Narrative:

I was operating flight as the first officer and pilot flying. Prior to departure; ca and I verified the FMS and completed the prescribed checklists. We pushed back and proceeded directly to the runway. All checklists were completed prior to takeoff. We were number one for departure; cleared to FL280; and cleared for takeoff. Md-80 is not an INS equipped aircraft; and no anomalies were noted prior to initiating the takeoff. Once in the air; we were flying runway heading; climbing to FL280. The departure controller cleared us on course. It was at this time ca noted the rnp displayed on the primary flight display/ADI. The course to our first fix was displayed as approximately 275 degrees. I initiated a turn to approximately 240 degrees to intercept. I noted the course line was continuously moving away; maintaining a 10 NM intercept; with increasing map shifting; and the displayed distance to the fix was increasing. The controller asked if we were deviating north; and we stated yes; as I had altered my intercept to maintain VFR in the mountainous terrain by about 3 degrees right as we sorted out the increasingly apparent malfunction and a moderate cu (cumulus) was at my 12 o'clock position. I then chose to remain at a lower airspeed to continue climbing at a higher rate. We were then handed off to the next ARTCC. I then took the radios; and ca began searching for an appropriate checklist. Ultimately; none were available in the QRH for a FMS failure as it pertained to our current situation. I advised the controller we were experiencing a navigational malfunction; immediately requesting a heading. We were issued a heading of 180 degrees. We were flying approximately 260 degrees at the time; as verified by ATC as I turned past 230 degrees. As we were issued the new heading; the wet compass was checked and agreed with the heading shown on the display. I then manually selected the next VOR frequency and proceeded directly to it via navigation 2 off of the assigned vector; initially navigating with use of the RMI. I noted the DME was continually increasing; starting at approximately 90 NM; and ultimately reaching a 310 NM easterly error. We were then cleared to FL340. Ca reminded me to ask for FL280 after being cleared due to rvsm requirements. FL280 was assigned and flown for the remainder of the flight. The aircraft equipment code was then amended as well. I switched to the rose mode which eliminated the erroneous GPS/DME reading; and was verified accurate by the next controller; at various cross radials/airway intersections. On course; communicating well with ATC; and the most appropriate and functional navigation system verified accurate; ca and I discussed continuing the flight or returning to the departure airport. We agreed to continue; as I now had the route plotted on the jeppesen charts; and the operational service volumes of the vors along our course were verified adequate. Only one exception was noted; and this was remedied by an amended clearance onto the arrival class B destination airport. I requested the controller contact our destination TRACON to verify they would accept us into the terminal airspace. The arrival didn't require GPS; but I wanted to make sure there wouldn't be any complications as we entered class B airspace. Our destination had no issue with our navigation status. We continued the flight with no other issues until my seat failed in the forward position. This was not a concern as it was the position used by me in takeoff and landing. All breaker panels were checked by ca; and no anomalies were found. Ca made multiple attempts to communicate with dispatch via SELCAL; but was unable to secure a clear and readable connection. I heard to poor clarity of the frequency first hand. Ca was able to verify our fuel was sufficient at 80 before the connection was terminated. As the flight progressed; the GPS began to slowly correct its position. Its latitude was never off by more than a mile; but longitude was; at its worst; 310 NM east of our actual position. I noted at one point in flight the coordinate disparity. The disparity continually lessened as we reached a point approximately 400 NM into our flight. Near the ZZZ VOR the latitude/longs were compared against the jeppesen charts and found to be accurate; and further verified at ZZZ1 VOR. The GPS was then used to supplement the VOR navigation for the remainder of the flight. Upon arrival in our destination; ca recorded to discrepancies in the aircraft maintenance log. Maintenance personnel were standing by at block-in and were thoroughly briefed. Maintenance stated this was not the first report of this type of failure recently. I will certainly take more care to verify the approximate heading to my first fix; prior to departure.

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Original NASA ASRS Text

Title: MD-80 crew discovered after takeoff a large GPS position error; which caused a track deviation but were given ATC vectors on course where VOR airway tracking continued. Later in the flight GPS accuracy was attained and therefore used to the destination.

Narrative: I was operating flight as the First Officer and Pilot Flying. Prior to departure; CA and I verified the FMS and completed the prescribed checklists. We pushed back and proceeded directly to the runway. All checklists were completed prior to takeoff. We were number one for departure; cleared to FL280; and cleared for takeoff. MD-80 is not an INS equipped aircraft; and no anomalies were noted prior to initiating the takeoff. Once in the air; we were flying runway heading; climbing to FL280. The departure controller cleared us on course. It was at this time CA noted the RNP displayed on the Primary Flight Display/ADI. The course to our first fix was displayed as approximately 275 degrees. I initiated a turn to approximately 240 degrees to intercept. I noted the course line was continuously moving away; maintaining a 10 NM intercept; with increasing map shifting; and the displayed distance to the fix was increasing. The controller asked if we were deviating north; and we stated yes; as I had altered my intercept to maintain VFR in the mountainous terrain by about 3 degrees right as we sorted out the increasingly apparent malfunction and a Moderate CU (Cumulus) was at my 12 o'clock position. I then chose to remain at a lower airspeed to continue climbing at a higher rate. We were then handed off to the next ARTCC. I then took the radios; and CA began searching for an appropriate checklist. Ultimately; none were available in the QRH for a FMS failure as it pertained to our current situation. I advised the controller we were experiencing a navigational malfunction; immediately requesting a heading. We were issued a heading of 180 degrees. We were flying approximately 260 degrees at the time; as verified by ATC as I turned past 230 degrees. As we were issued the new heading; the wet compass was checked and agreed with the heading shown on the display. I then manually selected the next VOR frequency and proceeded directly to it via NAV 2 off of the assigned vector; initially navigating WITH use of the RMI. I noted the DME was continually increasing; starting at approximately 90 NM; and ultimately reaching a 310 NM easterly error. We were then cleared to FL340. CA reminded me to ask for FL280 after being cleared due to RVSM requirements. FL280 was assigned and flown for the remainder of the flight. The aircraft equipment code was then amended as well. I switched to the ROSE mode which eliminated the erroneous GPS/DME reading; and was verified accurate by the next controller; at various cross radials/airway intersections. On course; communicating well with ATC; and the most appropriate and functional navigation system verified accurate; CA and I discussed continuing the flight or returning to the departure airport. We agreed to continue; as I now had the route plotted on the Jeppesen charts; and the operational service volumes of the VORs along our course were verified adequate. Only one exception was noted; and this was remedied by an amended clearance onto the arrival Class B destination airport. I requested the controller contact our destination TRACON to verify they would accept us into the terminal airspace. The arrival didn't require GPS; but I wanted to make sure there wouldn't be any complications as we entered Class B airspace. Our destination had no issue with our NAV status. We continued the flight with no other issues until my seat failed in the forward position. This was not a concern as it was the position used by me in takeoff and landing. All breaker panels were checked by CA; and no anomalies were found. CA made multiple attempts to communicate with Dispatch via SELCAL; but was unable to secure a clear and readable connection. I heard to poor clarity of the frequency first hand. CA was able to verify our fuel was sufficient at 80 before the connection was terminated. As the flight progressed; the GPS began to slowly correct its position. Its latitude was never off by more than a mile; but longitude was; at its worst; 310 NM east of our actual position. I noted at one point in flight the coordinate disparity. The disparity continually lessened as we reached a point approximately 400 NM into our flight. Near the ZZZ VOR the LAT/LONGS were compared against the Jeppesen charts and found to be accurate; and further verified at ZZZ1 VOR. The GPS was then used to supplement the VOR navigation for the remainder of the flight. Upon arrival in our destination; CA recorded to discrepancies in the Aircraft Maintenance Log. Maintenance personnel were standing by at block-in and were thoroughly briefed. Maintenance stated this was not the first report of this type of failure recently. I will certainly take more care to verify the approximate heading to my first fix; prior to departure.

Data retrieved from NASA's ASRS site 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.