One of the typical  but easier situation of separation is the one referring to same direction tracks within airways . They are actually aircraft flying on the same track between 2 points . 

The purpose is to descend / climb one aircraft through the level of the other when their present horizontal separation is not enough 

It is easy because controllers do not need to think of particular parameters and variations as the technique is standard and follows the simple steps displayed below : 

• Assign two diverging ( 'splitting' ) headings on opposite directions one to the left , the other to the right 

• Wait until their separation is at the minimum required and , obviously , increasing to execute the climb or descend 

• Then assign again the previous headings on opposite turns so as to put them on parallel tracks where the lateral separation is not obviously changed

• When the vertical separation is achieved then you may resume pilots' own navigation to their next way point 

Let us watch the following example 

Phase 1 : 

An aircraft wants to descend through the level of the other , say DLH234 at FL 330 , through the level of BAW215 at FL 310 ( Say RVSM is ...ON ! ) as being an arrival to a lower level , say FL 240,230 .

The controller has to assign two 'splitting headings' that is one to the right and the other to the left - either will do - considering the practical values of vectoring angles . In this case the 10,20 or may be 30 degrees  will not change anything except from time . In airways with both aircraft at higher speed like 450-460 Kt the 20 degrees are good for a rather quick split , 10 will do but separation lasts longer while the 30 degrees will drive you quickly to the airway boundary and will require fast corrections . 

Except if the speeds are different : then you assign the larger vector to the slower to achieve spacing almost at same time with the other .For instance if BAW215 was a slower turboprop doing 300 Kt (like an AT42) we would assign easily 30 degrees to achieve a deviation equal to the jet DLH234 in the same time 

Let us assume 20 degrees for every practical purpose in our example . We also assume a left turn for DLH234 and a right one for BAW215 

Phase 2 : 

While you vector to split , any descend to a level by just 1000 above the other ( FL 320) will only help you  in time . Do not wait for the vectoring split if you can already descend to any available safe level . The level on the 3rd label line just below the mode-C indicated level is the CLEARED LEVEL . You , or your assistant , must execute an entry command to your computer system to see it displayed . 

So the turn of DLH234 by 20 degrees to the left must be followed by a descend to FL 300 . In the same time BAW215 turns 20 degrees to the right .

We assume 1 min length speed vectors . Expect to watch the middle of the speed vectors crossing the boundary of the airway . Why the middle ? because the turn for correction to follow on the next step (up to vectoring angles of 30 degrees) is  completed within half a minute in such speeds . This is the time to turn back on the old headings , or the same , assign the opposite turns with the same vectoring angle  as before in order to put them on parallel tracks . So DLH234 turns now right 20 degrees and BAW215 left 20 degrees . Practically at this moment they are almost both touching the airway boundary and are separated 

Phase 3 : 

While now on parallel tracks and separated allow the full descend to DLH234 , say down to 230 . Watch however the present mode-C : It is just 314 , not yet separated vertically with  BAW215 . Just keep them waiting . Any request to the pilot to expedite descend ....is appreciated ! 

Phase 4 : 

When the vertical separation is at hand then resume both to the next navigation point ; lateral separation with vectoring is over  

Some considerations :

The side towards to turn although in theory unimportant , becomes extremely important when one of the aircraft has a routing splitting to another airway segment before the controller can resume to own navigation . Why ? because if say on the picture below the route of the DLH234 is via ABC-DEZ and for BAW215 the ABC-JKL , when aircraft arrive around ABC and BAW215 is on the right and  DLH 234 on the left you can not resume them to their next navigation point ! To do so you have to cross the tracks and if the separation is not over yet ....forget it ! 

So :

Decide the way to turn if tracks will split before vertical separation is achieved . Turn each aircraft to the side that favors the future 'resuming to own navigation'  .

Advantages : 

• Easy technique , requires less thinking

• Keeps tracks easily within airway limits or close to any desired track 

Dis-advantages : 

• Difficult if there are many opposite aircraft as they will need all of them to be displaced to one side while your traffic on climb / descend is on the other 

• When controllers have less time to spare on monitoring they usually forget to turn aircraft back on time to keep them within airway limits . At this point , all seconds do count ! 

Some simulator 'shots' :

 

	KLM951 (B744) at 380 is co-ordinated at FL 240 . AUA231 (A320) is an overflight to London steady at 340 Initial headings are 315 deg
	KLM951 is instructed  on left turn heading 305 and AUA231at right 330 The 20 degrees to AUA231 are justified by its lower speed as compared to the KLM B744
	The split is starting while we descend KLM to any available intermediate level like 360 to avoid delay
	Both aircraft instructed back to initial heading 315and KLM is further cleared down to 240 .Separation is already near 9 NM
	KLM  well below AUA is redirected to the next route point (HMM). See what was achieved : we avoided the also  conflicting AFR2332 converging from the left to HMM There was no other way to descend KLM951 on time over HMM !

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