B737-800 Flight Crew reported while navigation systems indicated the aircraft was on the approach course; actual position was a quarter mile right of center line. Flight crew conducted a go around. Flight crew stated at no time aircraft indicated off course.
Synopsis
B737-800 Flight Crew reported while navigation systems indicated the aircraft was on the approach course; actual position was a quarter mile right of center line. Flight crew conducted a go around. Flight crew stated at no time aircraft indicated off course.
Narrative
While operating ZZZ to LGA; we experienced an avionics anomaly that resulted in significant aircraft position error on the ILS to Runway 4.Initially; the FMS was loaded with the ILS 4 with no transition; however; during the arrival; we were given direct the IF (BENNG). With the CA (Captain) as PF (pilot Flying); I manually inserted BENNG on the LEGS page and manually added the 3000 ft. restriction. While enroute; we added the additional step down fixes outside of the FAF (PACHU and RAHEL). LNAV and VNAV were left as selected modes until inside RAHEL; then APP mode was selected. LOC and GS capture were shown; and ILGA was identified on frequency 110.50. Normal configuration to 40 flaps and before landing checks were completed just prior to the FAF (DNNIS). All indications in the cockpit were normal. Autopilot 1 was disconnected at the FAF. FD/GS/LOC were centered all the way down final.The reported weather was OVC006 2SM -RA. Once we broke out of the clouds; I transitioned from monitoring the PFD to looking for the runway. Initially; I saw a light and a dark area directly ahead (the visibility was limited) and thought that was the runway (it turned out to be the LGA Control Tower). As we continued; I picked up the runway approximately 1.25 miles away; and 30 degrees off to the left. I pointed it out and the CA immediately initiated a go around. I informed Tower of the go around; then told Approach about the erroneous indications. As a crew; we decided to try one additional approach; but if the indications were again erroneous; we would divert to a VMC airport (ZZZ1 was our initial choice). We chose the ILS again over RNAV thinking the INS solution may be unreliable; and ground navaids were our best choice. We re-verified that the correct ILS XX frequency was tuned; selected; and identified. We were told to expect outside of BENNG and while attempting to select BENNG (IF); the FMC produced a discontinuity and would not allow me to close the discontinuity between BENNG and PACHU; the next point on the approach. The CA elected to extend off of PACHU and place BENNG in the FIX page in order to identify the point along the approach course to which we were cleared.As we lined up for the second approach attempt; we serendipitously had a TCAS contact 8 miles ahead that superimposed our magenta final course line while zoomed in; which gave us a bit of confidence that the solution was correct. The second approach was nominal. No erroneous indications or position noted. Once visual; the runway was directly ahead. At no time did Approach or Tower note that we were off course. Based on external sources; we appeared to be nearly 3 miles offset east at BENNG; on a partially converging course that ended 0.4 miles west at the runway threshold. There were no errors reported internal to the aircraft. Internally; unknown. There were no indications in the cockpit that there was an error. Perhaps ATC may have been able to query our position and recommend a missed approach.
Second reporter narrative
On Date while flying Aircraft X ZZZ LGA; we experienced a significant navigational error while flying the ILS to Runway 4 placing the aircraft well right of course requiring a go around. There were no internal; flight deck; indications of the error or external; Approach Control and Tower; notifications of the error.The reported weather was OVC006 2SM -RA with gusty; right quartering headwinds. I recall no unusual anomalies associated with the approach and once set up and flown no indications that the aircraft was anywhere but appropriately lined up with the runway on the GS/LOC. As we continued down final all flight deck indications including my HUD were normal. At 1000 ft. I disconnected the autopilot and hand flew the approach. We broke out at approximately the reported ceiling and began our scan for the runway. I asked the FO (First Officer) for wipers to assist in picking up the visual. I stayed on instruments and remained on the flight directors as I began my cross check for the runway expecting it to be slightly off to the left with the associated winds. The FO was first to pick up the runway and call it out left about 30 degrees. It was obvious a landing could not be performed from our current position and I called for the go around. We discussed what we had just experienced and agreed to attempt one more approach; loading the approach from scratch and confirming our position on final with approach. If we experienced a similar result we would divert to ZZZ1 which I had previously coordinated with Dispatch as they were mostly VMC and NE of the weather system effecting New York. We chose the ILS once again over the RNAV since it was ground based and least susceptible to navigational issues. While setting up for the subsequent approach and attempting to select BENNG (IF) the FMC produced a discontinuity and would not allow closing the discontinuity between BENNG and PACHU; the next point on the approach. I elected to have the FO extend off of PACHU and place BENNG in the FIX page in order to identify the point along the approach course from which the approach was being cleared. Our position on final was verified by Approach and the TCAS indicated ahead on the ILS further supported our navigational position and that we were confident we were on the ILS final. The remainder of the approach and landing were uneventful. What is of serious concern is that during our initial approach and go around no indications on the flight deck indicated our actual position. We did not show full deflection on the LOC course or well right of the magenta line at any time during the approach; and at no time did Approach or Tower call out or question our position. The FO and I estimate we were approximately 1/4 mi right of the approach end of the runway when we initiated the Go Around. In my near XX years of flying and 14;000 hrs in type I have never experienced anything close to this event. Even an error on our part should have displayed a deviation with either the LOC or NAV display. Place this event in Central America in the mountains and the outcome could have been very different.I contacted Maintenance Control; and Maintenance and made an entry in the logbook. Maintenance performed their checks and found no anomalies or issues and placed it back in service.Suggestions - None that I'm aware of as we followed all known procedures and monitored appropriate navigational equipment.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.