Human-Machine Interface; Time Pressure; Distraction; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Took Evasive Action; Flight Crew Returned To Clearance; Flight Crew Regained Aircraft Control
Chart Or Publication; Aircraft; Procedure
While descending via the LTOWN6 in profile with the autopilot and autothrottles engaged; at the lead point for the turn at CLARK I was changing ATC frequency as PM when I felt the aircraft pitch over.  I looked up and saw the airspeed increasing and called out the deviation to the Captain.  He immediately threw out spoilers then turned off the autopilot and leveled off to correct the speed deviation.  At first we thought it might also be an altitude deviation; but we were at the lead point for the turn at CLARK; so the descent was OK; just a little too dramatic.  We built the LTOWN6 with one change.  At LTOWN we put in an altitude constraint of at 15;000 because the FMS was not calculating.  After making this one change the FMS appeared to be working just fine.  I have had this happen at least three times in the past but not so violently pitching and definitely not the dramatic increase in airspeed.I would suggest fly level change for the final portion of the LTOWN6 every time.  If a deviation such as this occurs quickly disconnect the autopilot/autothrottles and fly the airplane.  Never trust the automation.
The reporter stated that this event occurred while the aircraft was at 8;000 FT; 210 KTS beginning the turn at CLARK toward DIGLE. This RNAV arrival's final 3;000 FT DINKE constraint drives the entire arrival and in this case caused the aircraft's FMS to calculated a rapid descent in order to make the DINKE constraint. If the aircraft does not cross DIGLE at 6;000 FT it will not make the DINKE constraint. He has seen this in his widebody aircraft on several arrivals and has talked with other heavy jet pilots who have experienced the same FMS behavior. The reporter believes this RNAV and many others have too many constraints; too close together and that simpler would be better. The reporter also believes that the FMS computation ability along with the coupled automation have difficulty smoothly complying with the steep constraints.
During the LTOWN arrival about 2 NM prior to CLARK; the aircraft began a rapid pitch over and acceleration; causing a potential airspeed and altitude violation. Both Autopilot 1 and autothrottles were engaged. Assigned altitude and airspeed for CLARK is 210 KIAS and 8;000 MSL minimum. The aircraft was in Profile and was stable at 210 KIAS and 8;000 MSL with slats extended when it began to pitch over and accelerate just as it began turning south to intercept the next point on the arrival; DIGLE. Airspeed accelerated to 240 KIAS and the aircraft descended to 7;500 MSL. I disconnected the autopilot and extended the speedbrakes while pulling throttles back to idle manually to override the autothrottles to get back to 210 KIAS and stop the descent. By this time; we were abeam CLARK; so continued the descent down the 6;000 MSL min for DIGLE. We then re-engaged the autopilot and continued the descent using Level Change. Approach Control made no mention of either the airspeed or altitude deviation.The First Officer had recommended using a hard altitude 15;000 MSL at LTOWN; which we programmed into the FMS. Everything appeared to be operating normally until this time although we extended the slats a little earlier than normal to help maintain the required airspeed of 210 KIAS on this portion of the arrival. The First Officer was slightly distracted by a frequency change during all the excitement; but was still quick to call out the altitude deviation while I was trying to correct the airspeed problem. Only recommendation I can come up with is to use Level Change and Altitude Hold on the FCP for arrivals like the LTOWN with numerous altitude and airspeed restrictions. Airspeed and altitude control when using the autopilot and Profile are too imprecise for the restrictions on arrivals like the LTOWN.
An aircraft descending on the MEM LTOWN6 had an uncommanded descent while the aircraft was beginning the turn at CLARK toward DIGLE because the DINKE constraint compliance required rapid FMS calculations and automation response.