Basic Aeronautical Knowledge questions
Clinton, IAS is a very useful tool for separating aircraft that are travelling in trail and for sequencing as aircraft cruise and descend at a given Mach number or IAS. Ground speed decreases with altitude at a given Mach/IAS so to maintain spacing on descent I give M0.03 or M0.05 slower into 20kt IAS slower for the following aircraft over the lead (e.g lead M0.78 into 280kts, following M0.73 into 260kts). That maintains the spacing all the way down. There's no way I could achieve that with ground speed without constantly issuing new speeds.
Yes ground speed is how we establish that conflicts exist for crossing tracks and opposite direction traffic.
If we want to use speed control we almost always use Mach/IAS because that's what pilots fly on. Personally in over 25 years as a controller I never issued a speed control instruction based on ground speed.
Yes ground speed is how we establish that conflicts exist for crossing tracks and opposite direction traffic.
If we want to use speed control we almost always use Mach/IAS because that's what pilots fly on. Personally in over 25 years as a controller I never issued a speed control instruction based on ground speed.
Thread Starter
Thanks le P! I stand corrected.
My comment should have been confined to crossing tracks and opposite direction traffic. Chronic Snoozer's African Swallow made clear to me the usefulness of IAS in the 'trail' and other circumstances to which you refer.
As a matter of interest, how many aircraft 'squawk' their IAS instead of or as well as GS? As a rough percentage.
My comment should have been confined to crossing tracks and opposite direction traffic. Chronic Snoozer's African Swallow made clear to me the usefulness of IAS in the 'trail' and other circumstances to which you refer.
As a matter of interest, how many aircraft 'squawk' their IAS instead of or as well as GS? As a rough percentage.
We only see ground speed (measured direct by radar, reported by ADS-B or estimated for flight plan tracks). If we want IAS we ask or just assign a speed when we have a reasonable idea of what the aircraft will be doing.
Thread Starter
Thanks again, le P.
I note that in 36 years of flying so far, I've never been asked my IAS or to change it to something. The occasional 'go as fast as you can' or 'slow down' - yes.
But of course I'm not mixing it in the flights levels and airways that you're evidently dealing with. I'm generally down with the other plebs and the bugs.
I note that in 36 years of flying so far, I've never been asked my IAS or to change it to something. The occasional 'go as fast as you can' or 'slow down' - yes.
But of course I'm not mixing it in the flights levels and airways that you're evidently dealing with. I'm generally down with the other plebs and the bugs.
It only works with similar performance aircraft, so unless you're in-trail with another such aircraft precision speed control is never going to work. Vectoring for sequencing is effectively doing the same thing - giving you extra track miles so your speed along the track is reduced.
Thread Starter
Thanks again, le P. It’s always a good day when I learn something new.
I now realise that outside radar range and absent reliable and accurate real-time position, ground speed and track data, regulating a ‘known’ - IAS - for aircraft in airways would be a useful separation tool for ATC/S. I’d forgotten that some of the gizmos we now take for granted haven’t been around that long.
I now realise that outside radar range and absent reliable and accurate real-time position, ground speed and track data, regulating a ‘known’ - IAS - for aircraft in airways would be a useful separation tool for ATC/S. I’d forgotten that some of the gizmos we now take for granted haven’t been around that long.
A few times I've been trailing another type and the controller queries our indicated after being assigned IAS control. On a few occasions I've been nailing say 180 IAS as required to separate and been told I'm closing at 10 to 20 knots on an aircraft that's reported holding 190 IAS (less than 2000ft separation) could be positional errors between different types at lower airspeeds, or the other aircraft was a bit slow on their 190 I don't know the reasoning, usually resulted in a 'slow to minimum approach speed' shortly afterwards. So I don't hold much weight to the accuracy of IAS in different aircraft being much better than +-10 kts of indication, or maybe the preceding pilots ability to hold such speeds. In Sydney they just give you 210 kts and vector for separation so it's less apparent, if you are gaining they just widen you out. Melbourne it's like a jolting freight train of speed up and slow down so they can keep you on the star tracks. Outside RADAR/ADSB coverage you get simple maintain above or below X speed sometimes, but the separation they require for it is massive (20+nm), so you never get close. There used to be the old Perth to East coast battle between the Ansett A320 and 737s where the Airbus had to get above or in front of the 737 or face significant slow down to maintain behind the 737. The 737 had the climb advantage so if they departed similar times it could get up first and block the optimum air route altitudes, in the days when surveillance and available levels was less.
Last edited by 43Inches; 21st Apr 2022 at 00:37.
Thread Starter
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'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.)
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There is also significant safety issues with basing separation by controlling using GS. With IAS the pilot can immediately respond with 'negative' or 'unable' when it's below or above a safe speed for normal flight, with GS you might turn a corner and the wind change might put your IAS outside of a safe region to comply. In a busy environment that would lead to extra radio and compensation vectoring. With IAS all planes are 'roughly' doing the same thing, when they turn a corner they are all flying in the same wind packet so should not vary in GS significantly if the controller is doing their job well. Also no autopilot holds GS only IAS as far as I know, so it would cause issues there as well.
Clinton, how would you fly on groundspeed using steam driven instruments? Sure it could be done but not without you reaching for your wizz wheel. Everyone can fly on IAS and as 43Inches says airflow is IAS not groundspeed.
When there's a screaming northerly in Melbourne a jet might have a 300kt groundspeed on downwind and by the time they've rolled out on a 10 mile final it's back to 120kts. Alright, that's fairly extreme but it would be impossible to control using groundspeed. If both aircraft are flying more or less the same IAS profile they'll more or less be doing the same groundspeed at the same point, i.e. you have good idea what you're going to get.
I'd hand off a pair of 737s to approach with 12 miles at ARBEY both doing 280kts IAS into 250 IAS below A100 and they'd be a nice 5 miles as the front one touches down, untouched by the approach controller. With a screaming northerly you might need 15 miles at ARBEY to achieve the same result.
When there's a screaming northerly in Melbourne a jet might have a 300kt groundspeed on downwind and by the time they've rolled out on a 10 mile final it's back to 120kts. Alright, that's fairly extreme but it would be impossible to control using groundspeed. If both aircraft are flying more or less the same IAS profile they'll more or less be doing the same groundspeed at the same point, i.e. you have good idea what you're going to get.
I'd hand off a pair of 737s to approach with 12 miles at ARBEY both doing 280kts IAS into 250 IAS below A100 and they'd be a nice 5 miles as the front one touches down, untouched by the approach controller. With a screaming northerly you might need 15 miles at ARBEY to achieve the same result.
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'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.)
Depending on the level you are flying at often descent is initially done by constant mach, IAS increases until the mach/IAS crossover (eg FL317) then its a constant IAS descent to 10,000 ft. For example I can cruise at around 240 KIAS (M0.855) do a constant mach descent until hitting 310-320 KIAS to 10,000 ft, the 250 KIAS below 10,000 is a fairly normal profile.
Then there is the little old wives tale of a +/- 5kts buffer on ATC speed, so if ATC assigns 180 and min clean is 185, its okay to maintain 185. No ATC speed buffer is in the AIP.
Then there is the little old wives tale of a +/- 5kts buffer on ATC speed, so if ATC assigns 180 and min clean is 185, its okay to maintain 185. No ATC speed buffer is in the AIP.
There used to be an allowance of 10kts or .02 mach in ENR 1.6, but that has been removed. I think the +-5kts thing is just what the general requirement on a check is for holding a speed. That all said IAS indicators could be out by any factor as detailed in the FM depending on speed and config that figure floats due to position and angle. Then there's instrument error where the FO might have different speeds displayed to the Captain by up to 10kts depending on FM maximum again. So if both had max 10 kt differences and each plane flew off the extreme end indicator you could have a 20kt difference, before even accounting for calibration. Its all academic as the controller does not need to know that, they can ask what speed they are doing and then reduce that speed by x amount to balance it, if its still too fast, just drop it another 10 kts at a time until it works or the pilot says too slow, and then the vector trip starts. As Le P says knowing each type like having 2 737 would help and airspeed calibration errors would be similar on similar types. But say a Turboprop vs a jet, the jet is operating at much lower on its speed scale while its being mixed with TPs operating high on their speed scale meaning different positional errors possibly. As well as just purely different plumbing.
Airliners transmit the data, it’s up to individual states if their ATC units use it.
NATS for example publishes speed compliance reports to operators for LHR and DXB, they know what pilots have selected.
Thread Starter
Gosh, we’ve come quite a way from BAK in GA bugsmashers squawking altitude and groundspeed to transport category aircraft squawking every parameter including the galley sink level and the cockpit-selected IAS and FL/ALT.
I now get why pilot-reported IAS was useful before the advent of reliable EFIS/FMS and ADS systems, in airways. I’m still not quite sure why an instruction to ‘fly 10 knots IAS faster (if you can)’ or ‘fly 10 knots IAS slower (if you can)’ wouldn’t achieve the desired outcome, based on whatever information happens to be available to ATC these days. (Bit strange that you don’t get to see the selected IAS and FL/ALT data, le P?). If it’s about separation, that must be about predicting conflicts and I don’t understand how that can be done without an extrapolation from position, track and groundspeed (except in the case of aircraft flying e.g. flying the same airway and STAR at the same levels/altitudes - I get it that if each complies with an instruction to fly at or above or below a specified AIS you can predict whether they are all ‘catching up’ to, or ‘spreading apart’ from or maintaining ‘a’ distance from each other).
All interesting and important stuff - thanks.
I now get why pilot-reported IAS was useful before the advent of reliable EFIS/FMS and ADS systems, in airways. I’m still not quite sure why an instruction to ‘fly 10 knots IAS faster (if you can)’ or ‘fly 10 knots IAS slower (if you can)’ wouldn’t achieve the desired outcome, based on whatever information happens to be available to ATC these days. (Bit strange that you don’t get to see the selected IAS and FL/ALT data, le P?). If it’s about separation, that must be about predicting conflicts and I don’t understand how that can be done without an extrapolation from position, track and groundspeed (except in the case of aircraft flying e.g. flying the same airway and STAR at the same levels/altitudes - I get it that if each complies with an instruction to fly at or above or below a specified AIS you can predict whether they are all ‘catching up’ to, or ‘spreading apart’ from or maintaining ‘a’ distance from each other).
All interesting and important stuff - thanks.
To add the display functionality to TAAATS would likely cost a mozza and would be rather hard to justify given a new system isn't too far away - they're nice to haves rather than must haves.
Topsky has that functionality, for some ATC centers the most common issue is a level bust caused by pilots not setting the correct clearance, many places around the world use that feature as another layer to prevent level busts.
TAAATS alerts when there's a mismatch between the cleared level in the label and what the pilot has dialled in as the assigned level, it just doesn't show what the pilot has dialled in.