Honeywell AD-650 ADI Preventative Maintenance

September 2022

Have you ever waited too long to do a minor repair and had it turn into a costly, major event? This can also apply to aircraft instruments.

For example, we have seen this occur many times on a Honeywell AD-650 ADI where the filter capacitors in the power supply degrade and become “leaky,” indirectly causing damage to other components.

This particular ADI has multiple microservo assemblies that control the displays for pitch and roll command, glideslope, rate-of-turn, speed command and localizer.  These microservos work fine, as long as the instrument isn’t handled roughly and the power supply remains constant.

When filter capacitors get older, they sometimes begin to break down and are no longer able to filter the voltages for smooth outputs. If any of the capacitors were to short out, it could ground out its associated voltage, causing a microservo or several microservos, to slam to the limit of their travel.

This can crack a worm gear or break tiny gears within the microservo, rendering it useless.

Microservo assemblies cost $5,000-$15,000 each. A new power supply is about $1,000.

A smaller expenditure in parts now could save thousands of dollars down the road.

Visual Warnings 

The following are a few visual warnings of impending power supply failure. 
1.   Dimming of or brightness level changes on the digital displays 
2.   A flag that flutters in and out
3.   Pointers or command bars that seem to no longer be centered, when they should be
4.   R/T flag vibrating or whining, when it is pulled out of view
5.   General intermittent operation 

Example of a leaky capacitor

A quick visual evaluation could reveal leakage and/or discoloration of the capacitors, the presence of a fishy smell caused by leaking capacitors and a measurement of alternating current ripple will reveal the condition of your power supply.

This will allow the technician to chart the best course for preventative measures, to ensure no further damage occurs.

The adage is true. An ounce of prevention is worth a pound of cure.