A B757 Captain added fuel above his minimum dispatch load because if the flight were flown to its alternate the fuel would decrease below 4;000 pounds violating an aircraft limitation for low wing fuel and creating a fuel emergency.

Narrative

Dispatched into Low Fuel Emergency. On a flight to the west coast; my original flight plan would have me landing at my alternate airport with 3;842 LBS; if needed to fly as dispatched. If this action would been necessary; I would have flown the aircraft into a low fuel emergency. I added 2;500 LBS of fuel to prevent this scenario. All previous references appear to have been deleted from the 767/757 manual and the QRH concerning at what quantity the 'Low Fuel' EICAS message appears. The 767/757 fuel tab Fuel Quantity; states in part: Low fuel message is displayed when left or right tank fuel drops below approximately 2200 LBS. Our manual; Systems; 'Operations with Low Fuel Loads;' states in part: Operation with these boost pump indications is permitted for two minutes .... after two minutes accomplish appropriate Abnormal Procedure. The QRH states in part: Avoid high nose up attitude and excessive acceleration. Summary: How is flight legally dispatched in a situation like this. If flown as planned it results in a LOW FUEL EMERGENCY. As pilot in command under FAR 121; I have the responsibility to accept a flight plan with realistic contingencies not just paper legalities! With limited constraints in the ATC system; air traffic congestion; etc.; there appears to be a trend to dispatch paper legal. Is taking such a release illegal? A cockpit checklist for mechanics use only has a caution against operating engines with less than 4000 LBS of fuel because that condition could result in a hydraulic overheat. But can we dispatch a flight to land with less than 4000 LBS?