A319
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Training / Qualification; Confusion
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Chart Or Publication; Procedure; Human Factors
Cruising at FL240 on the Ivane5. ATC cleared us to descend via the IVANE RNAV STAR. Looked at the next fix Mayos; and there was not a restriction set in the FMS. Set 6;000 Ft in altitude alert window. Began to pick up a little chop; and I said to First Officer (FO): I think I'll take the descent now. Engaged managed descent; trusting the FMS; and FO said 'I'll let him know.' So he called ATC out of FL240; and he said; 'You have to cross Mayos at FL240.' He then asked if we could get cleared down to FL220; and ATC granted this request with descend to FL220 and then descend via the Ivane5.' We had started down; but were about 200 Ft below FL240; or 23;800 Ft when we intervened and leveled off.Because we were at a lower cruising altitude; FL240; I think that the FMS didn't accurately sense a crossing restriction at the next intersection on the arrival; and therefore ACCEPTED my input of a managed descent. There was not a window altitude at Mayos this time. This would have been contrary to the star which requires us to cross Mayos between FL260 and FL240. Certainly updating the software to retain crossing restrictions; and to honor them in managed descent would be nice; regardless if already at cruising altitude. Now I must check the arrival's Jepp page when initially beginning a descent; and then continuously check it during a descent against the glass. I remember when I first came over to the airbus from the 737; and the check airman said; 'The glass never lies.' In this case; I'm finding out more and more; that the glass does indeed lie.
While level at FL240 prior to MAYOS on the IVANE RNAV STAR to CLT the flight crew of an A319 opted to initiate their previously received 'descend via' clearance; set their 6000 MSL bottom altitude in the altitude alert and selected managed descent. As directed; the Flight Management Guidance Computer (FMGC) initiated the descent despite not yet having complied with the minimum FL240 altitude at MAYOS.