Date: 201507
Local time of day: 1201-1800
Location: PHX.Airport, AZ
Flight conditions: VMC
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B PHX
ATC / advisory: TRACON P50; Tower PHX
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Commercial
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Conflict Airborne Conflict
Detector: Automation Aircraft Other Automation
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action
Contributing factors: Airport; Human Factors; Airspace Structure
Primary problem: Human Factors

Narrative: We were on the EAGUL6 Arrival into PHX. After approach control asked if we had the airport in sight; we said we did. I was pilot flying. We were given a heading to intercept the localizer for RWY 26 in PHX; and cleared for the approach. We were in a descent and at approximately 4100 FT; we got a TCAS RA; climb; climb. I immediately followed the RA command and climbed.  There was a solid red box showing aircraft below us was approximately 400 FT below us. The First Officer advised PHX tower we were responding to a TCAS RA command and will not be able to continue the approach. Tower vectored around for another approach; and an uneventful landing a second time around.After landing the First Officer and I called PHX TRACON; to find out why we did not get any traffic advisory from approach or tower about the aircraft that seemed to have been over FFZ. The TRACON supervisor informed us that their radar initially picked up the aircraft in question's [transponder] on radar; but it then disappeared. He stated that the aircraft popped up again just before we got the RA command and disappeared again from their radar shortly afterwards. The aircraft was not in contact with any ATC facility. The First Officer and I were startled by this and gave our contact information to the TRACON supervisor to give us any update they obtained about the aircraft as it seemed the aircraft was flying erratically and unsafely along the approach path of PHX airport.As FFZ is close to the approach path of the PHX RWY 26 approach; maintaining the recommended altitude as we transition from the arrival to RWY 26 approach is very vital. However; even while above the recommended altitude; more vigilance is necessary as certain VFR traffic that are not in contact with ATC facility might still constitute a flight safety hazard as it did with our flight.The best suggestion is vigilance and awareness by flight crews; and improved traffic alerts by ATC of any traffic conflict that might occur. Having TCAS and following RA commands is also essential.


Second narrative: Once at gate we talked to PHX TRACON. He was reviewing the tape while we were on the phone and the target popped up out of nowhere; showed altitude then zero altitude and was gone. Possible drone I thought but the flight attendant saw what she referred to as a white crop duster while we were in the climb.


Synopsis: A321 flight crew experiences a TCAS RA at 4;500 feet over FFZ during a visual approach to Runway 26 at PHX. The RA commands a climb that leads to a go-around. The intruding aircraft is never sighted by the crew.