A319
Flight Crew Air Transport Pilot (ATP)
Confusion; Distraction; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Aircraft
On with Center into Las Vegas airport; we were descending via the Tyssn arrival and we were leveling off at a crossing restriction at 10;000 FT. At that time the aircraft went into ALT CONSTRAINT * as the bottom of the arrival with set for 8;000 FT. The aircraft activated the approach at that time as we passed a deceleration point and I reached up and selected 210 KTS for the next crossing restriction. When I looked back up the aircraft had reverted to VERTICAL SPEED from ALT CONSTRAINT and was descending below 10;000 FT. We were just prior to our 10;000 FT crossing point so I quickly selected 10;000 FT and returned us to our altitude I believe we had gotten down to 9;500 FT before the descent reversal. Upon reaching 10;000 FT Air Traffic Control cleared us down to 8;000 FT and we continued our flight with no disruption. After landing I talked to a couple of the instructors at the company and no one could figure out why the aircraft reverted to vertical speed. I guess we came to the conclusion that the aircraft Captured alt constraint star at the same time that it crossed the deceleration way point and this caused the aircraft to go into vertical speed.I guess we could have activated the approach our own to prevent this but I do not know if we would want to do that approaching a crossing restriction so we would have had to activate the approach sooner.
The reporter stated that the perfect storm of transitions apparently occurred at the 10;000 FT ALT CONSTRAINT * when the aircraft also began slowing for the 210 KTS restriction and simultaneously transitioned to Approach Activate. At that point; uncertain what its next action was; the FMGC transitioned to a VERT SPD default mode and began a descent to 8;000 FT. This confluence of events may be a design fault where the activation priorities are not clearly defined. This reporter's work around is to select Managed Speed prior to the deceleration point so that the automatic speed transition is not allowed.
An A319 flying the TYSSN RNAV STAR was leveling at 10;000 FT ALT CONSTRAINT * for SUZSI and began slowing to 210 KTS. As the 8;000 FT PRINO constraint was selected; the FMGC transitioned to VERT SPD and began a descent which the crew arrested.