Composites: Tips for working on Cirrus composite structures

There are some differences to be aware of when working on Cirrus aircraft.


Cirrus has made significant in-roads in the general aviation community. With more and more aircraft being purchased, more mechanics are working on these aircraft. But there are differences when working on this aircraft that we need to be aware of. We will look at some inspection and maintenance items specific to these aircraft.

Composite

The majority of the structure on a Cirrus aircraft is composite. If you ever get a chance to tour the assembly plant in Duluth, MN, you will see what looks like an assembly line for a large model airplane.

Most of the airframe structure is an eight-harness fiberglass composite. There are a few areas that are carbon composite, such as the doors. Most of the structure is closed-cell foam core for strength and stability.

Visual inspection

A major part of composite inspection is visual inspection. Although you are only looking at the outer (exposed) layer of the composite lay up during a visual inspection, it is an important part of the inspection process. A good visual inspection can help you identify areas that require further, more in-depth inspection. Look for any surface imperfections that could be the result of impact damage such as scuffing, chipping, surface cracking, or crazing. Visual inspection will also help identify near-surface delaminations (that appear as bulges) and severe disbonding. If the laminate is exposed, damage will appear white. With non-sandwiched laminate structure, if the surface is visible and you have access to the back side, then you can use the translucent nature of the fiberglass to help you with the inspection. If you use a light to illuminate the back side of the structure, any internal defects such as delaminations will be seen as a dark or grey area.

Different tools can be useful in visual inspections. Of course, a good flashlight is essential. A good tip to help locate defects such as slight bulges is to shine the flashlight at a low angle across the structure (versus shining it straight on). This angle will make surface defects pop out more.

Coin tap test

A coin tap test is used to determine laminate damage in the composite. Just as with performing a tap test on other composite components, the tap test uses acoustic sounds produced when a small metal object is tapped on the surface. Mechanics are looking for a clear, sharp sound. A dull thud would indicate a void or delamination. Here are a few things to consider when performing a tap test. Familiarize yourself with the structure you are inspecting. A change in the sound does not necessarily indicate a defect. For example, if tapping different areas of the wing, you would get a slightly different sound for areas that are sandwiched laminate, non-sandwiched laminate, and transition areas between the two. Corrie Volinkaty, a technical instructor for Cirrus, shares, "You will get slightly different sounds as you transition between different areas of the aircraft depending on the structure. The thing to remember is that a delamination will not be just a different sound, it will be a dull sound or a thud. Knowing the structure beneath is very helpful when performing tap tests."

  • When performing a tap test, locate an undamaged area with similar structure to that of the area with suspected damage. Tap the known good structure and use that as a reference when tapping the suspect area.
  • Use a consistent rate and force when tapping the structure.
  • When performing the tap test, mark the outer edge of the damaged area with a Sharpie.

Damage classification

Now that we have used visual and coin tap tests to evaluate the damaged area, we need to classify it. Damage can be classified into four classes: cosmetic, secondary bond, solid laminate, and sandwich structure damage. Always refer to the Cirrus maintenance manual for damage classification. Here are some highlights of the different classifications.

Cosmetic

Cosmetic damage is non-structural damage. It includes minor gouges, scratches, dents, or other defects. Cosmetic defects are defined to the surface protection of the laminate.

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