MD-11
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion; Situational Awareness
ATC Issue All Types
General None Reported / Taken
Chart Or Publication; Human Factors; Procedure
We filed to Memphis via LIT TAMMY 4 arrival. Enroute to LIT at FL350 we requested direct routing to TAMMY Intersection. The Memphis ARTCC Controller cleared us to descend to FL330 and then 'Pilot's discretion to FL240. Upon reaching FL330 cleared direct to TAMMY'. We complied with the clearance. Enroute to TAMMY; we were switched to Memphis Approach Control. The Approach Controller asked us to verify we were descending via the TAMMY 4 arrival; Landing North Transition. We responded with our previously cleared routing (direct TAMMY). (I don't recall if we were already at FL240; descending to FL240 or still at FL330) The Approach Controller then cleared us 'Direct TAMMY; descend via the TAMMY 4 arrival; landing north transition'. We assumed this meant to comply with the altitude restrictions at TAMMY depicted on the arrival (below 16;000 and at or above 10;000 FT). We therefore continued our descent to comply with the altitude restrictions at TAMMY. As an aircrew; we both believed this was the Controller's intent but I'm not sure this was technically correct because we weren't on the arrival until reaching TAMMY. It seems to me the Controller should have given us an altitude to reach at TAMMY and then cleared us to descend via the arrival. [Our confusion was the result of an] ambiguous clearance issued by the Controller [and an] expectation bias on the part of the aircrew.Clear; concise clearance by ATC; as with all ATC clearances involving SIDs and STARs; there is ALWAYS room for misinterpretation by the aircrew when cleared to climb or descend to a new altitude. This could easily be resolved by controllers using the word UNRESTRICTED; as: 'climb/descend UNRESTRICTED to...' We should have queried the Controller for a clearance to cross TAMMY Intersection at a specific altitude rather than continue descent based on our own assumption.
The flight crew of an MD-11 was confused by a clearance to proceed direct to an intermediate fix on and descend via the TAMMY RNAV STAR to MEM that did not include an altitude at which to cross the fix.