Date: 201509
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: FMS/FMC
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight; Pre-flight
Result: Flight Crew Became Reoriented; Flight Crew Took Evasive Action; General Maintenance Action; General Work Refused
Contributing factors: Aircraft; Company Policy; Manuals; Procedure
Primary problem: Procedure

Narrative: On the first leg of the day; on power up; the airplane gave us an 'invalid Nav database' message in the scratch pad of the MCDU. We decided to shut down and re power up the airplane to see if the problem would clear but it did not. So we contacted MOC and they sent a mechanic to meet us. We explained to the mechanic the issue. We also explained to him that we realized that we were not able to input anything into the MCDU except for radios. We could not put a flight plan; Zero Fuel Weight (ZFW); speeds or anything. The mechanic then thought it was 'Frozen' and to restart the plane again on APU power. So we shut down the plane for 3' and powered up on APU. Same issue. The mechanic then called MOC from the cockpit. He explained that we could not input anything into the MCDU. He then told us that MOC was telling him to just defer the database but we insisted that we thought they might be more wrong with the system. He said he was told to just defer both FMSs then. He left; and returned a few minutes later with MEL info; can filled out and we got our amendment for our release. Initially we thought that deferring was ok; the MEL looked correct. So we departed. We had no V speed indicated on the PFD; no flight plan programmed; no ZFW input but we had everything written down; briefed each other at length on everything we were going to do to replace automation. So we flew to our first destination; raw data; green needle and Heading. After a rushed quick turn due to arriving late and the amount of work required to substitute automation (amending clearance for no GPS; amending flight plan for no direct; calling dispatcher for takeoff numbers; ZFW...) we departed. About 20 minutes into the flight; I realized that the Landing Field Elevation (LFE) for the pressurization was not populated (associated to the lack of database) but I started questioning why the MEL wouldn't direct us to fill it in. So I pulled up the MEL book and started flipping around. On the previous page; I started reading 34-61-00-1 for just FMS deferral instead of the database and in the notes were all the related items that we had to do (writing down speeds; setting LFE...) and from that point on I truly believed that we all made a mistake and that it wasn't just an FMS database problem but both FMS were not working due to the lack of database. Also; under 34-61-00-1 at least one FMS had to be working.Upon landing; we contacted MOC and questioned all of our decision from earlier and MOC still insisted we were legal to which we did not agree. I subsequently advised them that I would contact the chief pilot. We talked at length about what was going on and that it was not just a database issue. [Another person] was also involved in the discussion for a short time and nobody seemed to come to a consensus on what was really going on with the plane. Whether we were legal or not; whether it was a database issue or FMS ISSUE.I believe that particular MEL is poorly worded and confusing since ALL involved still could come to an agreement on what was going on. I believe that 34-61-00-2 that we were dispatched under is for expired databases as in this case the FMSs still work. In our case; where the complete lack or invalid database prevented the FMS from working properly; should not be dispatched under 34-61-00-2 but it is not stated. If it was intended for us to be able to use in these circumstances; the notes should be adjusted to mirror -1 regarding writing down V speeds; imputing LFE before takeoff. As we had that.


Second narrative: [Report narrative contained no additional information.]


Synopsis: An ERJ175 flight crew reported Maintenance MEL'ed both FMS's found inoperative during preflight. Following the day's flight the crew questioned the MEL and believed 1 FMS was required for flight even though the database was out of data.