Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 23000; Flight Crew Type 13000
Situational Awareness; Human-Machine Interface; Distraction; Confusion; Workload; Communication Breakdown
ATC Issue All Types; Aircraft Equipment Problem Critical
Aircraft; Airspace Structure; Procedure; Human Factors; Chart Or Publication
We were flying at FL370 and filed via the GIBBZ1 Arrival. CLE Center cleared us to cross BRNDN at FL290. Approximately 10 miles from BRNDN we were handed off to Washington Center. We were then cleared direct to MOSLE to resume the arrival. We programmed direct to MOSLE and then asked if we were cleared to 'descend via' the arrival. The Controller affirmed we were cleared to descend via the arrival; but by then we were high on the VNAV path and went to FLCH to regain the VNAV path which we did. We then reset the MCP window for 7;000. We then were handed over to Potomac Approach who told us to cross GIBBZ at 6;000. We reset the MCP window to 6;000. VNAV went out of path to speed so we went to FLCH back up procedures and reset the MCP window to 11;000. We were in a fast descent to make the MOSLE crossing and both saw ALT CAP on the FMAs. The aircraft descended to approximately 10;700 before we corrected the altitude manually. In my opinion the controllers are trying to simplify the GIBBZ arrival with direct clearances that will eliminate some of the fixes. This is a very good idea but the phraseology has to be very precise to avoid confusion. We need to know if the direct to resume this arrival is for routing or also includes the altitudes (descend via). We also need to know if they want us to resume published speeds or stay with last assigned speed. The clearance to descend via the GIBBZ arrival comes at a Center hand off and any delay makes it very difficult to make crossing restrictions; especially with the normal tailwinds. We are being encouraged to fly in VNAV more often and this increases our dependence on autoflight. The work load on this arrival is such that I have no ability to do anything except monitor the arrival. No traffic watch; no weather awareness; no cabin coordination. I also believe that VNAV may be the only descent mode that will not work on this arrival and trying to use it results in navigation errors as we encountered. This arrival is overly complicated it has 11 fixes; nine altitude restrictions; including hard; at or above; at or below; below; and bracketed altitudes; and two speed reductions. This only gets you to the arrival gate and then we get 5 more fixes to downwind. This arrival is unsafe.
The Captain of an RNAV equipped Boeing found himself with more questions than answers when ATC attempted to minimize the workload associated with the complex GIBBZ RNAV STAR by giving clearances direct to down-line waypoints; bypassing multiple intervening fixes with complex crossing restrictions. Among the questions were uncertainty as to the status of 'descend via' procedures and the effect of being cleared to GIBZZ 'at' an assigned--but unpublished--altitude on their need to comply with the prior STAR crossing restrictions at MOSLE; MONNK and BMICH.