Narrative:

Aircraft turned south of terpz RNAV departure just after passing terpz vicinity of jocco cutting the corner off the departure and taking us south of the correct routing. ATC gave us a northwest heading then within 10-15 seconds cleared us to zackk then almost immediately recleared us to hafner. On departure we climbed well above the 900ft expect vector altitude before we were cleared direct terpz. The airplane went directly to the fix in the LNAV mode just as it should have with LNAV annunciated on the FMA. The departure was entered and verified by both pilots at the gate including the 11000 wonce restriction. After we were cleared directly to terpz to join the departure I did not reconfirm all the downrange fixes beyond terpz and jocco which were both there. We were just above 11000 ft when ATC caught the deviation and turned us right. At some point the wonce fix simply dropped out or I somehow deleted it and the airplane was navigating to zackk. For the life of me I can't think how I could have accidentally deleted wonce since in the starting display for terpz departure the fix is well down the page and we navigated correctly to the first fix which was terpz. There was a clue though that should have made me look harder. As we approached terpz still doing only 250 knots the light on the HSI indicating the arrival at an on course turn illuminated at 2.6 miles from terpz with a relatively shallow angle to the next fix (jocco). This seemed a bit far but I accepted it because the following fix was only 3 miles beyond terpz. I think that had I looked at the legs page beyond jocco I would have seen wonce was missing and we were going to go to zackk. I was complacent because the airplane navigated correctly to the fix that begins the departure. I need to verify every time we get a clearance from a vector to join an RNAV departure that the navigational fixes that we began with are still there because this time they weren't and we ended up with a navigational deviation. I sure hope somebody is tracking the lateral errors we're making on RNAV departures because there really are a few places where the B737 FMC does strange things. If the old software can make errors on RNAV arrivals I can't help but think it might do it on RNAV departures and if one keeps showing up maybe it is. We struggled to make our 737's do these things and finally after lots of trouble just gave up on the -300's and admitted they weren't capable of flying precisely enough to do RNAV departures. There are times in the older B737's; even when everything is done perfectly; that approach screams about us being off course. I don't want to be the one who discovers the problems because they are just barely good enough when you do everything correctly.

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Original NASA ASRS Text

Title: A B737 with an FMC but not GPS had a track deviation departing on the BWI TERPZ RNAV. A fix dropped from the FMC track and aircraft turned early. ATC issued an alert about the aircraft's performance.

Narrative: Aircraft turned south of TERPZ RNAV departure just after passing TERPZ vicinity of JOCCO cutting the corner off the departure and taking us south of the correct routing. ATC gave us a northwest heading then within 10-15 seconds cleared us to ZACKK then almost immediately recleared us to HAFNER. On departure we climbed well above the 900ft expect vector altitude before we were cleared direct TERPZ. The airplane went directly to the fix in the LNAV mode just as it should have with LNAV annunciated on the FMA. The departure was entered and verified by both pilots at the gate including the 11000 WONCE restriction. After we were cleared directly to TERPZ to join the departure I did not reconfirm all the downrange fixes beyond TERPZ and JOCCO which were both there. We were just above 11000 ft when ATC caught the deviation and turned us right. At some point the WONCE fix simply dropped out or I somehow deleted it and the airplane was navigating to ZACKK. For the life of me I can't think how I could have accidentally deleted WONCE since in the starting display for TERPZ departure the fix is well down the page and we navigated correctly to the first fix which was TERPZ. There was a clue though that should have made me look harder. As we approached TERPZ still doing only 250 knots the light on the HSI indicating the arrival at an on course turn illuminated at 2.6 miles from TERPZ with a relatively shallow angle to the next fix (JOCCO). This seemed a bit far but I accepted it because the following fix was only 3 miles beyond TERPZ. I think that had I looked at the legs page beyond JOCCO I would have seen WONCE was missing and we were going to go to ZACKK. I was complacent because the airplane navigated correctly to the fix that begins the departure. I need to verify every time we get a clearance from a vector to join an RNAV departure that the navigational fixes that we began with are still there because this time they weren't and we ended up with a navigational deviation. I sure hope somebody is tracking the lateral errors we're making on RNAV departures because there really are a few places where the B737 FMC does strange things. If the old software can make errors on RNAV arrivals I can't help but think it might do it on RNAV departures and if one keeps showing up maybe it is. We struggled to make our 737's do these things and finally after lots of trouble just gave up on the -300's and admitted they weren't capable of flying precisely enough to do RNAV departures. There are times in the older B737's; even when everything is done perfectly; that Approach screams about us being off course. I don't want to be the one who discovers the problems because they are just barely good enough when you do everything correctly.

Data retrieved from NASA's ASRS site as of April 2012 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.