DOUBLE INS FAILURE.

Narrative

AFTER DEP FROM HKG ON INS ACCURACY CHK WAS PERFORMED AT ELATO. #1 INS WAS WITHIN 2 MI; BUT #2 INS WAS ABOUT 10 MI NORTH OF THE KNOWN POS AND #3 WAS ABOUT 10 MI S OF THE KNOWN POS. CHKS WERE SUBSEQUENTLY PERFORMED OVER MAKUNG (MKG) AND ANPU (APU) ALONG A-1 AIRWAY. MEANWHILE; ATTEMPTS WERE MADE AT ESTABLISHING DME UPDATE AND RNAV UPDATE ON NUMBERS 2 AND 3 INS UNITS; BUT TO NO AVAIL DUE TO THE FACT THAT THAT PARTICULAR ACFT WAS NOT EQUIPPED WITH THOSE CAPABILITIES. ADDITIONALLY; AS TIME PASSED; IT BECAME CLR THAT NUMBERS 2 AND 3 INS WERE STEADILY DIVERGING FROM COURSE CTRLINE; EACH IN OPPOSITE DIRECTIONS FROM THE OTHER; IE; #2 WAS MOVING FURTHER N AND #3 MOVING FURTHER S AT ROUGHLY THE SAME RATE. #1 INS REMAINED CONSTANTLY ACCURATE COMPARED WITH KNOWN GND FIXES. WITH THE #2 AND #3 AT ABOUT 10-15 MI EITHER SIDE OF COURSE CTRLINE; MANUALLY UPDATING THEIR POSITIONS WAS ATTEMPTED SEVERAL TIMES. ONE TIME OUT OF THE MANY ATTEMPTS; BOTH INS'S ACCEPTED THE POS UPDATE AND OVER TIME WORKED THEIR WAY CLOSER AND CLOSER TO THE 'REAL' POS; HOWEVER WITHIN A FEW MINS THEREAFTER; PERFORMANCE BEGAN TO DECAY AGAIN ON #2 AND #3. SEVERAL ATTEMPTS WERE ALSO MADE AT INCREMENTALLY INSERTING UPDATES IN 5 NM STEPS; BUT HIS WAS TO NO AVAIL. SINCE AT LEAST 2 ACCURATE INS UNITS ARE REQUIRED PRIOR TO ENTRY INTO THE OCEANIC CTA; AND IT WAS CLR THAT #2 AND/OR #3 INS WERE GIVING NO INDICATIONS FOR THE PREVIOUS 3 1/2 HRS THAT THEY WERE RELIABLE; CONSIDERATION WAS GIVEN TO MAKING AN UNSCHEDULED LNDG. THE FLT HAD BEEN IN CONTACT WITH OUR FLT CTL DEPT FOR THE PREVIOUS 45 MINS TO 1 HR; AND IT WAS JOINTLY DECIDED TO LAND NORITA. #1 INS HAD BECOME SUSPECT BY NOW; THEREFORE AN EMER WAS DECLARED WHEN APPROX 30 NM ABEAM THE NORITA ARPT. FUEL WAS DUMPED. LNDG WAS MADE AT 587500 LBS TO A WT 2500 LBS ABOVE 585000 LBS (MAX STRUCTURAL WT) BUT IN ACCORDANCE WITH 1 PERCENT OVER MAX ALLOWABLE ALLOWANCE THAT DOES NOT REQUIRE AN OVERWT LNDG INSPECTION.