Date: 201310
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: IMC
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Training / Qualification; Human-Machine Interface; Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Executed Go Around / Missed Approach; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Manuals; Company Policy; Weather; Aircraft
Primary problem: Ambiguous

Narrative: While conducting the RNAV approach; we received conflicting vertical guidance from the FMS; while simultaneously receiving an altitude alert message from Tower (potentially 400 FT below altitude). We immediately executed a missed approach. We then took our time on an extended downwind to reprogram the FMS; and completed the approach and landing uneventfully. It's important to note we followed all applicable procedures as per our training and RNAV tab.Both flight crew members agreed upon setup of the FMS for the approach; and used the RNAV tab [in the handbook] to program the FMS. It would be beneficial to have a more structured 'checklist' for setting up an RNAV approach since we primarily conduct visual or ILS approaches. Also; it is worth mentioning that; while there were no ILS approaches currently available at ZZZ; there was a VOR that ATC suggested but for which we were not issued any approach plates and therefore had to use the RNAV again. Had the second approach resulted in another missed; we would have had to go to our alternate. If there is a way for the company to know when certain approaches will be unavailable; it would be helpful to have (at least temporarily) another form of approach to use that we may not otherwise need to carry.

Callback: The reporter advised he was only a couple of weeks removed from his initial qualification flights on the E-145 and also clarified that he was the Pilot Flying during the incident. He stressed that this heavily influenced his personal SOP to prepare for an RNAV approach literally 'by the book' utilizing a special section of the company's operating manual to assure it was done correctly. When the Captain flew the; successful; second approach he reprogrammed the FMS but neither pilot believed it had been done any differently than when the reporter had set it up for the first approach.The warnings that precipitated the event (which included an EGPWS 'terrain' warning) occurred on the descent to the DA(H) and their altitude at the time was about 500 AGL. He was uncertain of their distance from the threshold at the time of the alert and the initiation of the go-around. The 'conflicting guidance' to which he referred was not a difference between the Captain and First Officer's displays. He believes the CFTT was the result of the FMS incorrectly directing a much greater than required rate of descent; a rate with which he had complied.Those comments initiated a discussion of the FMS/autoflight system on the aircraft. He advised that there are no autothrottles and no 'unassisted' VNAV capability. Compliance with FMS generated vertical navigation requirements must be navigated via manual autothrottle manipulation and the use of the autopilot's vertical speed mode to descend. Computerized VNAV assistance is limited to an FMS generated VPI (Vertical Path Indicator) on the flight display which must be tracked; again; by manipulation of throttles and manual or autopilot vertical speed mode inputs. He felt he was competently following the VPI but that the resulting descent rate was excessive.The second approach was 'programmed' identically but no such abnormality occurred. The reporter didn't know whether the potential VPI anomaly was written up by the Captain.



Synopsis: An E-145 flight crew conducting an RNAV approach received low altitude alerts from the Tower and EGPWS terrain warning inside of the final approach fix. They initiated a missed approach; reprogrammed the FMS and conducted a second approach uneventfully.