Widebody; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Procedure
The initial preparation for the flight went well; and was completed properly and accurately In Accordance with SOP. This report pertains to an event with the navigation system/FMC. There were no prior issues with the system recently reported. The IRUs were initialized from OFF; and the ramp position was derived from the Airport Diagram; Electronic Flight Bag (EFB) [page] '10-9'; compared to the airport reference coordinates; and also; as a triple check; compared to the GPS position displayed on the EFBs. It was verified by both of us as being properly entered into the FMC during initial alignment. The FMC in this case is a non-GPS equipped 'Legacy Box'; which does not automatically update position to the selected runway when the 'N1/EPR button' is pushed at the beginning of the takeoff roll; often requiring a position update prior to entering the runway for takeoff.After a normal and early block out; we taxied out. I recall viewing the ND (Navigational Display); and we appeared to be in a reasonable position relative to the selected runway; 19. Prior to block out; in anticipation of possibly updating our position prior to takeoff; I retrieved the runway coordinates from the FMC database by selecting the LOC/BC 19 approach; line selecting the runway point from its list of waypoints onto the scratchpad; erasing the approach without executing it; pulling up the index page; selecting NAVIGATION DATA; and moving the runway point to the open blocks on that page. This presented the Runway 19 coordinates...; which I wrote down on my paper scratchpad; as well as onto the scratchpad of the CDU; and checked them against the Airport Diagram. Upon reaching the hold short point at the intersection of F/19; I stopped the aircraft. Although we appeared to have been still in a fairly accurate position situation on the ND; I had extra time; and I decided to update the FMC position prior to takeoff. I initiated a quick realign of the IRUs; and went to move the coordinates from the CDU scratchpad to the open aircraft position blocks on the Position page; when I noticed that the coordinates were no longer in the scratchpad. This is where things may have gone wrong; barring any deficiencies found post flight by Maintenance; which I am currently unaware of. I repeated the coordinate retrieval process described earlier; and typed those coordinates back into the CDU scratchpad. I returned to the Position page; showed the coordinates to the First Officer (FO); he agreed with me; and I entered and executed them; neglecting to also compare them to the coordinates I had written down on paper the first time. The Alignment completed seconds later; and Tower cleared us onto the runway; cleared for takeoff; direct to MZV VOR. I believe at this point; I rotated my ND range selector from the lowest range to a much further out range in order to mentally orient myself with the direction of MZV. This action may have prevented me from noticing a self-induced 'Map Shift' event; or the map just hadn't caught up yet. Either way; I had mentally already 'checked that box'; so to speak; but that was before the position update; so that was null and void. We initiated the takeoff roll with the FO as Pilot Flying; and all appeared normal. After liftoff; we received an aural 'Terrain' warning; in VMC conditions; where there was obviously no terrain or obstacle present. I immediately referenced the ND; and saw that the RFD airport was depicted on it directly in front of us. Obviously; it was actually behind us. We continued the climb out otherwise normally; and I immediately selected Manual on the VOR control head. I compared the DME to the mileages depicted on the LEGS page and the FIX page with MZV entered. The difference was about 8 miles. By the time I had processed this in formation; I was about to contact Center; and had just passed over the MZV VOR; according to the Navigation Radio. We had not yet reached it according to the FMC. He contacted us rightthen; and confirmed that we were just past it. I requested a radar vector; and informed him that we had a navigational problem; and we're working on it; and that we would like to remain on vectors for the time being. The controller was very understanding; and seemed eager to assist us.Although I am not certain of this; I will assume that the discrepancy was caused by operator error; mostly on my part. I did not stress enough to the FO the importance of cross checking my coordinate selections; or their input; or the verification of aircraft position afterwards. The FO may not have realized at that time that I had to re-type those coordinates into the scratch pad after they disappeared; so he may not have felt the need to re-check them. I may have typed in [the wrong coordinates] by accident the second time; and not caught it at that point. I may have selected the wrong waypoint from the list; which was very similar in name to the runway waypoint; and used those coordinates the second time around. I needed to take an extra moment before entering the runway for takeoff to verify our aircraft position on the ND; but had mentally checked that off already. Unfortunately; that mental check was completed before the position update. Being cleared for takeoff and directly to MZV; just before positioning; I increased the range out to view MZV on the ND; which degraded the apparent accuracy of that picture; making it more difficult to notice that we were not quite in the right place.
We did some troubleshooting and found that the IRSs were in a degraded situation; hence the inaccurate map position and false GPWS warning earlier.... We continued to [destination] on our filed jet route; tracking VORs along the way.
Air Carrier flight crew reported that a self-induced FMC position error was the likely cause of a false GPWS Terrain Warning and track deviation after takeoff.