Narrative:

I was the pilot flying; cleared to descend via the ltown RNAV STAR into memphis; landing north. I had 3;000 ft set in the fcp. The captain and I had previously had a detailed discussion about the arrival constraints and I elected to 'hard wire' some of the constraints to help get the aircraft down and avoid a potential 'slam dunk' by memphis approach. Specifically; I elected to hardwire the following points: dasac (instead of between 11;000 & 9;000 ft; I elected to make this crossing a hard altitude of 9;000 ft) clark (instead of at or above 8;000 ft; I elected to make this crossing a hard altitude of 8;000 ft) digle (instead of at or above 6000 ft; I elected to make this crossing a hard altitude of 6;000 ft) approximately one mile from clark; flying at 210 KTS indicated and with profile selected; the aircraft began to descend below the 8;000 ft altitude constraint and began picking up speed. I immediately punched off the autopilot; pulled the throttle back; began leveling the aircraft while also deploying the speedbrake to try and slow the aircraft. At it's fastest point; I think our airspeed momentarily reached around 230 KTS and at it's lowest point; I recall seeing the aircraft deviate around 150 ft low (approximately 7;850 ft). We leveled the aircraft back at 8;000 ft and set the fcp altitude to 8;000 ft with level change selected. With the captain's assistance; we then reautomated the aircraft and continued the arrival; meeting all subsequent restrictions at digle. We were turned to a 270 heading before ever reaching dinke and proceeded to fly our approach to runway 36R. As for why the aircraft began its descent below the 8;000 ft restriction at clark prematurely; I was not entirely sure why this occurred. After more closely examining the arrival; I think that the aircraft's increase in speed was due to its attempt to try and make the subsequent 'hard wired' restriction of 6;000 ft at digle. With only 5 miles between clark and digle and 2;000 ft of altitude to lose; as the aircraft nosed over to make the next restriction the speed built up faster than the auto-throttles could compensate for. In hindsight; the best course of action may have been to leave all of the restrictions as at or above restrictions and allow profile to manage the descent as it seems fit. Introducing hard altitude constraints into this arrival might have complicated the descent profile for the aircraft/automation. Another possible remedy would've been to leave the restriction at digle as at or above 6;000. This would've permitted a more gradual descent from 8;000 ft (at clark) to the final altitude of 3;000 ft at dinke; and might've prevented the aircraft from nosing over and gaining airspeed as quickly as it did. In either case; I have learned a valuable lesson about my responsibility to make timely inputs to the aircraft when I realize that it isn't doing what I expect it to do. I feel as if we had thoroughly reviewed and planned for the arrival into memphis and up until this point; was confident that profile was working as expected. Regardless; I will ensure that I maintain a closer watch on the aircraft while using profile.

Related reports:

Google
 

Original NASA ASRS Text

Title: When the flight crew of an A300 opted to make FMS crossing restrictions at DASAC; CLARK; and DIGLE on the LTOWN RNAV STAR to MEM hard altitudes vice at or above; as published; the autoflight initiated a descent and accelerated prior to crossing CLARK at 8;000/210 KTS.

Narrative: I was the pilot flying; cleared to descend via the LTOWN RNAV STAR into Memphis; landing north. I had 3;000 FT set in the FCP. The Captain and I had previously had a detailed discussion about the arrival constraints and I elected to 'Hard Wire' some of the constraints to help get the aircraft down and avoid a potential 'slam dunk' by Memphis Approach. Specifically; I elected to hardwire the following points: DASAC (Instead of BETWEEN 11;000 & 9;000 FT; I elected to make this crossing a hard altitude of 9;000 FT) CLARK (Instead of AT OR ABOVE 8;000 FT; I elected to make this crossing a hard altitude of 8;000 FT) DIGLE (Instead of AT OR ABOVE 6000 FT; I elected to make this crossing a hard altitude of 6;000 FT) Approximately one mile from CLARK; flying at 210 KTS indicated and with PROFILE selected; the aircraft began to descend below the 8;000 FT altitude constraint and began picking up speed. I immediately punched off the autopilot; pulled the throttle back; began leveling the aircraft while also deploying the speedbrake to try and slow the aircraft. At it's fastest point; I think our airspeed momentarily reached around 230 KTS and at it's lowest point; I recall seeing the aircraft deviate around 150 FT low (approximately 7;850 FT). We leveled the aircraft back at 8;000 FT and set the FCP altitude to 8;000 FT with Level Change selected. With the Captain's assistance; we then reautomated the aircraft and continued the arrival; meeting all subsequent restrictions at DIGLE. We were turned to a 270 heading before ever reaching DINKE and proceeded to fly our approach to Runway 36R. As for why the aircraft began its descent below the 8;000 FT restriction at CLARK prematurely; I was not entirely sure why this occurred. After more closely examining the arrival; I think that the aircraft's increase in speed was due to its attempt to try and make the subsequent 'Hard Wired' restriction of 6;000 FT at DIGLE. With only 5 miles between CLARK and DIGLE and 2;000 FT of altitude to lose; as the aircraft nosed over to make the next restriction the speed built up faster than the auto-throttles could compensate for. In hindsight; the best course of action may have been to leave all of the restrictions as AT OR ABOVE restrictions and allow profile to manage the descent as it seems fit. Introducing hard altitude constraints into this arrival might have complicated the descent profile for the aircraft/automation. Another possible remedy would've been to leave the restriction at DIGLE as AT OR ABOVE 6;000. This would've permitted a more gradual descent from 8;000 FT (at CLARK) to the final altitude of 3;000 FT at DINKE; and might've prevented the aircraft from nosing over and gaining airspeed as quickly as it did. In either case; I have learned a valuable lesson about my responsibility to make timely inputs to the aircraft when I realize that it isn't doing what I expect it to do. I feel as if we had thoroughly reviewed and planned for the arrival into Memphis and up until this point; was confident that Profile was working as expected. Regardless; I will ensure that I maintain a closer watch on the aircraft while using Profile.

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.