Narrative:

We were cleared for takeoff on runway 34R on the brycc 2 departure; RNAV to caazz. I read back the clearance; and then both the captain verified the runway on the FMS. The from waypoint said 34R; and the first fix was caazz. We said 'before takeoff checklist complete' and the captain initiated the takeoff roll. About halfway down the runway after the 80 KTS callout; I looked at my mfd and noticed that the airplane icon was offset from the 34R waypoint. I then looked down at my pink needles and noticed that the needle was pegged off to the left. I didn't say anything at that moment because we were in a high speed phase of takeoff and didn't want to cause an abort. As soon as we rotated; I mentioned that something weird was going on with the FMS. The captain stayed on heading and then tracked towards caazz when the FMS sequenced its leg. Once we contacted departure we were given direct to brycc; I think; and I began looking at the FMS to find out what was going on. I went to the position sensors page of the FMS and found that there was no location for either GPS 1 or GPS 2. On the GPS status page 2; it showed acquisition and that it was tracking 0 satellites. Moving to the VOR/DME page; I found that the FMS was using DME/DME data to determine its position. About 5 minutes into the flight; GPS 1 finally acquired a position; and the position sensors page said that the FMS position differed from GPS 1 by about 0.4 NM. A few seconds later the FMS updated and was then collocated with GPS 1. The rest of the flight proceeded without incident. The aircraft never got GPS signals before takeoff. We don't really do a check for that; and there's no annunciation that this has occurred. The FMS had a valid DME/DME position; so it didn't go into dgrad or dr mode; but its position was not accurate enough to place us on the runway when we were in fact on the runway. I made an error in only checking the FMS and the runway outside; and neglected to check my FMS needle centered before the takeoff roll; though that threat was mitigated by ATC's giving us a direct to shortly after takeoff; so the RNAV path was not critical like it normally would be. We are doing more and more of these RNAV off the ground procedures; but we just sort of assume that we have a GPS position and that the FMS is using a good location. But what happens when it doesn't have a GPS position? We're not trained; as far as I can tell; on ensuring that the FMS is using GPS data. We're not told; or it is not emphasized; that the FMS is perfectly happy using DME/DME data for its location and will not tell you about it in an obvious way; the only thing that I found after looking around was the GPS lrn = 2 on the progress page of the FMS; which changes to DME/DME lrn = 0 when it has no GPS signal. The MEL book indicates that without both GPS signals we're prohibited from operating on RNAV paths. This seems to suggest to me that even though the FMS happily uses DME/DME; the accuracy is suspect enough that we need to know not to use it. If GPS is required for RNAV paths; we need to incorporate a check that the GPS is in fact functioning prior to flight. In our case; we didn't find out about it until in the air; and that was after a confusing few minutes with unreliable FMS information. On a clear day in den with a non-critical RNAV path it was ok; but in IMC in atl; for instance; it could have drastically different results. In this case; an unannunciated system failure on a system that we do not check resulted in a 0.5 NM course offset. Can we do something about this?

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Original NASA ASRS Text

Title: EMB145 First Officer discovers a nav accuracy error during the takeoff roll. Once airborne it is found that neither GPS receiver is tracking any satellites. Five minutes into the flight GPS1 acquired a position and the remainder of the flight was normal.

Narrative: We were cleared for takeoff on Runway 34R on the BRYCC 2 departure; RNAV to CAAZZ. I read back the clearance; and then both the Captain verified the runway on the FMS. The FROM waypoint said 34R; and the first fix was CAAZZ. We said 'Before Takeoff Checklist complete' and the Captain initiated the takeoff roll. About halfway down the runway after the 80 KTS callout; I looked at my MFD and noticed that the airplane icon was offset from the 34R waypoint. I then looked down at my pink needles and noticed that the needle was pegged off to the left. I didn't say anything at that moment because we were in a high speed phase of takeoff and didn't want to cause an abort. As soon as we rotated; I mentioned that something weird was going on with the FMS. The Captain stayed on heading and then tracked towards CAAZZ when the FMS sequenced its leg. Once we contacted Departure we were given direct to BRYCC; I think; and I began looking at the FMS to find out what was going on. I went to the POS SENSORS page of the FMS and found that there was no location for either GPS 1 or GPS 2. On the GPS status page 2; it showed ACQUISITION and that it was tracking 0 satellites. Moving to the VOR/DME page; I found that the FMS was using DME/DME data to determine its position. About 5 minutes into the flight; GPS 1 finally acquired a position; and the POS SENSORS page said that the FMS position differed from GPS 1 by about 0.4 NM. A few seconds later the FMS updated and was then collocated with GPS 1. The rest of the flight proceeded without incident. The aircraft never got GPS signals before takeoff. We don't really do a check for that; and there's no annunciation that this has occurred. The FMS had a valid DME/DME position; so it didn't go into DGRAD or DR mode; but its position was not accurate enough to place us on the runway when we were in fact on the runway. I made an error in only checking the FMS and the runway outside; and neglected to check my FMS needle centered before the takeoff roll; though that threat was mitigated by ATC's giving us a direct to shortly after takeoff; so the RNAV path was not critical like it normally would be. We are doing more and more of these RNAV off the ground procedures; but we just sort of assume that we have a GPS position and that the FMS is using a good location. But what happens when it doesn't have a GPS position? We're not trained; as far as I can tell; on ensuring that the FMS is using GPS data. We're not told; or it is not emphasized; that the FMS is perfectly happy using DME/DME data for its location and will not tell you about it in an obvious way; the only thing that I found after looking around was the GPS LRN = 2 on the progress page of the FMS; which changes to DME/DME LRN = 0 when it has no GPS signal. The MEL book indicates that without both GPS signals we're prohibited from operating on RNAV paths. This seems to suggest to me that even though the FMS happily uses DME/DME; the accuracy is suspect enough that we need to know not to use it. If GPS is required for RNAV paths; we need to incorporate a check that the GPS is in fact functioning prior to flight. In our case; we didn't find out about it until in the air; and that was after a confusing few minutes with unreliable FMS information. On a clear day in DEN with a non-critical RNAV path it was OK; but in IMC in ATL; for instance; it could have drastically different results. In this case; an unannunciated system failure on a system that we do not check resulted in a 0.5 NM course offset. Can we do something about this?

Data retrieved from NASA's ASRS site as of July 2013 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.