Narrative:

Memphis has recently enacted several complicated and confusing RNAV stars that are utilized by ATC with descend via clearances. We were issued a descend via clearance into mem with the tammy four RNAV arrival (tammy.TAMMY4) with the el dorado transition. There were several thunderstorms along the arrival that required deviating off course. Mem was landing north so we loaded a 36L approach into mem with rocab being the final fly over waypoint on the arrival.we briefed before-hand the complexity of the arrival and the need to have the appropriate altitudes set. Of particular concern was the tammy fix which required an altitude between 16;000-10;000 ft MSL. We made note that we would want to be at 10;000 ft MSL at tammy rather than 16;000 ft MSL because rocab is only 2.0 NM from tammy and must be crossed at 10;000 MSL and 230 KTS. Obviously; it would be impossible to be at 16;000 MSL at tammy and then meet those restrictions at rocab. Even being aware of this obscure RNAV arrival requirements we still had difficulty meeting the 10;000 MSL at 230 KTS requirement at rocab. Our situation was further complicated with navigating around a thunderstorm outside of barff and also being directed by ATC to maintain a high airspeed for sequencing.one aspect of our operation that I was not aware of and contributed to our difficulty during this arrival was the VNAV guidance displayed on our mfd. When we loaded the arrival and approach; the FMS pre-loads all the RNAV waypoints and their individual altitude and airspeed requirements. During the descent I was following the VNAV guidance displayed in green on the mfd; but noticed it did not coincide with the white pfd snowflake which was showing a steeper descent required than did the mfd. Realizing that the VNAV information was incorrect; I had to increase my descent rate to around 4;000 FPM to accommodate the final rocab fix. I managed to get close to the requirement but my speed took a short time past rocab to slow. By then we were being vectored in by mem approach and the flight continued without incident. This is a complex RNAV arrival with demanding altitude and speed requirements that may conflict with one-another. Further issues; such as weather and/or ATC speed requests; may further complicate meeting those restrictions.I recommend we issue a bulletin regarding these arrivals in mem to increase crewmember awareness and discuss possible alternatives for FMS programming to provide correct VNAV guidance with these arrivals.

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Original NASA ASRS Text

Title: A CRJ-200 flight crew; after encountering weather avoidance vectors; ATC requests to keep their speed up for arrival sequencing. They receive conflicting vertical navigation guidance from their FMS and are unable to comply with the crossing restriction at ROCAB on the TAMMY RNAV STAR to MEM.

Narrative: Memphis has recently enacted several complicated and confusing RNAV STARs that are utilized by ATC with descend via clearances. We were issued a descend via clearance into MEM with the TAMMY FOUR RNAV ARRIVAL (TAMMY.TAMMY4) with the EL DORADO transition. There were several thunderstorms along the arrival that required deviating off course. MEM was landing north so we loaded a 36L approach into MEM with ROCAB being the final fly over waypoint on the arrival.We briefed before-hand the complexity of the arrival and the need to have the appropriate altitudes set. Of particular concern was the TAMMY fix which required an altitude between 16;000-10;000 FT MSL. We made note that we would want to be at 10;000 FT MSL at TAMMY rather than 16;000 FT MSL because ROCAB is only 2.0 NM from TAMMY and must be crossed at 10;000 MSL and 230 KTS. Obviously; it would be impossible to be at 16;000 MSL at TAMMY and then meet those restrictions at ROCAB. Even being aware of this obscure RNAV arrival requirements we still had difficulty meeting the 10;000 MSL at 230 KTS requirement at ROCAB. Our situation was further complicated with navigating around a thunderstorm outside of BARFF and also being directed by ATC to maintain a high airspeed for sequencing.One aspect of our operation that I was not aware of and contributed to our difficulty during this arrival was the VNAV guidance displayed on our MFD. When we loaded the arrival and approach; the FMS pre-loads all the RNAV waypoints and their individual altitude and airspeed requirements. During the descent I was following the VNAV guidance displayed in green on the MFD; but noticed it did not coincide with the white PFD snowflake which was showing a steeper descent required than did the MFD. Realizing that the VNAV information was incorrect; I had to increase my descent rate to around 4;000 FPM to accommodate the final ROCAB fix. I managed to get close to the requirement but my speed took a short time past ROCAB to slow. By then we were being vectored in by MEM Approach and the flight continued without incident. This is a complex RNAV arrival with demanding altitude and speed requirements that may conflict with one-another. Further issues; such as weather and/or ATC speed requests; may further complicate meeting those restrictions.I recommend we issue a bulletin regarding these arrivals in MEM to increase crewmember awareness and discuss possible alternatives for FMS programming to provide correct VNAV guidance with these arrivals.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.