The minimum separation required between aircraft depends on their aircraft types, the airspace that they are in, and whether they are IFR or VFR. There are two types of radar separation, lateral and vertical. In order for aircraft to be positively separated one of these must occur.
Lateral separation
Simplified for VATSIM use, these are the separation requirements without taking into account wake turbulence or visual separation:
Class B airspace
IFR-IFR: 3 miles, 1000ft
IFR/VFR-VFR and either aircraft is >19000 lbs or a jet: 1.5 miles, 500ft
IFR/VFR-VFR <19000 lbs: target resolution, 500ft
Class C airspace
IFR-IFR: 3 miles, 1000ft
IFR-VFR: target resolution, 500ft
VFR-VFR: sequencing to primary airport, traffic advisories and safety alerts
Class D airspace
IFR-IFR: 3 miles, 1000ft
IFR/VFR-VFR: sequencing, traffic advisories and safety alerts
Class E airspace
IFR-IFR: 3 miles, 1000ft
IFR/VFR-VFR: traffic advisories and safety alerts
TRSA airspace
IFR-IFR: 3 miles, 1000ft
IFR/VFR-VFR: target resolution, 500ft
Remember, that you must have aircraft at least half the required lateral separation from your sector edge. This is to ensure positive separation with the neighboring controller’s aircraft. An IFR aircraft must always be kept at least 1.5 miles from the sector edge to ensure separation from other aircraft an adjacent controller may be working with. This ensures the minimum 3 mile separation between the two aircraft.
In some of the requirements above, the term target resolution was mentioned. Target resolution procedures mean that the targets may not touch and that a mandatory traffic advisory must be issued.
Wake turbulence separation
Some aircraft require specific lateral separation depending on their wake turbulence category. Information regarding weight and wake turbulence category can be found in FAAO 7360.1
Separate aircraft operating directly behind, or directly behind and less than 1,000 feet below, or following an aircraft conducting an instrument approach by
Behind super:
Heavy - 6 miles
Large - 7 miles
Small - 8 miles
Behind heavy:
Heavy - 4 miles
Large or small - 5 miles
Small behind B757 - 4 miles, when operating less than 500 below.
In addition to that, you must separate an aircraft from another aircraft landing on the same runway by ensuring that the following separation minima exists at the time the preceding aircraft crosses the runway threshold:
Small behind large- 4 miles.
Small behind heavy- 6 miles.
Final approach course separation
You may use the minimum separation of 2.5 miles between aircraft established on the final approach course and within 10 miles from the landing runway provided that:
1.Both aircraft are of the same weight category
2. Super and heavy aircraft participate as the trailing aircraft only
3. The tower has a certified tower radar display
“{Reference: FAAO 7110.65 5-5-2 to 5-5-4}”
Vertical separation
You may separate aircraft vertically provided they both have valid Mode-C readouts. In order to separate vertically, you may assign altitudes to comply with the above separation requirements. However, vertical separation may only be used when the aircraft is level, in other words, if you may only issue a descent to an altitude when the previous aircraft at that altitude has started a descent.
Methods to vertically separate targets:
1.Assign altitudes to aircraft, provided valid Mode C altitude information is monitored and the applicable separation minima is maintained at all times.
2.Assign an altitude to an aircraft after the aircraft previously at that altitude has been issued a climb/descent clearance and is observed, or reports, leaving the altitude.
Example-
“American 123” is following “United 345” 2 miles in trail. American 123 is at 8000ft and United 345 is at 7000ft. They are both vertically separated.
LAS_APP: “United 345, descend and maintain 6000ft”
LAS_APP: “American 123, descend and maintain 7000ft”
You may think this is ok as both are separated by 1000ft but you are not complying with part 1 “the applicable separation minima is maintained at all times”. American may start its descent earlier and therefore you would have a separation bust as they are 2 miles apart. This would be the correct way to separate both aircraft vertically:
LAS_APP: “United 345, descend and maintain 6000ft”
UAL354 is observed leaving 7000ft
LAS_APP: “American 123, descend and maintain 7000ft”
This way you are complying with part 2 and you don’t have to worry about part 1
“{Reference: FAAO 7110.65 5-5-5}”
Visual separation
Visual separation takes the responsibility to separate aircraft from the controller to the pilot provided that there is standard radar separation before and after the application of visual separation.
You may use visual separation up to (but not including) FL180 provided that you inform the pilot of the traffic and he has the traffic in sight. However, you must:
1. Tell the pilot about the other aircraft including position, direction and, unless it is obvious, the other aircraft’s intention.
2. Obtain acknowledgment from the pilot that the other aircraft is in sight.
3. Instruct the pilot to maintain visual separation from that aircraft.
4. Advise the pilot if the radar targets appear likely to converge.
5. If the aircraft are on converging courses, inform the other aircraft of the traffic and that visual separation is being applied.
6. If the pilot advises he/she has the traffic in sight and will maintain visual separation from it (the pilot must use that entire phrase), the controller need only “approve” the operation instead of restating the instructions.
PHRASEOLOGY-
TRAFFIC, (clock position and distance), (direction)-BOUND, (type of aircraft), (intentions and other relevant information).
If applicable,
ON CONVERGING COURSE.
DO YOU HAVE IT IN SIGHT?
If the answer is in the affirmative,
MAINTAIN VISUAL SEPARATION.
If the pilot advises he/she has the traffic in sight and will maintain visual separation from it (pilot must use that entire phrase):
APPROVED.
If aircraft are on converging courses, advise the other aircraft:
TRAFFIC, (clock position and distance), (direction)-BOUND, (type of aircraft), HAS YOU IN SIGHT AND WILL MAINTAIN VISUAL SEPARATION.
Example-
LAS_APP: “American 123, traffic, 2 o’clock, 9 miles, southeast-bound, CRJ-200, level 13000, on converging course, do you have it in sight?”
AAL123: “Traffic in sight”
LAS_APP: “American 123, maintain visual separation”
LAS_APP: “Skywest 456, traffic, 10 o’clock, 9 miles, west-bound, Boeing 737, climbing through 11000 for 17000, has you in sight and will maintain visual separation”
LAS_APP: “American 123, traffic, 12 o’clock, 9 miles, west-bound, CRJ-200, level 13000, do you have it in sight?”
AAL123: “Traffic in sight and will maintain visual separation”
LAS_APP: “American 123, approved”
“{Reference: FAAO 7110.65 7-2-1}”