Narrative:

On high frequency requested a climb to FL390 at 20 west. Clearance came back, 'cleared to climb to FL390, be level by XB00Z.' the approximately time receiving clearance was XA20. In the clearance, no crossing restriction was mentioned at 20 west, whose ETA was XA43Z. At XA37Z, I reported out of FL350 for FL390 (ATC had requested a 'report leaving' report also). We climbed uneventfully to FL390 in approximately 7-8 mins, so we were still in a climb mode at 20 west. However due to frequency congestion, the copilot was not able to make the position report (or the 'report reaching' report) until 2 or 3 mins past 20 west. He gave a normal oceanic position report, except that he said we were level at FL390 at 20 west, when, in fact, we were probably passing through FL380. The controller acknowledged the position report, and we went into SELCAL mode. In a few mins we got a SELCAL message. On answering, the controller advised us that we had climbed early, he restated the clearance, then realized it was ambiguous. He said the early climb caused no traffic conflicts or compromised safety, in any way. I normally seek a clarification of any clearance not completely understood. It 'seemed' a straightforward clearance and not at all ambiguous the first time I heard it. It just so happened the copilot had gone to the bathroom, for about 5 mins, so I didn't have the benefit of another set of ears listening to the clearance. The next time I will ask for a clarification so at least I will get the benefit of hearing it twice.

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Original NASA ASRS Text

Title: MISSED ALT CLB CLRNC RESTRICTION.

Narrative: ON HIGH FREQ REQUESTED A CLB TO FL390 AT 20 W. CLRNC CAME BACK, 'CLRED TO CLB TO FL390, BE LEVEL BY XB00Z.' THE APPROX TIME RECEIVING CLRNC WAS XA20. IN THE CLRNC, NO XING RESTRICTION WAS MENTIONED AT 20 W, WHOSE ETA WAS XA43Z. AT XA37Z, I RPTED OUT OF FL350 FOR FL390 (ATC HAD REQUESTED A 'RPT LEAVING' RPT ALSO). WE CLBED UNEVENTFULLY TO FL390 IN APPROX 7-8 MINS, SO WE WERE STILL IN A CLB MODE AT 20 W. HOWEVER DUE TO FREQ CONGESTION, THE COPLT WAS NOT ABLE TO MAKE THE POS RPT (OR THE 'RPT REACHING' RPT) UNTIL 2 OR 3 MINS PAST 20 W. HE GAVE A NORMAL OCEANIC POS RPT, EXCEPT THAT HE SAID WE WERE LEVEL AT FL390 AT 20 W, WHEN, IN FACT, WE WERE PROBABLY PASSING THROUGH FL380. THE CTLR ACKNOWLEDGED THE POS RPT, AND WE WENT INTO SELCAL MODE. IN A FEW MINS WE GOT A SELCAL MESSAGE. ON ANSWERING, THE CTLR ADVISED US THAT WE HAD CLBED EARLY, HE RESTATED THE CLRNC, THEN REALIZED IT WAS AMBIGUOUS. HE SAID THE EARLY CLB CAUSED NO TFC CONFLICTS OR COMPROMISED SAFETY, IN ANY WAY. I NORMALLY SEEK A CLARIFICATION OF ANY CLRNC NOT COMPLETELY UNDERSTOOD. IT 'SEEMED' A STRAIGHTFORWARD CLRNC AND NOT AT ALL AMBIGUOUS THE FIRST TIME I HEARD IT. IT JUST SO HAPPENED THE COPLT HAD GONE TO THE BATHROOM, FOR ABOUT 5 MINS, SO I DIDN'T HAVE THE BENEFIT OF ANOTHER SET OF EARS LISTENING TO THE CLRNC. THE NEXT TIME I WILL ASK FOR A CLARIFICATION SO AT LEAST I WILL GET THE BENEFIT OF HEARING IT TWICE.

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.