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Fmc stab trim changes when using atm on the 737max

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Fmc stab trim changes when using atm on the 737max

Old 28th Sep 2022, 14:34
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Fmc stab trim changes when using atm on the 737max

Hi guys! Since the introduction of the MAX on our fleet Iíve noticed that the FMC stab trim changes when entering an assumed temperature (we donít use DERATE), whereas this doesnít happen on the NGís. Any thoughts why ?
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Old 30th Sep 2022, 18:39
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Originally Posted by RocknRola87 View Post
Hi guys! Since the introduction of the MAX on our fleet Iíve noticed that the FMC stab trim changes when entering an assumed temperature (we donít use DERATE), whereas this doesnít happen on the NGís. Any thoughts why ?
It also does so on NG
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Old 30th Sep 2022, 18:45
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Yep but later on and the change is only in the order of 0.01 unit , i guess had to do with APU fuel consumption which decreases a little bit the take off weight , on the MAX it happens instantly when you put an assumed temperature !
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Old 1st Oct 2022, 03:26
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Originally Posted by RocknRola87 View Post
Hi guys! Since the introduction of the MAX on our fleet Iíve noticed that the FMC stab trim changes when entering an assumed temperature (we donít use DERATE), whereas this doesnít happen on the NGís. Any thoughts why ?
Lower ANU moment with higher assumed temperature due to lower thrust? And calculations weren't as accurate back in the NG days, or NG has less thrust couple? Just taking a stab at it....
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Old 1st Oct 2022, 10:05
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Originally Posted by hans brinker View Post
Lower ANU moment with higher assumed temperature due to lower thrust? And calculations weren't as accurate back in the NG days, or NG has less thrust couple? Just taking a stab at it....
I think the geometric thrust line of the MAX is nearly a foot, vertically, closer to the fuselage center line and that drag=thrust means the couple should be less.


If there's an effect of engine I'd consider looking into if it's from the larger bypass ratio as more air will be passing through the fan and not having heat added to it making the thrust more sensitive to the density of the air. Also, in increased density conditions the CL required is lower so the trim should also change; if the density goes down then a higher CL is needed - also a trim change.


Perhaps the better question is why the delay on the NG? Maybe with the smaller bypass ratio the effects tended to cancel?


I know - more questions than answers.
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Old 1st Oct 2022, 10:40
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Originally Posted by RocknRola87 View Post
Hi guys! Since the introduction of the MAX on our fleet Iíve noticed that the FMC stab trim changes when entering an assumed temperature (we donít use DERATE), whereas this doesnít happen on the NGís. Any thoughts why ?
Donít worry, itís just the undocumented FMCAS doing itís job. Unless it goes all the way forward. Then worry.
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