Speed Separation

(in APP or Lower ACC to APP Pre-Sequencing)



There are two tools in  achieving radar separation at lower levels (below 20-24 000 feet ) ; vectoring the most important one but speed as well . Speed control may be complementary but not less important as it 'fine tunes' or corrects -and therefore reduces- vectoring  .

The important things to note down on speed , however , are the following : 

  1. The speed depends on the individual pilot - apart from the landing and take-off speed 

  2. The speed to control used is the indicated one (IAS) but the movement of the aircraft depends on the true air speed (TAS) 

To control the first you should keep your eyes open on the speed evolution of a flight despite the 'promise' of a pilot to comply or not with a restriction . You should additionally get familiar with the 'speed-attitude' of pilots in your area . 

TAS and IAS relation . The Speed Differential 

Where there is more thinking for the controller is the relation between IAS and TAS , as TAS derives from IAS if the flight level FL is taken into account 

Let us watch these two arriving flights : The one below  is the BAW6341 a typical AT42 just in contact at FL 090 ( mode-C +/- 200 ft ) making 189 kt of TAS reporting IAS=165 ., about 25 nm from the aerodrome   

The second is LGL335 is a F50 - this pilot likes to 'push-it' a bit -  he is descending to FL 140 , presently at 150 making TAS=242 reporting IAS=193


Can we identify a relation between IAS and TAS due to Flight Level  ? The general relation considered is 'linear' obeying to the general format : TAS = IAS + C x FL , where C is a coefficient to define . From the first relation we get : C1 = (242-193)/150 = 0.32 and  from the second C2 = (189-165)/90=0.26 . Yes , the relation is not a steady one but if we think of an "average" of 0.30 for simplicity then we have an easy way of calculating  quickly the speed effect . So : 

TAS = IAS + 0.3 ( or 30%) x FL  (unit is Knots)     [1] 

The controller has a tendency to impose same IAS in order to avoid the speed effect on separation . In this case the Speed Differential ( SD = TAS1 - TAS2 ) becomes important , where if IAS are the same then the speed difference between traffic - mainly succeeding - it is actually :

SD = TAS1 - TAS2 = 0.3 x (FL1 - FL2) | on same IAS  [2]

So practically a 1/3 of FL becomes the speed difference . For say a difference of some 2000 ft , this is 20/3 = 6 knots or a difference of 6 knots / 60 = 1/10 of nm per minute !! Really nothing ! It is obvious from the above relation that on the final with succeeding arrivals then equal IAS produce a practically zero SD . Therefore at this phase IAS control does everything ! 

Succeeding Traffic on final with same IAS do not loose separation due to speed 


Separation reduction due to  Speed Differential

What if the IAS are not maintained the same ? Then :

SD = TAS1 - TAS2 = (IAS1-IAS2) + 0.3 x (FL1 - FL2) | on IAS1<>IAS2  [3]

For an easy reference remember that a difference of 60 Kt loses 1 nm of separation per minute . If you assume that from the time between the first aircraft on sequence is aligned on the Localiser about 12 nm from the aerodrome until it lands is an average of 4-5 minutes , then  this is how many nm of separation you'll loose at the end 

See an example here :

The FBBGN is an AT42 about 15 nm from a/d TAS=160 , IAS = 146 . The second behind LGL1801 is a B735 on TAS=297, IAS=260 at 30 nm from a/d . Both a straight in not in a critical situation we do not ask any speed adjustment . How much will the separation be when FBBGN is landing ? 

Think :

FBBGN will need about 5 min to land so a total of something less than around 10  nm of separation reduction is expected


When FBBGN is half a mile from the RWY , the B735 is 7 nm behind him , on the simulator . We have lost 15 - 7 = 8 nm of the initial separation . At this time the AT42 has an IAS=135 and the B735 of 195 .  You may note that the IAS difference alone would suggest something just below 5 nm of separation reduction , but ...it is more than that ! You may also note that the very exact calculation in the  case above suggests with a 100 Kt differential in 5 min  a separation reduction of :

100 nm/h * 5 min / 60 min/h = 8.33 nm !!! Our model has a remarkable precision !! 

How to increase / decrease or adjust the separation using IAS changes ? Well ...have a look at our next chapter 


Speed Separation adjustment on same point converging traffic

This case is especially useful to the 'feeder' sectors ( APP or Lower ACC ) that deliver approaching traffic at same level but separated by a steady distance . Traffic enters from the upper sectors usually at FL 240 at various separations from  a given point ( IAF or hand over fix ) and after some distance of about 30-40 NM they must reach a level like FL 140 but all having a spacing of some , say 5-10 NM between them . For 'feeder' sectors from only one direction usually 10 NM are demanded as there are other feeders from other points delivering traffic to the APP ILS-sequencer 

The real 'difference' here is that most of traffic follow simultaneous descends and there is practically no FL difference - except if aircraft types are incompatible in speed . It remains therefore to see how much one could increase the separation by using the IAS changes .


If in [3] , FL1 = FL2 then the IAS difference IAS1 - IAS2 does the whole job . For jets these IAS values vary between 280 to 350 Kt ( at FL = 240 that makes about a TAS of 360 to 420 Kt ) . At those TAS the distance per minute ( have a look at the speed vectors ) is 6-7 NM and a distance of 30-40 NM is covered in about 6 minutes ! 

So if at average the trajectory is 6 min the time of the speed reduction should be around 5 min as no pilot would immediately cut down the speed . To make it more simple we assume 5 min available for speed reduction in all cases and then we ask how many NM of separation could we achieve at a given IAS difference . The answer is given by the :


Additional separation increase in NM = ( IAS1 - IAS2 Kt ) x 5 / 60 ~     0.1 * ( IAS1 - IAS2 Kt )   [4]

So it is practically the 1 / 10 of the IAS difference , or :

the IAS difference is 10 times the required separation !! 

in favor of the slower ahead - no doubt !


Example 1 : A B733 reports IAS=320 and the succeeding A320 reports IAS=340 . Present separation to hand over fix is 7 NM , what reduction can we suggest to achieve the additional 3 NM ? 

To get additional separation of 3 we need 30 Kt IAS difference . We may ask the B733 to increase to 330 and the A320 to reduce to 300 . It is better to divide the speed adjustment if we can .

Example 2 : A CARJ reports IAS=290 and the succeeding B762 reports IAS=360 . Present separation to hand over fix is 5 NM , what reduction can we suggest to achieve the additional 5 NM ? 

To get additional separation of 5 we need 50 Kt difference with the B762 as slower !! We may ask the CARJ to increase to 320 but then the B762 should reduce to  270 , a very hard situation for a B762 at this point ! We may improve it : The CARJ pilot can increase to 340 , practically close to its maximum so that the B762 will only reduce to 290 

Example 3 : On final about 15 NM from RWY aircraft are on ILS ( at below 5000 feet ) . A F50 reports IAS=150 and the succeeding A320 reports IAS=180 . Present separation is 4 NM we want to make them 5 . We need another 1 NM of separation therefore 10 IAS difference less for the jet behind for whom the less than 150 is already near stalling speed . We ask the F50 to keep up 180 Kt and suggest 170 Kt to the A320  . 

Note : considering the altitude difference is always safer to adjust further by 5 Kt , so I would have asked the F50 to make 185 . The reason is that overall the 1/3 of FL difference does give some value around 5-10 Kt in TAS .  

Example 4 : A AT42 reports IAS=210 and the succeeding A320 reports IAS=340 . Present separation to hand over fix is 6 NM , what reduction can we suggest to achieve the additional 4 NM ? 

Well ...we need a difference of 40 with the A320 as ..slower ! The AT42 may increase but no more than about 240 , in which case the A320 should do ...200 Kt !!! The A320 will probably reply Negative ! to such a request . The answer is ...the APP can not receive this situation with 10 NM . If the tracks are wide and allow some vectoring then we may get few miles but then only near the ILS we may force the A320 to accept something like 200 Kt or less . It is good to try to co-ordinate another type of spacing 



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