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What Happens During an L-3 Vertical Gyro Overhaul?

January 2015

A gyro is an essential piece of business aircraft avionics equipment. You invest a lot of money to keep it airworthy. And when it comes time for overhaul, you pack it up and send it in. But do you know what really happens when you send it in?

STOP! Before you run off to box and ship your gyro for overhaul, read this!

Why? Because if you don’t, this could happen!

And if it does…it could cost you lots of extra money. Here’s why.

Here’s what happens to your L-3 Vertical Gyro during an overhaul at Duncan Aviation.

Initial Function Test

what-happens-during-an-l-3-vertical-gyro-overhaul_clip_image002Every gyroscope that arrives is run through the traps of an initial function test. This provides us with a clear picture of the condition of the unit upon arrival and a baseline of information about the indicator, autopilot and radar. 




Prep, Evaluation & Customer Approval

what-happens-during-an-l-3-vertical-gyro-overhaul_clip_image004All decals and paint are removed from the unit and the chassis is unsoldered from the base assembly. During this step, your gyro receives a thorough evaluation and the results are communicated to you prior to any repair or overhaul. We do this so that you are aware of all identified squawks and / or damage sustained during shipping. 



Disassembly and Inspection

During the disassembly, every part is removed, cleaned and inspected for damage and proper function. At the end of this stage, the gyro is reduced to its most basic components.

Remove base assembly

Remove inner / outer gimbal

Remove inner gimbal assembly from outer assembly

Remove inner (pitch synchro) shaft assembly and rotor covers

Disassembly complete


New Replacement Parts

Per the L-3 Communications Vertical Gyro Component Maintenance Manual there is a required list of components that must be replaced with new OEM components. These parts include rotors, gimbal bearings and a spiral flex, to name a few. The rotors are rebuilt in-house at Duncan Aviation using our Rotor Pre-load Fixture and Rotor Balancing Machine.
If we were to stop with the required list above, the overhaul would be correct according to maintenance manual requirements and the vertical gyro could be returned to service with no issues. This is where most component service facilities stop. But at Duncan Aviation, we do more than what is required; we do the right thing.
Additional components, such as the Electrolytic Leveling Switches, are replaced. Because of our years of knowledge and experience repairing damaged gyros, we include the replacement of additional components as part of our overhaul. Doing only what is required is not always doing what’s right.


During the assembly phase of the overhaul, we put back together what we took apart, making sure all pieces and parts are clean and working properly.

Inspect inner gimbal housing. Remove any Loctite residue from the housing and cup. Treat metal housing and cup with primer. Seat bearings flush and secure in place with new Loctite. Cure a minimum of two hours in an oven. Apply barrier coating.

Attach circuit board; pitch synchro and rotor stator wires.

Install erection switches, bracket assembly and flex power board. Install cup on inner shaft assembly.

Install inner shaft assembly. Add dampening fluid to cup assembly.

Install inner gimbal assembly into outer assembly.
Install spiral flex lead and secure to inner gimbal housing. Solder spiral flex to outer and inner gimbals.

Install stators, rotors and covers. Attach stator wires and inner gimbal wires to cover terminal boards.

Align inner shaft assembly (pitch synchro), balance inner gimbal with outer gimbal, Hysol outer gimbal conical nuts. Cure in oven for two hours.

Static balance inner and outer gimbal assemblies.
Clean slip rings.

Insert gimbal assembly into outer frame assembly.

Secure plate to frame.
Solder roll synchro wires to circuit board.

Install bearing. Tighten nut and retaining ring.
Secure frame flex to frame.
Align brushes on slip ring.

Install retaining ring. Install torquer ring. Align brushes on slip ring. Place gyro into base assembly. Apply grease to both slip rings.


No overhaul is complete without proper calibration. Duncan Aviation performs the following calibrations:

  • Set the Pitch and Roll verticality
  • Pitch and Roll Erection Switches
  • Roll Cut-Out Switches
  • Pitch and Roll Free Drift Rates in North, South, East and West Headings
  • Pitch and Roll Slaving Rates
  • Measuring output voltages and operating currents.

After calibration we perform a 12 hour extended run and 12 hour cold soak cycle. Then we check the calibration again, making any necessary adjustments.

Final Assembly and Testing

After a final inspection, we finish the assembly by solder sealing the chassis cover to the base assembly, using a sealing band.
what-happens-during-an-l-3-vertical-gyro-overhaul_clip_image044 what-happens-during-an-l-3-vertical-gyro-overhaul_clip_image046
For the next two hours, your gyro is put in an oven under vacuum to remove any moisture inside, then backfilled with helium to atmospheric pressure per the manual. It is then placed in a bell jar (shown above) to test for leaks. This process is repeated until it passes specifications.
Only after your gyro passes a leak rate and function test, is your overhaul declared complete. It is then returned to you with a fresh coat of paint and new stickers!


Trivia Question

Why is the gyro backfilled with helium instead of regular air?
Two reasons:

  • To purge the gyro of all remaining moisture so all internal mechanisms do not corrode.
  • To increase the life of the gyro by reducing friction and heat generated by the rotor.  The helium molecule is smaller than the air molecule. The result is less drag on the rotors as they spin. This creates a more stable running gyro.