Narrative:

I was the pilot in command (PIC) and pilot flying (PF) for aircraft X. This was the last scheduled flight for myself and my first officer (first officer). We had been scheduled to swap aircraft by leaving the one in which we had been operating all day aircraft Y with the present aircraft X. This was our first flight of the day in aircraft X. We completed a thorough preflight upon aircraft acceptance and discovered that the previous crew neglected to note a severely bald tire when they had accepted the aircraft from a 100 hour inspection. This caused us concern; as we were unsure how thorough the acceptance preflight had been.weather was marginal in the area throughout the day; but legal at the time of departure and the intended time of arrival. As we approached ZZZ; however; visibility on the AWOS dropped precipitously. Upon our arrival; weather was below the required minimums to initiate an approach. I obtained a holding clearance and held at 9000 feet over the ZZZ VOR; as published. After approximately 30 minutes; a jet aircraft departed from ZZZ and reported that conditions were still deteriorating on the field.we prepared to initiate a diversion to our filed alternate. As we began to set up for this maneuver and before we requested clearance; I noted that that the lubber line of the needle of my HSI was fluctuating back and forth rapidly. I had my first officer confirm this observation. I confirmed that the instrument was set to 'GPS' on the garmin 530 and that a distant 'direct to' point was selected. I also had my first officer confirm this. The number 2 VOR omni was set identically and not having the same faulty indications. We discussed the situation and determined together that the HSI instrument could not be relied upon safely.because we were in actual IMC and on an instrument flight plan at this time; I directed my first officer to notify center of the loss of instrumentation; per far 91.183(c) and 91.187. Before discovering this loss of instrumentation; my copilot and I noted conditions at surrounding airports had deteriorated to low visibilities and that conditions were optimal for icing (+/- 5 degrees and periods of visible moisture). In fact; during the hold; we had developed approximately ?? Inch of rime ice on the wings. Although I knew it was warmer below us; we had slowly been adding power to maintain a constant airspeed. Based on the information available from our onboard weather; ZZZ1 was reporting 7-mile visibility and was close. All other reasonably nearby airports were now reporting?? Mile visibility or below. I instructed my first officer to simultaneously [advise ATC of our situation] based upon the need for urgency and to notify ATC that we intended to now land at ZZZ1. Factors were combining to produce a situation that was not immediately dangerous but which could result in a catastrophic event if not properly addressed.we proceeded to ZZZ1 as vectored for the RNAV (GPS) runway 25; which we could still execute on our number 2 VOR (which is connected to an instrument-approved garmin 430). The number 2 VOR omni has a glideslope as well. Although I removed the HSI from my scan; I did direct my first officer to activate the localizer on that instrument so I could determine if that part of it was working. At the same time; I had him pull the RNAV (GPS) runway 25 on the garmin 530 screen for situational awareness. We confirmed more faulty indications on the HSI when set to 'vloc' and tuned into the localizer. The HSI directed us far left of the RNAV (GPS) 25 centerline and significantly short of the actual runway and RNAV (GPS) 25 glideslope on VOR number 2. It is also possible that the HSI directional compass card was processing; even though it is an electrical instrument. At any rate; the HSI was entirely not reliable and classically misleading so it had to be faulty.we were able to fully and safely execute the RNAV (GPS) runway 25 approach on the VOR number 2 indications; substituting the secondin command's directional gyro for the HSI altogether; utilizing the 430 and 530 screens; and working together with effective CRM. Together; we were capable of completing the flight from the discovery of the problem to ZZZ1 without violating ATC assigned altitudes or limits.we notified ATC of our safe arrival; asked them for a contact number so we could reach them after this event and taxied to the ramp. After shutdown; we immediately contacted our operations and began making alternate travel arrangements for our passenger; who was unaware of the circumstances surrounding incident.as after action; I continue to press our ops department not to dispatch new aircraft to crews on the last flight of the day. While my crew and I preflighted the aircraft as good as possible; it is not possible to locate all faults prior to flight. Fatigue from being on the completion end of a [long] duty period as well as constant operations by my crew and I over the last several days also played a clear factor. Icing; although manageable; further complicated the options on the table. In retrospect; this was probably handled to the best capability of the crew under the operational circumstances present.

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Original NASA ASRS Text

Title: C208 pilot reported a loss of the HSI in IMC conditions resulting in a diversion.

Narrative: I was the Pilot in Command (PIC) and Pilot Flying (PF) for Aircraft X. This was the last scheduled flight for myself and my First Officer (FO). We had been scheduled to swap aircraft by leaving the one in which we had been operating all day Aircraft Y with the present Aircraft X. This was our first flight of the day in Aircraft X. We completed a thorough preflight upon aircraft acceptance and discovered that the previous crew neglected to note a severely bald tire when they had accepted the aircraft from a 100 hour inspection. This caused us concern; as we were unsure how thorough the acceptance preflight had been.Weather was marginal in the area throughout the day; but legal at the time of departure and the intended time of arrival. As we approached ZZZ; however; visibility on the AWOS dropped precipitously. Upon our arrival; weather was below the required minimums to initiate an approach. I obtained a holding clearance and held at 9000 feet over the ZZZ VOR; as published. After approximately 30 minutes; a jet aircraft departed from ZZZ and reported that conditions were still deteriorating on the field.We prepared to initiate a diversion to our filed alternate. As we began to set up for this maneuver and before we requested clearance; I noted that that the lubber line of the needle of my HSI was fluctuating back and forth rapidly. I had my First Officer confirm this observation. I confirmed that the instrument was set to 'GPS' on the Garmin 530 and that a distant 'direct to' point was selected. I also had my First Officer confirm this. The Number 2 VOR omni was set identically and not having the same faulty indications. We discussed the situation and determined together that the HSI instrument could not be relied upon safely.Because we were in actual IMC and on an instrument flight plan at this time; I directed my First Officer to notify Center of the loss of instrumentation; per FAR 91.183(c) and 91.187. Before discovering this loss of instrumentation; my copilot and I noted conditions at surrounding airports had deteriorated to low visibilities and that conditions were optimal for icing (+/- 5 degrees and periods of visible moisture). In fact; during the hold; we had developed approximately ?? inch of rime ice on the wings. Although I knew it was warmer below us; we had slowly been adding power to maintain a constant airspeed. Based on the information available from our onboard weather; ZZZ1 was reporting 7-mile visibility and was close. All other reasonably nearby airports were now reporting?? Mile visibility or below. I instructed my First Officer to simultaneously [advise ATC of our situation] based upon the need for urgency and to notify ATC that we intended to now land at ZZZ1. Factors were combining to produce a situation that was not immediately dangerous but which could result in a catastrophic event if not properly addressed.We proceeded to ZZZ1 as vectored for the RNAV (GPS) Runway 25; which we could still execute on our Number 2 VOR (which is connected to an instrument-approved Garmin 430). The Number 2 VOR omni has a glideslope as well. Although I removed the HSI from my scan; I did direct my FO to activate the localizer on that instrument so I could determine if that part of it was working. At the same time; I had him pull the RNAV (GPS) Runway 25 on the Garmin 530 screen for situational awareness. We confirmed more faulty indications on the HSI when set to 'VLOC' and tuned into the localizer. The HSI directed us far left of the RNAV (GPS) 25 centerline and significantly short of the actual runway and RNAV (GPS) 25 glideslope on VOR Number 2. It is also possible that the HSI directional compass card was processing; even though it is an electrical instrument. At any rate; the HSI was entirely not reliable and classically misleading so it had to be faulty.We were able to fully and safely execute the RNAV (GPS) Runway 25 approach on the VOR Number 2 indications; substituting the Secondin Command's directional gyro for the HSI altogether; utilizing the 430 and 530 screens; and working together with effective CRM. Together; we were capable of completing the flight from the discovery of the problem to ZZZ1 without violating ATC assigned altitudes or limits.We notified ATC of our safe arrival; asked them for a contact number so we could reach them after this event and taxied to the ramp. After shutdown; we immediately contacted our operations and began making alternate travel arrangements for our passenger; who was unaware of the circumstances surrounding incident.As after action; I continue to press our ops department not to dispatch new aircraft to crews on the last flight of the day. While my crew and I preflighted the aircraft as good as possible; it is not possible to locate all faults prior to flight. Fatigue from being on the completion end of a [long] duty period as well as constant operations by my crew and I over the last several days also played a clear factor. Icing; although manageable; further complicated the options on the table. In retrospect; this was probably handled to the best capability of the crew under the operational circumstances present.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.