilot - preselected speeds & altitudes

[Luke SkyToddler]

Question for controllers if anyone could help me out, kind of following on from another thread running in Tech Log with regard to A320's maintaining a selected speed.

Starting with a hypothetical scenario, an aircraft is descending towards 10,000 ft and maintaining 300 kts indicated speed (350 TAS). We have a 50 kt tailwind as well. As it approaches 10,000 ft, pilot selects 250 kt in the speed control window.

At that moment that we select the new speed, what is the aircraft speed that you guys see on your radar screens? Is it the pure groundspeed (400), the actual TAS (350), the actual IAS (300) or the selected IAS (250)?

How can you tell that we're actually doing 250 indicated knots at all, obviously on a particularly windy day the groundspeed could be anything up to +/- 100 kts on that?

In the same line of thought, when you clear us to a new level and we start to climb / descend, can you see what preselected altitude we've set? I'm sure I read somewhere that the technology was available or about to be introduced or something along those lines, would obviously be a pretty powerful tool to eliminate level busts?

[BackPacker]

I'm not a controller so feel free to ignore me, but part of the answer lies in Mode S. More specifically, the EHS variant. This variant, mandatory or becoming mandatory in anything big (GA will use ELS), will transmit some sort of "aircraft intentions", derived from, for instance, FMS. If the controllers equipment can decode this information I guess the controller can see it.

Good article in this months Flyer magazine about it.

[matsATC]

What we see on radar is the ground speed, calculated by the distance covered between 2 or 3 radar returns. There is no way we can see if you actually fly the assigned speed. If we want to know, we have to ask...

With mode S, things will change somewhat: there will be a downlink of a lot of parameters, including speed (don't know wether that will be IAS, selected IAS or TAS, but I suppose it will be IAS).

[tired-flyboy]

With Mode S we see what you select,

our display can show, GS, IAS and Selected FL /Alt

[loubylou]

Having said that - we can work out if you're not flying close to the assigned speed - ie you're doing 350 and the assigned speed is 250kts - just from the distance between the radar returns and taking into account the upper winds -
So to answer your question - we get ground speed readout on the radar - but if the speed assigned is 210kts, the upper wind is 40kts tailwind, then we should be seeing 260kts as the readout - so we CAN tell if you're still doing 280kts!!!

louby

[tired-flyboy]

loubylou

With Mode S if we assign 210kts that is what we see in the readout!! you even see the numbers change when you ask to confirm the speed that is being flown! :rolleyes:

No escape :p

[matsATC]

so we CAN tell if you're still doing 280kts!!!

Well... that's the theory, and only if you are referring to TAS. I'm not going to calculate the tail wind component for every altitude and any wind direction...

Since all speed instructions are referring to IAS, you'll also have to make an altitude correction, since 250 kts IAS in 0 wind will show up with a much higher ground speed at higher altitudes...

Anyway, with some experience, you can make a pretty good estimate of the speed, but not to to knot...:E