Narrative:

On the flight prior to this event; we flew an ILS approach to minimums at flo. At the glide slope intercept point; the aircraft encountered an extreme pitch deviation; downward. The first officer (pilot flying) disconnected the autopilot; and hand-flew the rest of the approach. I noted erratic flight director (FD) indications; but first officer continued to hand fly. The ILS indications appeared to be stable enough to continue; especially as we got lower. After landing; we verbally reported the problem with the ILS to flo tower. Since we'd had no problem with the ILS system for the preceding three days; and since both pilot and copilot indications were erroneous; we did not suspect that the problem was with our airplane. On the 'event' flight; we were performing a visual approach; 'backed up' by the ILS to runway 16 at ftw. We were assigned a heading to intercept the i-ftw localizer; some 25 NM from ftw. With both of us in 'green source;' I noted that neither of us had a localizer indication. The FMS moving map display showed us moving toward intercept; but I admonished myself not to use the moving map for navigation! So I disregarded it; and waited for the localizer to come alive; which I was certain; was imminent. Some time period later; regional informed us that we had flown through the localizer (which the moving map confirmed); and gave us another heading. Further in; they asked us if we were 'familiar with the navy base?' (meaning knfw.) it was clear to them that we were having navigational difficulty; at one point saying: 'C'mon guys. It's ten miles visibility out there!' the first officer and I began discussing which airport was which. As we passed alliance (afw); which I could identify as being next to texas motor speedway; we were more certain that we had the right runway. Our localizer indications were erratic; and inconsistent with our view of the runway. Green source (VOR/ILS based navigation) indications are very powerful in the mind of a pilot. I find it hard to conceive that two of them were erroneous; but clearly they were. On short final to runway 16; we had time to remark at the indications that we had. If I remember correctly; we had a centered localizer while angling in from the west. The glideslope indications were also inconsistent with our visual perception; and the PAPI. We landed safely at ftw; and concluded from the prior two flights (to include the ILS approach at flo) that we had a dual navigation failure.after consulting with maintenance; and the duty manager; we determined that the MEL allowed for a 'reposition' flight from ftw to ZZZ. During that flight; I had the opportunity to recheck the accuracy of the ILS system. I casually tested the localizer while passing over dfw; again; while passing the centerline of runway 13L at dal; and again on final at ZZZ. All indications were as expected. So the condition that we encountered was an intermittent one. Even further; I have learned that maintenance has failed to duplicate the malfunction. I write this report as a warning that this airplane has an extremely dangerous; intermittent condition! Since the ILS system takes us down almost all the way to the ground; its integrity is vital to safety. In my opinion; the limitation on the use of the moving map display should be clarified; conditioned; or removed. If I had followed the moving map display; I would not have gone through the localizer. At one point on this approach; we offered to go direct to the mufin LOM; which we could do on the same GPS navigation that got us from south carolina to texas. ATC turned us down; and provided another vector. I suggest that ATC become more familiar with requests to go directly to such fixes when pilots so request. Most important; again; I would ask that all subsequent crewmembers are alerted to the problems that we had with this airplane; and remain vigilant for erroneous ILS indications.

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Original NASA ASRS Text

Title: LR40 Captain experienced dual ILS receiver failures on two separate ILS approaches. These failures disappeared during a ferry flight for maintenance.

Narrative: On the flight prior to this event; we flew an ILS approach to minimums at FLO. At the glide slope intercept point; the aircraft encountered an extreme pitch deviation; downward. The First Officer (pilot flying) disconnected the autopilot; and hand-flew the rest of the approach. I noted erratic flight director (FD) indications; but First Officer continued to hand fly. The ILS indications appeared to be stable enough to continue; especially as we got lower. After landing; we verbally reported the problem with the ILS to FLO Tower. Since we'd had no problem with the ILS system for the preceding three days; and since both pilot and copilot indications were erroneous; we did not suspect that the problem was with our airplane. On the 'Event' flight; we were performing a visual approach; 'backed up' by the ILS to Runway 16 at FTW. We were assigned a heading to intercept the I-FTW localizer; some 25 NM from FTW. With both of us in 'green source;' I noted that neither of us had a localizer indication. The FMS moving map display showed us moving toward intercept; but I admonished myself NOT TO USE THE MOVING MAP FOR NAVIGATION! So I disregarded it; and waited for the localizer to come alive; which I was certain; was imminent. Some time period later; Regional informed us that we had flown through the localizer (which the moving map confirmed); and gave us another heading. Further in; they asked us if we were 'familiar with the Navy Base?' (Meaning KNFW.) It was clear to them that we were having navigational difficulty; at one point saying: 'C'mon guys. It's ten miles visibility out there!' The First Officer and I began discussing which airport was which. As we passed Alliance (AFW); which I could identify as being next to Texas Motor Speedway; we were more certain that we had the right runway. Our localizer indications were erratic; and inconsistent with our view of the runway. Green source (VOR/ILS based navigation) indications are very powerful in the mind of a pilot. I find it hard to conceive that TWO of them were erroneous; but clearly they were. On short final to Runway 16; we had time to remark at the indications that we had. If I remember correctly; we had a centered localizer while angling in from the west. The glideslope indications were also inconsistent with our visual perception; and the PAPI. We landed safely at FTW; and concluded from the prior two flights (to include the ILS approach at FLO) that we had a dual navigation failure.After consulting with Maintenance; and the Duty Manager; we determined that the MEL allowed for a 'reposition' flight from FTW to ZZZ. During that flight; I had the opportunity to recheck the accuracy of the ILS system. I casually tested the localizer while passing over DFW; again; while passing the centerline of Runway 13L at DAL; and again on final at ZZZ. All indications were as expected. So the condition that we encountered was an intermittent one. Even further; I have learned that Maintenance has failed to duplicate the malfunction. I write this Report as a warning that this airplane has an extremely dangerous; intermittent condition! Since the ILS system takes us down almost all the way to the ground; its integrity is vital to safety. In my opinion; the limitation on the use of the moving map display should be clarified; conditioned; or removed. If I had followed the moving map display; I would not have gone through the localizer. At one point on this approach; we offered to go direct to the MUFIN LOM; which we could do on the same GPS navigation that got us from South Carolina to Texas. ATC turned us down; and provided another vector. I suggest that ATC become more familiar with requests to go directly to such fixes when pilots so request. Most important; again; I would ask that all subsequent crewmembers are alerted to the problems that we had with this airplane; and remain vigilant for erroneous ILS indications.

Data retrieved from NASA's ASRS site as of April 2012 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.