Composites: Tips for working on Cirrus composite structures

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.

Secondary bond damage

This is when disbonding occurs between two pre-cured components. This could be structural or non-structural in nature.

Solid laminate damage

This is structural or non-structural damage which extends beneath the surface protection and affects the solid laminate structure underneath. This could be either penetrating or non-penetrating.

Penetrating. This includes fractures and penetration through the laminate.

Non-penetrating. This includes abrasions, delaminations, surface impact, or gouges through one laminate surface.

Sandwich structure damage

As the name implies, this is damage to composite sandwich structures.

Laminate only. This is damage to only one side of the sandwich structure with no core damage. This type of damage to the sandwich structure can be identified using the solid laminate damage classifications.

Laminate and core. This is when there is damage to one side of the sandwich construction with damage to the core. There is no damage to the opposite laminate surface.

Sandwich penetration. This is damage to both sides of sandwich construction. Both laminate surfaces are punctured and foam core is exposed.

Repair classification

Now that we know what type of damage we have, we need to determine what type of repair we are looking at. Cirrus breaks repairs down into four categories: cosmetic repair, minor repair, major repair, and restricted repair.

Cosmetic repair. Cirrus classifies a cosmetic repair as that which is designed to repair localized surface defects to the original profile and to prevent UV damage and moisture ingress. Cosmetic repairs relate to minor defects which have no significant effect on the structural strength of the structure.

Minor repair. Cirrus defines minor and major repairs the same as the FARs do. So, a minor repair would be a repair that is not a major repair.

Major repair. A major repair is a repair, that if done improperly might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or a repair that is not done according to accepted practices or cannot be done by elementary operations.

Cirrus allows major repairs in the field if the repair area does not fall into a no-repair zone, and it does not require non-standard repair procedures. In addition, field repair is allowed if the repair is specifically covered in the maintenance manual. All other major repairs need Cirrus Design engineering disposition.

Restricted. Any repairs that occur in no-repair zones as listed in the maintenance manual are restricted. The mechanic needs to contact Cirrus Design for disposition. Once we have gotten to this point, we know whether or not a repair is allowed in the area we are looking at. We can then begin the repair process. There are too many possible repair scenarios to cover in the limited space here, but we can cover some essential tips when performing the repair.

Repair environment

One thing you need to be aware of is the repair environment. For cosmetic repairs, the environment is not as critical. However, Cirrus has stringent environment requirements for minor and major repairs.

From our own safety perspective, we want to make sure the repair is carried out in an area that has adequate ventilation.

We also need to ensure that we are performing the repair in a controlled environment to ensure proper integrity of the repair and avoid any contamination issues. See the Repair Environment sidebar on this page for some guidelines on proper composite repair environment.

Ply orientation

When looking at a repair, we need to know the ply lay-up. The zone diagrams in chapter 51 of the maintenance manual provide that information. The charts contain information such as ply count and warp clocks. They show the zero-degree reference for each section of the airframe.

Here is a tip for determining ply count. If you’re looking at a solid laminate and don’t know the ply count, you can measure the thickness to determine it. The thickness of one ply is generally between 0.008 and 0.010 inch. So, if you measure the total thickness of the laminate and divide by 0.009, you can determine the ply count.

Wet lay-up

Cirrus has established a wet lay-up composite repair process. No vacuum system is necessary. Volinkaty says that the repair procedure was chosen so that shops were not forced to buy expensive curing equipment. Instead, the mechanic fabricates a tent to create an envelope around the repair area. A dryer is used with a thermostat to control the cure cycle of the repair.

Cirrus training

Cirrus wanted mechanics in the field to be knowledgeable about proper repair procedures when working on Cirrus aircraft. The company has created several training opportunities that can help increase a mechanic’s proficiency when it comes to inspecting and maintaining Cirrus aircraft.

Online training

Cirrus has developed an online training program called the SR Series AvMx course. It covers inspecting, repairing, troubleshooting, and maintaining Cirrus SR-series airplanes. The course features seven interactive lessons, each with several multi-step challenges, that simulate aircraft maintenance in real-world situations such as busy schedules, difficult customers, and unpredictable situations. This course is a prerequisite for anyone wishing to attend other Cirrus training.

“Our online program has several benefits,” says Doug Larson, Cirrus senior technical instructor. “First of all, while the first section of the course deals with technical publications, they are referenced throughout all seven lessons. The mechanic becomes proficient with using the manuals. Second, it allows us to do a lot of the initial training before a mechanic steps foot in the Duluth classroom.” As Larson shares, mechanics have a widely varying amount of knowledge and experience when it comes to composites. By covering the basics in an online course, everyone starts off on a level playing field when they step foot in the Duluth classroom.

Duluth training

Cirrus provides a one-week composite repair laboratory in composite inspection and repair. If you enjoy sitting in a classroom for hours on end, this course is not for you. After a few hours of review and a safety briefing, you are in the lab with your respirator, safety glasses, and Tyvek suit performing composite repairs. The composite repair lab has a large resource of composite pieces on hand to work on. That way, the student gets the opportunity to get his or her feet wet by working on the actual structure he would see in the field before doing it on an actual aircraft.

I had the opportunity to take both the online course and the composite repair laboratory. Having never worked on a Cirrus before, I came away with information that I could apply to repair or inspection situations in the field. These courses are a valuable resource for any mechanic who will be maintaining or inspecting Cirrus aircraft.

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