LACK OF GUIDANCE FOR ACFT EQUIP FAILURES AFTER GATE DEP FOUND TO BE UNCLR BY THE FLC WHO REQUESTS BETTER GUIDANCE BE PROVIDED.

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DURING TAXI OUT FROM THE GATE IN A B757; RUDDER RATIO EICAS MESSAGE WAS OBSERVED. AIRLINE POLICY REGARDING BLOCK TURN BACKS TO THE GATE IS DETERMINED BY CONSULTING 2 DOCUMENTS. ONE DOCUMENT IS THE FAA ISSUED MEL. THE OTHER DOCUMENT IS THE FAA APPROVED BUT COMPANY ISSUED QRH. AIRLINE POLICY IS THAT THE MEL IS THE DETERMINING DOCUMENT WHEN PARKED AT THE GATE. ONCE THE ACFT BEGINS TO TAXI ON ITS OWN PWR; THE QRH IS THE DETERMINING DOCUMENT; UNLESS THE MEL ITEM THAT IS INOP IS UNDERLINED IN THE MEL; THEN A BLOCK TURN BACK IS REQUIRED. I STOPPED ON THE TXWY; AND CONSULTED THE MEL FOR THE RUDDER RATIO. IT WAS NOT LISTED IN THE MEL; SO WE CONSULTED THE QRH. THE QRH STATES THAT FOR A RUDDER RATIO EICAS MESSAGE WE SHOULD: 'AVOID LARGE OR ABRUPT RUDDER CTL INPUTS ABOVE 160 KTS.' 'IF NORMAL HYD SYS PRESSURE IS AVAILABLE; XWIND LIMIT IS 15 KTS. DO NOT ATTEMPT AUTO LAND.' SINCE HYD PRESSURE WAS NORMAL AND THE WIND AT DEP AND DEST WAS DOWN THE RWY; I FELT THAT WE COULD COMPLY WITH ALL THE RESTRS; IMPOSED BY THE QRH. WE FLEW A NORMAL FLT TO THE DEST. UPON ARR; ACFT MAINT RESET THE FLT CTL COMPUTER AND PERFORMED A BUILT-IN TEST (BITE) CHK. NO ABNORMALITIES WERE NOTED. RECOMMENDATION: MEL GUIDANCE FROM THE FAA AND COMPANY POLICY IS CONFUSING AND CONTRADICTORY. THE FAA NEEDS TO FORCE THE AIRLINE TO SET A MORE DEFINITIVE POLICY. THIS GRAY AREA IS TOO CONFUSING.