Date: 201210
Local time of day: 0601-1200
Location: BOS.Airport, MA
Flight conditions: IMC
Light: Daylight
Aircraft: EMB ERJ 190/195 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
Airspace: Class E A90
ATC / advisory: TRACON A90
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

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.


Second narrative: On the KRANN 2 arrival we were cleared to cross KRANN at 11;000. Then we were cleared to descend via the 22L transition. 5;000 FT was selected as the bottom altitude. Later we were told after KRANN to maintain 9;000. I thought it was just for the next arrival segment. Descending below 9;000 ATC told us to turn left and to climb back to 9;000. So we did. ATC informed us to call after landing. ATC clearing us to an unpublished altitude after being cleared to a 'descend via procedure' without making clear that the 'descend via procedure' no longer applied. ATC should state very clearly 'Cancel descend via'; and that will trigger in the pilots that the bottom altitude is no longer what should be set in the ALT selector.


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