KI229 RMI failure - what could be the cause of this?

[Peter]

Hi,

I have a dual-needle RMI, the Honeywell/Bendix KI229.

The instrument has seen various failures over the 3 years I've had it
from new. Just the OBS not turning, just the needles not turning, the
whole lot gone. The instrument has been changed and also a 400Hz
inverter has been changed.

I now have an intermittent failure which always fixes itself once the
aircraft becomes airborne (but never on the ground) so it's probably
vibration related. The OBS does not turn but the needles work fine.

The RMI has its own 28V 400Hz inverter and the dealer reckons it is
that (again).

But surely the KI229 uses the synchro principle to rotate not just the
OBS but also both needles, and the same inverter's failure would
freeze the whole instrument?

The HSI, from which the RMI (I am told) gets the heading data) works
fine.

I am an electronics engineer but haven't got into avionics much; also
obtaining the schematics is a world-class challenge

Can anyone suggest where to look?


Peter.

 

[Andy_R]

Hi,

I have a dual-needle RMI, the Honeywell/Bendix KI229.

The instrument has seen various failures over the 3 years I've had it
from new. Just the OBS not turning, just the needles not turning, the
whole lot gone. The instrument has been changed and also a 400Hz
inverter has been changed.

I now have an intermittent failure which always fixes itself once the
aircraft becomes airborne (but never on the ground) so it's probably
vibration related. The OBS does not turn but the needles work fine.

The RMI has its own 28V 400Hz inverter and the dealer reckons it is
that (again).

But surely the KI229 uses the synchro principle to rotate not just the
OBS but also both needles, and the same inverter's failure would
freeze the whole instrument?

The HSI, from which the RMI (I am told) gets the heading data) works
fine.

I am an electronics engineer but haven't got into avionics much; also
obtaining the schematics is a world-class challenge

Can anyone suggest where to look?

As an electronics engineer you've probably got a lot more idea than I have.
The only things I can think of that happen in the air but not on the ground
are:
1. the engine alternator is running continually at a higher speed so there
is a decent 28v supply regardless of load.
2. the heater is on so any damp dries out

Anything more sophisticated is beyond me.

Rgds

Andy R