777's PFC reset (volcanic ash C/L)
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777's PFC reset (volcanic ash C/L)
I was recently taught that in the (unlikely ?) event of an "Airspeed Unreliable" within Volcanic Ash, it is better not to reset PFC at the end of the Volcanic Ash C/L (whereas it is explicitly written to do it in the FCOM) because the reset will be based on unreliable airspeed.
That makes sense to me but where it is written ? Where can I find information on PFC reset ?
Thanks
That makes sense to me but where it is written ? Where can I find information on PFC reset ?
Thanks
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When you disconnect the PFC, you are removing all envelope protections. (Direct Mode). Your control inputs are translated to a voltage to the ACE's (Actuator Control Electronic) which will move the relevant flight control as much as you command them , irregardless of airspeed, bank, etc.
I suspect the reason for the disconnecting for unreliable airspeed would be to do with stall warning/stick push or other floor protections which may limit/override your input.
Have a look in the SDS 27-02-00 page 15
I suspect the reason for the disconnecting for unreliable airspeed would be to do with stall warning/stick push or other floor protections which may limit/override your input.
Have a look in the SDS 27-02-00 page 15
Last edited by LME (GOD); 22nd Jan 2013 at 13:27.
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Robert, don't start inventing procedures. If a TRI/TRE instructed you in this manner, he/she should be removed from that position (contact your management). If the reset does not bring back normal mode, perhaps you will have secondary. And so what if you get into unreliable airspeed! You are already there! Didn't you get the extra training on that subject?
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777 PFC Reset enables return to Normal Mode
If the 777 control system detects unreliable air data it will drop from Normal Mode to Secondary Mode. Secondary Mode has the same functionality as Direct Mode (see earlier description in this thread). There are no envelope protections (bank angle, speed, AOA) in either Secondary or Direct Modes.
The logic behind cycling the PFC Disconnect switch following an ash cloud encounter that has dropped the system out of Normal Mode is that this may allow re-engaging Normal Mode. The system will not upmode to Normal on its own, but will attempt an upmode when the PFC Disconnect switch is moved from its disconnect position to it normal operation position.
If you are in Secondary Mode following an ash cloud encounter, the worst that cycling the PFC Disconnect switch will do is put you temporarily into Direct Mode and then back into Secondary Mode. The upside is that you will end up back in Normal Mode with all of its protections if the fault that cause the system to drop out of Normal Mode in the first place no longer exists.
The logic behind cycling the PFC Disconnect switch following an ash cloud encounter that has dropped the system out of Normal Mode is that this may allow re-engaging Normal Mode. The system will not upmode to Normal on its own, but will attempt an upmode when the PFC Disconnect switch is moved from its disconnect position to it normal operation position.
If you are in Secondary Mode following an ash cloud encounter, the worst that cycling the PFC Disconnect switch will do is put you temporarily into Direct Mode and then back into Secondary Mode. The upside is that you will end up back in Normal Mode with all of its protections if the fault that cause the system to drop out of Normal Mode in the first place no longer exists.
