B737NG VNAV speed restriction anomaly
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B737NG VNAV speed restriction anomaly
I had an odd experience with an NG on a STAR a few nights ago and I thought I'd post it here for comments.
B738 with U10.7 on the HAWKZ3 into Seattle. The FMC NAV database was updated the day before (JAN09-FEB15). DESC page TGT SPD .78/280. LNAV/VNAV PATH. Idle Descent. Cleared to "descend VIA" with 6000ft in the MCP.
Once we passed BTG, I noticed the speed went to 270. I commented that this was odd as I had .78/280 on the DESC. The DESC page showed the active restriction as 270/HAWKZ. The ND showed a DECEL point far in front of us.
We reselected the STAR, and the correct 280 showed again on the DESC page until we crossed PTERA. The FMC wanted us to slow to the 270 speed again for HAWKZ, even though it showed a DECEL point far in front of us.
Each time we crossed a fix, the FMC wanted to slow for a restriction that was far ahead, in spite of where it showed a DECEL.
We ended up flying the correct STAR speeds in VNAV PATH by simply controlling thrust on our own, of course.
There's something familiar about this behavior, but I can't put my finger on it.
Is it a coding error for the STAR? FMC issues? Someone threw the wrong switch down in the avionics bay? Gremlins?
B738 with U10.7 on the HAWKZ3 into Seattle. The FMC NAV database was updated the day before (JAN09-FEB15). DESC page TGT SPD .78/280. LNAV/VNAV PATH. Idle Descent. Cleared to "descend VIA" with 6000ft in the MCP.
Once we passed BTG, I noticed the speed went to 270. I commented that this was odd as I had .78/280 on the DESC. The DESC page showed the active restriction as 270/HAWKZ. The ND showed a DECEL point far in front of us.
We reselected the STAR, and the correct 280 showed again on the DESC page until we crossed PTERA. The FMC wanted us to slow to the 270 speed again for HAWKZ, even though it showed a DECEL point far in front of us.
Each time we crossed a fix, the FMC wanted to slow for a restriction that was far ahead, in spite of where it showed a DECEL.
We ended up flying the correct STAR speeds in VNAV PATH by simply controlling thrust on our own, of course.
There's something familiar about this behavior, but I can't put my finger on it.
Is it a coding error for the STAR? FMC issues? Someone threw the wrong switch down in the avionics bay? Gremlins?
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Yes. BTG transition was selected on the ground during preflight. We received one reroute inflight, but it still included BTG.
Initially the FMC had a an old co route with BTG.OLM7, but that was overwritten. Most of our old co routes have OLM7, but the correct STAR is entered immediately after. I don't think it's a source of the anomaly.
Initially the FMC had a an old co route with BTG.OLM7, but that was overwritten. Most of our old co routes have OLM7, but the correct STAR is entered immediately after. I don't think it's a source of the anomaly.
