Bits and Pieces
Aircraft Inspection Techniques for Homebuilders - Part 4
By Bill Evans, President - EAA Chapter 266, EAA 794228
Theory to Application - This part makes the move from inspection theory to practice: what to buy and what to do.
You do not need to buy a great deal of equipment to inspect your aircraft well, nor does the equipment need to be expensive. However some purchases are necessary. Don't buy excessively at first. You can always add an item if you see that you need it.
Lights - Never use a trouble light - a bulb on a cord - to inspect. Trouble lights are worse than useless. They blind you and burn you; hence the name trouble light.
Get a series of three or four battery-powered portable lights. They should go from a penlight to a camping lantern. I prefer halogen bulbs for them all, if I can get them.
An assortment of lights
Quite a few of the LED lights give off a soft blue light. You want really astonishingly bright lights. You want them to give off white light, not yellow or blue. Even two million candle power is not too much. With enough millions of lumens you can read the fine print on almost anything without glasses. There is no such thing as too much light when inspecting.
Oh yes, and change batteries often. Rechargeable batteries are okay, but have enough of them.
Mirrors - As with lights, you need an assortment of mirrors. Those very expensive bendable Snap-on mirrors on a metal tube are very nice to look at, and many mechanics have them. Many good inspectors do not. Like the trouble light, they are more trouble than simpler mirrors.
Mirrors and glasses
You want a couple of mirrors to be held in the hand. Use pieces of duct tape to cover the back and edges. When a mirror breaks you don't want shards of sharp glass all over or worse, inside your aircraft.
You also want a cheap dental mirror, though my dentist gave me one of his. There are standard sizes for flat mirrors, say 2 inches by 3 inches and another one, 3 by 5. These can be held in the hand or attached to a metal strip with carpet tape. I have seen plastic mirrors in pharmacies and even dollar stores. It's better to have five of these than one really expensive brand-name mirror. Mirrors on metal strips are usually bent at 30 degrees to the handle.
When you inspect structure in a mirror, the mirror and light have all your attention, and this is what you want. You'll often see inspectors moving beside a part so they can see it in their mirror. It helps maintain focus and concentration. If you are inspecting 1,000 pounds of thin metal, you want and need concentration.
Glasses - Even if you don't wear reading glasses, get a pair of 1.75 cheaters from the dollar store. Also buy a cheap 5X magnifying glass. Buy a flash glass. A flash glass can be obtained at a stamp collector store. It combines a flashlight, mirror, and magnifying glass. You place it on the item you wish to see in detail, and behold, all its faults are revealed. The aircraft parts suppliers carry jeweller's loupes in various magnifications. I buy 10X. Keep all your glasses in bubble wrap. You don't want them to break. If you have bench where you do such inspections, then buy some rubber mats for the adjacent floor. Mirrors and glasses love to fall to the floor.
Reading Glasses - One year my employer sent all the inspectors for an eye exam. Why would they do that, you ask? It beat me...the machine they used to test eyes had a focal length of 16 inches. All our reading glasses had a focal length of 24 inches. The company had asked us not to buy 16-inch focal length glasses. Every inspector I knew said, "They won't make me look blind next year. I'm going to let them test my eyes with 16-inch glasses and just not tell them." Within the year there were about 450 pairs of new prescription glasses. All had a focal length of 16 inches.
This little story is to say that if you are in the market for reading glasses, and you will also be inspecting aircraft, then you need to find a focal length that allows you to read (documents) and inspect your aircraft. For the record I have four pairs of prescription reading glasses. One pair is also safety glasses.
Scrapers - Having some form of scraper to remove loose paint or other coating is often necessary. However, steel scrapers, knives of any sort, will do damage, sometimes serious damage. Anything you are going to use to scrape structure must not damage the structure. Otherwise it defeats the whole point. There are two kinds of scrapers that are softer than aircraft metals. One is Lexan, another is Plexiglas. Usually they are 1/4-inch thick by about 11/2-inches wide and 6 inches long, sharpened to an edge and with some sort of handhold to protect your hands. Many layers of cheap tape work well enough as a handle.
The other type of scraper is some form of fibre. In aviation there is Micarta. It is a composite layup of linen and phenolic resin, almost certainly cured with vacuum and heat. Control pulleys are often made from Micarta. It has better wear characteristics than Plexiglas and, for other reasons, I prefer Micarta. It can also be 1/4 x 11/2 x 6 inches with some form of handle.
Tappers, Quarters, and Drill Bits - In their own way, each of these can be a useful inspection tool. Brass and aluminum round-rod of 1/2-inch diameter, 3 inches in length, make good tappers. One end is turned to 1/4-inch radius and polished to a good finish. I find them to be superior to quarters, which have edges and can leave marks. What you want from a tapper is to be able to listen to composites, foam cores, and wood for a hollow sound. Composites typically have a consistent sound; while different than aluminum, they nevertheless exhibit a solid sound. If the tone of the taps changes and sounds hollow, it is very possible that delamination exists.
By 1982, even Boeing aircraft saw extensive use of composites. At that time I took a week-long course in repairing composites in flight control surfaces. These were of Nomex core and usually graphite weave for skins. When repaired, it is important to be able to know whether the hardness of the repair meets the needs for primary flight controls flying at Mach .85. How do you tell in the field?
Enter the #20 drill bit. You need a brand new HSS or cobalt #20 drill bit. When the cutting face of the drill's business end is used to scratch the skin, the strength can be determined. At the time I had access to repairs that had been cured at 200°F, 250°F, 300°F, and 350°F, so I scratched away to determine how hard you had to press to scratch the graphite skin. Then I quantified it with a fish scale used in compression. You hold the drill bit between the thumb and index finger at the chuck end and press downwards with the ring finger, so that the whole face of one cutting fluke contacts the composite skin. If the pressure of your ring finger is five pounds, and the skin is not marked when scratched, then the repair hardness is adequate for service.
Obviously if you have a Barcol hardness tester with a suitable tip, then you can also measure composite hardness. I assume you will not have that tester.
Boroscopes - I picked up a Pentax endoscope, valued at $4,300, for $50 at auction. It is amazing.
Cameras and camcorders connect to it, so that video can be taken of boroscope inspections and sent back to the engine shop or manufacturer for evaluation. Air can be pumped through it to blow away dust. Today you can buy a scope and camera for hundreds of dollars as opposed to $125,000 just 20 years ago. I find that the cheap flex scopes can be made to work.
I recently inspected all the cylinders on a Rotax engine using a $20 flex scope. It was very difficult to actually shape the probe to see the piston head and both the valves. It took hours. But if you are patient it can be done. You can also take pictures. So keep your eyes open for deals, but don't let price deter you. Harbor Freight, King Tools, and others sell moderately priced flex scopes. Some have mirrors and various tips to increase accessibility. Our chapter is buying one for $200 as I write this.
Gauges - There is a variety of small gauges that can help you quantify wear and play on aircraft components. Feeler gauges, Hi-Lok gauges, go/no-go, depth gauges, 6-inch steel rules, and tape measures all have their place. I was asked last year what possible use a 25-foot tape measure could be. An aircraft has three axes - longitudinal, lateral, and vertical. If you find a hard-to-locate snag and will need to find it again, you can use a coloured label with the snag # on it to show you where it is. Many of you know that there are a dozen or more station numbering codes to identify structural locations. Fuselage Station 120, or FS 120, is 120 inches aft of datum. Wing Station WS 20L is 20 inches left of the fuselage centerline. Buttock Line BS 10 is Buttock Station 10. It is 10 inches above the aircraft water line. Write down all three on your job card, and you'll know exactly where the defect is. Your tape measure tells you those numbers. It will also allow you to measure your way back to that spot.
Go/No-Go Gauges - Sometimes you have a number of components to measure for wear. One day I was asked to measure the seat floor tracks on a Boeing 747. The tracks are 1,700 inches long and 8 inches across. So I invented a gauge that measured all the critical locations on the tracks for all the different types of wear. Using a gauge speeds up your inspection. It also lets you know exactly which locations pass inspection and which fail. The passengers like the inspector to get this right. When the pilot hits the brakes, paying passengers do not like to have the seats pull out of the floor and careen down the cabin, presumably into other passengers. It's a bit of a nuisance to make up gauges but sometimes necessary.
Screwdrivers - are used for removing screws, Cam-Loks, Dzus, and other fasteners. They are not scrapers or pry bars, at least not on aircraft. I find it helpful to have a set of bits handy for the aircraft I'm working on. The common bits are flat, #1, 2, 3 Phillips, Apex, and Tri-wing #3, 4, 5. For Jabiru engines you need Allen wrenches. Aircraft Spruce and others sell them. At inspection time you really need them. I find that air drills and some impact drivers damage more screws than they remove. At least use a big-handled screwdriver to loosen screws before applying power tools. Also start them by hand. When you are sure they are engaged, then run them in, but keep the torque on the screwdriver to a minimum or you'll strip the head and make them impossible to remove in future.
Wrench Sets - The process for inspecting my aircraft involves extensive disassembly. The wings and tail surfaces come off.
There will be hundreds of bolts, screws, and other connectors on your aircraft. You may remember that I said you inspect with your hands. Your hands will often hold those tools. While inspecting, you want to ensure that every fastener on your aircraft is wrench tight. Don't overtighten things, but do check that all the hardware is tight. So keep a socket set and a combination wrench set handy. I find the 3/8-inch drive socket sets to be the most versatile.
Crack Check Kits - Magnaflux Corporation sells three-part kits: cleaner, dye, and developer to identify cracks with some certainty. I can't show you this on paper, but I'll try. Perhaps 20 years ago turbine engine shops found that the crack check processes in use (dye check) caused false negative indications and some engine failures after overhaul resulted. This happened where hundreds or thousands of parts were being inspected on a production line. Thus dye check was banned from certified repair shops.
All this is to say that if you have a mandatory airworthiness directive (AD) for cracks, then you cannot use dye check any longer. There is nothing else for it, so you need to gain access for florescent particle inspection (FPI) or magnetic particle inspection, ultrasonic, eddy-current, x-ray or whatever it takes. You have no choice.
As I said, Magnaflux does sell fluorescent particle inspection kits. You need the ultraviolet light and goggles.
So what is a homebuilder to do? If you have a service bulletin before you, you must do it right. But say you are looking at your homebuilt, and you think you see a line or mark but can't be sure what it is. You could and maybe should buy a Magnaflux FPI Kit. Aircraft Spruce sells them.
Red Dye, Whiskey, and Talc - However I learned from an old timer that you can use rubbing alcohol and compressed air to clean the suspect spot. You want to wash out and dry the crack. Then you use some red food dye on an artist's brush to try to fill the crack with dye. Brush for three minutes. Sometimes tapping will allow the dye to work into the crack. Then you wipe off the crack with a dry paper towel and wipe it six more times with a slightly damp cloth (alcohol damp, white wiper, or shop towel; cheesecloth is also okay).
Finally you blow the tiniest film of talcum powder over the crack. Tiny film is the operative term. If the part is cracked, the powder will show a red line. No, it's not a casting mark, scribe line, setback mark, or anything else. If you damp-wiped it six times, and the talc turned red, the part is cracked. Period. If the crack is deep, you can wipe off that thin talc and blow another tiny bit over the crack and it will turn red again. It's cracked! It's also a very cheap inspection crack test. I reiterate: This is not for ADs.
Foolscap and Pencil - You'll recall that I mentioned job cards (Mentioned 'em? He never shuts up...) At times you'll have a snag such that the repair won't go onto one card. Rather than use six cards for one item, you get a piece of 8 x 11 cardstock and write your snag on that. Then you have roughly 30 lines of space to record the details for this long repair process. I hasten to add that 11-inch cards are not the way to go for a bunch of identical snags at different locations. It is better to use a card for each snag. Individual cards avoid confusion about what has been repaired and what has not.
That's it for inspection equipment, which at least covers the structure, attachments, and some components. Thus the stage is set. In the next part, we'll deal with inspecting structures of various types. We'll see how these tools are employed to reveal an aircraft's secrets. We're cooking now, ladies and gents!