Date: 201307
Local time of day: 0601-1200
Location: ZME.ARTCC, TN
Flight conditions: IMC
Weather elements / visibility: Turbulence; Thunderstorm
Light: Daylight
Aircraft: A300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZME
ATC / advisory: Center ZME
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Troubleshooting; Human-Machine Interface; Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Regained Aircraft Control; Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Chart Or Publication; Aircraft; Procedure
Primary problem: Aircraft

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.

Callback: The reporter advised that; when she started to describe the event to her safety department; they quickly interrupted and spoke words to the effect of '...wait; let us guess...' and then went on to tell her what had happened and advised there had been several of these events on their fleet of A300s.  The reporter expressed concern that the company had not; to her knowledge; passed that information on to the flight crews.  The subject of the capability of the FMS/autoflight system on this more or less 'first generation autoflight aircraft' was discussed.  She advised that while it is true that the FMS has problems with some of the more complex RNAV STARs her technique is simply to disconnect autoflight when she deems it no longer capable of compliance and to; instead; manually 'make the jet go where it needs to go.'  She did stress that; in her experience; the FMS will prioritize programmed VNAV path over any associated speed constraints.The reporter called back later and clarified the 'TACT' mode autothrottle function; advising it is not a 'conventional' glareshield based speed intervention tool but is; instead; one of two FMS programmable mach/airspeed inputs; Strategic or Tactical.  Strategic being a takeoff to touchdown speed strategy (cost index; minimum flight time; etc) and a Tactical mode designed for alterations within the strategic plan to address interim or short term needs due to ATC constraints; weather; turbulence etc.  She emphasized she felt it was a poor choice to address the already anomalous actions of the autoflight system.  Thus; she assumed control to arrest the descent.



Synopsis: 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.