Small Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1.5
Communication Breakdown; Situational Awareness; Distraction; Confusion
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance
Human Factors; Procedure; Weather
Rainy weather caused a 300 FT broken ceiling (1700 FT overcast) with 2 SM of visibility. I had just received a position relief briefing on Local Control; and the previous controller had cleared Aircraft Y to land on runway 12R. A CRJ had just departed runway 12L; and Aircraft X was approaching the hold short line for runway 12L. Even though Aircraft Y was about three miles out; I knew I wouldn't have visual separation; and because of aircraft performance characteristics; I knew I couldn't get '2 increasing to 3' mileage separation; so I told Aircraft X to line up and wait on runway 12L. Aircraft X read back 'line up and wait'; and started moving onto the runway; so I started looking out to the west to see when I could visually locate Aircraft Y. When Aircraft Y was about a mile out; I scanned the runway to make sure there were no conflicts. As part of my scan included checking the ASDE-X; I noticed that the intersection bars were lit green for runway 12L; indicating that Aircraft X was on takeoff roll. When I looked down at runway 12L; Aircraft X was already passing intersection Papa; which; historically for Aircraft X; is about where they reach V2. I didn't feel I could safely tell Aircraft X to stop before he was airborne. Adding to the predicament was Aircraft Y being a heavy; so telling him to go around would put me in a tough wake turbulence situation; since we couldn't see aircraft past the departure end of the runway; and since Aircraft X took off without a clearance; I wasn't sure of his intentions or where he was going.Once Aircraft X became airborne; I noticed a slight left turn; so I told him to turn left to 060 degrees to intercept the STL standard turboprop departure track; which he read back and did. My best guess is the pilot is so accustomed to flying in/out of here that he automatically started his left turn out of force of habit. I issued the Brasher warning and shipped Aircraft X to departure. Aircraft Y landed on runway 12R without incident.I did not give Aircraft X a reason why he was lining up to wait instead of being cleared for takeoff; since I thought it would be obvious to the pilot that a jet had just departed; but I should have told him why he wasn't cleared for takeoff. That may have prevented him from starting his takeoff roll before being cleared for takeoff.
STL Local Controller reports of an aircraft taking off without a clearance. Aircraft was told to line up and wait; but Controller didn't tell him it was for spacing on previous departure.