An aircraft descending on the mem LTOWN6 had an uncommanded descent while the aircraft was beginning...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

Attributes
ACN	1091776
Time
Date	201306
Local Time Of Day	0601-1200
Place
Locale Reference	MEM.Airport
State Reference	TN
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Widebody Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR LTOWN6
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Pilot Not Flying First Officer
Person 2
Function	Pilot Flying Captain
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Inflight Event / Encounter Loss Of Aircraft Control

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.