Date: 201709
Local time of day: 1201-1800
Location: SDF.Airport, KY
Flight conditions: VMC
Light: Daylight
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class C SDF
ATC / advisory: Tower SDF
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Air Traffic Control; Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew FLC Overrode Automation
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: SDF was down to one runway; 35R for arrivals and departures. The First Officer (FO) wanted to practice for his upcoming training by accomplishing the RNAV35R approach. The tower gave us a vector from the southeast of the arrival course to the northeast to join the localizer. We opted to do an intercept leg to the course once established on the heading so that we could NAV to the inbound course and shoot the RNAV approach in VFR conditions. I entered the LOC frequency in the nav/rad so that we could monitor the ILS indications to make sure the RNAV approach in NAV/PROF would be within the ILS parameters. Once on the intercept heading; we were cleared for the approach; told to contact tower and slow to 170 or less. We were descending and once NAV was engaged we opted to select the Decision Altitude (DA) of 909 in the altitude window. We were configuring at this point and trying to go down and slow to 170 or less. Once established inbound; descending and trying to slow the FO mentioned that the descent rate was too high and that it was honoring the hard altitudes that we had entered on the RNAV and that matched the indications I was showing on the glideslope (GS) that were showing below GS. We let it go for another couple of seconds to analyze what we had done wrong and the FO decided it wasn't going to stop the descent and that he was going to click off the Auto Pilot and climb back up to the GS and just abort the RNAV which was the correct maneuver. As he started to climb we got a landing gear warning; the tower announced an altitude alert; the gear was in transit and the FO was climbing back up visually to the glideslope. We rejoined the glideslope; flew the visual approach with the ILS backup and were stable by 1000 feet. I believe the aircraft came out of prof or it didn't quite know where it was and was diving for the DA of 909 which we recognized and corrected. I called the tower upon leaving the airport and discussed what we were doing and that corrective action was taken and asked if they had any issue or paperwork or needed any information. They said it was all computer generated and was more of a cautionary advisory to us as it was VFR and they saw the corrective action in taking place.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Air carrier flight crew reported difficulty in executing an RNAV approach using aircraft systems automation which resulted in going below glideslope mid-approach at SDF.