A319
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 80; Flight Crew Total 18000; Flight Crew Type 6000
Confusion; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Aircraft Equipment Problem Dissipated; Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Human Factors; Aircraft
We were initially cleared to FL330 by ATC. We requested to level off at FL320 due to winds and ride. We received clearance to maintain FL320 while in the climb at around FL323 and we started the decent by FL325. When we leveled off at FL320; the pilot flying (Captain) selected FL320 on the PROG page in the FMGC; in order to set the mode from CLB to CRZ and re-compute the fuel computations. After entering a new cruise ALT of FL320 vs FL340; the autothrottles pulled back to idle and speed bug unexpectedly dropped to 221 KTS. It did not make sense for the speed to drop this low; which was assumed to be near green dot. The speed selection knob was pulled and spun up; but nothing happened. (Note: It's possible the knob was not pulled sufficiently to force the automation to go from managed to selected mode). Nevertheless; the automation was not doing as expected. Since our airspeed was slowing rapidly; the autopilot/throttles were disengaged in an effort to manually control the unwanted speed reduction. The throttles were advanced into the FLX/MCT detent. Note: There was an estimated delay (8-12 seconds) spooling the engines back up from idle thrust. While interpreting the automation and assessing what is it doing and why; the main focus was on the speed tape. When the thrust started to respond; the aircraft made an unnoticed gradual climb; which was called out by the pilot not flying. Thus; we made an inadvertent altitude deviation of +400 FT before correcting back to our assigned altitude of FL320. ATC did query us but we were already correcting as he called 'check altitude'. The pilot not flying managed to get the speed reset set to 280 KTS-selected mode. After we were re-stabilized; the managed mode was re-selected and no further problems were encountered.
After entering a new cruise ALT of FL320 vs FL340 on the PROG Page; an A319 Captain reports the autothrottles pulled back to idle and speed bug unexpectedly dropped to 221 KTS. Initial attempt to bug the speed up has no effect and the autopilot and autothrust are disengaged to return the aircraft to normal speed and correct an altitude overshoot. Once stabilized the autothrust system functions normally.