Falcon 900
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 75; Flight Crew Total 15650; Flight Crew Type 2327
Situational Awareness; Human-Machine Interface; Confusion; Communication Breakdown; Time Pressure
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Chart Or Publication; Human Factors; Procedure
We were descending into IAD via the Morgantown transition of the GIBBZ1 RNAV STAR. ATC had cleared us 'Direct BRNDN; cross BRNDN at FL270'. As PNF I set it; changed the altitude in the FMS (from the charted FL290); pointed at it; the PF confirmed it and I sent it over to his FMS.We hadn't yet reached BRNDN when we were handed off to Potomac TRACON who cleared us to 'Descend via the GIBBZ One arrival.'  The PF was already VNAV-ing; so I set 7000 in the altitutde window; pointed to it; said it out loud and he confirmed it. At that point I realized we hadn't yet passed BRNDN and were descending to 2000 feet below the published crossing altitude of FL290. Reference NOTE 6 at the top of the STAR ['MAINTAIN last assigned ATC altitude until cleared to 'descend via the GIBBZ ONE'; then comply with altitude restrictions as published.'] and it would seem that under the circumstances we were required to climb to cross BRNDN at FL290 in spite of having checked in telling the controller we were descending to FL270. Opinion: This is a horribly coded arrival. We spent an inordinate amount of time verifying altitudes and airspeeds and briefing it. Our FMS is always going to VNAV to cross the soft altitudes at the lowest one published for any given fix. Perhaps there is an advantage if you're using a newer FMS; but I don't see it. Give me hard altitudes any day. NOTE 6 needs to be modified with wording to the effect that you shouldn't climb above your current or last assigned altitude to meet a published crossing restriction. Terrain is never a factor on this arrival.Our event seems like a Catch 22 to me -- while I don't think we did anything wrong; I can also see how we could have been violated no matter which course of action we pursued. By the time I figured out what altitude we were supposed to be at; it made more sense to cross BRNDN at FL270 as we had been previously cleared; which is what we did. There was no other traffic anywhere around. I will allow there's probably a rule I've never had to use which covers this contingency; but I haven't been able to find it.
After having been previously cleared to cross BRNDN waypoint on the GIBBZ RNAV STAR at FL270 vice the published FL290; the Flight Crew of a Falcon 900 were cleared to 'descend via' the STAR and were uncertain if that clearance required them to climb back up to FL290 to comply with the charted crossing.