WHEN A B747-400 FLYING TO AKL RECEIVES ITS CLRNC TO CLB AND SQUAWK A DIFFERENT CODE; FLC IS UNCERTAIN AS TO WHEN TO CHANGE SQUAWK. CLRNC WAS DELIVERED BY DATA LINK (CPDLC).

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AUG/XA/98 I FLEW AN ACR B747-400 TRIP FROM LAX TO AKL AND RETURN. DURING THE TRIP; WE RECEIVED A CLRNC LIKE: AT LONGITUDE XXX; CLB AND MAINTAIN FL330; SQUAWK XXXX. THIS LEFT A QUESTION IN OUR MINDS IF WE WERE SUPPOSED TO SQUAWK NOW; OR AT THE LONGITUDE. LATER IN THE TRIP; WE RECEIVED ANOTHER; MORE OBVIOUS CLRNC: SQUAWK XXXX; AT WAYPOINT XXX; CONTACT ZLA ON FREQ. THIS FORMAT SEEMED OPERATIONALLY CLR; WHERE THE FIRST DID NOT. SUGGESTION: SINCE CREWS CONTINUE TO MISUNDERSTAND CONDITIONAL CPDLC CLRNCS; AND AS SHOWN HERE; THAT THEY OFTEN ARE INHERENTLY AMBIGUOUS; THE USE OF THESE CONDITIONAL CLRNCS SHOULD BE VERY RESTR TO ONLY WHEN ABSOLUTELY NECESSARY AND THE FORMATS SHOULD BE REPROGRAMMED WITH MORE HUMAN FACTORS CONSIDERATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR PARTICIPATED IN FANS STRUCTURED CALLBACK QUESTIONNAIRE. RPTR IS VERY COMPETENT AND COMFORTABLE IN USING THE CPDLC. HIS ONLY PROB HE SEES IN USING IT IS THE WAY SOME CLRNCS ARE RECEIVED; THEY BECOME AMBIGUOUS. FOR EXAMPLE IN THIS CASE; CLB TO FL330; SQUAWK XXXX. WHEN SHOULD THE XPONDER CODE BE CHANGED? AT THE PRESENT TIME OR AFTER LEVELING AT FL330? RPTR WANTS EXACTNESS IN IMPLEMENTING CLRNCS. RPTR FEELS THE CPDLC SYS IS WONDERFUL AND IS VERY COMFORTABLE IN USING IT. IN THIS CASE; ONE CLRNC IS AMBIGUOUS AND ANOTHER CLRNC LEAVES NO DOUBT IN HIS MIND WHAT NEEDS TO BE DONE.