Date: 201809
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A320
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: FMS/FMC
Component problem: Improperly Operated; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Troubleshooting
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: Cleared and descending via [the arrival] landing north. Approaching turn [the] aircraft was tracking lateral course normally. Pre extended final approach and noticed route dropping out of FMS. Selected a parallel heading for the downwind while rebuilding arrival. After; engaged auto flight to intercept track for the downwind. Instead of a normal intercept; auto flight commanded at least a 60 degree intercept. While flying through published track and starting a correction; ATC queried. While attempting to become established on downwind track; ATC called out an altitude alert. We advised we were having issues with the FMS and were in VMC conditions with terrain in sight below us. ATC directed a heading eventually to base. All recovery actions were accomplished in VMC. Airport visually acquired and final approach and landing accomplished.


Second narrative: Upon descent into ZZZ on the [arrival]; we were descending VIA and had passed 12000 feet and were rounding the corner to the downwind and were issued a new altitude to descend to. I can't recall which ALT; possibly 6000 feet or 9000 feet but whatever the altitude; it was the correct one. Around the turn in the STAR is when the captain inadvertently deleted the waypoints on the downwind segment of the STAR. He was trying to add points to the approach segment I believe and then I heard; 'oops' or something to that effect. We continued the turn in HDG mode towards ZZZZZ Intersection while I manually rebuilt the arrival. I entered in direct ZZZZZ1 Intersection and then rebuilt ZZZZZ Intersection behind it as the A320 navigates FROM not TO and we needed to be on the correct 164-degree course. We armed the NAV and the aircraft took an almost 90-degree heading to intercept the course. So the aircraft was east of course headed south and then took up the unrealistic intercept pointing the aircraft West. We executed a manual turn south towards ZZZZZ1 Intersection and around that time ATC called to query. We advised her of an 'issue with the FMS'. We had previously been cleared to descend to lower altitude and ATC told us she was getting a low altitude warning and advised us we could climb a thousand feet if we needed to. We declined. This was puzzling since we were VMC; the EGPWS terrain on the NAV DISPLAY was all green and it was clear visually to both myself and the captain that we were in no danger of terrain. She gave us a base turn heading and we executed the visual approach to Runway 34L without incident.To be absolutely clear: while the aircraft was in a [unintended state] due to the lateral navigation error and the ATC-perceived altitude deviation it was absolutely never in any danger of hitting terrain either from visual cues or aircraft warning systems.


Synopsis: A320 flight crew reported FMC issues resulted in an unstabilized approach and low altitude alert in visual conditions.