KI229 RMI Failure

This intermittent RMI failure has been observed once or twice shortly after the aircraft was new in 2002, and again during 2004. The original failure was permanent once observed and was "fixed", possibly by the replacement of an inverter or by the replacement of the KI229 RMI instrument (both of these were definitely done in 2002 but I don't know if they were closely related in time). The 2004 failures have always been intermittent, occur once every 5-10 flights, when they are seen they are always seen at startup, and always fix themselves shortly after the aircraft gets airborne, after a maximum time of 10 minutes.

The initial manifestation is that the RMI rotating compass scale does its own thing, rotating in the same direction very slowly (of the order of 1 degree per second, much more slowly than in either normal operation or under the FREE test mode) regardless of anything that happens in the aircraft. The two RMI needles continue to work correctly. The following image shows the RMI heading being entirely different from the HSI heading (you can click on the images to see enlarged versions):

If a flight plan has been entered into the KLN94B and has been activated so it shows on the KMD550 map display, the stornscope screen doesn't show it and shows "ERRORS 22,23" at the top:

When the fault disappears, typically after some minutes, it disappears very suddenly and completely and one gets the normal RMI operation:

and the normal flight plan display on the stormscope screen:

The cause of this was a 400Hz inverter which drives the KI-229. This is a particularly unreliable item which has failed about once a year.

Last edited 29th November 2004


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