EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion; Human-Machine Interface; Situational Awareness
ATC Issue All Types
Flight Crew Requested ATC Assistance / Clarification
Chart Or Publication; Human Factors; Procedure
We were descending on the GIBBZ1 RNAV into IAD between BBONE and KILMR when ATC assigned us a 020 heading for traffic. Our descent clearance was 'descend via the GIBBZ arrival; with no specific altitude given. I told my First Officer to ask ATC what altitude he wanted us at; and we got no reply. After 2 more unsuccessful tries to get an answer (the Controller was busy); I leveled the aircraft at 11;000 FT; which was the altitude for the next fix. We finally got a reply to our questions and were told to continue to descend to 6;000 FT. We complied with no further issues.As we had [previously] been cleared to 'descend via' the STAR and; when vectored off of it; were no longer on the arrival; we had [no guidance as to our cleared] altitude. Ive had this happen a couple times; with different altitudes given each time; some the same as the bottom altitude [on the STAR]; some different. Anytime an aircraft has been given a 'descend via' clearance and is later turned off the arrival; ATC needs to assign an altitude.
When they were vectored off the GIBBZ RNAV STAR; by which they had previously been cleared to 'descend via;' the flight crew of an ERJ-170 were uncertain as to their cleared altitude. Difficulty communicating with the busy Controller forced a brief level off at an intermediate altitude until a new clearance could be provided.