Small Transport
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Commercial
Flight Crew Last 90 Days 200; Flight Crew Total 8000; Flight Crew Type 4000
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action
Human Factors
The SIC (Second in Command) and I thoroughly briefed the approach; RNAV GPS 22 approach to BHB. We were cleared to cross CARDA at 3000 and cleared for the approach. Autopilot was on. I selected Approach mode and do not recall if I selected VNAV. We dialed the altitude down to the Missed Approach Altitude. Between CARDA and BECRA; ATC called with cancelling instructions. The SIC had his head down writing the Instructions. I looked over to see what the SIC was doing.When the aircraft turned at BECRA; I compared the altitude to the chart and noticed immediately we were descending through 1700. I knew something wasn't right so I leveled the wings; arrested the descent and asked the SIC to help me sort this out. Initiated a climb and at that exact moment ATC asked our altitude. Our altitude should have been 2100. SIC called missed approach. We climbed to 3000 on a heading per ATC instructions. We reloaded the approach and came back to a successful approach and landing. We did not get any TAWS (Terrain Awareness and Warning System) alerts.
We had completed our descent checklist and approach checklist. We had a thorough briefing on what the RNAV 22 approach into BHB would be; including altitudes and tracks as the weather was 500 OVC (Overcast) and there would be a possibility of a go around. At 5000; ATC cleared us for the approach to include crossing CARDA at 3000. We had selected APPR (Approach Mode). We were established on the inbound leg to BECRA at 3000. At interception of the inbound course; ATC gave me IFR flight plan cancellation instructions. I dropped my head down to write the frequency and instructions.The PIC (Pilot in Command) got my attention to evaluate the situational awareness of the airplane. I noticed that we were slightly left of course and descending below 1700 MSL. The PIC commented; we are low. ATC called an altitude alert and I responded that we are going around. I saw GA on [the] Flight Guidance Panel; and requested to fly out on our current heading. ATC said climb to 3000; and level off. I requested radar vectors for the approach once we got established. We commented the approach with a successful outcome. At no time did we get TAWS (Terrain Awareness Warning System) alerts. I should have verified that the corrected flight mode for the VNAV profile was selected and that I should have told ATC to standby until we intercepted in the inbound course. I should have been there to help the PIC monitor the proper sequence of events on the approach.
Flight crew reported descending below charted altitude on an instrument approach to destination airport.