Eeper,
At my facility, we have parallels 9,000' and 11,000'. The shorter runway is a Cat III runway. Standard in-trail minimum is three or more miles (wake turbulence) in trail. We have a waiver which allows us to use 2 1/2 miles in trail because we've demonstrated a typical runway occupancy time of less than 50 seconds. See FAA Order 7210.3 :
10-4-7. REDUCED SEPARATION ON FINAL
Separation between aircraft may be reduced to 2.5 NM in-trail separation on the final approach course within 10 NM's of the runway provided an average Runway Occupancy Time (ROT) of
50 seconds or less is documented for each runway. ROT is the length of time required for an arriving aircraft to proceed from over the runway threshold to a point clear of the runway. The average ROT is calculated by using the average of the ROT of no less than 250 arrivals. The
250 arrivals need not be consecutive, but shall contain a representative sample of the types of aircraft that use the runway. Average ROT documentation shall be revalidated, within 30 days, if there is a significant change in runway/taxiway configuration, fleet mix, or other factors that may increase ROT. Revalidation need not be done for situations that are temporary in nature. Only the ROT for the affected runway(s) will need to be revalidated. The revalidation documentation shall contain the following information for each arrival:
a. Aircraft call sign.
b. Aircraft type.
c. Time across the threshold.
d. Time clear of the runway.
e. Items c and d above may be omitted if using a stopwatch. Record the total number of seconds required for an aircraft to proceed from over the landing threshold to a point clear of the runway when using a stopwatch.
Note, however, that we can only use this reduced separation
When we can see the aircraft exiting the runway.
When visibility prevents us from seeing the aircraft exit, we normally run our in-trail separation up to about 4 miles, and that typically gives us plenty of "cushion" to get the report clear of runway from the preceeding aircraft.
As for the remarks about aircraft being fully clear, when the weather is low enough that we're below Cat I minima, it's a pretty safe bet that only professionals are still landing, and I trust a
Professional Pilot's report that he is in fact, clear. Additionally, we have very good, well lighted high-speed turnoffs with plenty of room between the runway and parallel taxiways, so it's never been a problem here with the pros flying. I am aware there are runway/taxiway configurations which do not allow aircraft to clear so easily and quickly. They wouldn't get the 2 1/2 mile waiver of course....
