Narrative:

We were coming in well rested and having planned ahead. Everything seemed to be going fine. We were filed for the tammy RNAV STAR but because of weather were given direct fnchr for the fnchr arrival. We plugged everything in [and were] seeming to make our targets well. We planned a hard 10[000] at fnchr to give us a lead and be stable since there was weather was the airport as it appeared on radar and give us time to best plan. As we crossed fnchr the airplane suddenly dived and picked up speed to about 320 KTS. I was the pilot monitoring and called airspeed. We had 4;000 plugged in as we were cleared to descend [via] the fnchr arrival at fnchr. As we were accelerating the first officer (pilot flying) reached down to [set] tact [an FMS speed intervention mode] 290 KTS (we were around 310-320 KTS at this point). [At this time] I took the airplane; disconnecting the autopilot and pulling the nose up. The next crossing restriction was 9;000 [but we crossed it] at 8;500. The controller evidently saw us dive through 9;000 [and cleared us to] maintain 8;000. We proceeded without incident for the rest of the flight. I have 15 years in the jet and have never seen it dive on its own like that. It can get confused if above profile but I thought we were fine. Many lessons learned.if I had intervened sooner and just hit altitude hold; this would have been a non-event. The first officer is very diligent; helpful; no reason to expect he would not have corrected as soon as possible. I was surprised when the solution he came up [with] for the diving and increasing speed was to go to the box. I took it at that point but with the weather; controller; surprise; and trying to coach too long about the speed I was [task] saturated and missed the altitude of 9;000. I should have taken it sooner. I am very watchful and aggressive in the approach environment and level offs but got caught by total surprise with the dive in the arrival environment; coming out of--what I thought was--nowhere. I take full responsibility. I will be more aggressive and forward thinking in all phases in the future. I briefed the first officer thoroughly on never flying the airplane via the box when it isn't doing what you want. Ask for what you want in the descent instead of reaching down. Fly. Rapidly correct by the most expeditious means safely possible. Aviate; navigate; communicate. I am sure he heard me. As I said; he is a really diligent guy. I've heard it said the good ones are the ones that get you. As for me I just came back from vacation and had a four day layover in phx. I was a little rusty. I assure you I will change habits to prevent this from happening again.

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Original NASA ASRS Text

Title: An A300-600 flight crew was caught by surprise when the autoflight system initiated an abrupt descent and accelerated after crossing FNCHR waypoint on the FNCHR RNAV STAR into MEM. The jet descended ~500 FT below the next minimum crossing restriction of 9;000 and accelerated to about 320 KTS before the autopilot was disconnected and a recovery completed.

Narrative: We were coming in well rested and having planned ahead. Everything seemed to be going fine. We were filed for the TAMMY RNAV STAR but because of weather were given direct FNCHR for the FNCHR arrival. We plugged everything in [and were] seeming to make our targets well. We planned a hard 10[000] at FNCHR to give us a lead and be stable since there was weather was the airport as it appeared on radar and give us time to best plan. As we crossed FNCHR the airplane suddenly dived and picked up speed to about 320 KTS. I was the pilot monitoring and called airspeed. We had 4;000 plugged in as we were cleared to descend [via] the FNCHR arrival at FNCHR. As we were accelerating the First Officer (pilot flying) reached down to [set] TACT [an FMS speed intervention mode] 290 KTS (we were around 310-320 KTS at this point). [At this time] I took the airplane; disconnecting the autopilot and pulling the nose up. The next crossing restriction was 9;000 [but we crossed it] at 8;500. The Controller evidently saw us dive through 9;000 [and cleared us to] maintain 8;000. We proceeded without incident for the rest of the flight. I have 15 years in the jet and have never seen it dive on its own like that. It can get confused if above profile but I thought we were fine. Many lessons learned.If I had intervened sooner and just hit altitude hold; this would have been a non-event. The First Officer is very diligent; helpful; no reason to expect he would not have corrected ASAP. I was surprised when the solution he came up [with] for the diving and increasing speed was to go to the box. I took it at that point but with the weather; Controller; surprise; and trying to coach too long about the speed I was [task] saturated and missed the altitude of 9;000. I should have taken it sooner. I am very watchful and aggressive in the approach environment and level offs but got caught by total surprise with the dive in the arrival environment; coming out of--what I thought was--nowhere. I take full responsibility. I will be more aggressive and forward thinking in all phases in the future. I briefed the First Officer thoroughly on never flying the airplane via the box when it isn't doing what you want. Ask for what you want in the descent instead of reaching down. FLY. Rapidly correct by the most expeditious means safely possible. Aviate; navigate; communicate. I am sure he heard me. As I said; he is a really diligent guy. I've heard it said the good ones are the ones that get you. As for me I just came back from vacation and had a four day layover in PHX. I was a little rusty. I assure you I will change habits to prevent this from happening again.

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.