FLT ATTENDANT WAS LEFT BEHIND AT GATE WHEN ACFT DEP WAS DELAYED AFTER PUSHBACK FROM GATE DUE TO A START VALVE MALFUNCTION. MINIMUM CABIN CREW NOT ON BOARD AT TKOF.

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I WAS INADVERTENTLY LEFT BEHIND AT THE GATE FOR FLT. THE AIC NOTIFIED THE TWR IMMEDIATELY BY THE JETWAY PHONE BUT THE PLANE HAD RPTEDLY TAKEN OFF. SUPPLEMENTAL INFO FROM ACN 392790: I WAS UNAWARE WE WERE BELOW FAA MINIMUM STAFFING WHEN WE LEFT THE GATE. I DID NOT REALIZE UNTIL WE WERE IN THE AIR. THE COCKPIT WAS NOTIFIED IMMEDIATELY. SUPPLEMENTAL INFO FROM ACN 392544: AFTER NORMAL PREFLT; THE R-HAND START VALVE FAILED AND WE RETURNED TO THE GATE. MAINT CAME ABOARD AND STATED THEY WOULD HAVE A NEW VALVE INSTALLED IN 30 MINS. I MADE A PA ANNOUNCEMENT THAT AS WE HAD NO GATE AGENT TO PROVIDE SECURITY I COULD NOT ALLOW ANYONE TO DEPART THE ACFT; AS IT WOULD CAUSE AN ADDITIONAL DELAY. MAINT CAME ABOARD AND SAID IT MIGHT BE A LITTLE LONGER AS THEY HAD A 'CONNECTOR' PROB. AT XA11 AFTER THE MECH CLOSED THE L1 DOOR AND PULLED THE JETWAY; WE DEPARTED. AFTER A SHORT TAXI AND ROUTINE FLT AT APPROX XB00 THE L1 FLT ATTENDANT CAME TO THE FLT DECK AND INFORMED ME THAT SHE HAD ALLOWED A FLT ATTENDANT TO GET OFF THE PLANE TO USE THE PHONE AND WE HAD LEFT HER BEHIND. AS I HAD ASKED EVERYONE TO REMAIN ABOARD; I WAS QUITE SURPRISED. THE REMAINING PORTION OF THE FLT WAS OPERATED AS IF WE HAD BEEN DISPATCHED WITH ONLY 2 FLT ATTENDANTS.