Date: 201606
Local time of day: 1801-2400
Location: DFW.Airport, TX
Flight conditions: Mixed
Weather elements / visibility: Thunderstorm
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ZFW
ATC / advisory: Center ZFW
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: While being vectored around an area of thunderstorms ATC assigned a heading which put us directly in the path of a large cell. As we approached within 20 NM of the cell we advised ATC that we would need to turn toward the arrival track. The Airbus at altitude was making [a] slow 15 degree turn. I was approaching the cell so turned off autopilot to increase the bank angle to properly avoid the weather. ATC assigned us a lower altitude and another heading and then said to level at 17000 ft make a left 360 for traffic spacing. They quickly gave us a number of instructions including a 180 degree turn with a 10 mile leg and to increase speed to 280. We talked about creating a present position holding pattern to comply with ATCs instructions. ATC's continuous change of plans during all of this was distracting. As we were briefing the arrival and runway path again because they had changed us from 36L to 35C; ATC was continuing to interrupt us with more instructions. It was clear that the corridor was a mess with weather being so close to DFW. With the changes to speed; heading and altitude I was the pilot flying and making the changes to the flight mode panel. I thought I had reengaged the autopilot; but we were both unaware that it was still disconnected. Once cleared to VIKRY with a solid green line we assumed the aircraft would go there. It didn't. We both realized something was wrong. The AC was level near FL190 and 250 Kts and we were in a gentle turn away from where ATC had cleared us. The AC was not responding to my inputs to the Flight Mode Panel. We both said it felt like an Airbus moment... 'what's it doing now.' We then realized the autopilot was off and reengaged it. ATC then made a comment about being off heading and said to hold present heading and they would have to resequence us. There was never a known or reported conflict with another AC. We complied with all clearances and were resequenced back in line to DFW. We landed on 35C without incident.Fly The Airplane. When distractions are higher than normal extra diligence is need. Brief color yellow so the pilot monitoring is more aware of distractions.


Second narrative: Fly the plane! We should have more clearly delineated who was flying and who was doing the rest. Pilot flying; pilot monitoring 'stuff.' Lesson learned.


Synopsis: A321 flight crew reported a track deviation resulted when distractions prevented them from noticing the autopilot was not engaged as they had assumed.