Narrative:

I was acting as check airman instructing a captain on his first trip on the north atlantic. Shortly after passing dovey we were asked by ATC; via cpdlc; what altitude and mach we were requesting at 50 west. We replied; via cpdlc; FL360; mach.78; which were the altitude and speed we were already using. A moment later we received a clearance to maintain FL360; mach.78. This was uplinked into the FMC via cpdlc. We checked then loaded and executed the clearance. I'm not sure why we did this since we were already at that speed and altitude. As soon as we executed the clearance; I sensed something was wrong. I checked the legs page and was shocked to discover that all waypoints from N42 W60 to atsur were gone; with a route discontinuity after 60W. I have no idea how this happened; everything up to this point was normal and by the book. After a few choice words we set about fixing this. We had time since we were some distance from 60W. I started to load waypoints manually but decided maybe it would be best to uplink the route; which we did. This worked fine but before I activated and executed; I put the plane in heading hold. It was at this point that we received another cpdlc message from ATC telling us to re sequence our waypoints and verify our next after 60W. Once executed and back in LNAV I made the mistake of starting the route at 60W which probably would have been fine except we had been on a one mile right offset; so now the offset was slowly coming out over 150 miles or so. I am quite sure we never moved left of track or more than one mile right offset. After 60W we were on course and all was normal. I can only think that when we loaded the uplinked cpdlc clearance it somehow affected the FMS route. This seems strange since the uplink did not include routing.

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Original NASA ASRS Text

Title: An air carrier flight crew on an eastbound north Atlantic flight suffered the loss of route/legs data in the FMS when they uploaded an overwater mach and altitude clearance from ATC via CPDLC.

Narrative: I was acting as Check Airman instructing a Captain on his first trip on the North Atlantic. Shortly after passing DOVEY we were asked by ATC; via CPDLC; what altitude and mach we were requesting at 50 west. We replied; via CPDLC; FL360; mach.78; which were the altitude and speed we were already using. A moment later we received a clearance to maintain FL360; mach.78. This was uplinked into the FMC via CPDLC. We checked then loaded and executed the clearance. I'm not sure why we did this since we were already at that speed and altitude. As soon as we executed the clearance; I sensed something was wrong. I checked the legs page and was shocked to discover that all waypoints from N42 W60 to Atsur were gone; with a route discontinuity after 60W. I have no idea how this happened; everything up to this point was normal and by the book. After a few choice words we set about fixing this. We had time since we were some distance from 60W. I started to load waypoints manually but decided maybe it would be best to uplink the route; which we did. This worked fine but before I activated and executed; I put the plane in heading hold. It was at this point that we received another CPDLC message from ATC telling us to re sequence our waypoints and verify our next after 60W. Once executed and back in LNAV I made the mistake of starting the route at 60W which probably would have been fine except we had been on a one mile right offset; so now the offset was slowly coming out over 150 miles or so. I am quite sure we never moved left of track or more than one mile right offset. After 60W we were on course and all was normal. I can only think that when we loaded the uplinked CPDLC clearance it somehow affected the FMS route. This seems strange since the uplink did not include routing.

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.