B-737-300 ACFT DECLARES A FUEL EMER AS FLC REALIZES THAT FUEL RESERVES PLUS ALTERNATE FUEL IS BEING ERODED.

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FLT WAS A NONSTOP FROM CHARLOTTE; NC; TO SAN FRANCISCO; CA. FLT PLAN FUEL FIGURES INDICATED THAT ARR FUEL IN SFO WOULD BE RESERVE AND ALTERNATE FUEL PLUS 2000 LBS. DUE TO EXTENDED TIME AT LOWER THAN PLANNED CRUISE ALT (FOR TURB); REROUTING; HIGHER THAN FORECAST WINDS AND MULTIPLE VECTORS FOR SEQUENCING; THAT 2000 LB 'CUSHION' WAS REDUCED TO 600 LBS (PER FMS FUEL COMPUTATION) AT FMG VOR (RENO). FLT WAS CLRED FOR GOLDEN GATE ARR TO SFO (FMG - PYE - SFO). ATC HAD PREVIOUSLY BEEN ADVISED THAT FLT WAS APCHING 'MINIMUM FUEL.' AFTER A SHORT SEQUENCE VECTOR AND WHILE PASSING SACRAMENTO; 'CUSHION' PER FMS WAS 500 LBS. AS DSCNT TO PYE CONTINUED; FMS INDICATED THIS 500 LB TO BE GRADUALLY DECREASING AND 'MINIMUM FUEL' WAS DECLARED 50 NM NE PYE VOR. APPROX 40 NM NE PYE; HOLDING INSTRUCTIONS WERE ISSUED (HOLD AT PYE; EFC IN 15 MINS) AND ACKNOWLEDGED. IT BECAME APPARENT THAT IF THE FLT HELD AND BECAUSE ADDITIONAL DELAYS WERE POSSIBLE AFTER HOLDING (AND BECAUSE THE FUEL GAUGES INDICATED 300 LBS LESS THAN THE FMS FUEL QUANTITY); A SAFE LNDG WOULD BE JEOPARDIZED IF THE FLT WERE NOT GIVEN PRIORITY. THEREFORE; AN EMER WAS DECLARED 30 NE PYE. THE FLT WAS CLRED DIRECTLY TO THE ARPT AND AN UNEVENTFUL LNDG FOLLOWED. FUEL QUANTITY ON TOUCHDOWN WAS 4700 LBS (RESERVE PLUS 130 LBS). THIS EXPERIENCE POINTS OUT THE NECESSITY OF KNOWING WHAT 'MINIMUM FUEL' IS AND WHAT IT DOES/DOESN'T DO FOR YOU: I.E.; IT IS A FUEL STAGE WHERE; UPON REACHING DEST; NO UNDUE DELAY CAN BE ACCEPTED AND IT IS ADVISORY ONLY AND DOES IMPLY A NEED FOR TFC PRIORITY. IF; AFTER DECLARING MINIMUM FUEL; FURTHER DELAYS ARE ENCOUNTERED AND IT APPEARS TFC PRIORITY IS NECESSARY TO ENSURE A SAFE LNDG; 'EMER FUEL' SHOULD BE DECLARED. THESE FUEL STATES ARE NOT NECESSARILY DERIVED FROM SOME FORMULA; BUT RATHER ARE SUBJECTIVE IN NATURE AND ARE BASED ON THE CONDITIONS AT HAND. THE PRESSURES A PLT FACES IN DETERMINING THESE FIGURES ARE HAVING THEM TOO HIGH AND POSSIBLY DIVERTING UNNECESSARILY (OR DECLARING AN EMER AND LNDG WITH AN EMBARRASSINGLY HIGH FUEL STATE) OR THE OTHER EXTREME OF HAVING THESE FIGURES TOO LOW AND LNDG WITH A DANGEROUSLY LOW FUEL STATE WHEN HIGHER FIGURES OR AN EARLY DIVERT MIGHT HAVE BEEN SAFER ALTERNATIVES. THE THOUGHT PROCESS AND CREW COM USED IN MAKING THESE DECISIONS SHOULD TAKE PLACE WELL IN ADVANCE AND WHILE A SUITABLE DIVERT FIELD IS WITHIN RANGE AS AN ALTERNATIVE.