Another principle of air traffic control.
It is an agreement between two sector controllers which allows control of an aircraft to be transferred from one to the other, when otherwise it would be unnecessarily delayed.
As I have discussed elsewhere, a 'release' is a set of conditions under which responsibility for control of an aircraft is transferred from one sector controller to another. A 'radar release' is a specialised version of this. It allows radar separation to be used as a condition of the release. Stay with me.
Conditions are often attached to releases; i.e. "I will take your <aircraft x>, as long as you <do stuff>". For instance, if two aircraft were waiting to depart from an airfield and were both flying straight into a sector of controlled airspace, the control tower would need a 'release' from that sector's controller before launching them. When granting permission - releasing - those aircraft to depart, the sector controller would instruct the tower controller to apply a minimum spacing between the two aircraft - for instance, that one takes off two minutes after the other. This is usually to allow for wake turbulence and/or noise restrictions that might exist.
With adjacent sectors of controlled airspace, it is a virtually-universal rule that responsibility for control of an aircraft cannot be transferred between sectors if that aircraft is not "clean": i.e. if it is not safely separated from all others. It is the remaining small percentage of transferals that this writeup covers.
Here's some examples around this, using an airway crossing between the by-now-infamous BLOCK and STACKER sectors:
BLOCK STACKER
---------------------------------+---------------------------------
\ AAL12 | /
/ 100 | *---- \
\ \ | \ /
/ ----* | VIR003 \
\ | 90 /
---------------------------------+---------------------------------
American Airlines 12 (AAL12) is being controlled by BLOCK sector, and is on its way to an airfield in STACKER sector. Virgin 003 (VIR003) is being controlled by STACKER sector, and has just departed the same airfield. Both aircraft are presently separated vertically by 1,000ft.
As I've mentioned before there are things called standing agreements which cut down on the time needed to coordinate aircraft passing between sectors, and I've contrived a couple here.
In VIR003's case, the agreement is that it is presented to BLOCK sector at flight level 90 (FL90). BLOCK sector will have control of the aircraft as soon as it calls them, at which point it is released for climb.
AAL12, on the other hand, is expected by STACKER sector at FL100. STACKER sector will, again, have control of the aircraft when it calls them, at which point it will be released for descent.
Fair enough so far.
However, although these aircraft are presently "clean", that could quite easily be subverted by, shall we say, a mistake of timing. Again, both aircraft are currently under control of the sectors that they are in. Let's say the STACKER controller decides they don't need to do anything else with VIR003, and instructs the pilot to contact BLOCK sector. Once the pilot does so, BLOCK sector has control of both aircraft.
The correct thing for the BLOCK sector controller to do now is to wait for the two aircraft to pass each other, then instruct AAL12 to contact STACKER sector. But if for whatever reason the controller doesn't wait, and transfers AAL12 before the two aircraft have passed each other, then AAL12 is "dirty."
Why? Because the standing agreement - which AAL12 is subject to - states that it is released for descent once it has been transferred to STACKER sector. Now, ideally the STACKER controller, upon receiving AAL12, would look at their radar screen, mutter something about what an idiot the BLOCK sector controller is for transferring the aircraft too early (and perhaps make a "what the hell is this?" phone call to them), and wait for the two to pass each other before descending it. But the agreement assumes the aircraft will be transferred "clean" (in fact, it most likely states it as a requirement), and therefore doesn't preclude AAL12 from being descended straight through VIR003. AAL12, therefore, isn't "clean."
The situation would be similar if AAL12 had been transferred to STACKER sector, then VIR003 were transferred to BLOCK sector before the two passed each other. Except in that case it would be the STACKER controller's fault.
Now, there are a couple of ways this can be avoided. The first is the flight progress strips - paper strips with details on each flight (including its assigned altitude) that are held by the controller for each sector. Usually when a controller transfers an aircraft to another sector they will throw the strip for it away, but if an aircraft is transferred before it has actually left the sector, the controller may keep the strip until the aircraft actually leaves, so that they can check its level against other aircraft entering their airspace from the same direction.
It's worth noting, though, that the above shouldn't be necessary in our example. It allows for recovery of a mistake, but that doesn't alter the fact that the controller made a mistake in transferring the aircraft when they did.
The way this can legitimately be avoided is using radar. Yes, it'd be handy if those spinning bread-baskets could be used for something now and again.
Clearly, in the situation above, both aircraft will be visible to both controllers involved: it is common for controllers to be able to see a user-definable amount of the airspace surrounding their sector. This allows fine-tuning of planning -- for example, looking ahead of the path of an aircraft that wants a climb to see if there is anything flying in the opposite direction that could get in the way.
This capability can be taken advantage of. Since both controllers can see both aircraft, they can agree a course of action between them that allows aircraft to be transferred early. Since the job of any self-respecting controller is to get rid of all of the aeroplanes under their control as early as possible, anything that allows them to dump an aeroplane onto another controller when they don't have anything left to do with it is a good thing.
In the situation above, which I'll reproduce to save you scrolling:
BLOCK STACKER
---------------------------------+---------------------------------
\ AAL12 | /
/ 100 | *---- \
\ \ | \ /
/ ----* | VIR003 \
\ | 90 /
---------------------------------+---------------------------------
...it may well be the case that both sectors have 'finished with' their respective aircraft: they are clear of any other traffic and the controller just wants to get rid of them. It becomes a case of 'who gets there first': whomever transfers their aircraft first won't get the other aircraft until the two have passed, whereas the 'slow' controller has to have both of them until they've passed.
The way to avoid a situation where a controller has to hang onto both aircraft is to use a radar release. In this case, the controller that employs it is adding a condition to the standing agreement, for this specific situation.
Here's how it might play out. In this contrived situation, VIR003 has been transferred to BLOCK sector. The BLOCK controller - the 'slow' one, in the context of the earlier paragraph - therefore has control of both aircraft, but equally has nothing else to do with AAL12 and wants to get rid of it.
(ring ring)
STACKER: Stacker sector?
BLOCK: Yeah, Block sector here, reference American 12...
S: Pass your message.
B: He is released to you now, subject Virgin 003, which is 20 miles
from the sector boundary, tracking West, at flight level nine-zero.
S: American 12 radar released subject Virgin 003 at flight level nine-zero.
B: Correct. Cheers.(click)
Here, the BLOCK sector controller is saying that they will transfer control of AAL12 to STACKER sector, subject to VIR003. The BLOCK sector controller - who currently has control of both aircraft - uses their radar to give the STACKER sector controller information about the position and level of VIR003. Once AAL12 is transferred to STACKER sector, it is then that controller's responsibility - using their radar - to keep it clear of VIR003. This prevents the BLOCK sector controller from having to hang onto AAL12 for an unnecessarily long time.
One final thing to point out is that under a radar release, it is required that the level or track (direction across the ground) of the conflicting aircraft - in the above example, VIR003 - is not to be changed without further coordination between the two controllers. A radar release does permit control of the conflicting aircraft to be transferred to the other controller, but that wouldn't make much sense in this case.
There are many other circumstances under which this might be used (to give another example, if an aircraft were overflying the approach sector for an airfield, which would otherwise impede the descent of aircraft approaching the airfield) but in all cases a receiving controller uses radar to separate one of their aircraft from conflicting traffic which is under control of another sector, once that conflicting aircraft has been identified and a radar release has been agreed between the controllers concerned.
See also:
Source: CAA CAP 493 (Manual of Air Traffic Services Part 1) Section 1, Chapter 5, Part 12 - rules for radar releases