Narrative:

The flight departed without incident; and while enroute at cruise altitude; the #1 FMS failed to respond to keypad entries and shortly after that displayed a red 'CDU bus fail' message on the #1 FMS screen. The first officer was the pilot flying; so it was a simple thing for me to switch to the #2 FMS for LNAV function on both my pfd and mfd displays. We were vectored for the ILS runway 30R into ZZZ and everything was in order all the way up to shortly after passing the final approach fix. We were still in IMC conditions; and after passing approximately 800-500 ft AGL; we received a 'terrain; terrain' egpws aural warning along with both our mfd's switching to terrain display mode. Not wanting to delay into possible CFIT; I commanded the first officer to do an immediate go around; which he did; and I advised tower of our missed approach. They queried as to the need for the missed approach and if we needed any assistance; and I advised them of the ground proximity warning during the approach and stated that we could take vectors for another approach to runway 30R. First officer and I discussed the ground proximity warning; and we both agreed that the aircraft appeared to be properly established and stabilized on course and glide path; and that most likely our #1 FMS CDU bus fail was probably feeding false information to the egpws. I pressed the 'terrain override' button on the egpws panel and we commenced and completed a second ILS approach to runway 30R without further incident. As I was pulling up to the gate in ZZZ; the #2 FMS gave us a message indicating that there was more than a 24nm difference in position between the #1 and #2 FMS; which was consistent with our feeling that inaccurate information from the #1 FMS had contributed to the ground proximity warning on our first approach. A maintenance location entry was made in ZZZ and in coordination with dispatch and maintenance control; we crew placarded the #1 FMS; #1 GPS and the egpws. The flight continued to ZZZ1 our next scheduled destination without incident. Several concerns have occurred out of this incident and I feel need to be addressed by both embraer and universal concerning this dual installation FMS on the emb regional jet. First of all; there is no abnormal or emergency procedure established for a red 'CDU bus fail' message on either FMS. Second; this crew received no FMS messages prior to commencement of our ILS runway 30R approach that would have clued us into the possibility that #1 FMS would be feeding inaccurate position data to the egpws; and even if it had; no procedure for this message prompts the crew to either turn off or pull the circuit breaker for #1 FMS to preclude this erroneous data to continue to flow to the egpws. This lack of procedural guidance is in conflict with safe practices we have maintained over the years in airline flight operations. I have personally experienced several CDU bus fail events in line operations and not until now have I had an event that caused a failure of the egpws system. In some cases the keypad on the associated FMS is frozen; with no further responses to keypad entries by the unit. Logic would tell you to ignore information from an FMS that has this type of failure; and the human machines in this event did just that; but it seems; in this case; that the electronic egpws did not ignore an unreliable source of navigational information. This could have profound results in another scenario such as when an aircraft is being either vectored or is flying off of established airways or published approach procedures and approaches terrain which would otherwise trigger an egpws warning. Without reliable position information being sent to the egpws; it could be argued that a flight crew would be denied this important piece of flight safety equipment because it is receiving its information from an unreliable source. Lack of an abnormal or emergency procedure for the precipitating event; the CDU bus fail; led to this incident; which thankf

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Original NASA ASRS Text

Title: An EMB-145's #1 FMS failed followed by CDU BUS FAIL alert displayed in the #1 FMS screen. On the flight's IMC approach an EGPWS 'TERRAIN' warning activated based on the #1 FMS's erroneous position. No procedures were available to deactivate the faulty FMS and to permit valid data for system's operation.

Narrative: The flight departed without incident; and while enroute at cruise altitude; the #1 FMS failed to respond to keypad entries and shortly after that displayed a red 'CDU BUS FAIL' message on the #1 FMS screen. The First Officer was the pilot flying; so it was a simple thing for me to switch to the #2 FMS for LNAV function on both my PFD and MFD displays. We were vectored for the ILS Runway 30R into ZZZ and everything was in order all the way up to shortly after passing the final approach fix. We were still in IMC conditions; and after passing approximately 800-500 FT AGL; we received a 'Terrain; Terrain' EGPWS aural warning along with both our MFD's switching to terrain display mode. Not wanting to delay into possible CFIT; I commanded the First Officer to do an immediate go around; which he did; and I advised Tower of our missed approach. They queried as to the need for the missed approach and if we needed any assistance; and I advised them of the ground proximity warning during the approach and stated that we could take vectors for another approach to Runway 30R. First Officer and I discussed the ground proximity warning; and we both agreed that the aircraft appeared to be properly established and stabilized on course and glide path; and that most likely our #1 FMS CDU BUS FAIL was probably feeding false information to the EGPWS. I pressed the 'Terrain Override' button on the EGPWS panel and we commenced and completed a second ILS approach to Runway 30R without further incident. As I was pulling up to the gate in ZZZ; the #2 FMS gave us a message indicating that there was more than a 24nm difference in position between the #1 and #2 FMS; which was consistent with our feeling that inaccurate information from the #1 FMS had contributed to the ground proximity warning on our first approach. A maintenance location entry was made in ZZZ and in coordination with dispatch and maintenance control; we crew placarded the #1 FMS; #1 GPS and the EGPWS. The flight continued to ZZZ1 our next scheduled destination without incident. Several concerns have occurred out of this incident and I feel need to be addressed by both Embraer and Universal concerning this dual installation FMS on the EMB Regional Jet. First of all; there is no abnormal or emergency procedure established for a RED 'CDU BUS FAIL' message on either FMS. Second; this crew received no FMS messages prior to commencement of our ILS Runway 30R approach that would have clued us into the possibility that #1 FMS would be feeding inaccurate position data to the EGPWS; and even if it had; no procedure for this message prompts the crew to either turn off or pull the CB for #1 FMS to preclude this erroneous data to continue to flow to the EGPWS. This lack of procedural guidance is in conflict with safe practices we have maintained over the years in airline flight operations. I have personally experienced several CDU BUS FAIL events in line operations and not until now have I had an event that caused a failure of the EGPWS system. In some cases the keypad on the associated FMS is frozen; with no further responses to keypad entries by the unit. Logic would tell you to ignore information from an FMS that has this type of failure; and the human machines in this event did just that; but it seems; in this case; that the electronic EGPWS did NOT ignore an unreliable source of navigational information. This could have profound results in another scenario such as when an aircraft is being either vectored or is flying off of established airways or published approach procedures and approaches terrain which would otherwise trigger an EGPWS warning. Without reliable position information being sent to the EGPWS; it could be argued that a flight crew would be denied this important piece of flight safety equipment because it is receiving its information from an unreliable source. Lack of an abnormal or emergency procedure for the precipitating event; the CDU BUS FAIL; led to this incident; which thankf

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.