Date: 202109
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: Marginal
Weather elements / visibility: Haze / Smoke; 3
Light: Daylight
Aircraft: Citationjet (C525/C526) - CJ I / II / III / IV
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Final Approach
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Fatigue; Situational Awareness
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was the pilot flying on this empty leg; Part 135 repositioning flight to ZZZ.  I was flying with a XX-year Part 121 pilot who was a recent upgrade and very new to Part 135 flying.  We were on our second week-long rotation together and had been working exceptionally well together on all our previous flights.  Because I had been to ZZZ before; i was pointing out all the important details of the area and what I knew of the airport and local ops prior to descending for the approach.  On this morning; we arrived over the ZZZ area to find a thick morning fog/haze layer over the entire area.  We were in hopes of visually acquiring the airport and making a visual approach; but were unable to see the airport until just before passing overhead; so we elected to shoot the RNAV (GPS) XX.  We had been cleared down to 10;000 feet or 11;000 feet before requesting the RNAV approach; but were instructed to climb and maintain 12;000 feet for the approach.  We were kept at 12;000 feet until crossing ZZZZZ intersection (the IAF).  Twelve thousand feet was significantly higher than the 10;600 feet the procedure calls for prior to ZZZZZ (from that direction) and the 7;500 foot minimum on the leg immediately to follow.  The MSA in the area is 12;400 feet; however.  We initiated a 2;000 FPM descent from 12;000 feet to get established.  I believe we crossed ZZZZZ1 at (or near) the published 7;500 feet. Because we picked up the field visually when crossing overhead and I knew the area (once established inbound) was flat and free of obstructions; coupled with the relatively good visibility near ZZZZZ2 intersection (GS intercept at 6;800 feet); I suppose I transitioned (erroneously) into a 'visual approach; backed up with the RNAV' mentality and; as a result; continued the steeper descent beyond ZZZZZ2 and wound up at least 500 feet low.  I believe the early morning hour; featureless terrain; relatively good (though reduced) visibility and knowledge that the mountainous areas were now behind us lulled me into a more relaxed; visual approach mindset.  The PM was heads-down and neither of us recognized how low we really were until we got an altitude alert from the aircraft.  At that point; I leveled off; recognized the fact we were just under 1;000 feet AGL at approximately 4 NM from the field climbed to correct the deficiency and elected to continue the approach; as we were now re-established with the field clearly in sight.  We immediately called the field and continued visually.  We waited until we were parked at the FBO before debriefing the flight.




Synopsis: Air Taxi First Officer reported not recognizing they had descended below final approach altitude. The flight crew returned to the correct altitude and continuing to landing.