2009-07 · NASA ASRS report 844033
An MD80 flight crew experienced a navigation error departing DFW on the NOBLY3 approaching NAVYE. An IRS malfunction was suspected as the cause; although all systems appear to be normal.
Departing DFW on the NOBLY3 at 1000 FT the EGPWS system said 'terrain terrain too low' and we dismissed this as an anomaly. The autopilot was on tracking to NAVYE; both displays were showing that we had about two miles to go to NAVYE when ATC said turn left to a heading 080. She asked what departure we were on and then what do you show? My response was the NOBLY3 NAVYE and then I read the next three fixes. ATC said that we were two miles south of NAVYE. We said that our display showed that we were two miles north of NAVYE. My First Officer and I discussed what could have possibly gone wrong. The NAV data dates were correct; the departure runway was correct; and there were no discontinuities the route. We informed ATC that our RNAV was inoperative and continued the flight. Upon arriving I called ATC Departure Control; and expressed that we could not figure out what had gone wrong. All of the inputs into the FMS were correct and that the autopilot was on. His response was that we would follow it up with a report. I then called Maintenance Control. He said that the 1000 FT terrain terrain to low was a sign the FMS did not know were it was. We did a report to maintenance explaining what had happened then we crew placarded the FMS and the EGPWS.
Both RMI NAV displays showed correct; activated; magenta route in front of us but Departure Control indicated we had flown 2 miles past first turn point (NAVYE) on the NOBLY3 departure. Preflight: Logbook showed IRS #2 battery change from previous flight. Loaded parking spot for gate and noticed only the North position was .2 off of current position in box. ADI/RMI came on line as expected. Noticed green RMI needles #1; #2 showing on RMI plus TTT NAVAID symbol with green hashed line leading from aircraft symbol to TTT symbol. Brief discussion with Captain as to why we were seeing this. Nothing in FIX page and TTT not on our route. No select buttons for 'airport; way points; NAVAIDs;' etc were selected at either crew position. Taxi: Cleared into position Runway 17R. Taxi checklist item 'ND: completed and showed Runway 17R on both displays with proper heading and picture. Both displays in lowest range distance position. In position/hold approximately 15 seconds with power slightly above idle for added cooling. Takeoff: Normal First Officer takeoff with slight rotation delay. At 1000 FT heard; 'terrain; too low' call. Confirmed climb/clear of terrain and dismissed as anomaly. Departure: Displaying approximately 2 miles to go to NAVYE point; autopilot coupled; NAV Cap displayed; IAS 230 selected; slats extended and approximately 2500 FT MSL; Departure Control directed left turn to heading 080 degrees. Departure Control asked what departure we were flying and we confirmed NOBLY 3. Departure Control said we had overflown NAVYE by 2 miles. We informed them we showed 2 miles to go to NAVYE. Both RMIs showed us on course with full magenta route in from of us. Departure Control gave phone number to call after landing. Cruise: Determined all pilot inputs to NAV box were correct and could not determine what had gone wrong with the departure. We elected to continue using VOR/jet route navigation versus the box; informed Center.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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