A320
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 220; Flight Crew Total 19500; Flight Crew Type 4000
Time Pressure; Workload; Distraction; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance; General Maintenance Action
Chart Or Publication; Human Factors; Procedure; Aircraft
Leaving FL300 we were cleared to descend via the FRDMM STAR into DCA. The First Officer engaged the DES mode and managed speed after I set the bottom altitude in the MCP. We verified the restrictions on the MCDU and on the ND. After making the first several crossings with no problem; the First Officer and I briefed the approach. At this time the autopilot seemed to 'go dumb' and continued descending below the next crossing altitude. The FMA still indicated DES mode engaged; but the autopilot was descending below profile. The First Officer disengaged the autopilot and climbed the jet to meet the restriction. The doughnut and the PROG page displayed correct information; but the autopilot was not maintaining the profile. We then reverted to the old way of setting the most restrictive altitude and the cockpit workload went through the roof as there are 15 altitude and 5 airspeed restrictions on this arrival. We attempted DES mode again and it made the altitude crossings; but would not make the airspeed restrictions even though we were in managed speed. We used selected speed to comply with the restrictions.Upon landing; we entered an item for the malfunction and called Maintenance Control. A Mechanic reset the FMGCs and we flew the airplane again; but did not have the opportunity to use DES mode on the the following flight. I have seen this problem several times on the Airbus; the autopilot disregards FMGC profiles.
The reporter advised there were no adverse wind or weather effects to complicate the profile.  He stated the initial crossing restrictions were complied with in harmony with the FMGC profile.  He did believe the errors began when they were programming a runway change and speculated that doing so may have overloaded the processing capabilities of the FMGs and suggested the commands to the autoflight system (vertical navigation and authothrottles) may have suffered as a result although they received no ECAM warnings to that effect.  He stressed that the FMGs continued to display appropriate 'descent doughnut' and PROGRESS PAGE predictive altitude displays.  The only computer generated alert was an amber circle around the next waypoint displayed when their actual altitude was below the minimum crossing at that fix.He further speculated on the processing demands of multiple crossing airspeed and altitude windows often spaced only a few miles apart which require the calculations for each approaching window to include computations of what has to be considered for each of the subsequent windows far down stream.  Finally; he stressed his belief that it was the autoflight system; not the FMGs; that erred...with the caveat that the demands on the FMGs may have been such that commands to the autoflight system were compromised.
An A319/20 Captain reported the failure of the aircraft's FMGC and autoflight systems to comply with the altitude and airspeed restrictions while conducting the FRDMM RNAV STAR to DCA.