How to Evaluate a Fabric-Covered Aircraft

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Fabric-covered planes in good condition are available, but you need to know what to look for.

Aircraft have been covered in cloth since the Wright Brothers took flight, and the material had to be as light as possible yet strong enough to withstand the demands of flight. 

The standard material used in the early days was cotton or linen. Vintage aircraft typically had wood wings and steel tubing used in the fuselage. 

This is a good example of rib lacing cord on an antique aircraft.
Wrinkles are an indication of possible damage to underlying structure. So are blisters or any rough areas, which could be signs of rust in the steel tubing.
The materials

The use of cotton or linen cloth is still approved; however, it is rarely used today because synthetic materials and improved processes are available. 

Synthetic materials and associated application processes not only reduce the amount of labor required, but also provide longer life, resistance to rot and fungus, and are safer in the case of fire (during material application, and while in flight). 

Polyester cloth specific to aviation applications is almost exclusively used in the recovering (or initial covering) of an aircraft today. Fiberglass cloth has been used as well, and other synthetic materials have been experimented with and/or are in development. 

The most important difference between newer synthetic materials and the original cotton and linen cloth is the fact that cotton is more difficult to work with. In addition, cotton is subject to attacks by fungus, mildew, chemicals (such as acid rain) and is susceptible to damage from rodents and sunlight. 

While synthetic fabric is deteriorated by sunlight too, it has better resistance to the effects of ultraviolet light. Synthetic fabric is also resistant to fungus attack, and while it can be damaged by chemicals, it is more resistant to damage than cotton. 

Cotton and the compatible nitrocellulose dope used to stiffen the fabric in the recovering process are flammable. Nitrate-based dope is extremely flammable even after it dries, and is seldom used today.

Synthetic fabrics sometimes call for cellulose acetate butyrate dope according to the STC, but oftentimes a material that is less flammable and more suitable to the synthetic fabric process is used. 

A significant factor regarding polyester cloth is that the tautness of the fabric
is controlled by heating the fabric with
a temperature-regulated device similar to a clothes iron. Application of dope or sealant materials will not appreciably shrink polyester, as is the case with
cotton fabric. 

Aviation-specific synthetic fabric can be much stronger than cotton fabric. This is a key issue in the pull testing (strength) of the raw fabric to determine continued airworthiness years after the initial fabric application process has been completed. Fabric is considered airworthy until the strength degrades to less than 70 percent of the original design strength. 

The FAA testing specification has always been in reference to the original material the aircraft was designed and certified with. Aircraft produced under the CAR 3 rules were approved with cotton or linen cloth of different grades depending on wing loading and maximum airspeed limitation. For example, aircraft could be certified with grade A cotton, intermediate cloth or glider cloth, depending on the never exceed speeds and wing loading, and then could be later recovered with a fabric of a higher rating. 

Drain grommets should be installed and clear of any obstructions so that air may circulate and moisture can escape.
The process

Working with cotton or linen requires special techniques and processes for a good-looking and airworthy cover job. When recovering an aircraft, the structure has to be carefully inspected and all defects repaired; then it can be primed and protected prior applying the fabric. 

The fabric has to be cut and sewn to the shape of the wing or fuselage and cemented or tacked into position. After the fabric is installed and secured to the frame, it’s permanently attached to the wing ribs with a special lacing cord using a designated knot. 

The spacing of the rib stitches varies according to the VNE (never exceed) speed of the aircraft and if the area is in the propeller slipstream or not. Some aircraft, like the Cessna 120 and early 140, use screws or fabric clips in place of the rib stitching. 

After the rib stitching, the next procedure is the application of cloth tape to cover the stitching and the installation of inspection rings, grommets and patches in various locations to protect the underlying fabric. 

A plasticized liquid lacquer (i.e., dope) is applied to the fabric in several applications initially by brush and then by spray gun to form an airtight and waterproof bond that also tightens and stiffens the fabric materials. 

The proper fit of cotton or linen fabric prior to doping is important, as extremely taut fabric caused by multiple applications of dope will shrink and distort or damage the underlying structure requiring removal, repairs and reapplication of the fabric. 

Proper health precautions must be followed when applying doping agents, especially when applying urethane in a spray form as it is extremely toxic. 

Multiple applications of various mixtures of dope are applied generally by spray gun. Mixtures may include dope with silver metallic compounds for resistance to light, dope with fungicide for resistance to fungus, and pigmented dope for the final color applications. 

After reviewing an airplane’s logbooks and AD compliance, carefully inspect the condition of the fabric. Check for cracked and missing paint or dope, and note the condition of the underlying structure.
Purchase considerations

An aircraft covered with polyester fabric—if it is applied according to STC, properly maintained and kept in a hangar—can have an almost indefinite life. However, when considering the purchase of a fabric-covered airplane, it is important to seek a mechanic that is familiar with this type of aircraft and knows what to look for. 

With the cost of a complete recover job for a tube-and-fabric airplane in the $30,000 to $40,000 range, you must be certain of the condition of not only the fabric, but what lies underneath. The cost to recover Cessna 120 and early 140 aircraft which have fabric-covered wings and a metal fuselage would be significantly less—$15,000 to $20,000—if no major repairs are required. 

As with most airplane purchases, it is always good to look for an aircraft that is in excellent condition and pay the asking price rather than look for the bargain. That bargain plane could require recovering that would make the final cost exceed the value of the aircraft. 

Prior to contracting with a mechanic to do a pre-purchase inspection, there are areas which you can check yourself just to see if the fabric-covered aircraft is in a condition that you would consider purchasing it. 

Keep in mind that vintage tailwheel aircraft probably have had a few ground loops, with airframe and/or engine damage and major repairs. Damage history is almost a given—but what this means for the purchaser is that the repairs must have been done correctly and that the aircraft flies like it should. 

The first order of business is to check the aircraft records, including any FAA Form 337 documents, to get an idea of the history of the repairs done to the airframe and engine. 

After checking the aircraft records, including compliance with all ADs, it would be wise to make up a written list of items to check on a pre-purchase walkaround. Make notes of anything you have a question about. 

Start with the condition of the fabric, and what the finish looks like. Check for cracked and missing paint or dope that would allow sunlight to directly access the fabric. Look for ringworm in the fabric; this indicates that the paint job is failing and will cause the cloth to deteriorate in a short time if exposed to direct sunlight. 

Check for patches, noting any especially large patch areas—these would require a logbook entry, or possibly a 337 form indicating a major repair. If there is no logbook entry indicating a repair was made where a large patch is located, be suspicious. There could have been major damage to the airframe structure that was repaired improperly, or not at all. 

Wrinkles or sags in the fabric most likely point to structural damage. For example, a dent in the metal leading edge of a wing would cause a sag or wrinkle in the fabric that would be visible from the outside. 

Blisters or rough areas under the fabric along lower longerons are an indication of rust in the steel tubing. Other areas could also have blisters or rough spots, such as the horizontal stabilizer, elevator or rudder; water is often trapped in these areas and eventually causes rust or corrosion. 

A fabric-covered aircraft should have sufficient drain holes or grommets installed—not only to allow moisture to escape, but also allow air to circulate and expel any moisture created by condensation. 

With the owner’s permission, pull a few inspection plates off from under the wings, especially in the area where the lift struts attach to the spar. Use a flashlight to take a good look at the wooden spar around the bolt holes, checking for obvious defects such as cracks or splits in the wood.

Move the strut at the upper end and see if there is any evidence of movement between the spar and the lift strut attachment fitting. Whether the spar is wood or metal, any movement is not good and could cause the spar to crack in this location, which would be an expensive repair or replacement. 

While the inspection plates are off, take a look up through the wing. Sunlight is the number-one enemy of fabric, and any daylight showing through the upper wing surface means a reduction in the useful life of the fabric. A very dull indication of light is okay, but if you can see a shadow of a person’s hand blocking the sunlight, then there probably isn’t enough light-resistant silver or pigmented dope remaining on the fabric. 

While the inspection plates are off, take a look at the rib stitching to see if the lacing cord is intact. Rodents have been known to get into a wing and chew the lacing cords, requiring expensive repairs. Rodents and birds can destroy an aircraft, especially if the structure is compromised by droppings or if drain holes are plugged with debris. 

While on the subject of wing ribs, note that over the years, several aircraft accidents (and at least one fatality) have occurred as a result of missing rib nails that secure the rib to a wooden spar. 

Another problem with wings and ribs is that of dissimilar metal corrosion when steel clips are used to secure fabric to the individual aluminum wing ribs—such as those used in the fabric-covered Cessna 120 and 140 aircraft. 

Since tailwheel equipped aircraft are sometimes involved in ground loops, check the wingtips for damage. Look at the fabric to see that there are no scrapes or tears, and check the wingtip for cracks or damage by looking up and out toward the tip through an inspection hole near the wingtip. 

Take a look at the lower rudder area and tail post for signs of damage such as loose fabric, wrinkles or sags, and possibly bent tubing from a hard landing on the tail. 

Get up on a stepladder and check the center section and inboard wing fabric directly in the propeller slipstream. This area sees a lot of vibration and heavy airstream deflection from the propeller, which induces wear/chafing and weakening of the fabric. 

The use of a suction cup on the fabric—attempting to pull up on the fabric in this area—is a simple test to see if the fabric is weak and/or not secure, requiring repair or replacement.
Final thoughts

When evaluating a fabric-covered aircraft, you really need to take enough time to go over the paperwork and the aircraft completely. Repairs to structure or a complete recover job are considered major repairs; they are expensive, and legally must be done by (or supervised by) an experienced and licensed mechanic with inspector status to complete the FAA Form 337. 

Recovering a Type Certificated aircraft is a job I recommend you leave to the experts. Errors in the fabric replacement process are easily made—and these can be difficult and costly to correct. Mistakes may even require starting the job over. 

Replacement of aircraft fabric is a big job because it never is just a plain recover job—there may be repairs required along with the preparation involved, such as completely disassembling the fuselage frame and sand blasting the fuselage, inspecting for damage and rust, and applying dope proof primer. 

Woodwork requires proper preparation with cleaning, sanding and application of special dope proof sealer. 

Multiple repairs to the structure, to include welding prior to the recover process, are more common than one may anticipate. These repairs can become overwhelming unless the job is properly planned and executed by an experienced person. 

How much a recover job costs depends on the process used, how many repairs are required prior to covering, and if you are able to assist in the process. Published cost and time estimates can be overly optimistic, especially if you don’t have experience or close supervision. 

When considering the purchase of a fabric-covered aircraft, look for a well-maintained aircraft with a quality fabric cover job. A quality job should last 20 years or more, depending on environmental conditions and exposure to sunlight. Any bargain-priced fabric-covered plane will most likely cost more to own. 

Vintage tailwheel aircraft can be a joy to own and fly. Enjoy the experience, buy the best—and leave the recover job to someone else!   

Tubing repairs or manufacture require a skilled welder to do the job correctly.

Michael Berry is a former aircraft repair shop owner. He is also a multi-engine rated ATP (757/727), A&P/IA, airplane owner, turbojet flight engineer and Part 121 air carrier captain. Berry has over 15,000 pilot hours. Send questions or comments to .

Resources

Consolidated Aircraft Coatings
(Poly-Fiber)