Date: 201707
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Daylight
Aircraft: Regional Jet 700 ER/LR (CRJ700)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Traffic Collision Avoidance System (TCAS)
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Anomaly: ATC Issue All Types; Conflict Airborne Conflict
Detector: Automation Aircraft TA; Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Aircraft; Airspace Structure; Human Factors
Primary problem: Ambiguous

Narrative: Level at FL 120 and slowing to 250 knots per Approach request when amber solid block TCAS traffic appeared on display 11 -12 o'clock at approx. 5 NM range and 500 feet of separation.  Verbalized traffic to FO (First Officer) to ensure he was aware that it had popped up on TCAS. Both FO and I visually scanned for traffic.  Soon thereafter TCAS 'TRAFFIC; TRAFFIC' aural. TCAS RA commanded 'CLIMB; CLIMB'. At some time here I noticed only 400 feet of separation now displayed; indicating conflicting traffic possibly in a climb. After initiating climb TCAS quickly thereafter commanded 'INCREASE CLIMB'. I pitched to the top of the VSI scale which had the small green block at the very top of the scale; with the remainder in red. Very soon after achieving pitch at top of VSI; the TCAS commanded 'DESCEND; DESCEND'. This was a big surprise. Had the other aircraft climbed also? Was there another aircraft above us now?  As I pitched down I asked the FO if he had the aircraft visually. I believe he may have said no.  I scanned the VSI and all I could interpret from a brief look was Red outline. Where was the green?  I looked at the MFD and the target was close but still to our left. To aid the rate of descent and provide necessary vertical separation in a deteriorating situation I elected to turn to the right. As I started the turn I caught sight of the high-wing small aircraft at 10-11 o'clock and below our altitude so I stopped descending. A second or two later ATC issued us a 'right turn heading 270 deg.' clearance so I kept the airplane turning to that heading while maintaining vertical separation. When I queried Approach after the event they confirmed traffic was VFR.  At no time were we issued a traffic advisory for this traffic prior to the event.  A minute or so after event [the small aircraft] contacted Approach in a manner that suggested that they may have already established radio contact. '[Approach] is everything OK now?' or words to that effect. Approach just replied 'Yes'.  The same controller asked us if we had responded to a TCAS alert. Within a few minutes this male controller's voice was replaced by a female controller for the remainder of our transition to Tower control for landing. If possible I would like to get feedback of any further information that may be available to help explain the TCAS commanding a climb and then a descent.




Synopsis: CRJ-700 Captain at 12;000 feet experienced a TCAS RA with a small aircraft 500 feet below. The TCAS commanded a Climb Climb then an 'Increase climb'; followed quickly by 'Descend; Descend.'  In the end it appeared that the light aircraft had never changed altitude.