Narrative:

Coming into bos on the krann 2 RNAV arrival bos approach first told us to cross krann at 11;000 ft. Then while descending they told us to descend via the krann 2 runway 22L transition. So we set 5;000 ft in the alt. Selector. After reaching and leveling off at 11;000 ft I thought bos approach said continue down to 9;000 ft so I set 9;000 in the mcdu at cradl and hit des now. After reaching 9;000 ft and hitting the TOD for the rest of the arrival the aircraft started to descend to 5;000 ft. At approximately 8;700 ft; bos approach told us to immediately climb back to 9;000 ft and turn left to I think 030. After we stabilized at 9;000 ft bos said we were told to maintain 9;000 ft and asked us to call after landing; which I did. The approach supervisor asked me what happened and I told him again that we understood that we were cleared via the 22L transition and that approach just wanted us at 9;000 ft early so we just continued via VNAV to 9;000 and never changed the bottom of 5;000 ft. He said that; no we were told to maintain 9;000 ft which deleted the rest. I told him again I did not remember being told to maintain 9;000 and that I thought they just wanted us at 9;000 early because; had they not said anything we would not have started down so early. [We were confused] being given different clearances in close succession without making clear that the previous clearance no longer applied. I think ATC should say that they do not want you to continue via a profile descent after they have already cleared you to do so; instead of saying that the new clearance implies that the previous clearance is deleted.

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Original NASA ASRS Text

Title: EMB190 flight crew reports misunderstanding an ATC altitude clearance after beig initially assigned to descend via the KRANN 2 arrival.

Narrative: Coming into BOS on the KRANN 2 RNAV arrival BOS Approach first told us to cross KRANN at 11;000 FT. then while descending they told us to descend VIA the KRANN 2 Runway 22L transition. So we set 5;000 FT in the alt. selector. After reaching and leveling off at 11;000 FT I thought BOS Approach said continue down to 9;000 FT so I set 9;000 in the MCDU at CRADL and hit DES now. After reaching 9;000 FT and hitting the TOD for the rest of the arrival the aircraft started to descend to 5;000 FT. At approximately 8;700 FT; BOS Approach told us to immediately climb back to 9;000 FT and turn left to I think 030. After we stabilized at 9;000 FT BOS said we were told to maintain 9;000 FT and asked us to call after landing; which I did. The Approach Supervisor asked me what happened and I told him again that we understood that we were cleared via the 22L transition and that approach just wanted us at 9;000 FT early so we just continued via VNAV to 9;000 and never changed the bottom of 5;000 FT. He said that; no we were told to maintain 9;000 FT which deleted the rest. I told him again I did not remember being told to maintain 9;000 and that I thought they just wanted us at 9;000 early because; had they not said anything we would not have started down so early. [We were confused] being given different clearances in close succession without making clear that the previous clearance no longer applied. I think ATC should say that they do not want you to continue via a profile descent after they have already cleared you to do so; instead of saying that the new clearance implies that the previous clearance is deleted.

Data retrieved from NASA's ASRS site as of July 2013 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.