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787 in VNAV went into degraded mode

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787 in VNAV went into degraded mode

Old 22nd Aug 2023, 12:14
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RMC
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787 in VNAV went into degraded mode

For no apparent reason in a VNAV climb the autopilot caution (along with an amber line through VNAV FMA etc) presented.
Went into FLC back into VNAV and reengaged OK. Engineers have no idea. Any thoughts?
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Old 22nd Aug 2023, 16:29
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When was the last full electrical power down?
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Old 22nd Aug 2023, 19:56
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Tend to keep them alive

B2N2 Last full electrical power down was likely to have been almost two days beforehand.

Really interested to know what your thoughts are on that. Thanks
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Old 23rd Aug 2023, 17:45
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Iíve seen a number of issues related to VNAV on our 787 fleet, including an uncommanded descent in the cruise and the amber lines through VNAV with an autopilot EICAS caution.

Follow-up suggested that it was caused by a software error on one of the GPMs, resolved with a reload.

I suspect that there are effectively two different autopilots on the 787. The ďnormalĒ one which is hosted on a virtual machine and handles all flying other than auto lands.

For autoland I believe the FCE computers take over with a much more robust and proven design (you can feel the clunk in the controls when LAND 3 or LAND 2 shows on the PFD).

This would explain some of the erratic behaviour of the autopilot in climb / cruise / descent phases.

I would be interested to learn how it actually works if any Boeing engineers read this forum.
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Old 26th Aug 2023, 22:09
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Originally Posted by RMC
B2N2 Last full electrical power down was likely to have been almost two days beforehand.

Really interested to know what your thoughts are on that. Thanks
Nothing eloquent, more like Boeing recommends (?) a full electrical shutdown to clear all the electrical gremlins every 5- 7(?) days.
Donít mind me, Iíve been drinking and donít have the references at hand.
I fly a semi - modern Boeing and mysterious glitches usually canít be duplicated anymore after a full shut eye and wake up.
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Old 31st Aug 2023, 14:16
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VNAV issues more likley to be FMC related than Autopilot related, amber line through FMA means the autopilot is still engaged but the function (VNAV) is degraded.
FMC transfer master to hot spare?
Did you get an FMC L/C/R status message?
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