777 Run/Cutoff Relay Operation Query
Thread Starter
Joined: Feb 2004
Posts: 1,410
Likes: 37
From: Australia
777 Run/Cutoff Relay Operation Query
Gentlefolk...
I should know how this works, but my memory fails me...
The 777 RUN/CUTOFF relays are dual coil and appear to have two energised states
777 Fuel Control Diagram
If there is no "neutral"/normally open position, are the relays magnetically latched in the last position with permanent magnets ... or are they rotary-type relays which latch in position.. or none of the above?
Thank you!
I should know how this works, but my memory fails me...
The 777 RUN/CUTOFF relays are dual coil and appear to have two energised states
777 Fuel Control Diagram
If there is no "neutral"/normally open position, are the relays magnetically latched in the last position with permanent magnets ... or are they rotary-type relays which latch in position.. or none of the above?
Thank you!
Joined: Jan 2025
Aviation Qualifications: Non-Aircrew
Posts: 640
Likes: 782
From: New Zealand
Probably so-called 'bistable' relays. I think they use springs and an over-centre mechanism to latch in position, similar to a toggle switch. Just the actuation is by two coils (or one coil operated in two polarities), rather than by fingers.
Thread Starter
Joined: Feb 2004
Posts: 1,410
Likes: 37
From: Australia
https://www.leachint.com/wp-content/...tique_M402.pdf
Not sure I understand the symbology in that document, either.
I see the fuel spar valve has 3 levels of "latching". One of the latching fuel relays shown above controls a spar valve latching relay. The spar valve latching relay controls the polarity of the spar valve DC motor. If power is lost from the spar valve latching relay, the motor is obviously not going to move, keeping the valve in its last commanded position.
