MD-11 flight crew reported progressive degradation of all navigation systems after departure. The flight crew determined that continuing to destination airport was not possible. The flight crew requested vectors to return to departure airport. Fuel was jettisoned to get below maximum landing weight and the flight crew landed uneventfully.
Synopsis
MD-11 flight crew reported progressive degradation of all navigation systems after departure. The flight crew determined that continuing to destination airport was not possible. The flight crew requested vectors to return to departure airport. Fuel was jettisoned to get below maximum landing weight and the flight crew landed uneventfully.
Narrative
Prior to push back we had a message of unable RNP. We referenced the Company chart stating that there is the possibility of magnetic interference at Gate X and Y. The positions looked in tolerance and we thought the message would clear after push back. The aircraft was showing RNP1. After taxing clear of the ramp the unable RNP message had not cleared. We pulled over and called Dispatch and Maintenance; the RNP was now showing RNP4. We decided that the RNP was sufficient and thought the message should clear after takeoff. Takeoff and initial climb seemed normal. When cleared to our first way point the turn point was coming from the North and the aircraft turned to intercept. The course was obviously in error; so we tried going direct to the way point again without success. I changed the scale on the Navigation Display (ND) to max; I could see ZZZ and our first way point but could not see the start of the turn point. I asked for a heading and to continue the climb while we troubleshot a navigational issue. On climb we figured out that our heading was not matching the magnetic compass and got an message for GPS failure. The Captain then transferred control of the aircraft to me and looked up the checklist as well as contacted Dispatch and Maintenance. It was determined that we were not going to be able to continue the flight. We coordinated with ATC for a fuel dump to get back to landing weight. On the turn back to ZZZ the heading processed another 30 degrees from the actual magnetic heading. The map on the ND during the turn also shifted and started showing airports for ZZZZ1 matching the position. After landing in ZZZ we started the APU and left all navigational systems on for Maintenance to troubleshoot. While stationary the navigation system continued to slowly change.
Second reporter narrative
We had UNABLE RNP message during taxi out increasing to RNP 4. We taxied to and held position on taxiway x then the Captain called Dispatch and spoke with Maintenance; the decision was made to proceed with the flight. On climbout around 5;000 ft; ATC cleared us direct ZZZZZ and when it was entered; a magenta line was drawn to the North instead of our westbound course. We continued the climb with radar vectors on course westbound while we were troubleshooting the problem. Our Navigation Display (ND) heading was initially off about 20 degrees of the standby compass then eventually processed to 45 degrees off. Sensor status showed Global Navigation Satelie (GNS) 1/2 Fail. Inertial Reference System (IRS) status showed blank for drift rate and ground speed. GNS 1/2 appeared to show the correct position but ND showed we were near ZZZZ1. We attempted to call Dispatch with the satellite phone but it did not work. We then made contact with Dispatch and Maintenance for further troubleshooting. When we could not fix the navigation problem the decision was made to return to base. We made the turn back to ZZZ and coordinated with ATC to dump fuel to make landing weight; approximately 44000 lbs. dumped. The Captain flew a visual approach to an uneventful landing at ZZZ Runway XXR.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.