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737NG - FMC input to determine TO thrust - (what faults will cause a split?)

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737NG - FMC input to determine TO thrust - (what faults will cause a split?)

Old 23rd Jul 2013, 11:17
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737NG - FMC input to determine TO thrust - (what faults will cause a split?)

Hello all.

I have a question regarding the 737-800 autothrottle thrust limit on takeoff, as computed by the FMC.

The other day, when entering takeoff performance data into the FMC, the AT thrust limit showed a split of about 0.8pp. N1. I can't recall the specific numbers, but something like a TO-2 and some assumed temp, gave us, say, a thrust limit of 90.0 and 90.8.

From the ensuing discussion, we observed from the QRH-PI that variations in bleeds, packs or anti-ice all could give differences in takeoff N1 of this magnitude.

However, for this time, all systems were "normal", ie bleeds on, packs auto and antiice off. This leads me to assume there might have been some kind of sensing fault somewhere, leading the FMC to "think" that one of the sides were configured differently from the other.

(Unfortunately I didn't write down the specific numbers, and I can't recall if the split was on the high or low side of the expected N1. It went into the log and I haven't been in this aircraft since so I don't know how maintenance solved it).


The question then becomes; what data and sensors are input to the FMC, to allow it to decide a taget takeoff N1?


Thanks a lot
bfisk is offline  
Old 23rd Jul 2013, 12:05
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While doing perf. calcs with APU feeding aircon packs (xvalve open/ both packs auto) I've often seen 0.8% difference to max N1 (26K) even though both packs should be on in normal flow. I usually take lower figure. You'll probably get a better answer from a trainer.
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Old 23rd Jul 2013, 15:42
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Sounds to me like you had the isolation valve switch closed. The difference between bleeds ON & bleeds OFF at 26k is 8%. The hi N1% reading would have been the right side with APU feeding left duct only.
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Old 23rd Jul 2013, 21:12
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I've seen splits before, as well. I did hear a plane conversing with MX over the company frequency with this problem just a few days ago. They ended back at the gate. Not sure if it was a sensing issue or a valve issue, but the switches were supposedly in the normal positions.
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