B737 Undifferentiated or Other Model
Flight Crew Last 90 Days 169; Flight Crew Total 15872; Flight Crew Type 11664
Communication Breakdown; Confusion; Human-Machine Interface
ATC Issue All Types
Procedure; Chart Or Publication; Human Factors
I am submitting this to highlight a threat that is built into the new RNAV arrivals into IAH in hopes that we can permanently mitigate them for future crews. There were no incidents on this flight and all threats were mitigated successfully.The flight was in clear evening skies with not many airplanes in the pattern. We were cleared to descend via the DRLLR2 into IAH. I asked for the runway transition; so we would know the bottom altitude to set into the MCP window and were told that Approach would give us that assignment. We debated what altitude to put in the MCP. We agreed on 7;000 until we received a runway assignment that would dictate something different; as that is the highest of the three runways.I see on the arrival chart that it says to 'EXPECT runway assignment from Houston TRACON upon initial contact.' I question whether a 'descend via' clearance is legal when there is no prescribed bottom altitude.' Are we supposed to put 8;000 ([all DRLLR arrivals must cross] DOMNO between 10;000 and 8;000) until we get a runway assignment; or do we put the highest of the runway transition altitudes until we receive an assignment? I would think that Center would clear flights to 'descend via the DRLLR2 except maintain 7;000 [the highest of the runway transition 'bottom' altitudes]' when the bottom altitude is based on a runway assignment that Center does not have. The MSCOT RNAV STAR has the same problem. The changes are new and the controllers and pilots are working well together to make sure there are no errors. There is the possibility that there is a standard way to figure the bottom altitude that I am missing; in which case; please let me know.
A B737 First Officer questions the legitimacy of Center clearing flights to 'descend via' the DRLLR (and MSCOT) RNAV STARS to IAH without also providing a bottom altitude to which cleared; the reason being that there are three runway transitions beyond DOMNO; the last common fix; which include two different bottom altitudes.