Inspection points for tubular structural pieces
Now that that the engine mount has been sandblasted, what do we look for. For starters, the tubular structural pieces should be inspected for corrosion and pitting. Tube sections with pitting and voids exceeding 10 percent in relation to the overall tube thickness should be replaced. Weld joints should be inspected for cracks. Follow-on dye penetrant inspection will uncover cracks, voids and pinholes not detected by visual inspection.
Check for chafing and misalignment
Next, the mount should be inspected for visible signs of wear due to chafing. Wear spots resulting from chafing are caused by the contact of cables, wire harnesses, tubing or other moving metal parts will weaken the structural integrity of the Chromaloy 4130 tubing. Tubes displaying any type of chafe marks or wear spots should be replaced.
Finally, the mount should be inspected for bent tubes. The most common causes of bent or misaligned engine mount structures are landing mishaps quite often involving the landing gear.
Performing a fixture check
A fixture check of the engine mount will help determine whether the engine mount is out of alignment or if there may be an interference problem with the aircraft. The use of fixtures is a very important element of the entire repair process. Fixtures ensure that tube replacement and other repairs will be within design specifications minimizing any potential for misalignment. Without the use of the proper fixture designed for the specific aircraft model, the mechanic is taking a huge gamble that the mount will attach correctly to the aircraft and allow for the proper clearances and fit when the engine is reinstalled. Fixture checks are part of the standard inspection and repair process at an FAA licensed repair station.
Assessing needed repairs
Now that the preliminary inspection process has been completed, it is now time to contemplate the repairs that may be needed. The sand blast operation has cleaned the areas requiring repair or replacement. It is very important to note that any tube sections requiring repair must be replaced using Chromaloy 4130 tubing. Do not substitute any other type of steel tubing. Aircraft engine mount assemblies are fabricated using Chromaloy 4130 with tube thickness ranging from .049 to .065 inches.
Never replace a tube section with material having a tube thickness less than the design specification.
Replacement tubing should always have the same or greater tube thickness. It is recommended that Chromaloy 4130 filler rod be used when welding replacement tubes and making other mount repairs. Any other type of filler material, as well as tube material, can degrade the reliability of the engine mount..
Weld repair of aircraft engine mount assemblies is accomplished using the TIG welding method. Engine mount weldments require clean, precise welded joints. TIG welding allows a fine, precision beaded weld seam to be applied to the tube joint sections. TIG welding of engine mounts requires skill and patience. Another reason to consider the services of a FAA repair station licensed to inspect and repair engine mounts.
No preheating required
Engine mount assemblies do not require a preheat operation prior to commencing repair. However, repaired areas requiring holes to be drilled must be annealed, otherwise the point of connection may crack under stress. We also recommend that all firewall and engine attach points be stress relieved.
Dye penetrant inspection
With the repairs completed, the weldment should undergo a dye penetrant inspection to ensure that all weld joints have been made correctly and to detect any hairline cracks, voids ,or pinholes that may not be visible to the eye. Applying the dye and the subsequent inspection should be performed in a well-vented area with good lighting. Dye penetrant inspection is added insurance that the repair has been completed correctly and that a good quality engine mount is being returned to service.
A second sandblast
After the dye penetrant inspection, the engine mount must undergo a second sandblast operation to prep it for painting.
We recommend a good quality zinc chromate primer be applied prior to applying paint. After spending thousands of dollars on an engine replacement, you will want it to be supported with a freshly painted, good-looking engine mount assembly. After painting the engine mount in a well-ventilated paint booth, the mount must be allowed to dry at least four hours before reinstallation in the aircraft.
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