Hi Claire....hope I am able to provide some answers to your questions from the U.S. criteria!
Separation standards are set and must be adhered to or the controllers get time in the chapel so to speak....the FAA is not too lenient with errors within the system, so most of the standards are maintained very close, and in some facilities there is software that will detect less than standard separation for you, so that tap on your shoulder could be the start of listening to tapes, watching replays of your position, and much more fun. All of the enroute facilities have the software and some terminal facilities are getting it soon to test and then be put in place all over the joint.
You are correct, there are different separation requirements for aircraft in the enroute environment (center airspace) than there is in the terminal environment (approach controls). Then there is the wake turbulence issues with the different aircraft weight classes for the terminal controllers.
The density of traffic does not change the separation standard however it might and often does generate a greater distance for aircraft arriving over a specific fix to an airport or route which could be due to weather, amount of traffic in a particular sector, an airport capacity issue, or any number of reasons. That restriction would be a traffic management initiative put in place where the traffic management folks (they are not controlling a particular sector) may say to an adjacent facility, provide 10 miles in trail for the next hour for all arrivals over XXX....but that is not changing the separation standard set in the controller handbook, 7110.65.
So having said aaaaaaaaaaalll that......centers use 5 NM
at the same altitude in almost every case, there are some places that are able to reduce that down to 3 NM
with certain RADAR conditions being met but for the sake of confusion on my part let's stay with the 5 NM
at the same altitude. With reduced vertical separation from FL410 and below it is 1,000' vertical separation if less than 5 NM
. It is very difficult to guess what 5 NM
looks like sitting in an airplane at FL370 and determing if the plane you see off the right side is even at your altitude or could be at FL380 or FL360....that 1,000' is sure deceptive.
The approach controls (including departure control) use 3 NM
at the same altitude, 1,000' vertical if less than 3 NM
. There are situations when landing that some airports are able to use 2.5 NM
on final when the aircraft are within 10 NM
of the landing runway threshold and certain conditions have been met, that is called reduced separation on final. Then you have the requirements for aircraft making approaches to parallel runway either dual or triple and those requirements which essentially allow aircraft to be side by side by side on final, again if certain requirements have been met.....a few airports that are able to conduct triple simultaneous approaches would be DFW
Visual separation is another tool for the controllers to use, basically used in terminal facilities with provisions for enroute facilities to use it as well, but visual is used primarily in the terminals with a number of ways to utilize visual separation to reduce the standard separation that would be required.
You should be able to go to faa.gov and search for the 7110.65, go to chapter 5 which addresses RADAR separation standards........remember you asked for it!
Sure hope this helps answer your question.
Any views shared are strictly my own and do not a represent those of any former employer.