Date: 201812
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: Super King Air 300
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Final Approach
ATC / advisory: TRACON ZZZ
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor
Human factors: Situational Awareness; Workload; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Procedure; Human Factors; Weather
Primary problem: Human Factors

Narrative: We were doing a reposition flight with no passengers from ZZZ1 to ZZZ. Prior to this flight I was mainly focused on our next leg which was supposed to be from ZZZ to ZZZ2 but was switched last minute to ZZZ3 due to weather. I was in constant communication with Dispatch while preflighting as ZZZ2 and ZZZ3 were both siting right around approach minimums and there were complications arranging the guests' ground transportation as this was a very busy travel day. My focus was definitely on the second flight rather than the 10 min repo to ZZZ. I had checked the weather earlier that day for ZZZ and saw VFR all day however I did not check the METAR there before I took off. There wasn't a cloud in the sky at ZZZ1 and they were departing north. Because of that; I made the mistake of assuming that we would be getting a visual to one of the north runways at ZZZ. We got the ZZZ ATIS as soon as possible and were very surprised to hear that it was reporting IFR at ZZZ. I could actually see the runway while being vectored. There was a strange cloud bank at ZZZ that was sitting over only the north half of the airport but the approach end and area where we were being vector was completely VMC. We checked on to the ZZZ Approach frequency handling planes on final and were told 'Fly heading 090; vectors for the ILS Runway XXL.' My SIC who was Pilot Monitoring loaded up the approach in the FMS and selected 'vectors' as the Initial Approach Fix. This put ZZZZZ (the final app fix) and ZZZZZ1 (one fix further out) into our FMS. As we were completing the In Range Checklist and about to start briefing the approach; we were given a poor vector to intercept final. It was such a sharp turn that I had to kick off the autopilot to avoid drastically over shooting final; potentially into traffic on the parallel ILS for XXR. Right about the time that we intercepted the localizer; ATC instructed us to 'Maintain 10;000 until ZZZZZ2 and cleared for the ILS Runway XXL'. At that point; we realized we didn't have ZZZZZ2 in the FMS as it was outside ZZZZZ1 further back on the final approach course. Shortly after; I saw the FMS' descent profile telling me to descend in order to be at ZZZZZ1 at 9;000 per the approach chart. In my rushed and slightly confused state; I mistook it this FMS reading for the actual glideslope and started descending. I knew we were cleared on the approach and I assumed we had already passed ZZZZZ2. As we approached about 9;500 MSL; ATC called and said 'Your clearance was to maintain 10;000 until ZZZZZ2; I'm showing you at 9500'. My SIC responded 'Correcting' and I brought the plane back up to 10;000 feet. Shortly after we intercepted the actual glideslope and landed. We were completely VMC throughout the entire approach. After landing; I spoke with ATC via phone and he informed that there was a regional jet below us at 9;000 feet that had to be vectored because we broke the required separation for an ILS approach. We never received a traffic alert or even noticed him on our TCAS display. The controller mentioned to me that if we were on the visual rather than ILS; it would not have been a factor. I think there were several factors that led to this event. First and foremost; I did not prepare for this flight as much as I should have. I was much more focused on the difficult second leg with passengers to ZZZ3. In my mind; I thought this was going to be an easy 10 minute flight with a visual approach into ZZZ; something I had done dozens of times at my previous job. I was not in the proper mindset for a true ILS approach. The fact that we were VMC and I had the runway in sight definitely added to this. Second; it was such a short flight that we really didn't have time to properly brief the approach. I think about a minute and half passed between the time we were told to expect the ILS XXL and when we were getting a vector to intercept the localizer. Third; it was a poor vector to intercept and required me to start hand flying which definitely took away from situational awareness of where we were on the approach. Fourth; there was a breakdown in our usual crew duties of Pilot Flying and Pilot Monitoring. The SIC (Pilot Monitoring) was very overloaded with work doing the comms and loading the FMS and I thought it would be helpful if I called the FBO to let them know we were 5 minutes out. I made this call right about the time we got the vector to intercept which was foolish and unnecessary. I also did a poor job communicating with the SIC and did not announce that I was leaving 10;000 which is standard protocol. Lastly; I think it would have been helpful for the controller to say vectors to ZZZZZ2 for the ILS rather than just vectors to the ILS. This would have made it easier to properly program in the FMS in a very short period of time. Going forward; I will make sure I am completely focused on the current flight; no matter how short and seemingly routine. I will also strive to always maintain the proper Pilot Flying and Pilot Monitoring roles and to strictly stick to SOPs no matter how work saturated I become. This has been a major learning moment for me to understand that you can never become complacent in the cockpit; especially in an IFR environment.




Synopsis: King Air Captain reported a loss of separation due to an altitude excursion and not being completely prepared for the short flight with unexpected weather conditions.