UNAUTH RWY ENTRY OP XING.

Narrative

CHKED IN AT AM05 LCL FOR TRIP. DUE TO SEVERE WX IN AREA; WE WERE ULTIMATELY REASSIGNED TO FLY DTW-MSN-MSP-RST-LSE. WE DEPARTED THE GATE AT DTW AT PM09 EDT. INSTEAD OF 'STANDARD TAXI' CLRNC TO RWY 21C; CLRNC WAS TO EXIT THE RAMP AT 06 VIA THE OUTER TO RWY 21C (NOT AN UNUSUAL OR DIFFICULT VARIATION). FOR NO KNOWN REASON; WHEN I EXITED THE RAMP AT 06; I CONTINUED STRAIGHT AHEAD ON TXWY F TOWARD RWY 3C (PART OF 'STANDARD TAXI' RTE TO THAT RWY) IN PROCEEDING ON THAT RTE; I CROSSED RWY 9L-27R. WHEN I BECAME AWARE OF WHAT I HAD DONE; WE CONTACTED GND TO ADVISE OUR POS AND PROCEEDED FROM THERE TO RWY 21C. GND TFC WAS LIGHT AND RWY 9L-27R WAS NOT IN USE; BUT I HAD IN FACT CROSSED A RWY WITHOUT A CLRNC (ORIGINAL TAXI CLRNC TO RWY 21C WOULD NOT HAVE CROSSED RWY 9L-27R). FACTORS: FATIGUE DUE TO A DOLEFUL AFTERNOON OF SITTING AROUND. DISTR OF ANTICIPATING A FAIRLY LONG EVENING OF FLYING AN UNFAMILIAR TRIP UNTIL AFTER I AM EDT. INFREQUENT USE OF RWY 9L-27R FOR OTHER THAN TAXIING MAKES AN INCURSION MORE LIKELY. SIMILARITY OF ORIGINAL TAXI CLRNC TO 'STANDARD TAXI' RTE TO 3C. CONCLUSION: AIN'T NO SUBSTITUTE FOR PAYING CLOSE ATTN TO THE BUSINESS AT HAND.