I've learned about Phase 1, Phase 2, and Phase 3 electrical systems. But recently I've faced an issue about replacement of Phase 3 Common Display System (CDS) Display Units (DUs) with new Phase 3A CDS DUs.

My question is what does it mean actually regarding Phase 3A? And what are the differences between Phase 3 and Phase 3A?

  • $\begingroup$ As there is an upgrade from 3 to 3A possible with a kit and an upgrade, and the FAA notice that orders the phasing out (no pun intended) of the phase 3 displays was about possible WiFi interference, my guess is that 3A is a emissions-hardened variant of 3 $\endgroup$
    – bukwyrm
    May 30, 2018 at 11:23

1 Answer 1


It is just a matter of technology, it turned out that phase 3 are subject to interferences from wireless devises, resulting in disappearance of displayed data.

We have been alerted of this some years ago and as long as several similar equipments are simultaneously functional, it is probabilistically nearly impossible to loose the parameters on all these equipments simultaneously.

I found something about this on the following website, but may be on Honeywell website you will get more details


So the trouble of interferences is specific to phase 3 DU, phase 3A corrects this effect.


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