Super King Air 300
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Confusion; Distraction; Situational Awareness; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Flight Crew Returned To Clearance
Human Factors
While on RNAV RWY 4 approach into UIN; between the initial fix (JISNA) and the FAF (WUSIS); we descended below the minimum altitude prescribed for that segment of the approach; which was 2100 ft. MSL . I did not recognize that we were below until about 1600 ft. MSL and by the time I had corrected our pitch and added power we between 1500 ft. and 1600 ft. MSL. We immediately climbed back up to 2100'; and at about 2 miles prior to the FAF; we re-evaluated our position; flight guidance panel and configuration and felt that we could resume the approach as normal; which was an autopilot coupled LPV approach. Part of the cause of this issue was my set up of the flight guidance panel. We had been descending from about 7000 ft. and navigating direct to the initial fix of JISNA. At some point in the descent; ATC cleared us for the approach and told us to maintain 2700 ft. until JISNA. We were given the frequency change from Center to CTAF at this point as well. At that point; I thought I was in VNAV mode and descending. I hit APPR (Approach); since we were cleared for the approach; and dialed my altitude selector to the bottom of the approach; which was 960 ft. MSL for the minimums. My assumption was that we would then VNAV to 2700 ft. at JISNA; or essentially just VNAV to 2100 ft. by WUSIS; grab the glideslope and continue the descent. I felt I had set it up so that all stepdown altitudes would be met. At that point; I may have become distracted with other radio calls and prepping for landing. During this time; the SIC (Second In Command) was running checklists; trying to reach the FBO  on unicom; and also conferring with ATC to ensure we had a good way to cancel IFR  once we were on the ground. Neither of us were totally focused on the task at hand; which was to monitor the plane as we were beginning this approach. I estimate we were doing about an 800 fpm descent rate. We may have been in VNAV mode; but I was not coupled to the glide path; so we were descending in such a way that would not allow the autopilot to level us off at each fix. 1. PIC/SIC should have been totally focused on the primary task at hand: entering; establishing; and flying the approach as published. This would have caught any errors in my programming of the descent and flight guidance panel before we descended below published altitude. 2. Both pilots should be looking at the flight guidance panel in these critical times to verify proper mode; and calling out altitudes around these fixes. 3. Once the altitude deviation was realized; I should have climbed up; executed a missed approach; and called ATC to vector us back around for the approach. 4. My decision not to go missed was influenced by my desire to keep my passenger's schedule. That is a hazardous attitude that I want others to be aware of and learn from.
As our passengers were boarding the plane; they decided to change the destination to UIN due to the fact we were unlikely to be able to get into ZZZ due to low ceilings. The altitude deviation occurred on the RNAV (Area Navigation) approach to RW 4 at UIN. We had been cleared for the approach and we were to instructed to maintain 2700 ft. until established on the approach and had been approved to change to advisory frequency. I was the nonflying pilot and was busy verifying everything on the checklist had been completed and communicating with KC center and UNICOM at UIN to make position reports. I also verified with KC center that we would be able to contact them on the ground using that frequency or did we need to cancel our IFR through flight service. This took a little extra time to complete due to my lack of experience with the Proline Fusion avionics so I was not monitoring the PFD as closely as I should have. We were inside JISNA (IAF)and I was trying to determine our position from the airport in order to give an accurate position report when the pilot flying said something to the effect of wait why are we low. He immediately added power selected vertical speed up and set our altitude selector to 2100 ft. I was focused on other tasks in the cockpit so I cannot say with 100% certainty what autopilot modes were selected previously. We were somewhere between 1600 ft. and 1500 ft. MSL when he started the recovery. We reached 2100 ft. two miles prior to the WUSIS (FAF) intercepted the glide path and landed without further issue. During the debrief with my captain after the flight we discussed the need to treat such a critical phase of flight with the utmost respect and after getting established on the approach to keep all attention on the task at hand which is flying/monitoring the approach. We also discussed the need to make a verbal call out of all autopilot/ flight director mode changes to make sure both pilots are on the same page and are totally aware of what the automation is doing if it is engaged.
BE-300 flight crew descended below minimum altitude on approach.