2 aircraft flying same IAS
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2 aircraft flying same IAS
Simple (?) question...
If 2 aircraft (let's say an ATR 42 and an Airbus 320) are flying the same IAS (say 220KIAS) at relatively low altitudes (say 6000ft) in the same weather conditions (same temp, pressure, density, etc and assume nil wind), will they both take the same amount of time to cover 60nm?
Is the difference between the 2 aircraft's groundspeeds negligible over this distance?
If 2 aircraft (let's say an ATR 42 and an Airbus 320) are flying the same IAS (say 220KIAS) at relatively low altitudes (say 6000ft) in the same weather conditions (same temp, pressure, density, etc and assume nil wind), will they both take the same amount of time to cover 60nm?
Is the difference between the 2 aircraft's groundspeeds negligible over this distance?
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I am not being facetious, but I phoned a friend, took a 50/50, removed the first answer I thought of and we all need up with the same answer. Apart from the minutiae of PDR's offering we still came to the same conclusion.
We then started wondering what is behind the question. Perhaps there is more to this than we first assume. So, Good Egg, please enlighten us.
We then started wondering what is behind the question. Perhaps there is more to this than we first assume. So, Good Egg, please enlighten us.
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I was, many moons ago, (1980) duty bound to take a PA-31 into FRA at 07.00. My Ops guy told me he had to guarantee I could do 150kts to 4nm. Evidently he said yes. So there I was, coming onto approach freq at crz speed 150kts. I was told to maintain and position to a base leg orbit. ATC would try & slot me in. Problems due wake etc. When asked for IAS it was always 150kts. Clb power, 300 fpm ROD and it was just about possible. Orbiting at 150kts is not a good idea, so asked to slow down to keep it tighter. Agreed, but be ready to resume 150kts when given clearance to slot in visually following given target traffic. Last message from ATC at 2nm with landing clearance was that my wheels were still up. Too damn right if I didn't want to tear them off. Good job my apron was at the far end of a long runway.
No-one lies about speed capabilities, surely.
No-one lies about speed capabilities, surely.
Last edited by RAT 5; 10th Mar 2017 at 16:24.
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Ah, but with Mode S we (controllers) know what IAS the aircraft are flying...
Obviously in ATC we apply speed control using IAS...but even with same IAS for 2 aircraft there are differences in ground speed. I just wondered how significant these differences are?
Perhaps they are so negligible that it doesn't really matter right now but, with an ever increasing demand for increasing airspace/airport capacity, I foresee time based separation becoming more and more common at airport approaches worldwide.
I was just looking to quantify how big/small the difference in time was...
(Perhaps I should get out more...)
Obviously in ATC we apply speed control using IAS...but even with same IAS for 2 aircraft there are differences in ground speed. I just wondered how significant these differences are?
Perhaps they are so negligible that it doesn't really matter right now but, with an ever increasing demand for increasing airspace/airport capacity, I foresee time based separation becoming more and more common at airport approaches worldwide.
I was just looking to quantify how big/small the difference in time was...
(Perhaps I should get out more...)
A quick look at the ETSO's (ETSO-c2d) I think would give the required tolerances to be met/allowable (well, for aircraft later than 2003 anyway), dependent on age and equipment fit etc., but this then refers you on to an SAE Standards document (AS8019A), which you have to pay to download....which is nice 
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Ok, so tolerance is +/- 3kts on IAS pointer....that's a good start.
I guess that means, for 2 aircraft, a variation of up to 6 knots....not including any instrument corrections, etc, etc.
That would make up to 20 seconds difference in the above scenario...interesting (well it sort of is interesting, if you're into that!
)
I guess that means, for 2 aircraft, a variation of up to 6 knots....not including any instrument corrections, etc, etc.
That would make up to 20 seconds difference in the above scenario...interesting (well it sort of is interesting, if you're into that!
)
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I think in reality the two aircraft should have had same ground speed. There will be some minor differences but at 220 kts CAS you should have a TAS of 240 kts and it should take both aircraft 15 minutes to cover 60nm. But reality means 220 kts will not always be delivered. Yesterday I was flying a plane that always flew three or four knots faster than was bugged. Just out of interest, is close enough good enough?
