PC-12 reported an inflight failure of the standby electrical bus; which resulted in multiple navigational and communications issues; necessitating an immediate diversion to a suitable airport.

Narrative

During an IFR leg ZZZ to ZZZ1 in Aircraft X being operated under a company callsign the aircraft experienced a failure of the standby bus. This failure occurred at 16;000 ft prompting the crew to notify ATC we would like to remain at our current altitude while we troubleshoot the issue. It was not immediately apparent what systems had failed even with the CAS (Crew Alert System) alert as the only immediately noticeable failure was the loss of display unit 3 and pilot to pilot communications; however after using our QRH on each item within the CAS alert system we quickly realized this failure was far larger then a DU3 (Display Unit) fail. By losing the standby bus we lost all equipment tied to it including (DU3; LH PFD (Primary Function Display) controller; LH audio control panel; GPS1; pilot to pilot communication; TSC and CCD (Cursor Control Device)). After completion of the QRH CAS related items the decision was made to [request priority handling] our primary reasons being the loss of DU 3; TSC and CCD this resulted in us being unable to use the aircraft FMS system; causing a cascade of issues including but not limited to (The inability to program approaches within the FMS; Inability to bring up our synoptic display; inability to program FMS routing). Additionally the combined loss of pilot to pilot communications whilst also being limited the only one operational audio control panel and PFD controller made effective use of CRM in this situation extremely difficult; lastly the knowledge that an IFR approach and landing was required to get into any airport within a reasonable distance. After [requesting priority handling] we began working with ATC to find an aircraft with suitable conditions and a ILS approach as we where limited to ground based NAV facilities that could be manually tuned with our NAV radios due to the loss of the FMS. ATC advised us that ZZZ2 had improving conditions and was equipped with a ILS approach; ATC began giving us vectors to the localizer course when the crew realized the ILS DME for ZZZ was inaccurate; luckily the approach was equipped with waypoints identifiable through the use of VOR radials. As a crew we then set up all frequencies and radials for the approach and briefed how it would be flown as well as received alternate missed instructions from ATC before being vectored onto the localizer course for runway XX at ZZZ2. We broke out of the cloud bases at 1;000ft MSL and continued to land on runway XX for ZZZ2.