Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness; Human-Machine Interface; Fatigue; Confusion
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Chart Or Publication; Human Factors; Procedure
We were on the TAMMY4 arrival ELD transition into MEM. We were given a 'descend via' clearance. Everything was going fine until the descent to cross TAMMY between 16;000 and 10;000 FT; followed 2 miles later by ROCAB at 10;000 FT and 230 KTS. Three things happened in fairly quick succession that contributed to me missing the crossing restriction. I was looking at the vertical guidance supplied on the MFD (the green data) and it was showing the descent rate that I was currently doing. It didn't seem right so I did some mental math and increased my descent rate also checking for the snowflake which wasn't showing (found out later that it had fallen out when the FMS had briefly gone into roll mode).Then ATC gave us a runway change which distracted us and when we had answered we both realized that we would not make the crossing restriction and notified ATC immediately. I believe the root of the problem was that: 1) I didn't realize the snowflake was disabled; 2) I originally thought the green VNAV info on the MFD would give me the bottom of the 16;000 to 10;000 FT envelope since ROCAB is so close to TAMMY and 3) I was distracted with the runway change and lost awareness for enough time to not make the restriction. Contributing factors included: hurrying because we had been delayed; there were new arrivals in Memphis--this was our first time on this arrival landing to the North; some weather deviations; and an early morning report for duty. Our errors included: not remembering that the snowflake will become disabled if the aircraft goes into the roll mode; not catching that the snowflake wasn't coming up because it was disabled; not using the range to altitude (banana bar) to it's fullest extent; not holding off on worrying about the runway change until past the crossing restriction; not realizing that the green VNAV data on the MFD wouldn't necessarily give me the bottom of the altitude envelope at TAMMY even with ROCAB being so close; finally; not backing up the automation with good old math. As a result we crossed ROCAB on speed but about 4;000 FT high. I do not believe that our briefing was to blame because we completely briefed and cross checked the arrival with the FMS but the automation was where I made my mistake. In the future I will make sure that the primary sources for descending via an arrival are the snowflake and mental math--not the green MFD VNAV--because that is giving information for the next fix only and with an altitude block not necessarily the bottom of said block.
The Captain advised she had been commuting into MEM on another air carrier jump seat shortly after this event and was told that carrier had bulletined the flight crews not to accept the TAMMY arrival because of the same issue; the FMS' failure to 'look ahead' to the more restrictive fix and instead simply provide guidance to the higher altitude in the 'crossing window.'
On descent into MEM we were assigned to descend via the TAMMY ONE arrival. The Captain placed 10;000 FT in the altitude alerter so that we would have vertical guidance on the way down. Approaching TAMMY Intersection to cross between 16;000 and 10;000 FT we noticed that we were going to be too high to cross ROCAB Intersection at 10;000 FT. We immediately advised ATC and were told that it was no problem and to further descend for the approach into MEM. The descent info provided to us via the green descent box on the MFD only gave us descent info to reach TAMMY at 16;000 and not ROCAB at 10;000 FT.We believe the major contributing factor to this event was the complexity and vagueness of the arrivals into MEM. The descent info provided to the crew was inaccurate due to the FMS wanting to cross TAMMY at 16;000 and not ROCAB at 10;000 FT. These arrivals are unlike any descend via's I have flown as they allow too much leeway and too many areas to mess up. Also on the MFD the Green Attitude information next to TAMMY Intersection seems to cover up the altitude information next to ROCAB. These charts are difficult to read as well and with the controllers having issues as well it is causing a much higher workload than should be required to achieve a successful outcome.
While flying the TAMMY RNAV STAR to land North at MEM; the flight crew of a CRJ-200 arrived at TAMMY too high to make the crossing restriction of 10;000 FT and 230 KTS at ROCAB; only two miles beyond TAMMY.