Air Traffic Control Fully Certified
Other / Unknown
ATC Issue All Types
General None Reported / Taken
Human Factors; ATC Equipment / Nav Facility / Buildings; Procedure
A Cherokee was on about a 5 mile final for Runway 30R with a ground speed of 120 KTS. An MD80 was switched to my frequency by Approach Control. The MD80 was about 4.5 miles behind the Cherokee with a 60 KT overtake. I issued instructions to the MD80 to make an S turn for spacing with the slower Cherokee. Weather was good VFR and the MD80 was on a Visual Approach. The MD80 complied and made a good S turn. I advised the Cherokee to plan an early turn off at Taxiway J if possible for the traffic following. The Cherokee said they would comply. The Cherokee landed and was able to turn off at Taxiway J without difficulty; made a prompt turn off and did not stop as he was issued taxi instructions to parking. The MD80 was cleared to land as the Cherokee was rolling out on the runway as I anticipated he would make Taxiway J without problem. The Cherokee was off the runway completely; and taxiing at a good rate; when the MD80 crossed the landing threshold. As the MD80 was in the flair and about to touch down the ASDE alerted; 'Runway 30R; Go Around'. It was too late to issue a go around in my opinion; and so I did not. The ASDE indicated the conflict was in fact between the two concerned aircraft (red flashing circle observed around the Cherokee). I don't believe adequate spacing was provided behind the Cherokee to start with; as an S turn by the MD80 and an early turn off were both necessary to make it work; but it did work. The reason I did not send the MD80 around was twofold.  1. The MD80 was too close to touchdown and; 2. The Cherokee was clear; and I had just scanned the runway so I knew it was clear; and the ASDE was indicating the Cherokee was the conflict which I knew was incorrect. This last part all happened pretty quickly; as you can imagine. I really don't know; unless something can be done to tighten up ASDE parameters to avoid alerts that aren't necessary. Less than 5 minutes later I had another ASDE alert for a closed runway; telling me to send an aircraft around that was on final for a completely different runway.
STL Controller described his/her reasoning for not complying with an ASDE-X commanded go around; the reporter noting the 'conflicting' traffic was observed completely clear of the landing runway.