Narrative:

Aug/xa/98 I flew an air carrier B747-400 trip from lax to akl and return. During the trip, we received a clearance like: at longitude XXX, climb 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 clearance: squawk XXXX, at waypoint XXX, contact ZLA on frequency. This format seemed operationally clear, 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 reporter revealed the following information: reporter participated in fans structured callback questionnaire. Reporter is very competent and comfortable in using the cpdlc. His only problem he sees in using it is the way some clrncs are received, they become ambiguous. For example in this case, climb to FL330, squawk XXXX. When should the transponder code be changed? At the present time or after leveling at FL330? Reporter wants exactness in implementing clrncs. Reporter feels the cpdlc system is wonderful and is very comfortable in using it. In this case, one clearance is ambiguous and another clearance leaves no doubt in his mind what needs to be done.

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Original NASA ASRS Text

Title: 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).

Narrative: 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.

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.