A320
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 25700; Flight Crew Type 6900
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Human Factors
Due to moderate to severe turbulence at higher altitudes; this flight was flown at FL240. We were cleared to descend via the EAGUL5 arrival into PHX; at which time the final altitude of the arrival was dialed into the window of the autopilot. Perhaps because we were starting at a lower altitude than usual for this arrival; the FMGC had eliminated the FL240 restriction at PAYSO. It was initially confusing; because the speed restriction (270) was not eliminated. I have recently had many issues with altitude restrictions not displaying properly in the new FMGC software (no ring displayed; particularly the red ring); and have attributed it to the software upgrade; though there has been no consistency from aircraft to aircraft with this anomaly. At any rate; and prior to PAYSO; the aircraft began a descent out of FL240 when it should not have. Rather than climb back up; I called ATC and got a clearance to FL220 so we could let the FMGC continue (I wanted to see what direction it would follow). Though below the 'dot'; it continued down; passing FL230 when the next fix constraint was FL180-230. The descent dot showed the box to know that it was below profile the entire time. I did not let the box descend below FL220 until passing PAYSO; at which point it did continue to descend below the dot; but remained within the follow on ALT constraint at the next fix. There were no other aircraft in the area; and separation was not a factor. I am going to play with lower altitude STAR intercepts to determine what the FMGC does to handle the intercept constraints which are not below the initial STAR intercept altitude.
The reporter believes; after thinking back on the event; that a managed descent was initiated prior to reaching PAYSO.
A320 Captain cruising at FL240 and cleared to descend via the EAGLE5 RNAV arrival to PHX reports his aircraft descending of its' own volition prior to PAYSO with a minimum altitude of FL240.