This is what a compression spar crack looks like

This one was caused by a hard  ground loop


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What you need to know about a FIELD APPROVAL
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Fabric Covering for Dummies
Week ONE

Well another year has come and gone!  I was contacted by many folks last year who was doing a recovering job on their airplane, some were first timers at it and some were old timers at it or they had done fabric covering in the past but did not remember some basic things.  So I decided to sit down and scratch out a few things about just some basic fabric covering problem areas and problems that I was ask about.

The most ask question about fabric covering an aircraft is which process should I use?  So with that being said, I will try not to answer this question showing too much favoritism toward the Air-Tech  Fabric Covering Process, because that is the only process that we use here at Special Products Aviation, even though we have used other processes many years ago.

All fabric covering process have the same basic ingredients and all most identical installation steps. The first basic ingredient we have to have is an approved fabric, and there is only one kind of fabric that we are all familiar with and that is DACRON fabric.  There are several brand of Dacron and all are manufactured and distributed to only a few companies which in turn offer three different weights of fabric ranging from 1.4 ounce to 3.6 ounce.  As far as I know there is no certified FAA/PMA approved grade “A” available today.  The major brands of FAA/PMA approved dacron fabrics are Ceconite, Stits, and Super Flite.

The second basic ingredient is an adhesive.  Now this is where it get a little touched when it comes to what the FAA considers legal and not legal.  Each dacron fabric covering process system owner, such as, Air-Tech, Ceconite, Stits and Super Flite for example, has designed their own  attaching adhesive for the  attachment of the FAA approved dacron fabric to a  structure and must prove to the FAA, by actual testing in their presents, that their adhesive meets the minimum FAA required breaking strength for safety and longevity as well as being able to prove to the FAA that they have the ability and facility to manufacture, produce, catalog, trace, record, lot label, can, store and maintain production consistency in accordance with a PMA, (Parts Manufacturing Authorization), as required by the Federal Aviation Regulations.  In short an FAA approved fabric adhesive must meet the same approval process as an approved aircraft fastener, such as an AN bolt or AN nut, because it is a fastener also.  Catch number two, be sure that this FAA approved adhesive is only used with the STC’d fabric covering process for which it is associated with.   You cannot mix fabric covering processes and be legal, you  must, follow the STC holders FAA approved fabric covering procedures manual unless you get an FAA field approval to deviate, and this very rarely happens.  Packaged adhesives that is legal will be properly marked “FAA/PMA approved”, with an STC number, “Part Number ###-##”, “Batch Number”, Shelf life or use before date, and reference to an eligibility list, as well as any warnings and instructions.  Be aware that there are several fabric adhesives in the market place that do not meet these basic guidelines and you as the end user is responsible to see that they meet the FAA requirements as being an approved airworthy fastener and used in accordance with an approved procedure.  One big question that always comes up, with some seasoned aircraft mechanics, “ I  always use Mil. Spec. products  when I do my fabric job”!  This is good because you are using products that meets a standard that is acceptable, but, that does not mean that these products are approved for the STC’d fabric covering process you are using.  One must use the products and materials that are spelled out in the STC holders instruction manual and if they called out using a particular Mil. Spec. product then that would be legal.--- The use of products or materials that is not approved by the STC holder for their installation and or application is considered by the FAA as an unapproved part.

I have had several conversations with mechanics about the use of fabric attaching rivets.  “ I can buy these same rivets locally why cant I use them”?  I had a complaint from a mechanic once who said  his fabric was coming loose around his fabric attaching rivets and he did not know why.  Close examination of his assembly we found an acceptable fabric covering job, but his fabric was pulled through around the fabric attaching rivets.  After we removed several rivets and examined the rivets, and checking with the fabric process supplier, we found out that he had purchased off the shelf big headed rivets that looked like an  approved rivet, but it was concaved around the edges and when pulled the outer edges of the rivet would act as a hole punch and was cutting through the reinforcement tape as well as the fabric.  Remember the fabric attaching rivets used for fabric attachment has a different design than most rivets.  The fabric attaching rivet spelled out in the STC holders procedure manual is the rivet to use.  All fabric covering procedures manual states you must attach the new fabric the same way it was done when the aircraft was manufactured.  Now there is exceptions, in some cases the FAA will allow a field approval to be implemented to use an approved fabric rivet in place of pk screws/washer and martin clips, but it is up to the mechanic to prove to his FAA inspector that this is equal to or better than.

Fabric shrinking is virtually the same on all aircraft dacron fabric, some procedure manuals may vary a few degrees one way or the other, but they all have to be heat shrunk in accordance with the FAA approved STC holders procedures manual.  Follow the directions of the manual and you cant go wrong.  Shrinking the fabric properly is an age old problem because this is how the dacron fabric remains stabilized for life.  Low heat shrinking leaves the fabric loose for life, or you could over heat it and the dacron fabric will go from tight to loose again and be unstable for life.

As you can see, first we have to have a fabric, next we must attach it with an adhesive and now we shrink it.  All fabric covering processes are the same up to this point.  Of course there is some in between knowledge we need to know such as where to apply the adhesive, how thick, what kind of adhesive reducer? What kind of brush?

I know of  people who give top dollar for a fabric covering kit and then they go and buy a $.29 cent paint brush to do the job.  Get a good brush! A good brush! I prefer a 1½ inch  professional oil base paint brush, which will cost you about $7 dollars  give or take a buck.  I have been using the same one for several fabric covering jobs. Where do you brush foundation adhesive?  Here is how I would answer that --  any place that the fabric will touch, just that simple.  How thick do I mix my attaching adhesive --- about like hot pancake syrup or varnish.

Adhesive reducers is another problem area.  I have people call me and say -- can I reduce my adhesive with lacquer thinner or enamel reducer cause I can get that locally?  No No No --- remember here again the adhesive reducer is an FAA/PMA part also, just check out the container and consult you STC application manual and it will tell you what the part number is and a basic mixing ratio’s.  Several adhesive reducers are of the MEK, (methyl Ethel keytone), base family,  be sure and use caution here because MEK is hazardous to your health and causes liver problems ---- read the label.  Air-Tech adhesive uses an acetone based reducer for their adhesive and acetone is not at the top of the EPA list.  All chemicals are to be used with caution and according to the label or directions.  FAA/PMA approved adhesive reducers that are manufactured by the STC holder use special blends of such things as drying agents, flow inhibitors, retards and etc..  Locally bought Acetone or MEK will not give you the performance and longevity necessary to do the job correctly.  These products are great for cleaning up you brushes and containers but do not use or mix them in your FAA/PMA approved adhesives.

Chemical technology has come a long way in the last 10 years.  Advances in having the ability to test these chemicals make it possible for the real serious companies to continue to improve their products while other companies do not want to change from the old ways.  I find it very sad that someone who is doing a complete fabric recovering of their aircraft does not properly repair or preserve the super structure to the best degree possible so it will last at least as long as the fabric covering system that they are installing.   With metal and wood preservatives chemicals and products that are available in the market place today, there is no excuse not to demand the best.

Should I use a pre-sewed envelope or the blanket method?  This is your judgment call because both are acceptable standards.  The only reason that we have used the slip covers in the past is because our fabric only came 48” wide and we had to sew it together to have enough fabric to do the job.  It is legal for the mechanic to sew up his or her own fabric, but be sure and use the correct stitching techniques and threads listed in AC 43.13.  I have seen several marketed pre sewn slip covers that is not sewn up using the correct sewing techniques as required,  so stay on guard for this.   With the 70” fabric, that is available today, the blanket covering method is more widely used and does not leave the unsightly sewn seams that we all hated, plus it allows us to double cover the leading edges.

The question has been ask “when I removed my old fabric, I noticed  all the ribs were “X” tied with some kind of  flat fabric material, what is this for”?  This flat fabric material is used to keep the ribs lined up from front to rear.  Years ago when grade A cotton was used the cotton was sized snugly over the assemblies to be covered, then toughening dope was applied to shrink the cotton.  As the dopes dried it had a tendency to pull the ribs one way or another because the cotton glued itself to the top of the ribs and could not slide around under the cotton.  Today with the modern dacron fabric this is not such an issue because as the fabric is heat shrunk properly the dacron will slide over the ribs and not have the tendency to carry the ribs with it.  It is still common practice to use the rib tie to hold your ribs into place and does offer some rigidity and uniformity before the dacron is installed.  Noting is more detracting than ribs that have an “S” look when finished.  In some cases I have had to make a complete fabric bay repair and I found that the rib tie helped hold my good bays of fabric in place while the repair was being made.  For all practical reasoning once the ribs have been properly rib stitched, the rib cannot move.

“Which tape do you like the best, straight or pinked edge.”?  I would say I like the pinked edge tape over the straight tapes because the pinked edge tape has better edge holding power than the straight tapes.  If you could stretch out the edge of a pinked edge tape you would have more glue area so therefore the pinked edge tapes has better edge holding qualities because the adhesive saturated more area.  Pinked edge tapes will not revel out and has a smooth edge, where straight tapes are heat cut and they have a hard edge.  Straight tapes will give satisfactory results but pinked edge tapes hold better and just look correct, of course that is my opinion and yours may differ.  While we are on the subject of tapes, I have been ask which tape works the best, light weight tapes or the standard tapes?  Both works just great,  the light weight tapes takes less adhesive to use and lays down much flatter. I  prefer the heavier pinked edge tapes  when I use the heavy weight fabric on utility type aircraft, such as  ag planes, bush planes, or heavy duty operation aircraft.   If you have decided to paint your aircraft, when completed using a dark color, it is highly suggested that you use the regular tapes over the light weight tapes, or preshrink your light weight tapes with an iron to eliminate the possibility of heat absorption by the sun which may cause a slight shrinking of the light weight tapes. (see M & D - Light Weight Tapes --click here ) All fabric covering process manuals or bulletins gives written cautions about this as well as preventive measures you should consider.

The following is a recap of a few things that people have called me about when they  have an emergency and don’t know what to do next or what to do.

? “I mixed up too much Air-Tech primer/filler and I don’t have any thing to spray it on and I hate to dump it”.   I have had this happen to me when using the Air-Tech primer/filler as well as Air-Tech paint.  There are two ways to keep catalyzed liquids from setting up, (1) keep it agitated or  (2) refrigerate it.  Put the left over primer/filler or paint in the freezer compartment of a refrigerator or put it in a deep freeze, then when I am ready to use it  stir it real well and just add a little more reducer.  Do not make a habit using this tip as I have had success most of the time but after a long time the material will still go ahead and set up.

? “I understand that there are flexing chemicals that can be added to paints to help keep them  "flexible”  Yes several paint companies sell a flexing additive for their paints, but when this material is used it makes the dried finish very vulnerable to scratching and with age the paint will chalk up and oxidize much quicker, if you don’t use it, the dried finish becomes brittle and causes cracks when the fabric is flexed.  Air-Tech Coatings, inc. is the only company, that I know of, that manufactures a flexing chemical into their paints and primers/fillers.

? “I  can see light through my Air-Tech primer/filler/uv and it is not silver like other primer fillers”?  There are two ways to take care of the ultra violet light problem, one is to reflect it and this is why you remember seeing fabric covered components silver in color, the silver would reflect the sun light.  The second way to take care of the ultra violet problem is to block it.  To give you an example --- when you apply a sun blocker on your body it is not silver, but you could also spray or paint your body silver and do the same thing --- which would you do?  Ultra violet was a big problem when we used grade A cotton, but synthetic dacron fabrics are much more stable when subjected to the ultra violet rays of the sun and is not near the issue as it use to be.

? “Is there a chemical adhesive product that will permanently stick to dacron fabric”  No nothing actually sticks to dacron fabric, some products adhere to dacron better than others and that is why your major fabric coating STC holders have put so much research into their products.  Dacron peel-ply is used in the fiberglass airplane building process to wick and shape out with epoxies and when the epoxy is cured you just peel it off, so as you can see epoxy adhesives will not stick to it.

? FUNNY – Phone Call --“I painted my wing three days ago and it is not dry yet!!?”  answer -“How much catalyst did you add to the paint”.  “BY”.

? “The company that cuts my “N” numbers using the exterior peel and stick plastic lettering do not have the color I want, where can I find the color I need”?  The lettering company can supply you peel and stick product that you can paint.  We just get 3 or 4 foot of this material and paint it as we are doing our painting, then we return it to them, and they cut out what ever lettering we need, now you have the exact color.

? “I can’t get my masking tape to stick to my paint very well and I have bought good tape”?  Two part polyurethane paints have a tendency as they are curing to have a slight boil off oily residue and you will need to wipe this invisible residue off with alcohol.

Here is a trick to sharpen you scissors ---- use a glass coke bottle and act as if you are trying to cut the top off the coke bottle with the scissors ---- this will reshape the cutting edges of the scissors   ---- give this a try and you will find that it sure does help.  There are special scissors for cutting Dacron material and they have a different angle cut on the scissor blades, but they are expensive also.  Also it helps to clean the glue off the scissors from time to time.

Tip !!!  With a good foundation of adhesive on the assembly you are going to cover, lay out you dacron on the assembly and then using straight Air-Tech adhesive REDUCER and a clean cloth and dampen just the edges you want glued down----- this will cause the dacron to adhere to the assembly and not move around as you size it, or hold it so you can turn it over..

Tip !!! Don't throw away that dried Air-Tech Adhesive that has attached itself to the plastic container you are using. ----- peel it off the container and put it into a fresh container with some straight Air-Tech Reducer and the next day it will be dissolve and you are ready to use it again.   (CHEAP Skate TRICK)

Tip !!!!  Make your own bias tape ----- lay out a piece of fabric and mark and cut out your tape at a 45 degree angle to the weave.

Tip !!! Make your own rib lacing needles using a 1/16" welding rod.  Just hammer a flat spot on one end and drill it and sharpen the other end ----- This is a throw away model---.

NO ! NO !  Ball point ink pens.  Lead pencil only.

Tip !!! Keep a roll of wax paper around for laying over glued on reinforcements that refuses to lay down, then stack some weight on top of the wax paper and let it dry.

Tip !!!  Keep a cheap pencil point soldering iron around to burn you drain grommet holes with, or use a high speed dremil and a small pointed rock grinder or do both.  I like to melt the raw edges of the dacron hole to element fuzzing.

"Question"  How big do I need to make my drain holes ?
"Answer"  Kill a Dirt Dobber, (a spieces that lives in your area --- some areas have smaller or larger ones) then using a  micrometer measure his head and make you drain hole is aproximately .003 thousands smaller.  If he  can't get  his head in then he can't get in.  (How is that for being technical)!!

Tip !!! If you are not sure where to locate the inspection rings on a wing  ---- cover the bottom first using the blanket method and then turn you wing over and looking through the uncovered side you can locate all your areas that may need to have an inspection ring installed.  Remember it is better to have an inspection ring and not use it, than later need it and not have it.
 
 

Generating the F.A.A. Form 337 An authorized signed copy of the original STC must be attached to the FAA copy of the form 337 that is sent to your local FAA office for filing in Oklahoma City, Ok. aircraft records section.  Just a STC number in the remarks section, of the form 337 is not sufficient.

The FAA form 337 has to be completed--that is the law.
As a rule the front portion of the form 337 is self explanatory, just fill in the blanks, but  remember in block (4) to check "Alteration" next to AIRFRAME.  Where the problem begins is in Block (8),  "Description of Work Accomplished" section on the back.  The following is an example of what needs to be addressed here.  The FAA now requires that an authorized signed copy of the STC be attached to the completed form 337 that is sent to the local FAA.  This is your written permission allowing you to use this STC by the owner of the STC.  Most STC holders will stamp your original  STC you are using with the date, aircraft serial number and registration number and put you on file this in their records as a legal user of the STC.

You just completed a full cover job on, say, a Piper J-3 Cub and now you have to explain what you did in Block (8)  section on the form 337.

EXAMPLE **
8.   Description of Work Accomplished
Removed all fabric from all fabric covered assemblies.  Cleaned and inspected all uncovered assemblies.  Made minor repair as necessary.   Inspected all assemblies for corrosion and treated as necessary I/A/W A.C. 43.13-1A Chapter 1, Section 2., Chapter 3, Section 1, Chapter 6., and applicable sections of A.C. 43-4A entitled "Corrosion Control For Aircraft".

Completed final Pre-Covering inspection of all assemblies and found all assemblies satisfactory to accept new fabric covering.

Recovered all assemblies using F.A.A. P/M/A approved --(Ceconite, Stits, Superflight) dacron fabric - (D-101, D-102,---part#?-etc.) in accordance with Air-Tech fabric covering procedures manual entitled "Fabric Covering Information Manual" AT 101 Revision (8 or later) dated (date mm/dd/yy).  This alteration is approved under F.A.A. S.T.C. SA xxxxxxx.  Applied Air-Tech flexible polyurethane color coat, color number (CBP xxxxxxx  yellow) as the base and decorated out using color number (CBP xxxxxx black).  This alteration is found to be compatible with any other installed  and approved modification.

Make: Piper
Model: J3-C
Serial: -----------
"N": N******
Date: mm/dd/yy
Tack Time:  hh/mm
---------------------------------------End ----------------------------------------------
Note ***
Make out 3 copies of this form 337, keep one copy for your file, send one copy to the FAA with a copy of the STC attached and put the original, with the original STC attached in the aircraft permanent records.

Suppose you just completed covering a wing for a, say, J-3 Cub and you are not going to reinstall it on the aircraft, but someone else will.  The components, (wing, aileron, etc...) has to be returned to service in  Block (8) "Description of Work Accomplished", may read something like this-----

EXAMPLE #2
8. Description of Work Accomplished
Removed fabric from right wing and aileron.  Cleaned, inspected and made necessary minor repairs.  Treated as necessary for corrosion I/A/W A.C. 43.13-1A Chapter 1 Section 2., Chapter 3, Section 1, Chapter 6., and AC 43.13-4 entitled "CORROSION CONTROL FOR AIRCRAFT".

Completed final Pre-Covering inspection and found right wing and aileron satisfactory to accept new fabric covering.

Recovered right wing and aileron using FAA P/M/A approved (Ceconite--Stitts--Superflight) fabric (Part no D-101-D-102, ##,etc.) in accordance with Air Tech covering procedures manual entitled "Fabric Covering Information Manual", AT 101, Dated mm/dd/yy, Rev: (7 or later).  This alteration is approved under FAA S.T.C. No. SA xxxxxxx.  Applied Air-Tech flexible polyurethane color number (CBP xxxxxxx yellow).  This alteration is found to be compatible with any other installed and approved modification.

Make:  Piper
Model: J3
Serial No.: *******
"N": ******
Date: dd/mm/yy
Tack Time:  hh/mm

Note**The above listed assemblies were not reinstalled on aircraft, block (1) and block (2) of this form must be completed when they are reinstalled.
      Mail a copy of this form with a copy of the signed STC attached to local FAA .   Retain the original form 337 with original STC attached in the permanent aircraft records.
---------------------------------------- End ----------------------------------------------

Field approvals

     Give me an example of a continued airworthiness statement for a field approval.

     Back side of  Form 337

8.  Description of Work Accomplished       (if more space is required, attach additional sheets.  Identify with aircraft nationality and registration mark and date ..... Removed Hays 6.50 x 10 wheels and expander brakes --- (repair parts no longer available).  Installed Cleveland wheel and brakes 199-64 conversion kit using drawing 50-58 dated March 17, 1981 as a guide.  Kit 199-64 consist of main wheel 40-40B (2), brake 30-28 D (2). No special parts or spacer had to be built for this installation.  Reinstalled original 6.50 x 10 - 6 ply tires.  Complied with Product Reference Memo (metallic brake lining conditioning procedure) PRM14A. Continued airworthiness and inspection of this alteration will be accomplished using Cleveland Wheel and Brake Component Maintenance Manual and PRM64 and 14A. Aircraft:  abcedfg Model:  xyz-55 N: N12345X Date: dd/mm/yy Tack Time: 555555                -------------------------------------------END--------------------------------------

Note *** Be sure and make this same continued airworthiness statement in the aircraft log book and/or refer back to this form 337.-----this statement will alert the next technician who will be performing maintenance on this aircraft on what should be inspected and how the alteration will be maintained.

I have made up a statement for the  buyer to sign after you have completed a pre purchase inspection.
     To access this statement, click on,  PRE PURCHASE STATEMENT, then click the print button on your browser.                                                                        click here !

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