Date: 201807
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Cloudy; 10
Light: Daylight
Aircraft: Learjet 45
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Aircraft; Weather; MEL; Human Factors
Primary problem: Human Factors

Narrative: This specific aircraft had a database problem with both UNS-1 FMS (Flight Management System) units. After a previous software update; it reset the date to a future date; and as a result; the FMS was unusable for RNAV/GPS approaches. A GPS approach could be loaded in the FMS; but the message 'No RAIM at FAF (Final Approach Fix)' would display; and all guidance would be lost after crossing the FAF on an IAP (Instrument Approach Procedures).As a result; the company issued an email explaining the problem; and stating that GPS approaches could not be flown; as there would be no guidance after the FAF; and that ground-based approaches were to be used until the problem could be solved; as per manufacturer's instructions on the original squawk.The gist of it was this: The FMS could guide you on a RNAV/GPS approach up to the FAF; but after that point; all lateral and vertical guidance would be lost.We flew the arrival into ZZZ and were handed off from Center to Approach Control. I was the PM (Pilot Monitoring) and was responsible for the radio communication. The PIC was the PF (Pilot Flying) on this leg. We had discussed the problem with the FMS en route; and had agreed on the fact that there would be no guidance on an RNAV/GPS approach after the FAF; and that we would plan on the ILS approach.ATIS reported the winds as calm; overcast at around 6;000/6;500 feet; and FEW clouds at 600 feet. It also reported that the Runway 9 was in use; and to expect the RNAV/GPS or visual approach to Runway 9.Upon handoff to Approach Control; I informed them that we were unable any RNAV approaches; and he responded that he assumed that meant that we were looking for the ILS; which I confirmed. He came back at some point saying that the ILS was out of order and asked what we wanted to do. I had already given the PIC the ATIS report; and he instructed me to tell them that we could accept the RNAV/GPS to Runway 9. The controller asked us to confirm that we were indeed able to accept the RNAV approach; and the PIC nodded his head and said yes; so I replied with an affirmative. We were cleared direct to IAF (Initial Approach Fix). I had loaded a different waypoint in the FMS as an IAF; so had to reload the approach into the FMS. The PIC requested a vector to fly towards [IAF] while we were getting the approach loaded into the FMS; and ATC gave us a heading to fly; and when able- direct to [IAF]. Once loaded; we navigated to [IAF]; and turned inbound onto the final approach course at [waypoint]; towards the FAF. I was busy with the before landing configuration of the plane; and the checklist- with my head down; and glanced momentarily outside and saw that the reported FEW 600 feet was a fairly solid layer underneath with patches of ground contact; closer to BKN in my opinion.Approach handed us off to the Tower; but the PIC asked for confirmation that we had indeed been cleared for the approach. The Tower Controller asked us to standby for a second while he confirmed- he confirmed that we were. I was busy completing the BEFORE LANDING checklist; and when I looked up- saw that we had flown past the FAF and were descending in IMC conditions into the layer of cloud. I looked over at the PIC to ask him what his intentions were; and to see if he had some form of guidance on his PFD (Primary Flight Display) when the Tower Controller called on the radio saying : 'Stop your descent immediately!' as we had triggered a ground proximity warning for him. I looked up; and saw that we had just broken out below the cloud layer; and that there was a ridge line between us and the runway.We continued visually; and landed on Runway 9.In hindsight; there should not have been any assumptions made on my part that the PIC and I were on the same page regarding the functionality of the FMS. I should have clarified with the approach controller that we would only be able to accept the approach up until the FAF; and if we were not in VMC at that point; that we would be diverting to [an alternate airport]; or another alternate- if they were not willing to allow us to fly the ILS approach into ZZZ. I should have been more assertive towards the PIC; insisting on a missed approach when we reached the FAF without the airport in sight.




Synopsis: Learjet 45 pilot reported flying a RNAV/GPS approach when it was not authorized.