Date: 201504
Local time of day: 0601-1200
Location: ZAU.ARTCC, IL
Flight conditions: IMC
Weather elements / visibility: Cloudy
Light: Daylight
Aircraft: A319
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZAU
ATC / advisory: Center ZAU
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Confusion; Communication Breakdown; Workload
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Returned To Clearance
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

Narrative: We were giving a descent clearance to FL240 by ATC. The captain then heard back from the Flight Attendant to go back and use the lavatory.  Shortly after; Chicago center cleared me direct to PRISE intersection for the WATSN Two RNAV arrival and descend to; I thought; FL220 even though we were on the VEECK One arrival.  At the same time the FA who had entered the cockpit was asking me a question.  I tried to read back the clearance but got it wrong because I was unfamiliar with the new arrival and at the same time tried to get a read back of the altitude.  I don't remember clearly but I think I read back FL220.  I also tried to get a heading while I figured out what the new fix and arrival was.  It took a while but I got a heading while I searched for the arrival. This was happening while the Captain was buzzing to reenter the cockpit. I dialed in the new heading and then talked to the captain to let him back into the cockpit.  As he came in and I was passing through some altitude below FL240 center said 'I'm not sure what you're doing but descend to FL220.'  I then looked up and saw 310 set in the alert window.  I then redialed down to FL220.  When I tried again to clarify our new RNAV arrival; center cleared us back onto our original fix and on the original arrival. I then thought about it as we made the arrival and wasn't sure if something had happened or not.  Looking back I think I might have moved the altitude dial when I meant to move the heading bug.  Also I am not sure about the altitude given by ATC.  On the ground I let the captain know of my concerns and suggestion to write an ASAP.I should have not moved the altitude alerter until ATC gave me a more affirmative answer to my query about my read back. When the captain called to reenter the cockpit I should have momentarily ignored it while stopping my decent to clarify the altitude clearance. ATC should have given me a heads up of a clearance change before giving me the new clearance.  It was too much information to all of a sudden be given the new arrival; the new fix; and new altitude at the same time. It should be easier to clarify clearances.  I've noticed often times when I make a query it takes a while for ATC to reply back if at all.


Second narrative: After block in at the gate in ORD the First Officer (FO) indicated that he was unsure if the descent clearance on our arrival was handled properly during my absence from the flight deck on a lavatory break. We had started the descent from cruise altitude when I left the flight deck. Upon returning from the short lavatory break the FO was acknowledging a new clearance to resume the cleared arrival and descend to FL220. This was accomplished and we proceeded to the airport uneventfully. After blocking in the FO was unsure if he had improperly descended and indicated to me his recollection of events during my absence. I do not know if the clearance was followed incorrectly. Accomplish physiological breaks prior to initiating descent.


Synopsis: A319 First Officer reports being assigned direct to a fix on a new arrival and a descent clearance as the Captain is leaving the cockpit for a lavatory break. A request for a confirmation of the altitude assignment and a heading to the new fix is made. During the confusion; the altitude knob on the FCU may have been turned instead of the heading knob resulting in track and altitude deviations. ATC issues a new altitude assignment and back to the previous arrival as the Captain is returning to the cockpit.