A fractional jet flight crew reported a failure of their Inertial navigation system.
Synopsis
A fractional jet flight crew reported a failure of their Inertial navigation system.
Narrative
PIC was Pilot Monitoring; SIC was Pilot Flying. Second leg of the day; first leg was ZZZ1 to ZZZ. In ZZZ was cleared the ZZZZZX.ZZZ transition as filed climb via SID. During the 'before takeoff to the line' checklist it was verified by pressing 'departure' on the FMS flight plan page. Screen stated ZZZ; Runway XX; ZZZZZX.ZZZ". On the initial climb out; I saw the nav needles turn to the left and waited until 400 AGL to make that turn. As we were following the nav; Tower confirmed if we were on the ZZZZZX. CA replied with yes. At this time Tower switched us over to Departure and controller stated we were not flying towards 'ZZZZZ1'. Controller stated we had an altitude alert and to max climb. Autopilot was engaged and I selected 'FLC 170'; take off power and watching the AOA gauge. We were in VMC and clear of terrain. No terrain alert came on. We asked for a heading to flying to get back on track because direct to a fix was not working. After given a heading we discovered that what they saw on radar and what we saw on both PFD (Primary Flight Display) and standby were different. We were switched over to Center and cleared up to FL330. Plane was under control at all times of flight. Controller gave us a heading of 180. Once we leveled off the controlled stated we were on a heading of 125 and not heading towards ZZZ2. Both PFD'S; standby and compass was all showing 180; wind indications were not working and terrain was inop per the MFD (Multi-Function Display) bottom right box. CA and I were discussing best plan of action. We confirmed that our TA/RA was working by calling out a traffic and confirming with ATC that it was correct. ATC stated they were okay with us to be vectored to ZZZ3. After some trouble shooting the GPS nav needles were responding correctly; given if we had to fly a heading of 125 our heading had to be 180. We also discovered with my fore flight the GPS heading on there was matching what ATC saw on radar. Was cleared up to FL450 and for safety told ATC we were negative RVSM for extra spacing between other traffic. Switched the FMS from dual to single operations and was going through the sensors on the non-controlling FMS; CA side. Discovered that IRS 1 and 2 status stated 'attitude mode' in yellow with an option to set the magnetic heading. We agreed we were in a safe and steady phase of flight. We plugged in 128 from my GPS heading on fore flight; confirmed with ATC that we were flying the right direction and they confirmed; yes. Systems were working as normal at this time; but still did not have wind indications. ATC confirmed we were on track and we replied we would still like vectors to final. Landed safely at ZZZ3.Cause: not sure what caused this situation and still puzzled that both PFD'S; standby and compass were on showing the same heading but it was not correct. Suggestions: could be beneficial to have the moving map on the MFD zoomed out enough to see the first fix 'ZZZZZ1' but could see that being an issue in busy airspace. Also; if that would have been done in this situation; it would have been the same problem; the nav needles would have been working correct but would have had a deviation getting wider as we were flying."
Second reporter narrative
Second leg of the day. The issues we dealt with in this leg were not present in the first leg. We were cleared to ZZZ via the ZZZZZ departure- - ZZZ1 transition.; as filed; climb via SID. Departure was briefed by both pilots as well as verified in the FMS; with all waypoints and altitudes being checked as well. After takeoff the magenta NAV needles turned immediately to the left; and the PF; First Officer began a turn to the left at 400 ft.; which should have been direct to the first fix on the departure; ZZZZZ1. The Tower immediately called to ask if we were on the ZZZZZ; and we were flying towards terrain; and immediately initiate a Max Climb. We confirmed the departure; increased our climb; and were then notified that we were not going direct ZZZZZ1; and gave us the heading we were flying; which was not what our indications were showing. They then handed us off to ATC and we continued sorting out the issue. The airplane was never unstable or in danger of CFIT. We were always wings level; and in control of the aircraft; except for the false readings of our instruments. We asked for a heading to the next fix because we could not depend on our NAV mode. This when we realized that the headings we were flying was not what ATC was seeing. We made note of the differences and made the appropriate adjustments to our heading to stay on course. After some quick troubleshooting; checking the weather at destination (which was visual conditions); some in depth discussion; and phone calls to the Maintenance Facility on duty and Maintenance; we determined we were safe to continue to ZZZ and land there; getting our clients to their destination. We were in a climb to FL450 and remained on course the entire time.Now that things had calmed down; we began trying to determine what was actually going on and how can we correct it; if possible. We knew that in order to fly a heading of 128 deg we had to fly a heading of about 175 degrees. About a 45-degree difference. The magenta needles were pointing at the correct heading of 128. What was inconceivable was that both heading indicators on our PFD (Primary Flight Display) and MFD (Multi-Function Display); plus the standby; plus the compass were all in agreement with each other; but not ATC. We also were able to use our iPads for navigation assistance. The headings shown on there matched what ATC was saying. We also alerted ATC that we were negative RVSM due to our situation; and provide extra spacing from traffic.While we were troubleshooting we checked the GPS status and discovered that both IRS 1&2 status stated in yellow letters 'ATTITUDE MODE' with an option to enter a magnetic heading. We had both never seen that before and thought that could be the problem. We entered the heading ATC said we were flying of 128. The heading indicators immediately spun to 128; the plane turned slightly to the left; and we confirmed with ATC that we were on course. We were able to change to NAV mode and the airplane seemed to be correctly tracking correctly; which ATC confirmed. Despite the situation seemed to be resolved; we continued to ZZZ via vectors and landed safely.Cause: Not really sure. As mentioned above; the yellow ATTITUDE MODE when checking the IRS status had something to do with it; but how it happened I can't say. Incredibly strange that 4 different heading indicators on 3 different system in the plane were in agreement with each other but not ATC. Suggestions: Although not required by any of our preflight checklists or aircraft setup/configurations; it might be helpful after initializing the FMS to check the IRS status to confirm it is proper; but that's not required. I; however; to avoid this; will quickly check from time to time that I don't see this again. Nothing we could have done within our normal operational checklists would have prevented this from happening.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.