2011-06 · NASA ASRS report 952769
A B737-500 flight crew had a track deviation when their attempt to program a 'direct to a fix with intervening abeam points' went awry. The lack of a moving map Navigation Display was cited as a contributing factor.
ATC vectored us to 030 degrees for traffic; then re-cleared us direct BOWEN. I selected 'BOWEN; Page1; ABEAM POINTS; Execute (I saw the abeam points repopulated) then LNAV; and VNAV switches down.' The aircraft turned back onto course 30 degrees back to heading prior to 30 degree vector. This was the wrong direction. What I believe happened is the FMC did not receive the 'execute' command either because I did not hit it or it did not accept the BOWEN abeam points. Regardless; the aircraft turned the wrong direction when I selected LNAV. ATC queried our turn. We again line selected BOWEN without the abeam points and turned back direct BOWEN. I was not spatially aware that BOWEN was another 30 degrees to the left of course; so when the aircraft turned right; it did not raise my suspicions because I thought we were turning back on course toward our previous heading. Since I believed I hit all the right buttons and saw the abeams populate to BOWEN; when the aircraft turned; I felt I was turning in the correct direction. I need to confirm the heading to the next point I line select to the top of Page 1 and before hitting abeam point note the heading to ensure the aircraft turns to the correct heading.
A few minutes later; Boston called out traffic climbing on the right side. The remainder of the flight was uneventful. My assumption was that we were going direct BOWEN upon the FMC execution that turned right from the 030 vector. I did not know that BOWEN was to the left of the vector. It appears that that as the new course was not actually selected in the FMC; the aircraft simply turned toward the previous LNAV FMC point from before the vector into the traffic it appears they wanted to vector us from. After today; I recommend no longer using abeam points; as the primary point often ends up on Legs Page 2 (or more); as it did today. For those that insist on using abeam points; the course must be confirmed to the point before executing; followed by confirming the '01' courses are similar. The lack of a map display was a primary factor in our lack of situational awareness of the fix position.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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