A B757 CREW DEPARTED WITH THE APU TURNED OFF AFTER DISCONNECTING A MALFUNCTIONING ENG GENERATOR FOLLOWING ENG START BUT PRIOR TO TKOF.

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AFTER PUSHBACK AND STARTING BOTH ENGS; THE L GENERATOR TRIPPED OFF. I ATTEMPTED 1 RESET; WHICH DID NOT CORRECT THE PROB. CONCURRENTLY; THE FO WAS RUNNING THE AFTER START CHKLIST. HE INITIALLY TURNED THE APU OFF; THEN IMMEDIATELY CANCELED THE APU SHUTDOWN WHEN THE GENERATOR PROB SURFACED. WE WENT THROUGH THE MEL DRILL: CALLING DISPATCH; TALKING TO MAINT; DISCONNECTING THE GENERATOR PER THE MEL; AND INFORMING FLT ATTENDANTS AND PAX ABOUT THE STATUS OF THE ACFT. AFTER COMPLETING THESE ITEMS; WE CONTINUED OUR TAXI BELIEVING THAT WE HAD MET THE APPLICABLE MEL DISPATCH REQUIREMENTS; WITH BOTH OF US CONVINCED THE APU WAS RUNNING. SHORTLY AFTER TKOF; THE FO REALIZED THAT THE APU; IN FACT; WAS NOT RUNNING; DESPITE THE APU SELECTOR BEING IN THE 'ON' POS. HE IMMEDIATELY RESTARTED THE APU AND WE CONTINUED TO OUR DEST WITHOUT FURTHER INCIDENT. PROBABILITY SUGGESTS THAT THE APU WAS ACTUALLY NOT RUNNING PRIOR TO TKOF. ASSUMING THAT TO BE THE CASE; HOW COULD 2 EXPERIENCED PLTS BELIEVE OTHERWISE? I THINK THERE ARE SEVERAL CONTRIBUTING FACTORS. FIRST; ADDRESSING MEL ITEMS; ESPECIALLY THOSE THAT REQUIRE PLTS TO FOLLOW (M) PROCS THAT MAY BE NEW OR CONFUSING TO THEM; WHILE PARKED ON AN ACTIVE TXWY IS ONE OF THE MOST DISTRACTING AND TIME SENSITIVE TASKS WE FACE. SECOND; PLTS TEND TO ASSUME THAT A SYS; ONCE TURNED ON AND OPERATING NORMALLY; CONTINUES IN THAT STATE UNTIL PURPOSELY TURNED OFF OR SOME DIRECT INDICATOR APPEARS THAT INDICATES A 'FAULT' HAS OCCURRED. TO THE BEST OF OUR RECOLLECTION; WE NEVER RECEIVED ANY APU FAULT EICAS MESSAGE. THIRD; OPERATING THE APU; OR ANY SYS FOR THAT MATTER; IN AN UNUSUAL SEQUENCE (CANCELING THE APU SHUTDOWN VERSUS THE NORMAL 'ON THEN OFF;' 'OFF THEN ON' SEQUENCE OR AS MANDATED BY AN ABNORMAL OR MAINT CHKLIST) MAY VERY WELL MASK INDICATIONS THAT 'NORMAL' OPS WOULD PRODUCE AND 'NORMAL' HABIT PATTERNS WOULD IDENT. I; AND MOST PLTS I ASSUME; USE THE 'TRUST BUT VERIFY' METHOD WHEN STARTING THE APU. I NOW WILL USE THAT SAME PHILOSOPHY WHENEVER THE SHUTDOWN CANCELLATION FEATURE IS USED AS WELL.