Narrative:

After departure from hkg on INS accuracy check was performed at elato. #1 INS was within 2 mi, but #2 INS was about 10 mi north of the known position and #3 was about 10 mi south of the known position. Checks 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 aircraft was not equipped with those capabilities. Additionally, as time passed, it became clear that numbers 2 and 3 INS were steadily diverging from course centerline, each in opposite directions from the other, ie, #2 was moving further north and #3 moving further south at roughly the same rate. #1 INS remained constantly accurate compared with known ground fixes. With the #2 and #3 at about 10-15 mi either side of course centerline, manually updating their positions was attempted several times. One time out of the many attempts, both INS's accepted the position update and over time worked their way closer and closer to the 'real' position, 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 clear that #2 and/or #3 INS were giving no indications for the previous 3 1/2 hours that they were reliable, consideration was given to making an unscheduled landing. The flight had been in contact with our flight control department for the previous 45 mins to 1 hour, and it was jointly decided to land norita. #1 INS had become suspect by now, therefore an emergency was declared when approximately 30 NM abeam the norita airport. Fuel was dumped. Landing was made at 587500 pounds to a weight 2500 pounds above 585000 pounds (maximum structural weight) but in accordance with 1 percent over maximum allowable allowance that does not require an overweight landing inspection.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.