Maintenance of General Aviation Engine Mounts - Noise, Vibration & Harshness Division | Parker US

Maintaining General Aviation Engine Mounts

Bonded sandwich assemblies are natural rubber or specially blended synthetic compounds bonded to two plates. The normal installation requires four assemblies, each consisting of two sandwich mountings and one spacer. Satisfactory performance requires that the spacer be designed to the correct length to precompress each mounting to guarantee proper positioning of the mountings. When properly installed, these mountings provide excellent isolation of engine vibration, resulting in smoother, quieter flight. Always order replacement mountings by the mounting assembly number to ensure the correct spacer.

At each annual inspection and at FAA-required inspections of the aircraft’s engines, all mounting assemblies should be visually inspected in their installed state while supporting the engine weight. The correct visual inspection procedure is as follows:

To remove mounting assemblies from the installation, it is necessary to remove the engine weight from the mountings, by the use of a chain hoist or equivalent, and disconnect the nuts and bolts which hold the mountings in place. In general, removal of one mounting at a time can usually be accomplished, if necessary, without removing the engine weight from the mountings.

Visual Inspection of Installed Mountings

At each annual inspection and at every FAA-required engine inspection, visually examine the engine mounting assemblies while installed and supporting the engine weight.

Examine the bonded sandwich mountings for:

  • separation between the elastomer and the metal part to which it is bonded
  • elastomer surface deterioration, such as noticeably soft or gummy elastomer; noticeably stiff or hardened elastomer; cracks; cuts or gouges
  • drift, which is shown by the drift indicators, if they are part of the mounting
  • excessive sag, when the bonded plate pulls away from the attaching structure

Examine all metal components for:

  • cracks
  • large or multiple nicks, scratches or gouges
  • deformed edges

When you see any of the conditions described above, follow the instructions in Removal and Cleaning and Inspection.

Examine all lockwires and attaching items. Follow the instructions in the engine or aircraft manual to tighten any loose parts. Replace any damaged parts

CAUTION: Cleaning fluids, solvents, and engine lubricants can cause deterioration of the elastomer. When you clean or service the aircraft’s engines, put a cover on each engine mounting assembly to make sure that the fluids do not touch the elastomer. If these fluids touch the elastomer, use a clean, dry cloth to remove them.

When you clean or service the aircraft’s engines, put a cover on each engine mounting assembly to make sure that the fluids do not touch the elastomer. If these fluids touch the elastomer, use a clean, dry cloth to remove them.

Removal

CAUTION: You can damage the engine mounting assemblies if you use prying tools to remove them from the installation. They can cut the elastomer or make dents in the metals.

CAUTION: You can remove only one engine mounting assembly without removing the engine weight. Do this only if necessary because this can easily cause damage to the engine mounting assembly.

Use a chain hoist or equivalent to remove the engine weight from the engine mounting assemblies. Remove the nuts and bolts that connect the engine mounting assemblies to the engine and structure. Remove the engine mounting assembly components with your hands.

Cleaning & Inspection

CAUTION: Do not put bonded elastomer components into cleaning solvents. Cleaning solvents can cause damage to elastomer.

Use a clean dry cloth to remove oil and dirt from the engine mounting components. You can moisten the cloth with methyl alcohol, if necessary, to remove the debris.

  • Examine bolts and nuts for any damage. Follow the instructions in the aircraft or engine manual for the inspection of these items.
  • Examine the spacers. If the spacers have cracks, large dings, or damaged surfaces (caused by over-tightened bolts), replace them.
  • Examine the bonded components.
  • Replace the engine mounting assembly if any of the elastomer is very soft, swollen, or gummy (caused by fluid exposure); cracked (other than ozone cracks – see below); separated from the metal plates; ozone cracked, and the depth of the ozone cracks is greater than 0.020 inch

    Measure the dimensions A and B as shown in the diagram below. Replace the engine mounting assembly if any A dimension is greater than the limit shown or if any B dimension is less than the limit shown.

    NOTE: Ozone damage will give the elastomer a hazy surface, with shallow (the depth is less than 0.020 inch) cracks that occur in a grid (checkered/lattice) pattern. It is not necessary to replace assemblies with only ozone damage that is also less than 0.020 inch deep. However, ozone cracks can grow to become flex cracks with continued service so you must monitor the condition of any engine mounting assembly reinstalled with known ozone damage.

     

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