Narrative:

Portland tower cleared me 'taxi into position and hold' on runway 18. Thinking I had been cleared to go, I took off. Another aircraft Y was making a low pass right to left on runway 11-29 and I climbed out over him at the runway intersection by approximately 400-500'. Tower corrected me after occurrence. Cause was being preoccupied with troublesome strobe switch along with allowing myself to mentally be at my EST. Inadvertent and momentary lapse of concentration caused me to act as if I had been cleared for takeoff. Corrective action: I am attending FAA wings program on ATC procedures and have engaged my CFI-I for remedial training.

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Original NASA ASRS Text

Title: SMA PLT TAKES OFF FROM PWM WITHOUT CLRNC, HAS NMAC WITH ANOTHER ACFT Y DOING A LOW AND OVER ON AN INTERSECTING RWY.

Narrative: PORTLAND TWR CLRED ME 'TAXI INTO POS AND HOLD' ON RWY 18. THINKING I HAD BEEN CLRED TO GO, I TOOK OFF. ANOTHER ACFT Y WAS MAKING A LOW PASS RIGHT TO LEFT ON RWY 11-29 AND I CLBED OUT OVER HIM AT THE RWY INTXN BY APPROX 400-500'. TWR CORRECTED ME AFTER OCCURRENCE. CAUSE WAS BEING PREOCCUPIED WITH TROUBLESOME STROBE SWITCH ALONG WITH ALLOWING MYSELF TO MENTALLY BE AT MY EST. INADVERTENT AND MOMENTARY LAPSE OF CONCENTRATION CAUSED ME TO ACT AS IF I HAD BEEN CLRED FOR TKOF. CORRECTIVE ACTION: I AM ATTENDING FAA WINGS PROGRAM ON ATC PROCS AND HAVE ENGAGED MY CFI-I FOR REMEDIAL TRNING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.