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cslawrence

VNAV Descent Throttle Issue

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I've had it happen a few times recently, but the 777 on P3D (haven't tried in FSX) will suddenly go full throttle not too long after I initiate a descent. VNAV will be enabled, I'll set an altitude in the MCP, and push the ALT knob to initiate the descent. Not too much later (within a minute or so) the throttle will advance to full on its own and I have to bring the throttles back manually. I looked in the FCOM and couldn't find anything describing this behavior, but I may not have been looking in the right place. Am I hitting a bug or is this expected behavior? If it's the latter, what's going on here?

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I've had it happen a few times recently, but the 777 on P3D (haven't tried in FSX) will suddenly go full throttle not too long after I initiate a descent. VNAV will be enabled, I'll set an altitude in the MCP, and push the ALT knob to initiate the descent. Not too much later (within a minute or so) the throttle will advance to full on its own and I have to bring the throttles back manually. I looked in the FCOM and couldn't find anything describing this behavior, but I may not have been looking in the right place. Am I hitting a bug or is this expected behavior? If it's the latter, what's going on here?

 

Your issue sounds very similar to the one described in this thread:

 

http://www.avsim.com/topic/472279-rapid-throttles-acceleration-on-approach-p3d-25-here/page-3#entry3286343

 

It would be useful to let everyone know that you are experiencing a similar problem.  I don't have P3D so I can't be of further help.

 

Mike

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Am I hitting a bug or is this expected behavior? If it's the latter, what's going on here?

 

Expected behavior.

 

You're likely using the hardware setting to override your hardware in all situations except HOLD. The initial descent in VNAV is done in HOLD mode. As such, if your throttle spikes some, the sim will "wake up" to the fact that the hardware and the in-sim throttle are in different positions. Since you likely left your hardware at nearly full thrust at takeoff, the sim starts paying attention to your hardware and sets your in-sim throttle to your hardware position.

 

I made a video about this a while ago to explain everything with visuals:

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I like the override in hold setting, so I am in the habit of pulling throttles back to idle before 80 kts on takeoff roll with TOGA active and leaving them there until there's a reason.

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I'll set an altitude in the MCP, and push the ALT knob to initiate the descent.

A small tip regarding the T7, it is a Boeing, not an Airbus. You don't need to push Alt intervene to initiate the descent. Just set a lower altitude on the MCP and the aircraft will start descending by herself once reaching the calculated TOD.

(just in case you wouldn't know already).

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I like the override in hold setting, so I am in the habit of pulling throttles back to idle before 80 kts on takeoff roll with TOGA active and leaving them there until there's a reason.

With that option set, in HOLD mode you ideally want the throttle input to match the simulated lever position. I'd leave them at TO thrust until in the climb at least in case of the need to RTO. Closing the throttle will initiate that automatically. My habit is to retard the throttle just before top of descent when I set the MCP ALT.

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