Originally Posted by
Captain Tony
I have had this happen many, many times.
I think most controllers don't understand that TCAS devices give us a traffic message if we are less than 1000' vertically and less than ~2 miles. We get an RA if we are 500' apart and less than ~1 mile. We are REQUIRED by SOP to comply with all RAs, even in VFR with traffic in sight. So while it's "legal" for you to provide 500' separation with a nonparticipating VFR aircraft, it causes both us and you considerable workload when we have to comply and report. It might be advantageous for you to just provide the vector or altitide spacing.
We don't know everyone's SOPs, but are bound by the 7110.65 when we're separating aircraft. I don't know which airlines follow their TCAS when the traffic is in sight, and which follow the TCAS in which phases of flight.
At MDW, for example, the tower keeps all their VFRs at 2000' or below within so many miles of the airport, while approach uses 2500' all the time for IFR arrivals on visual approaches (and 2500' is the FAF altitude on the runway 4R ILS; we use it for 31C usually, too). I've never had an IFR aircraft report a TCAS event when passing 500' above these VFRs in these situations -- but 30 miles away from the airport, it has happened on occasion.
Because VFRs are usually at 3500', 4500', 5500', etc, I try to give 1500' separation when possible, especially when I don't have the room to vector around them . . . but that's not always possible either.
Your best bet is to ask for a vector around the guy if you anticipate an RA. Some controllers would take your advice and would (or do already) try to vector you around, but there are always going to be some that do what the .65 says and nothing else, period.
Someone else had mentioned low ceilings affecting the arrival rate at an airport. Two big reasons --
1) When the tower can't provide visual separation between successive arrivals, controllers have to provide 2.5 or 3 miles (depending on the runway) all the way to touchdown. When the tower *can* provide visual, the distance between airplanes can go down to the acceptance rate of the runway, which can be more like 2.5 to 2 miles on final on runways with well-placed turnoffs.
2) This may only be true at some airports. Certain runway configurations might require 800' ceilings, or 1500' ceilings, or 2000' ceilings . . . the configurations that require higher ceilings are usually the ones that have the higher acceptance rate. I believe it is generally because of go-arounds. At ORD, we can run arrivals to 10, 14R and 22R -- none of the runways intersect, but if 2 airplanes went around at the same time, you can see it being a problem with a low overcast vs. a higher overcast day.
To sum up compression very easily -- Aircraft on final are doing 180 to the FAF at 4 miles-in-trail. Aircraft in the marker are slowing to a ground speed of 120 inside the FAF, which causes compression to reduce the separation to 3 miles or less. Some runways compress more than others due to wind -- maybe deflecting off of buildings or local phenomena, so they may run 5 miles instead of 4 on final.