AA,
Here is the actual rule from the U.S. ATC Handbook:
3-10-6. ANTICIPATING SEPARATION
Landing clearance to succeeding aircraft in a landing sequence need not be withheld if you observe the positions of the aircraft and determine that prescribed runway separation will exist when the aircraft cross the landing threshold. Issue traffic information to the succeeding aircraft if not previously reported and appropriate traffic holding in position or departing prior to their arrival.
EXAMPLE-
"American Two Forty-Five cleared to land, number two following United Boeing
Seven-Thirty-Seven two mile final, traffic will depart prior to your arrival."
"American Two Forty-Five cleared to land, number two following United Boeing
Seven-Thirty-Seven two mile final, traffic will be an MD 88 holding in position."
"American Two Forty-Five cleared to land, following United Boeing Seven-Thirty-Seven
two mile final, traffic will depart prior to your arrival."
NOTE-
Landing sequence number is optional at tower facilities where arrivals are sequenced by the approach control.
I work at KAUS, we have two parallel runways, both of which are used for both departures and arrivals in a normal flow. We are an "up/down" facility, that is all controllers are rated in both the Tower and Approach Control, and rotate through all positions. SOP at my facility is that arrivals be turned over to the Tower between 15 and 5 miles from the airport, and, as Mike said, Approach is responsible for the approach sequence, Tower sets the landing sequence, blending any local pattern traffic into the flow of arriving traffic.
You should notice that the U.S. rule requires that we inform the pilot what and where the preceding aircraft is. We must also inform the arrival closest to the runway of any traffic we have holding in position on a runway for departure. Likewise, we must tell any departure holding in position about the closest arrival for that runway. The professional pilots I work with (and who are used to the U.S. system) seldom express any difficulty maintaining situational awareness of their position in the landing sequence or with departing traffic ahead. Of course, some of the newer pilots are a bit overwhelmed by all the chatter. It would be quite common here to have two or perhaps three aircraft cleared to land on each runway during busy traffic in VMC weather.
During IMC wx, we are required to maintain a stagger between arrivals on the parallel ILS approaches, so it would be uncommon for Tower to be talking to more than three or four (total) aircraft on approach for both runways. When the visibility deteriorates to the point we cannot physically see aircraft on the runways, or exiting the runways, then naturally, most controllers will get a bit more cautious. I certainly do. As yet, we do not have an ASDE system, but are (were) scheduled to receive one in a couple years. (Never believe good news until it's seen walking in the front door.)
The majority of the time, in VMC wx, the arrivals will already have the preceding aircraft in sight when contacting the Tower, OR, the spacing is great enough that separation is not a factor, providing the first aircraft is still mechanicaly capable of taxiing under it's own power after landing... Since in the U.S., Pilots are equally responsible for maintaining safe and proper spacing with another aircraft ahead, IF he has it in sight, , then the procedure is really seldom a problem. Obviously, the controllers bear most all the responsibility in IMC wx, but with proper spacing and appropriate groundspeeds and closure rates, again, we seldom have a problem with the procedure. If all else fails, send somebody around!
Don
ps The entire U.S. ATC handbook is available here:
http://www1.faa.gov/ATpubs/ATC/index.htm
edited in vain attempt to improve my grammar....
[Last edited by vector4fun on 21st February 2004 at 01:13