Date: 202109
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Marginal
Weather elements / visibility: Haze / Smoke; 6
Light: Daylight
Aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Fractional
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Human-Machine Interface; Situational Awareness; Workload
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Overrode Automation
Contributing factors: Software and Automation; Weather; Procedure; Airspace Structure; Human Factors
Primary problem: Human Factors

Narrative: Center descended us to ZZZ in the hopes we would see the field; but the visibility was inadequate. We were then instructed to climb and received vectors for the GPS approach. The approach had been briefed and loaded into the FMS; but we weren't expecting to fly it.  Due to terrain; we were cleared for the approach from a high altitude requiring a steep descent to achieve the FAF altitude. We left center frequency and I began making position reports on CTAF.  I was heads up for most of it; and failed to realize the PF continued the high descent rate through the FAF altitude.  The altitude alerter was set to the approach minimums; and the flight director did command a level off; but the PM continued hand flying and disregarded the flight director.  I saw the ground throughout the entire approach; and felt we should have been close enough to see the runway and began to feel uneasy about how low we were with no runway in sight.  I looked at my PFD and realized the VPI was pegged full up.  I asked why we were so low; then I realized his mistake and directed him to level off.  We were about 800 AGL (5800 MSL); out side of the FAF which should have been crossed at 6800 MSL.  Then we saw the runway.  I called center back; cancelled IFR and we landed without further incident. Descending below a charted altitude on an approach is a big killer.  As a former sim instructor; check airman and ERC (Event Review Committee) ASAP rep; I've seen it happen several times to flight crews; and it almost always involves high workload and poor workload/automation management. I should have been a better PM by only taking occasional looks out the windshield; and backing him up inside. I shouldn't have set the approach mins in the altitude selector until level at the FAF altitude.  I am also new in type; and not as familiar with the automation as I needed to be.  The GPWS did verbalize a 'Caution Terrain'; but no 'Pull Up' command was received. What I feel worst about though is not commanding a missed approach; we had the gas for it.




Synopsis: Small transport Captain reported the pilot flying ignored the Flight Director command to level off and descended below the Final Approach Fix altitude prior to the fix resulting in a Ground Proximity alert.