Originally Posted by
Loose rivets
Single relay!? That would seem to obviate the safety factor that the series switch modification gives. Well, at least in part. I would be horrified to learn there was truly only one point of cut off to that vital flying surface motor.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.
There is one main (3 phase) power relay to the the trim motor block, on the MAX at least it is shown feeding a AC/DC/ converter so not an inductive load.
The NG diagram has less detail in the motor block.
This relay is controlled by the cutout switches but is normally on (energized).
There are also a number of 'enable' as well as direction input shown connected to the motor block, on the NG the autopilot signals are interrupted by the right hand switch. The left switch kills power to the power relay. On MAX either switch kills power (and control inputs)..
Note both of these are labelled "functional description" so don't include all details.