SR.............not entirely correct
quote ECD Data -
Attitude and Heading Reference Unit (AHRU)
The Attitude and Heading Reference Units AHRU 1 and 2 (Attitude and Heading Reference Units 1 and 2) use sensors to measure the primary reference flight data such as: attitude, rotational speed, (angular speed or turn rate) and acceleration values for linear acceleration of the helicopter. This data is provided across an ARINC 429 Bus for processing in electronic flight control display equipment and the autopilot computer.
The AHRUs also supply data for the inertial baro-anemometric vertical speed, as set by the ADC Air Data Computer, i.e. the measured data must be sent by the (ADC Air Data Computer) to the AHRUs.
Each AHRU consists of one inertial platform with 3 fibre optic gyros and a miniature silicon accelerometer.
Allowing for the translation to English of course!
It only performs the "I" part of the IVSI function, carried out by the vertical acceleration pump in an analogue instrument.
I don't think there is an annunciation of discrepancy between the VSI's.
I'm sticking to my original post.