A320
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion; Distraction; Human-Machine Interface; Situational Awareness; Workload
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew FLC Overrode Automation
Aircraft; Chart Or Publication; Human Factors; Procedure
This event occurred during descent on the SUNST RNAV arrival to LAS. I was pilot flying and the Captain the pilot not flying. We had been cleared and descended to cross FUZZY at 16;000 FT and 250 KTS when I noticed; several miles outside of IPUMY; that we hadn't heard from ATC in a while and were rapidly becoming too high to comply with the 11;000 FT and 230 KTS crossing at IPUMY (we still had not yet been given the 'descend via' clearance). When we received no response to our inquiry to ATC we went back to the last assigned frequency and were told to switch to another frequency. When the Captain checked in and advised of our altitude we were cleared to descend to 10;000 FT and to delete the restriction at IPUMY. I selected open descent but; before we leveled off at 10;000 FT; ATC cleared us to 'descend via' the SUNST RNAV arrival. I selected 8;000 FT in the altitude but forgot to push 'altitude' for a managed descent. I announced the next crossing; which was NIPZO at 9;000 FT; but neither of us realized we were still in Open descent. Shortly after passing 9;000 FT we received traffic advisory and both of us focused on this. The traffic was 2;500 FT below us; but this also contributed to our not noticing the altitude deviation. We realized we had gone approximately 500 FT below the NIPZO crossing when ATC advised us. I deselected the autopilot and climbed back up to 9;000 FT and no other disruptions occurred. Several factors led to this event: the confusion and distraction from the lost communications; receiving a non-standard/out-of-the-ordinary clearance to descend to 10;000 FT rather than to 'descend via the SUNST RNAV arrival;' and being distracted with a traffic advisory; all which took place during a high workload environment.To prevent this from happening again; I will be more alert and prepared to confirm automation when a change in descent clearance is given other than what we are used to receive in a multiple crossing RNAV arrival. I would also be quicker to query ATC when I notice approaching our next descent point rather than waiting to query after well crossing it and being further behind and being put in a catch up mode.
[No substantive additional information was provided by the secondary narrative.]
A temporary loss of communication with ATC and non standard clearances while flying the SUNST RNAV STAR to LAS resulted in two deviations from charted crossing restrictions for the reporting A320 flight crew.