Date: 201503
Local time of day: 0601-1200
Location: STL.Airport, MO
Weather elements / visibility: Rain; 2
Light: Daylight
Aircraft: Small Transport
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower STL
Second aircraft: Widebody; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness; Distraction; Confusion
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance
Contributing factors: Human Factors; Procedure; Weather
Primary problem: Procedure

Narrative: 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.




Synopsis: 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.