Recommended Service Bulletins for Honeywell Vertical and Directional Gyros

June 2022

At Duncan Aviation, we continue to see Honeywell vertical gyros come to us for overhaul without being in compliance with an OEM-recommended SB (Service Bulletin). We encourage all operators to include the relevant SB as a part of the next gyro overhaul.

Service Bulletins Replacing Gimbal Bearings

These modifications replace the 4 gimbal bearings with 4 new bearings with a titanium carbide coating. This coating prevents microwelding of the balls to the raceway, limiting premature failure.

SB A21-1992-029, Mod P: Applicable to VG-14 Vertical Gyro, P/N: 4001808-VAR (VG-401 Attitude Reference, P/N: 2593691-901). It applies to units that do not have the letter P (L for VG-401) marked out on the MOD block of the identification place.

SB A21-1992-030, Mod G: Applicable to VG-14A Vertical Gyro, P/N: 7000622-VAR (VG-401 Attitude Reference, P/N: 2593691-901). It applies to units that do not have the letter G (L for VG-401) marked out on the mod block of the identification plate.

SB A21-1992-033, Mod E: Applicable to C-14D Directional Gyro, P/N 4020577-VAR (DG-401 Gyro Synchronizer, P/N: 4019190-VAR. It applies to units that do not have the letter E (C for DG-401) marked out on the MOD block of the identification plate.

Service Bulletins Replacing Spin Bearings

These modifications replace 2 spin bearings with a new technology that improves the life characteristics of cold-start performance of the VG-14A rotor assembly. The new bearings are coated with titanium carbide and use an oil lubricant, allowing them to operate almost wear-free with easier starts at low temperatures. They show a life span 3 times longer than the old bearings.

SB A21-1990-176, Mod E: Applicable to VG-14A Vertical Gyro, P/N 7000622-VAR (VG-401 Attitude Reference, P/N: 2593691). It applies to units that do not have the letter E (K for VG-401) marked out on the MOD block of the identification plate.

SB A21-1990-175, Mod N: Applicable to VG-14A Vertical Gyro, P/N 4001808-VAR (VG-401 Attitude Reference, P/N: 2593691, -901). It applies to units that do not have the letter N (K for VG-401) marked out on the MOD block of the identification plate.

SB A21-1990-179, Mod D: Applicable to C-14D Directional Gyro, P/N 4020577-3, -6, -8 (C-14A Gyro Synchronizer, P/N: 4019190-VAR). It applies to units that do not have the letter D (B for C-14A) marked out on the MOD block of the identification plate.

Don’t Take the Shortcut. Do these Service Bulletins

These are recommended SBs by the OEM and can be performed as a part of your Honeywell gyro overhaul. Duncan Aviation includes incorporating these SBs as a standard part of gyro overhauls rather than trying to clean and relubricate the existing bearings.

NOTE OF CAUTION: If the bearings are cleaned and not lubricated properly, they will begin to bind and not move freely, causing the gyro to be slow to erect. Bearings are easily damaged during cleaning. Special care is always necessary.