Bits and Pieces
Aircraft Inspection Techniques for Homebuilders - Part 5
By Bill Evans, President - EAA Chapter 266, EAA 794228
Last month, Part 4 of this series completed the study of inspection tools. This month in Part 5 we continue the study of aircraft inspections with the structural inspection process. There are three main levels of inspection: surveillance inspection, external inspection, and internal inspection.
Surveillance inspection involves looking at structure and control surfaces from the hangar floor, a ladder, or a stand. You might not be able to get closer than 2 feet. You are looking for obvious damage, or defects, dents, lightning strikes, or corrosion.
Often a detailed inspection is needed. For example, in certain structures the skin will crack under the head of countersunk rivets. In pressurized aircraft, once the crack has reached the point where it is visible, the situation is becoming critical. A pressurized fuselage may burst if this fault is missed.
Detailed means every rivet. In composite structures, tiny fissures in the paint will allow substantial moisture to ingress the structure. With the human eye, nothing will be seen. With 20 times magnification, thousands of these fissures may be seen. If undetected, this water will eventually freeze or otherwise seriously damage the wing's internal structure. Can you see that detection matters and a very close, detailed inspection is required to see these defects?
External inspection - The above paragraphs have focused on inspecting the aircraft's external surfaces. Internal damage may also be found to start here.
Internal inspection - To accomplish this work, it will be necessary to open the canopy and all the access panels. Remove the cowlings. On my aircraft it is also necessary to remove the rudder, elevator, horizontal stabilizers, and all the fairings on the wings and tail areas. These removals also permit comprehensive cleaning and lubrication. On some aircraft there are wheel-well doors and instrument panel access doors on the firewall and other locations. You need to find and remove them all.
The most serious accusation that can be brought against you as an inspector is that you did not accomplish a complete inspection, that components, linkages, and electrical connections remain unseen.
One of my colleagues detected a crack in an aft pressure bulkhead on a jet transport, looking forward from the tail compartment. Within days other bulkhead cracks had been found. It was realized that the bulkhead crack was progressing 1/4 inch with each pressurization cycle. There were thousands of these airplanes flying.
These cracks were located in a part called a bulkhead T-ring. By using a long flex borescope, it was possible to see these cracks from locations 10 feet away. It was also possible to photograph them. These photographs resulted in a worldwide airworthiness directive, which mandated bulkhead inspections before further flight. There are serious snags out there. Are you determined to find them?
This defect was in a stressed skin aluminum structure. Had this been a composite aircraft, it might have only been possible to determine the extent of damage using X-ray, eddy current, ultrasonic, or thermal imaging technologies. All of these are recognized inspection technologies. Operators have specific training and calibrated equipment. Thus we say again, if you see something obviously wrong but don't have the skills and equipment to inspect it adequately, ask your local AME to recommend someone who does. It may cost you money, but nothing like what it will cost you to ignore such defects to the primary structure.
Types of Structure
At one time, say, 30 years ago, two-thirds of homebuilt aircraft were comprised of wood structure wings and steel tube fuselages, and both were covered with some form of Dacron polyester. Ninety years ago aircraft structures were covered in Egyptian cotton or fine grades of linen. Some brands of Dacron are FAA certified such as Poly-Fiber, which is still what we prefer. So we'll begin here just to have some chronology or time line. Wilbur Wright once used a pair of satin bloomers.
Fabric Coverings and Coatings - It may be that your fabric paint will withstand oil and dirt for a long time. But that time ends on inspection day. AeroShell makes a comprehensive line of cleaning products called Flight Jacket. However, we learned something else from someone who performs aerobatics for air shows in the summer. Presently we use kerosene (outdoors) in an air can to spray oily areas of the lower wing and fuselage and carefully wipe it all off. We use good rags, such as jersey, to avoid scratches, turning these rags often. Kerosene is less likely to damage paint than other chemicals.
To restore the shine, we use the Mother's line of waxes. We will say this: Only paste waxes result in a finish where the wax remains on the paint. A good paste wax shine will resist UV sunlight far more than others. So buy the best paste wax you can. Do not use a spinning buffer as it will remove much of the wax. If your arm joints do not allow a hand shine, then use an orbital polisher operated in a figure eight pattern. Simonize makes one. It will shine the wax while allowing most of it to remain. In my opinion the Mother's carnauba paste wax is as good as any and better than most. Your aircraft will even smell good.
Today homebuilders are often using 2 ounce/yard Ceconite on their fuselages and 6 ounce/yard on the wings, controls, and tail surfaces.
The protection of the fabric from ultraviolet sunlight is provided by the aluminum powder in the coating that is applied over dope. Poly-brush dope is pink in colour. The aluminized protective coating goes over the dope, followed by primer and colour coat.
Use the brightest light you have in a dim hangar or garage to determine where light shines through. It will be necessary to repair the aluminum coating everywhere light shines through. In my opinion, use a million candlepower or more.
Why? If the aluminized coat is intact, then no further testing of the fabric is required. It must be visually inspected and touched for tension, but that is all.
If the coating has seams or areas where light penetrates, then over time ultraviolet degradation of the fabric occurs. Yearly inspections and maintenance will ensure the life of your fabric may be almost indefinite.
Once the fabric has deteriorated/weakened, then it will be necessary to use a fabric tester, which punches a tiny hole in the fabric to determine its strength. One fabric tester is the Shure Hardness tester. Many AMEs still know how to use them. Afterwards the hole needs to be patched and painted. If the fabric fails, then it needs to be repaired or replaced.
In the case of cotton or linen coverings, there are time limits after which these fabrics must be tested. Yes, there are still such coverings out there, but they're mostly confined to museums. Which fabric do you have? Fabric coverings usually are stamped inside such that they may be easily read. Mine has an FAA TSO number and date.
Once we found a loose spot in the Poly-Fiber fabric on my aircraft. So we called Poly-Fiber and discussed the matter with its technician. He asked us to get a really great heat gun, like an 1875-watt hair drier. With it turned on max, make one pass at 2-inch intervals across the wrinkled area and stop. The operative term is one slow pass. We did this and the wrinkle was removed. In seven years the problem has not resurfaced. The Poly-Fiber system lends itself to repairs and rough usage, say, a little more than others.
Likewise tears and cuts may also be repaired with the same original materials. We have once or twice purchased a practice kit that contains enough of the fabric and tapes and four or five chemicals to perform two patches of about a foot each. The cost is around $20 for the kits. Same place: Poly-Fiber.
Two winters ago we were invited to the Canadian Aircraft Heritage Centre west of Montreal to help do the rib stitching on a replica Bleriot being completed for display in the Montreal City Hall. This coincided with the 100th Canadian Anniversary of Flight. It was nice to be a part of that.
Apparently rib stitching was a "black art" there, and those who knew did not share, but worked in secret. In our case we were to work with a fellow named Eric, maybe 7 feet tall - really a giant and a big man in every sense.
Now, for me rib stitching is just rib stitching. We learned from a very senior EAA member at a Sun 'n Fun forum. The EAA teaches its own method of rib stitching, similar to what Ray Stits pioneered. It involves a modified seine knot, and if done properly, results in very taut stitching. Taut is what you want.
On your aircraft you might have a modern airfoil, symmetrical or a nearly flat lower wing profile. However, the Bleriot does not. It has a convex lower wing surface. If the stitching is not close and tight, the fabric may pull away from the structure when shrunk. It could also pull away in flight and flap.
The EAA technique involves a number of steps that others do not use, but together they result in very taut rib stitching, which is very capable of holding fabric to convex wings.
Now, we won't go over each step here, because you have to see it done. At each point where our method would vary with the "black art" guys, Eric would say, "We don't do that here," and we two would roar with laughter. In the end we were able to show him that the flat fibreglass thread stitched this way produces a stitch that will sound like a bass guitar string if plucked. The other methods do not so resonate.
Why are we telling you this? Good rib stitching obviously should be taut, and the threads should not be broken. In your inspection, you want to determine that the fabric is taut to the structure and none of the stitching is broken. It is possible for loose fabric and broken stitching to come loose from the wing structure or tear in flight. If you lose your wing fabric in flight, you are probably done for. This inspection is to ensure you are not done for.
The inspection of fabric coverings involves the search for defects, wear, thinness, tears, staining, paint damage, and looseness of seams or tapes. In some designs, fabric is retained by screws or rivets. In these cases, the security of those fasteners is also inspected.
If fabric is struck at internal members, the fabric may be visually okay, but internal damage may exist beneath the fabric. Thus fabric is cleaned and then inspected with both eyes and hands. Does your wing structure give or move beneath your hands? Write down whatever you find. We'll see how to record defects in a later part.
Stressed Aluminum Skin - Where alloy aluminum sheet is stretched over frames, stringers, and longerons, the resulting structure is called stressed aluminum structure. In some cases it is possible to get slight compound curves just by proper attachment. In most cases compound curves require either hydraulic forming or the use of bead bags or an English wheel or both. Cowlings are an example. If aluminum in soft condition is used, then it must be heat treated.
Defects are more likely to occur at stress points or otherwise where loads are applied. Defects are also common at attachments and at fasteners such as rivets. We have shown in a previous part that one defect is very often a link in a chain of failures. You must find them all.
What are we inspecting structures for? We want to find cracks, dents, or bent attachments, internal structure, and joints.
Attachments are also inspected for corrosion, fretting, looseness, or having become adrift. The whole world of aluminum structures has now aged to some extent, and some aircraft have continued to fly since World War II. Thus an Ageing Aircraft program has been developed to deal with aircraft, say 25 years and older. How old is your aircraft?
Forms of Aluminum Corrosion
We have seen a heavy jet flap-beam removed because it was time-expired. Upon removal it was set down on a wooden bench. When it touched the bench the beam broke in half, releasing a litre of white and gray powder. The beam had been subjected to a form of internal or intergranular corrosion.
No exterior cracks or perforations could be seen with the naked eye. The exterior appeared to be intact, though the interior was totally compromised.
There are other more common forms of corrosion. Where aluminum alloy is exposed to the elements over time, surface corrosion may occur. It starts looking similar to the tarnish of the family silver; but over time surface corrosion becomes deeper, and eventually pits will form. It may be possible to use abrasives to remove the products of corrosion, after which processes a coating may be employed to prevent future corrosion.
As a rule, and there are exceptions, one may say that the removal of material to remove corrosion should not exceed 15% of the original thickness. If the component is primary structure - a spar, ribs, frames - these limits should be further reduced. On some components, no wear or corrosion is permitted: Spar pins, trunnion bolts, and axles are examples. If you don't know the limits, it's wise to ask a structural AME.
Exfoliation corrosion often starts as surface corrosion, but then the aluminum opens up much as you would open a book, exposing leaves, layered by the products of corrosion, i.e. aluminum oxide. On homebuilt aircraft with such very light components and skins, exfoliation would virtually always require repair or replacement.
Chemical corrosion - If you use a lead-acid battery in your aircraft and it is damaged or frozen hard, it may leak acid. Most battery boxes are stainless and will resist some chemical corrosion. But if acid, brine, or what's in a urine container leaks onto an aluminum structure, then chemical corrosion will result and almost certainly do substantial damage. There are some other chemicals you may carry which can also damage your aircraft. Double containers or shockproof/unbreakable containers are also important to prevent such damage. Think about what is at stake.
Galvanic corrosion - Dissimilar metals can also result in an increased rate of corrosion. Our own first aircraft had some magnesium components on certain flight control surfaces. While lighter, they were also more subject to this form of corrosion. The paint would blister and a certain form of magnesium oxide would be evident. In most cases, corroded components were replaced by aluminum parts covered by both an AD and service bulletin. Where (if at all) does your aircraft have dissimilar metals in contact?
There are several excellent discourses on galvanic corrosion available on the Internet, including this one on the NASA website.
Fibreglass and composites - Composite structure will be defined here as some form of relatively light, soft core, either wood, honeycomb, or foam, which is sandwiched between stronger layers of reinforced fibreglass or other synthetic skin.
Properly done, a composite wing may have twice the strength yet half the weight of the stressed aluminum wing on my homebuilt. With certain techniques the wing may be completed by a team of two in a week.
(Epervier is such an aircraft. It won the prize for the best undergraduate engineering student project in Canada in 2009, our flight centennial. The wing on Epervier was tested to +14g but only weighs 33 pounds. It presently resides in the National Collection in Ottawa; we think for one more year. EAA 266 Montreal donated the fuel for 10 hours of flight testing in 2009.)
Composite structures are inspected for obvious defects, dents, cracks, softness, delamination, discolouration, minute cracks in the paint, and signs of moisture absorption, e.g. into honeycomb cores. One key point for composites is that appearances are deceiving; the two students who designed and built the Epervier wings made a test wing to prove their concept.
A 16-ounce ball-peen hammer was used to punch a 1-inch hole in the upper wing. This test wing was then cut in two, through the 1-inch puncture. The composite wing was found to have the skin delaminated over an area 24 inches in diameter in all directions. The 24-inch delamination could also be heard by means of a tap test to attuned listeners. Small dents may mean much larger areas of damage than is apparent.
It was explained in our previous article on inspection tools how a new drill bit can be employed to evaluate the hardness of composite skin repairs. We won't explain that here again, but it also may be used to test in-service components for strength.
A tapper is preferable to tap for delamination in composites and plastics. Tappers do not leave marks.
Composite flight controls may sometimes absorb moisture, litres of it. Sometimes it may be felt by moving the now heavier control by hand. If the aircraft has been outside in cold weather, the water will still be cold when the rest of the structure has warmed.
Thermal imaging cameras are now more affordable and will reveal in blue the existence of water, where other areas may be a different colour such as green. Where could you borrow one?
A thermal image of a composite wing
Ultrasonic inspections are able to see hidden flaws. Hospitals use them for many things, often to determine the health of unborn babies.
In all sorts of aircraft structures, ultrasonics can allow you to see defects at various depths below the surface of composite skins. For example, if you had used a vacuum pump to remove water from a composite flight control, then an ultrasonic inspection would help you to determine what residual damage existed.
Hopefully you will never get to this point with your homebuilt, but if you do it's good to know.
Chromalloy steel tube - It may be fair to say that aviation pioneered the use and repair of chrome-molybdenum steel in truss type structures. For example, the Linde Welding Company's book has a section on gas welding 4130 steel tubing. We find their knowledge of the subject to be far ahead of what others have written, even in much more recent works. While it was not written for homebuilders or hobby welders, the wealth of information and advanced techniques contained is remarkable. This textbook is not new by any means. Used copies may exist online.
4130 is an advanced alloy steel. For more than 50 years, exceptional aircraft strength requirements have been met by 4130 steel. At its ultimate strength 4130 steel is very brittle, since its ultimate tensile strength can reach 385,000 psi. You will never see that hardness in a commercial product, but it is possible. Usually 4130 is welded with lesser strength steel rods. Some certified code welders prefer to use a rod just one number above mild steel in order to get purity in the rod, while avoiding stronger alloys prone to crack. Purity in welds is very important. Never use a coat hangar as a welding rod.
When 4130 tubing is inspected, we are looking for a number of possible defects: cracks in welds and joints or rust beside welds. Rust is a subject of some concern since rust in 4130 often accompanies cracks. It is preferable to use a tapper to determine rust perforation starting from the inside since a punch and hammer will damage the tubing. If the paint is punctured, re-prime at once.
If the exterior of tubing is rusted and/or pitted to the point that it is visible, then measuring tools, feeler gauges, micrometers, telescopic gauges, and verniers are used to determine the remaining wall thickness. Usually, steel tube structures are flooded with linseed and preservative oils, then drained to reduce internal corrosion (after welding of course). The writer has seen 4130 tubing stored in less than ideal circumstances for 30 years or more. After the rust was removed, the tubing was measured both inside and outside. It was determined that the rust had penetrated to a depth of 0.003 inch at the worst place. In my opinion, in the case of 0.049-inch wall tubing, the pits (once removed) pose no threat to the strength of the tubing. If you do find rust, don't panic too soon.
4130 steel and hydrogen embrittlement - The chemical hardening of 4130 steel is another matter. There are two known ways in which 4130 steel may become excessively brittle from hydrogen.
The first is ammonia NH4 in paint stripper. Paint on 4130 steel must never be removed by paint stripper. As the stripper works, one can smell the ammonia, NH4. The H (hydrogen) is coming out of the stripper, and it will cause hydrogen embrittlement of the tube structure. If paint must be removed, it must be removed by mechanical means such as hand sanding. Use increasingly fine grades as you approach the parent metal. Scratches result in stress risers which may result in cracks.
The other threat is water. Again, water in contact with 4130 steel will allow some hydrogen from the water to penetrate the steel over time. This is proven.
Why are we telling you this? All the 4130 components on your aircraft must be painted immediately after mechanical cleaning, and the paint must be maintained.
Possibly our earliest aircraft structures were made of wood. Birch has been used for skins, sheets and plywood. Sitka spruce is used for spars, ribs, and frames because it is light. Balsa is still viable where a wood core/aluminum skin composite is desired. It is very light and less expensive than a Nomex core. We have even seen oak used in aircraft floors beneath galleys and washrooms. One example is the DC-8-40. No, it's not a homebuilt.
At annual inspection time it will be necessary to open all the wing access panels. If two panels are adjacent, then a strong light through one and mirror on a metal strip in the other may be adequate to see all the ribs and spars. If not, then either more access panels are needed or you will need the use of a Boroscope flex scope. For structures, it may not need to be as complex (read expensive) as needed for engine inspections. It may be run through ribs, allowing inspection deeper into the wing's interior. Some $20 scopes may be adequate.
What are you looking for? Wooden structure wings are subject to failed glue joints especially where Aerolite type glues are used. Certainly modern epoxy glues are superior.
Accidental damage can and does occur to wood components that may be cracked or broken outright. It is not at all uncommon for spruce spars to crack. Such cracks often occur where spar attachment plates are bolted together through wooden spars.
If you think for a moment, you can probably recall the musty smell of rotten wood. While you are inspecting wood structures, be on the lookout for that smell. Also be looking for stains and any other evidence of water damage to the wood. While wood may get wet and dry normally, if it remains wet, such as through the seasons in storage, wood can crack if frozen wet, and rot in the summer. Wood/fabric aircraft should be flown often and kept dry when not flown.
That Boroscope should be used to see the end of the spars, especially at the fuselage. This is more difficult because that is where the steel attachment plates are located. The Boroscope allows you to see between those plates.
Sometimes a spar can receive shock loads such that it will have a crack like a broken branch. Look carefully for this sort of damage. If you see something suspicious, take a photo if at all possible. While you are inspecting inside the wings, look for broken rib stitching and evidence of loose fabric. Write all defects down as you find them.
How do you inspect wood? You want to handle as much or the wing interior structure as possible. Feel for cracks, wetness, loose rib parts, and anything else that feels strange. At each access panel, you want to sniff for rot. Some people are asthmatic and do not inhale any fungus well. Don't induce an asthma attack, but do be aware of what you smell. Visually inspecting inside a wing, whether by long mirrors and lights or Boroscope, is time consuming. It simply cannot be rushed, or components will be missed. The first time I inspected my homebuilt I found 94 snags/defects. Virtually every aircraft has snags. What have you found in your inspection?
Canopies and Windows
Usually canopies and windshields are made of thicker material (Perspex acrylic sheet) than side windows for reasons of occupant protection and noise. This material, while more brittle than Lexan, is preferable because it resists scratches better. Admittedly Lexan is much tougher than Plexiglas if subjected to FOD or a bird strike.
We will limit ourselves to inspecting these components since the removal of scratches is another subject in its own right.
We will say that using a stream of clear water, with your hands, to remove dirt and bugs from your canopy is probably the single best thing you can do to prevent scratches. Fingernails are softer than Plexiglas. Using a good canopy cover to prevent the sun from attacking your canopy is perhaps the best thing you can do to prevent crazing, cracking, and discolouration.
Cracks - If canopies and windows are properly installed and sealed, they are much less likely to crack, as pressure points are hopefully eliminated.
We have seen canopies, as you may have, with numerous cracks that have been stop-drilled and copper-wired several times. In a previous section we covered limits. It was stated earlier that it's probably a good rule to limit the number of times a crack may be stop-drilled to one, and that the crack should not affect the viewing area. We'll stay with that.
Delamination is a defect in plastics similar to exfoliation corrosion in aluminum. It usually starts at the edge of the Plexiglas surface and spreads inwards. Often ultraviolet (UV) rays cause delamination, though other causes are possible.
It will not be possible for a homebuilder to adequately repair a delaminated canopy. If it is large enough to necessitate repair, the canopy is probably decades old or was left in the sun. A canopy for our homebuilt costs around $400. With a little saving, most of us should be able to replace a delaminated canopy the following winter. If you are interested in a deeper understanding of how to arrest delamination, check out this presentation.
Crazing is also caused by UV rays. Crazing consists of hundreds or thousands of tiny cracks in the surface of the canopy. In minor cases it may be possible to polish them out. But in all cases of polishing, heat is your enemy. The canopy will distort or even melt a little if heated by polishing. It might take days or weeks to remove crazing and repair the surface to be optically perfect.
Discolouration - UV rays can also cause Plexiglas to discolour or become milky in colour. It is probably the case that the discolouration will penetrate right through the window or canopy.
Scratches - We see from our notes that AeroShell Flight Jacket has a kit for scratch removal. We expect that other parts suppliers for homebuilders have kits as well. We have not used any of them, so we won't comment.
Attachments - Canopies and windows may have attachments. Some designers install the compass on the windscreen. Many windows have hinges and latches. Inspect these very carefully for cracks at the edges and at fasteners.
In a homebuilt aircraft, a broken or damaged latch or hinge in a canopy is cause to ground the aircraft. Over the years a few hinged canopies have opened in flight. Only one pilot that we know of had the physical strength to close it again. His name may be Atlas. Most of the rest are done. Our canopy is secured by two latches and a lanyard. Consider this yourself.
Protection - As with the case of fabric paints, paste wax is a good protection against dirt, most scratches, and UV radiation. The canopy needs to be spotless. Use microfibre cloths, and turn them often. Use a new can of wax to avoid contamination. Apply the wax by hand and polish the canopy with new cloth wipers while turning them often. Cheesecloth works okay as well. Don't even think of the power buffer on your canopy or windshield.
Finally, "gentle" is the operative work with Plexiglas.
This completes the section on inspecting structures. The next part should close out the series, for structures, anyway. We will look at recording the defects you find, how to organize and plan your work, and how will you return your aircraft to flight after this annual is completed.
I am hoping that you can see by now that a complete inspection is not a small thing. Neither is it beyond you if you prepare properly - and work to an inspection document, item by item.
Why would you do all this work? Your AME is not the only person who should know your aircraft's exact condition. You should know. A complete knowledge of your aircraft can provide valuable information as a resource for flight planning, route choices, and flight limits. If your aircraft is in top condition, it can remove the fear, at least the fear of not knowing the condition of your aircraft. If your aircraft is also maintained to a high standard, you can also have confidence in its reliability in flight. That's worth a lot.