Gulfstream Summer 2008

• Gulfstream Vertical Stabilizer Unscheduled Removals–Part One

Gulfstream Vertical Stabilizer Unscheduled Removals–Part One

—James Overheul

In recent months we have had to remove several Gulfstream vertical stabilizers, most of them unscheduled. The models affected were the GII, GIII and the GIV. The reasons for removal ranged from corrosion in the vertical attach fitting pivot bores, a vertical attach fitting condition caused by a previous lightning strike, drill starts/holes in the center vertical attach fittings and F.S. 793 cap angle replacements.

Let’s start with the corrosion in the vertical attach fitting pivot bores. We had identified several levels of corrosion in these bores requiring portions of the fitting to be replaced. In several cases, what appeared to be small pitting corrosion (0.002” in depth) in the bore turned out to be a more severe level of corrosion once we machined the bore to remove the corrosion. In this case, the bore could not be repaired because the corrosion exceeded the maximum repair limits. Other aircraft with similar conditions were able to be returned to service after repairing per Gulfstream repair drawing SE05455904 STD REPAIR-VERTICAL FIN ATTACH FITTINGS.

This repair requires the defective vertical attach fitting to be removed. To remove the attach fitting, the horizontal and vertical stabilizers and vertical shear deck have to be removed for access. Additionally, we have had to take several of the repaired fittings to the maximum repair limits. We think the root cause can be traced to the removal of the cadmium in the bores of the fittings and installation of the wrong bushings or hardware. We are still finding CRES bushings and hardware being installed in low-alloy steel attach fittings.

In these cases, the problem was found during a 72 month Vertical Stabilizer Attach Fitting-Alternate Inspection, which used to be performed during the 144 month interval but was reduced to a 72 month after the incorporation of ASC 431 GII/GIIB, ASC235 GIII and ASC230 GIV. Most of these fittings are on their third inspection and we think that light surface corrosion may have been previously identified and removed with some mechanical means, removing the cadmium. The cadmium cannot be reapplied with the fitting installed because of their fail-safe design. This leaves the type VI CIC as the only protection for the vertical attach fitting bores.

Note: The above condition only applies to aircraft with the Low-Alloy steel (4340) Vertical Fin to fuselage attach fittings installed (GII/GIIB/GIII and GIV S/N 1000-1171.) Several of these models may have been retrofitted with the Stainless Steel (CRES) fittings, with which we have not seen any corrosion issues. The CRES vertical attach fittings are production-installed on GIV S/N 1172 and subsequent.

On one occasion the vertical stabilizer had to be removed because of the damage affected by a lightning strike to one of the vertical attach fitting. During a routine Vertical Stabilizer Attach Fitting-Alternate Inspection one of the attach fitting bushings took more effort than normal to remove. On further investigation it was noted the bushing was electrically welded to the fitting. The welding of the bushing took place at the outboard center link. As the bushing was being removed it scored the outboard bores of the inner and outer fittings because of the material transfer affected by the lightning strike.

If you think you may encounter any of the issues like the examples noted here, we encourage you to work with a shop with the experience and special tooling necessary to perform unscheduled removals of the vertical stabilizer.

More information on other vertical stabilizer removals we’ve seen recently will follow in the next edition of this publication. If you have any questions please contact me at 269.969.8477.

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