Universal FMS Battery Issues: CPU/Data Bus Failures & Replacement

May 2025

If you are experiencing frustrating CPU Fail, Data Bus Fail, or even RAM Fail messages during the initial startup of your Universal FMS (Flight Management System), you are not alone.  

These seemingly complex errors often stem from a simple source—the batteries.

The Universal FMS relies on two dedicated batteries to safeguard critical data. The battery located on the CPU board ensures the retention of your programmed flight plans and navigation data. Simultaneously, a separate battery residing on the GPS board specifically supports the almanac data, which is essential for accurate GPS positioning.

Signs Of a Failing FMS Battery

The FMC BATTERY LOW message indicates the battery on the internal CPU Board has depleted to the minimum threshold. This message can be viewed only before the FMS initialization data is accepted. It is difficult to judge how much longer the FMS will operate before being rendered inoperative and posting CDU DATABUS FAIL.  

Critical Failure: If a battery is allowed to fully deplete, it will halt the FMS during its critical self-test phase, displaying the red CPU Data Bus Fail message. At this point, the system will refuse to proceed, rendering the FMS unusable until a new battery is installed, potentially causing significant delays and disruptions to your operational schedule.

Older FMS Systems (UNS-1x)

In our experience, batteries in the older Universal UNS-1x FMS units provide about three years of useful service. They can fail suddenly without warning, leading to the CDU (Control Display Unit) Data Bus Failure message. We highly recommend replacing these batteries within this three-year window to mitigate the risk of in-flight failures, unscheduled maintenance, and costly downtime.

Newer FMS Systems

Newer Universal FMS use a different type of battery that has a longer useful service life of approximately 10 years. However, just like the UNS-1 systems, these batteries can fail without notice when they reach that threshold.

Battery Location Based on FMS Configuration:

•  Single-box FMS with integral NCU and CDU (UNS-1C, UNS-1Csp, UNS-1C+, UNS-1Csp+, UNS-1E, UNS-1Esp, UNS-1EW, UNS-1EspW): In this configuration, the Flight Plan and Almanac batteries are located within the pedestal-mounted unit. These units will have a GPS antenna coaxial cable connection on the rear of the pedestal-mounted unit.
• Two-box FMS with a remote-mounted NCU (Navigation Computer Unit) (UNS-1D, UNS-1D+, UNS-1K, UNS-1K+, UNS-1F, UNS-1L, UNS-1FW, UNS-1LW): When your FMS includes a separate NCU, both the Flight Plan and Almanac batteries will be found inside the NCU.

NOTE: The CPU and GPS batteries are surface mounted onto internal boards and can only be replaced by a Universal Avionics Repair Station or Authorized Service Center.

More Information

Don't wait for a system failure. Contact Duncan Aviation's experienced Avionics Instrument Technical Representatives today at 800.562.6377 for expert assistance and to schedule your battery replacement.