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Aircraft Inspection Techniques For Homebuilders - Part 1

Principles of Inspection A

By Bill Evans, President - EAA Chapter 266, EAA 794228

We chose to do this part now because, in a sense, the rest can be bought or learned; attitude cannot.

One of the most helpful things we learn is that inspectors are really suspectors. You suspect everything. Nothing is okay until proven to be okay.

Lists - We deal with lists in a separate part of this series. It is enough that you know that you must inspect according to a list. Never "wing it". Plan to work methodically. This will be covered as well. Anything that flies people around deserves a full day's inspection. Systems described as retractable, constant speed, and flaps will require more time. Plan ahead.

When you finish inspecting every part and system, sit down with a coffee and ask yourself, what have you missed? Two of us did a pre-buy inspection on my present homebuilt. Once we got it home we opened all the panels and cowlings to have a deeper look. The result was a few shop days with a light in one hand and a mirror in the other. In that time, 94 snags were found, ones not seen before. "What have you somehow missed?" is a very real question. Your task is to find them all.

Key Points - To begin with, any defect that will take your life becomes a key point by definition.

You have read that accidents are a series of events, conditions, errors, and failures that lead to a crash site. It's not nice to think about, but better now than when on that course.

Aircraft defects are like that. When you find, for example, a part of structure that is bent, cracked, broken, adrift, or in some other way defective, what should you do?

Stop right there. Determine exactly what you have found. Is it a crack, crevice, casting line, setback line, fretting mark, notch, or score mark? All these are different things. Just six of them are snags.

Structural crack in a spar
Structural crack in a spar

This 2003 photograph shows a slightly jagged line from a radius and is very typical of crack development. There is some staining but no real fretting marks. Thus, the crack may be fairly new. There are other things that look similar: paint lines, a hair from a paint brush, a casting line, a score mark, or perhaps less likely, a setback line. A 10X jeweler's loupe helps make the identification clear.

More important, structural snags do not stand alone. Structural components fail because loads were exerted on them. When a component fails, those loads did not just go away; they were transferred to the next part in the chain and so on. Where are the other snags?

Thus our next question is this: Where exactly does this defect you have found fit into what is very likely a chain of failures? Is it the first failure, second, or a failure affecting another system, or is it the final link in this chain of failures? Stop everything else and be certain that you have tracked this snag down from beginning to end.

Does this failure also affect controls, landing gear, steering, the electrical system, or your expensive avionics? Your future may well depend upon you being thorough here. If you have a hunch that you are in over your head, then so be it. Get help. There are not many AMEs around who would refuse you a look with their probably very skilled eyes. It may not be before work or day's end, but most will take a look.


Pride is your enemy here. It may be that you built this aircraft and know it better than anyone else. Hopefully there is now a realization that building requires a different skill set from maintenance and inspection. Many of the designs really try to keep it simple. However, pilots increasingly demand faster, higher, further, and more sophisticated designs. How many such new composite designs are there? We can't remember them all. But we will need to do justice to them if they are to remain safe. In future articles we plan to dig deep here.

Most aircraft carry four fluids: fuel, hydraulic (brake) fluid, engine oil, and battery acid. We'll allow that your compass fluid hopefully does not remain a great safety threat.

Fluids leak, corrode, burn in various ways, and dissolve glues and sealants. If you find a low fluid level or a leak, find out where that fluid has been and what it has damaged. Fluids also represent fire hazards. Fuel, oil, and hydraulic fluid all burn as an example. Before you replenish and move on, be certain that this leak will not result in an in-flight fire on a future flight.

Attachments - One sunny day, we were turning final at about 85 mph. After the turn when slowed to 80, there was just a slightest twitch in the control stick. It was like a minute ticking. So I e-mailed the moderator of the builders group. The reply was that those aircraft with an elevator trim tab have a metal clamp to retain the Bowden cable. If this clamp is anything less than tight such that the cable cannot move, then it will start to tick. As it ticks it loosens more, and the tick soon becomes a flutter where the tab goes full travel one way, driving the elevator far the other way; and so the flutter progresses. Should that metal clamp crack, it cannot be replaced with a plastic- or rubber-covered one of the right size.

This anecdote is here to say, "Don't take little things lightly at inspection time." Attachments are virtually all key points since the attachment of almost anything on your aircraft's structure, engine, or controls is important. If there is play, why is it there? What is the risk of failure, or worse, flutter?

When you inspect your aircraft, your task is to inspect and write down everything. The write-it-down part will be explained later and in detail. For now, know that you need to inspect everything and write everything down that doesn't look just right.

Prioritize - Once your aircraft is inspected, it becomes important to prioritize the snags you find.

The question once again is what can take your life? If an engine quits you might make a field, road, beach or even put a jet down at Gimli. They have all been done.

If, however, you found fuel leaking from your wing, and upon closer inspection saw that the wing skin was cracked near the trailing edge, then upon further inspection saw that at the same wing station number as the crack there was a vertical crack in the rear spar, what are you to do? In 1984 I asked the chief engineer for an airline, "What is the most serious snag that can exist?" It's a cracked spar. I saw this snag at Dusseldorf.

For our purposes, there is no such thing as a ferry permit, or a quick-and-dirty fix. Maybe you'll trailer it home. Maybe you'll rent hangar space to replace it. Possibly you could fabricate and install a channel doubler. It might be adequate to get you home if all the other conditions were ideal, but first call the designer.

While not essential to flight itself, most of us covet a reliable engine. We watch the condition and performance of my engine like a hawk. It has been said that if your engine has all cylinders with good compressions, good oil pressure at idle and cruise, a nice smooth sound when operating, no leaks of any kind, no unusual vibration, and produces its full rated takeoff rpm, then as far as possible the engine may be considered reliable.

The very lovely editor of our local EAA newsletter recounted to us how the 50-hour engine in her C-150 went from smooth as silk to the big bang theory in just a few seconds. The instructor made the plowed field landing of his career. The engine overhaul shop replaced the engine at once and without a murmur. Elvis sang a song about suspicion. Well, so do inspectors, and at annual inspection time, so should you.

What else can take your life? One year we went to Sun 'n Fun and asked Forums Director Bob Curtis what could be done to reduce the number of Sun 'n Fun Forums about Airparks. Bob replied that one could give him better choices but we should present something about which we were knowledgeable.

Not long after that a very expensive Lancair with a new TSIO 520 went in from a stall spin on its first flight. The local EAA chapter president and his best friend died then. We live 45 minutes north of this chapter and took it hard. These pilots were our friends.

It turns out the injection fuel pump was not torqued nor safetied to the engine. It wiggled adrift just enough to stop the fuel flow. There is a Mandatory AD CF-2007-27R1 for this defect. We were told that they'd assumed the factory delivered the engine with all the accessories ready to run. Obviously not so. We also learned they had not done a fuel flow test.

TCM Fuel Pump
The TCM mechanical fuel pump

The AD mentioned above is a repetitive AD mandating the detailed inspection of Teledyne Continental Motors IO 520 engines for proper installation of the engine fuel pump. Specifically it must be inspected to make certain the proper engagement of the engine and pump splined shafts. The engagement is of a type which requires only 1/4-inch engagement. Further, the two attachment nuts must be correctly torqued and safetied. The example we've seen used tab washers with provision for lock wiring. Experience has shown that if any of these items are incorrect the pump will become adrift and disengage. Since the engine is fuel injected, it requires pressure to operate. Thus disengagement results in loss of power.

Key Points

  • The flanges must fully mate, ensuring that full spline engagement exists.
  • At the left of the photo you can see the oval flange that mates to the engine. It has two holes, one at each end visible. Whether bolts or studs and nuts are used, the two fasteners must be correctly torqued and lock-wired.

This instance inspired this article and the forum we present at Sun 'n Fun. Why? Because two pilots died as a result of this snag.

As we say, the more sophisticated aircraft and systems become, the greater the demands on inspection. Suspicion in this work is your friend.

Bogus Parts - Now, when some of us shop for hardware we look for bogus parts, and they are out there. Seven years ago we installed a cable-driven stabilizer trim system on my homebuilt. It worked like a charm for five years. Of course, I needed a cable and hardware to join the ends together. As we went over the turnbuckles (aviation term is turn-barrels), we saw one that was just a tiny bit different than the others. Instead of having the little bell symbol stamped on, as virtually all of them have, this one had four diamonds formed into a "T".

In 1965 there was a repetitive AD mandating the removal and destruction of those T-stamped turn-barrels. At anytime into perpetuity they are to be destroyed when found. There are lots of them out there. The story is that a helicopter lost control due to a broken control, traced to the T.

Anytime you are inspecting your aircraft or buying new parts, the signs of bogus parts should be in your mind. Here are some red flags you should look out for:

Price - Too good to be true.
Supplier - There is no paper trail for this part to the maker.
NOS (new old stock) - Various organizations warehouse aircraft parts. There have been parts stored in otherwise dry conditions since World War II. Beware! In the world of aircraft parts, just like antique cars, there are new tires and "fresh" tires. Fresh means made during the last quarter. Thus a part can be "new", technically meaning unused, but it can also be old. Metal parts may age well and could be a bargain if not corroded. However, parts with gaskets, sealant, O-rings, or diaphragms, etc., do not age well and should be replaced with fresh parts. We say that tubes and tires have a shelf life of three years. Vacuum pumps have a service life of five years, as do many carburetors and fuel pumps. Because there is so much at stake with fuel components, buying new old stock is seldom a good choice.

Further, a certified part may be subject to ADs and other recalls over time. Yesterday's new part may be bogus today.

Condition - It's very nice to think you have a bargain on a part. Get out your inspection equipment and have a very good look at what you've bought. Also assemble it or try to install it. See if it fits properly and works correctly. No, don't inspect just the release note and the stamps; inspect the part.

Next month: Part 2 - Principles of Inspection B


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