IAS and HDG rather than....
 

Doubtless this question has been asked before but,sorry I've not seen it on here.

It is this.Why do Controllers want to know one's IAS rather than GS and similarly HDG rather than TRK?

Two a/c's IAS will differ at differing levels for a given GS due differing temperature and density and likewise so will thier HDGs for a given Track obviously due to wind differences at different levels.Surely to Control targets on an MTI where everything is relative,GS and TRK is the most important information? Why then are we not asked for the latter?

Put me out of my misery,please!
:rolleyes:

[ 18 October 2001: Message edited by: Betty Swalls ]

We have the groundspeed of targets displayed, so don't have to ask. You are correct about relativity, but that is part of the art. Not all types can fly a groundspeed and track, and everyone has to be on the same page. When all aircraft can, maybe we will change the way we do it. Don't hold your breath- this is a very conservative profession slow to accept new ideas. :rolleyes:

Groundspeed on the radar: that would be nice! Oh yes, I forgot, that's what we will be getting for £700m when the new centre opens!

Seriously, though, we live in a somewhat archaic world in UK ATC where aircraft FMSs can fly a track (eg 3nm offset) far more accurately than we can steer them using radar headings, but we are still required to use radar headings to provide separation between traffic at or crossing the same level. The reason for asking for IAS and HDG is that then, as my UAE colleague points out, we can be sure that everybody can do that. We can always adjust the heading as necessary. That's why you often get 5 degree adjustments. I think most controllers are capable of recognising that the track and GS will change as altitude changes.

One thing which does astound me, however, is how many people ask an aircraft its IAS at, say, FL350 and then tells it to maintain that in the descent without realising that this is VERY slow once you get lower down. It is for that reason that I use the phrase: "When using IAS, speed no more/less than xxx kts". Then everybody knows where they are. The use of mach numbers for speed control in the climb/descent is source of hours of amusement!

Interesting,guys.Thankyou.

Only ever worked down low myself (FL245-) but when sequencing traffic IAS is far more useful than GS at a level and direction at which you're no longer flying.

A few questions:

Does an a/c know what it's GS is?

If not, is it not better where two a/c are on the same HDG and FL to ask the first a/c his IAS, and get the second a/c to maintain that IAS?

Does UK ATC not have GS on their datablocks? If not, how do you know what GS an a/c is doing????

Speaking only for LATCC, GS is available to Terminal Control but not to Area Control (different processing system).

If the pilot doesn't know his GS then he had better keep his fingers crossed he doesn't run out of fuel because he will have no idea how he's getting on along his route or when he will get there!

Usually yes, if you want to keep two aircraft apart it's easy to do what you say. I'm not sure quite what other way you would suggest.

eyeinthesky,

What I was getting at is whether it is possible for a pilot to maintain a particular GS if requested to do so, namely, does he have an instrument that would tell him (rather that calculating between points)?

SWM, etc. I believe modern nav systems provide a constant read-out of G/S - they can certainly provide instantaneous readout of wind (for which we're very grateful).

Regarding G/S on TC radars - somewhat less than reliable, as anyone who has seen it will confirm.

Many thanks for the info...in which case would it not be better to instruct a climbing/descending a/c to maintain a GS (rather than IAS) or is it the case that until ALL a/c are suitably equipped, it is better to follow ONE procedure for all (as I would suspect)?

Sticking to one procedure is best until everyone can do it, although there are a few who now utilise the capabilities of offset flying and ground speed readouts to separate, unfortunately very few though, as its not taught or written anywhere.

In AC at Latcc, the only way of guessing a ground speed is by watching the gaps between the returns, we normally have the last 5 displayed to give a direction too. The bigger the gaps the greater the speed. Obviously no separation can be given by this, just monitoring to check that its whats expected.

Going back to the first point, when given a IAS to fly at top of descent, the other a/c behind will be given the same IAS if about 20 miles or more behind, but the closer the two are, then the second just gets a lesser speed to a) build a gap and b)account for the height difference.
I believe the difference is about 10kts GS per 2000ft for a/c flying exactly the same IAS/Mach, anyone have the correct amount??? :confused: :confused:

One other issue to deal with too with ground speed. As you climb or go down the upper winds change and make it a bit difficult to remain at a constant ground speed. If you are using IAS and Mach everyone is playing on the same field...

regards

SWMAE03, it's impossible to climb safely on set groundspeed. Most aircraft types have set performance climb criteria based on specific IAS against aircraft weight; this speed converts to a Mach Number, generally around 27000 - 29000 feet (depending on certain atmospheric conditions). Yes, most modern (or even older) aircraft have some form of navigational display for groundspeed, but again, it is not always practical to fly by groundspeed alone. Often, for the first aircraft to arrive in a TMA (after shift change, or at airfield opening time, etc), the astute controller will ask for a wind check at different levels, so that he/she can adjust headings/speeds to take the wind into account.