MD-80 Series (DC-9-80) Undifferentiated or Other Model
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft; Procedure
Upon arrival at the aircraft there were some issues that were still being worked on and eventually the mechanics were able to resolve the FMS position issues. The aircraft pre-flight was completed and the aircraft was ready for flight. We took off and within the first hour ATC began to query about our heading. After checking our heading with the mag compass we could see there was a discrepancy. After consulting the QRH it was clear this was not a heading fail situation. We checked the FMS and it had lost its position. From that point we changed over to flying from VOR to VOR and disregarded the FMS indication. We had full map and heading information but it was incorrect. Eventually ATC called about our heading again and we checked the heading with the wet compass and it was off. We placed both Captain and First Officer compasses in Non-Slaved mode and made adjustments in relation to the wet compass. After navigating through a line of weather we contacted Dispatch and informed them of the issues and their instructions were to turn around and go back to [departure airport]. We advised them we could make [destination] but were told to turn around and go back. We made the 180 turn and asked for the fuel burn at our present position. Dispatch informed us that we would have approximately 6;800 lbs upon landing. [Later] I called dispatch again for an update and they informed us that we would be landing with 5;600 lbs. In addition to this we noticed that our wind indicator was showing 191 kts Headwind (HW) and when we made the 180 back the indicator still showed 191 kts HW.Plan that a plane coming out of extensive maintenance will most likely have issues. It is better to work out the issues rather than expecting to use the aircraft. Assign a Team to shake down the aircraft issues quickly. Don't release the aircraft to the line until the Team has given the green light.
A MD-80 Captain reported returning to departure airport after loss of the FMS.