CTLR QUESTIONED CAPT ABOUT WAKE VORTEX SEPARATION REQUEST.

Narrative

OUR FLT WAS CLRED TO TAXI TO RWY 25R. AS WE STARTED DOWN THE TXWY; A B747-400 WAS APCHING THE DEP END OF RWY 25R. SINCE WE WERE TO DEPART BEHIND THE B747-400; I REALIZED THAT WE MIGHT WANT TO REQUEST ADDITIONAL SEPARATION INSTEAD OF THE FAA MINIMUM OF 5 MI. TAKING INTO CONSIDERATION THE WIND DIRECTION AND SPD; THE B747-400 ROTATION POINT AND OUR ROTATION POINT; I ASKED THE FO TO ASK FOR 3 MIN SEPARATION BTWN THE B747-400 AND OUR DEP FOR WAKE TURB AVOIDANCE. AS WE APCHED TXWY 8-J; THE TWR CTLR; CTLR #1; CLRED US TO TAXI FULL LENGTH AND TO TAXI INTO POS AND HOLD ON RWY 25R. AT THAT TIME; THE FO ACKNOWLEDGED THE CLRNC AND REQUESTED 3 MINS BTWN US AND THE DEPARTED TFC FOR WAKE TURB AVOIDANCE. THE TWR CTLR CAME BACK AND CANCELED THE CLRNC FOR POS AND HOLD; AND TO TAXI ONTO RWY 25R AND EXIT AT TXWY 8-J; BECAUSE THERE WAS NO HOLDING PAD AT THE DEP END OF RWY 25R. WE ACKNOWLEDGED THE CLRNC AND CONTINUED TAXIING TOWARD THE DEP END OF RWY 25R. THE B747-400 HAD DEPARTED JUST 30 SECONDS AGO. AS WE WERE READY TO TAXI ONTO THE RWY; CTLR #2; A SUPVR OR INSTRUCTOR; CAME ON THE FREQ AND INFORMED US THAT THE LAX TWR USES 5 MI FOR WAKE TURB SEPARATION. THE FO ACKNOWLEDGED THE XMISSION; AND AGAIN REQUESTED THE 3 MIN SEPARATION. CTLR #2 THEN CAME BACK WITH THE OLD 'CALL THIS NUMBER WHEN YOU LAND.' WITH OUR LATE ARR IN CHICAGO AND ALREADY FAMILIAR WITH THE FAA WAKE VORTEX SEPARATION CRITERIA; I TOLD CTLR #2 THAT I WOULD NOT CALL HIM AFTER LNDG BUT I WOULD HAVE THE CHIEF PLT FROM CHICAGO RETURN HIS CALL IN THE MORNING. AGAIN CTLR #2 ASKED FOR US TO CALL HIM; AND MY RESPONSE WAS THE SAME AS FIRST TIME. NOTHING ELSE WAS SAID. WE CLRED RWY 25R WITHIN 30 SECONDS; FALLING IN LINE BEHIND 2 COMMUTER ACFT AND A B737. THE FLT DEPARTED AT XX21 UCT AND ARRIVED AT THE GATE IN CHICAGO 2 MINS EARLY. PROB: WHY DOES THE CAPT HAVE TO EXPLAIN HIS REASONING TO THE LAX TWR WHEN HE OR SHE REQUESTS ADDITIONAL SEPARATION FOR WAKE TURB AVOIDANCE. OUR FOM; PAGE ATC-11 CLRLY STATES; UNDER RADAR CTL; PLTS OF DEPARTING AIRPLANES MAY REQUEST ADDITIONAL SEPARATION FOR WAKE TURB AVOIDANCE. THIS REQUEST SHOULD BE MADE AS SOON AS PRACTICAL ON GND CTL AND AT LEAST BEFORE TAXIING ONTO THE RWY.