Mini,
The "official" rules for instrument approaches are thus:
5-9-1. VECTORS TO FINAL APPROACH COURSE
Except as provided in para 7-4-2, Vectors for Visual Approach, vector arriving aircraft to intercept the final approach course:
a. At least 2 miles outside the approach gate unless one of the following exists:
1. When the reported ceiling is at least 500 feet above the MVA/MIA and the visibility is at least 3 miles (report may be a PIREP if no weather is reported for the airport), aircraft may be vectored to intercept the final approach course closer than 2 miles outside the approach gate but no closer than the approach gate.
2. If specifically requested by the pilot, aircraft may be vectored to intercept the final approach course inside the approach gate but no closer than the final approach fix.
EXCEPTION. Conditions 1 and 2 above do not apply to RNAV aircraft being vectored for a GPS or RNAV approach.
b. For a precision approach, at an altitude not above the glideslope/glidepath or below the minimum glideslope intercept altitude specified on the approach procedure chart.
c. For a nonprecision approach, at an altitude which will allow descent in accordance with the published procedure.
NOTE-
A pilot request for an "evaluation approach," or a "coupled approach," or use of a similar term, indicates the pilot desires the application of subparas a and b.
d. EN ROUTE. The following provisions are required before an aircraft may be vectored to the final approach course:
1. The approach gate and a line (solid or broken), depicting the final approach course starting at or passing through the approach gate and extending away from the airport, be displayed on the radar scope; for a precision approach, the line length shall extend at least the maximum range of the localizer; for a nonprecision approach, the line length shall extend at least 10NM outside the approach gate; and
2. The maximum range selected on the radar display is 150 NM; or
3. An adjacent radar display is set at 125 NM or less, configured for the approach in use, and is utilized for the vector to the final approach course.
4. If unable to comply with subparas 1, 2, or 3 above, issue the clearance in accordance with para 4-8-1, Approach Clearance.
REFERENCE-
FAAO 7110.65, Approach Clearance, Para 4-8-1.
FAAO 7110.65, Final Approach Course Interception, Para 5-9-2.
5-9-2. FINAL APPROACH COURSE INTERCEPTION
a. Assign headings that will permit final approach course interception on a track that does not exceed the interception angles specified in the TBL 5-9-1.
TBL 5-9-1
Approach Course Interception Angle
Distance from interception point to approach gate
Maximum interception angle
Less than 2 miles or triple simultaneous ILS/MLS approaches in use
20 degrees
2 miles or more
30 degrees
(45 degrees for helicopters)
b. If deviations from the final approach course are observed after initial course interception, apply the following:
1. Outside the approach gate: apply procedures in accordance with subpara a, if necessary, vector the aircraft for another approach.
2. Inside the approach gate: inform the pilot of the aircraft's position and ask intentions.
PHRASEOLOGY-
(Ident) (distance) MILE(S) FROM THE AIRPORT, (distance) MILE(S) RIGHT/LEFT OF COURSE, SAY INTENTIONS.
NOTE-
The intent is to provide for a track course intercept angle judged by the controller to be no greater than specified by this procedure.
REFERENCE-
FAAO 7110.65, Chapter 5, Section 9, Radar Arrivals, and Section 10, Radar Approaches- Terminal.
c. EN ROUTE. When using a radar scope range above 125 NM, the controller shall solicit and receive a pilot report that the aircraft is established on the final approach course. If the pilot has not reported established by the final approach gate, inform the pilot of his/her observed position and ask intentions.
I dare say that these rules do get bent a bit when trying to fill a final.
When vectoring for a Visual, the reported ceiling must be at least 500" above the lowest MVA. However, if we're vectoring for any type approach, and the WX is at least VFR, if the pilot requests a Visual and has the field or preceeding traffic in sight, we can still clear him. It just has to be 500' above for the controller to initiate a vector.
Some airlines, as a general rule, don't like turning final closer than the FAF, especially at night, even on a visual. That doesn't mean there aren't exceptions. Often they'll turn closer for me in the day if I ask them in advance before I try stuffing them in on a 2 mile base.
Now, here's your homework: If there's a Cessna doing 80 kts IAS on ILS final 6 miles out, and a B737-800 behind doing 200 kts IAS; and both have a 20 kt headwind, how far behind the Cessna does the B738 have to be in order to have 3 miles spacing at the Cessna's touchdown?
(assume the Boeing will stay at 200 kts until the 3 mile final.)
