Ep3man

FMC Message "CUTBACK DISARMED"

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Today, I departed RJAA/16R (P3Dv4.3 default) enroute to WSSS.  Climbing through 1600ft I received a "CUTBACK DISARMED" message on the FMC.  The remainder of the climb went fine (except the now expected CLB 1 not engaging).  This one was new for me so I checked the docs.  I could only find in the FCOM 11.60.3 that it was due to a runway change while on the ground.  After verifying the correct runway was loaded into the FMC (and that I didn't foolishly launch from the wrong runway), I was somewhat at a loss.  I loaded the FMC while at gate 22 (according to P3D) and taxied to 16R via A.  I was on the SAKUR4 departure.  I received no notification during taxi or takeoff roll.

I was hoping someone could help point me in the right direction as to what might cause this.  Not discounting any error between the chair and yoke.

Thanks!

Edited by Ep3man
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I see that too, the FCOM mentions only runway change.  Same for the NG FMS manual that I have.... I assume we are talking about the 747-8 and you had Q Climb armed?  There is a short list of reasons why the cutback thrust might become unavailable, probably in this case the FMS was unable to achieve the cutback goal of maintaining a 1% climb gradient (I am going from memory here) with a thrust cutback.  You should have had access to CLB 1 from the THR REF page.  Also, Q Climb is a VNAV mode, so did VNAV function otherwise as expected?

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Thanks Dan,

You gave me an idea.  I didn't arm Q Climb during preflight as I'm not yet familiar with it.  I just armed CLB1.  At the time of the message, hadn't yet engaged VNAV.  After looking at things again, I reviewed the SID and may have connected a few more dots.  After takeoff from 16R the SID requires a teardrop back over the NRE VOR, then shortly after at TETRA it requires +12000 ft.  I was only at 706K GW but that's still a lot of weight to get up that high in a relatively short distance.

I'm just guessing here.  Could it be possible that the system recognized this and automatically disarmed the cutback in order to provide enough thrust to meet the alt restriction thus triggering the CUTBACK DISARMED message?

 

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14 hours ago, Ep3man said:

I didn't arm Q Climb during preflight as I'm not yet familiar with it.  I just armed CLB1. 

Ah... the words acquire certain meanings.  Cutback would refer to a temporary reduction in thrust whereas a derate refers to CLB1 and CLB2 thrust derates.  That was until you posted this about the CUTBACK DISARMED message.  I'm with you on this one in that it is a puzzle.

I cannot find anything in the manuals that suggest the FMS will cancel a CLB1 10% derate of climb thrust.  If there is an altitude constraint such as your AT OR ABOVE 12000 than is not going to be achieved with CLB1 then the pilot needs to cancel the derate using the THRUST LIMIT page.

Can you reproduce the events that result in a CUTBACK DISARMED message, take careful notes of things like TOGW, V2 and takeoff configuration?

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Sure.  I could reproduce it using the same and perhaps a couple of different configurations to see if I get the same result.  Unfortunately, I'm away from home ATM and won't be able to try until I get back tomorrow.

Curiously though, based on the information I could find in the FCOM, I would think this message might appear while on the ground.  

 

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43 minutes ago, Ep3man said:

Unfortunately, I'm away from home ATM and won't be able to try until I get back tomorrow.

Great... when able please do and then I'll use your steps to see if I can reproduce it.  I suspect this will end up on a trouble ticket for the bug tracking system if we can reliably reproduce it.

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