A B747 ENRTE TOKYO TO SINGAPORE ENCOUNTERS MODERATE TO SEVERE TURB THAT WAS NOT FORECAST BY THEIR COMPANY'S DISPATCH OFFICE. CREW LEARNED LATER THAT THE DISPATCHER HAD 4 SIGMETS OF MODERATE TO SEVERE TURB FORECAST FOR THE RTE AND TIME OF THE FLT BUT HAD NOT INCLUDED THEM IN THE FLT PAPERS BECAUSE OF SCHEDULING LEGALITY REASONS.

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SCHEDULED FOR FROM NARITA; JAPAN TO SINGAPORE. WE WERE DISPATCHED WITH NO SIGMETS FOR OUR RTE OF FLT; ONLY CODE 1-2 TURB. WE RECEIVED A RPT FROM THE FOREIGN ACR Y FLT IN FRONT OF US FOR MODERATE TO SEVERE AT OUR ALT. WE REQUESTED LOWER; BUT WE WERE NOT ALLOWED TO DSND DUE TO TFC. WE TURNED OFF ALL AUTOFLT AIDS UPON ENCOUNTERING THE FIRST MODERATE TO SEVERE BUMPS. (ALL PAX AND FLT ATTENDANTS WERE SEATED WITH BELTS FASTENED AS SOON AS WE RECEIVED FLT Y'S WARNING.) FOR THE NEXT 1 1/2 HRS WE RECEIVED +/-15 KTS; AIRSPD DEVS; +/-2000 FPM ON THE VSI AND SIDE/VERT LOADS WHICH WOULD HAVE INJURED ANYONE MOVING ABOUT IN THE BACK. WE LATER LEARNED THAT THERE WERE 4 SIGMETS OF MODERATE TO SEVERE TURB ISSUED FOR OUR FLT TIME AND RTE OF FLT. OUR TOKYO DISPATCH MAINTAINED THAT 'ACR Y RPTS SEVERE ALL THE TIME WHEN IT IS REALLY ONLY MODERATE;' SO THEY WERE NOT CONCERNED. SINCE WE ARE NOT ALLOWED TO BE DISPATCHED INTO KNOWN AREAS OF SEVERE TURB; THIS WOULD SHUT DOWN MANY FLTS. WE NEED A CLR; ICAO APPROVED DEFINITION OF 'SEVERE' TURB THAT ALL AIRLINES 'USE.' IN THIS CASE ACR Y WAS CORRECT -- THE TURB WAS SEVERE (WE CALLED IT CODE 5; ALTHOUGH PART OF THE CODE 6 DEFINITION ALSO APPLIED). WHEN DISPATCHERS BEGIN TO DISREGARD RPTS BASED UPON THEIR EXPERIENCE (WHICH IN THE PAST HAS BEEN THAT CODE 5 IS REALLY CODE 3-4); WE HAVE A PROB. MISRPTING COULD SHUT OUR OPS DOWN EFFECTIVELY. DISREGARDING SIGMETS COULD GET PEOPLE HURT/KILLED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO CALLED BACK AND SAID THAT THE CAPT HAD A MEETING WITH THE UNION SAFETY CHAIR PERSON; WITH A REPRESENTATIVE FROM THE COMPANY BEING PRESENT. THIS INCIDENT WAS ALLEGED TO HAVE OCCURRED TO 5-6 OTHER CREWS. THE FO HEARD THAT THE COMPANY AND UNION WAS GOING TO TALK WITH THE DISPATCHER'S OFFICE TO PREVENT A RECURRENCE OF THIS TYPE OF INCIDENT. THE DISPATCHER WAS QUOTED AS STATING THAT THE FLTS 'WOULDN'T BE FLYING IF THEY PAID ATTN TO EVERY RPT THAT THIS FOREIGN AIRLINE PUT OUT.' THE PLT RPTR DID NOT KNOW WHERE THESE SIGMETS ORIGINATED OR HOW THEY WERE DISSEMINATED; EXCEPT THAT COMPANY DISPATCH WAS THE FINAL ASSESSOR BEFORE DISTRIBUTION TO THE FLCS. AS A SIDE ISSUE; THE RPTR COMPLAINED THAT THE COMPANY MAINT OFFICE CALLED THE CAPT TWICE IN AN ATTEMPT TO GET HIM TO 'CANCEL' HIS WRITE-UP IN THE LOGBOOK SO THAT THEY WOULD NOT HAVE TO PERFORM AN ACFT INSPECTION FOR ENCOUNTERING SEVERE TURB.