Gents
Fully access to Manufacture documents . It looks like the IMU is equipped with single axis Sensors working as Gyrometers.
An IRU detects it's position in space and movement through it, an attitude indicator only it's position.
Standby horizon function
The ISIS indicator can operate from -180 to +180 deg. in pitch and roll without deterioration.
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gyrometers (3): a monitoring is triggered when the angular speed in x, y or z axis becomes greater than 95 deg./s. ISIS can follow a 10,000 deg./square second angular acceleration.
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accelerometers (2): the associated monitoring is released when the constant acceleration in x and/or y axis becomes greater than 3.25 g.
If this monitoring is triggered, the "WAIT ATT" message is displayed.
If the conditions become normal again and if the attitude performance remains adequate for 10 seconds , the "WAIT ATT" message disappears and is replaced by the horizon display.
Otherwise, after 10 seconds or if the attitude performance is affected, the attitude function is considered as failed and the "ATT:RST" message is displayed in place of the "WAIT ATT" message
Pressing the RST pushbutton switch more than 2 seconds in steady flight quickly resets the attitude information.
Anyhow, it is an seperate standby unit, even when connected to several computers, it acts alone and gets it's physical infos from it's on sources.
BOAC , I hope this Info about the quick reset possible in flight answers yr question. I think steady flight means in this philosophie less than 3.25 g.
kind regards