From Matronics

Thoughts on Living with Ageing Aircraft

Bob Nuckolls
AeroElectric Connection
1 September 2004

There is a lot of discussion on the problem of “ageing aircraft”. There have been numerous incidents in the air transport industry wherein root cause of the event was attributed to wiring. The wiring was, “In service beyond expected service life”. A few years back, a major portion of cabin top separated from an airplane in Hawaii. The spontaneous addition of a sunroof to the passenger transport was attributed to unnoticed, or ignored cracks in the skin.

Numerous groups in aviation’s regulatory, manufacturing, maintenance and users of such airplanes are spending tens of $millions$ to gage the magnitude of the problem and decide what should/can be done about it. Anecdotes from various deliberations provide a wealth of material upon which “worry stories” may be considered worthy of the front page of Today or 15 seconds notice on the nightly news.

Without a doubt, those who own and operate large, heavily utilized aircraft are faced with tough questions. The machines are expensive to buy and expensive to maintain. The notion that one should periodically return the airplane’s condition to factory-green status (fully functional airplane with no interior) for total refurbishment of major portions of airframe or systems is a horrible thing to contemplate. None-the-less, this is the simple, obviously correct answer.

It’s unlikely this will happen very often in the air-transport or military worlds . . . but it’s a sure bet that meetings of lots of people will go on for a long time to craft rules and regulations that upset as few bureaucrats as possible while maximizing probability of hiring more staff to oversee new rules and insuring jobs for however many folks it takes to refurbish a fleet of aging aircraft.

It will be up to the marketplace to decide whether refurbishment or scrapping old airplanes is the best thing to do. It’s a demonstrable fact that the most competitive products are those touched least by human hands. A device built by machine is less expensive to build in the future. Further, it’s easier to evolve machine-built products to stay abreast of latest technologies. The efficiency of machines gets better as the technology evolves.

Conversely, a product fabricated and maintained with hands-on labor is more expensive to build in the future; the cost of labor always rises over time. Further, high-labor products are least likely to enjoy the benefits of improved technology. It seems that there’s a fundamental human resistance to change. At some point in the life of large aircraft, return-on-investment for labor to refurbish the machine is too low and will not be considered.

What’s all this aging aircraft stuff for small airplanes? Very few small aircraft serve as capital equipment in for-profit ventures. Virtually all small aircraft are owned and operated by individuals who make their return-on-investment decisions on a very small scale. Let’s consider the concept of aging . . .

Suppose in 1949 someone had asked Walter Beech, “How long do you expect this new Bonanza to last?” What might he have said? Folks would have been incredulous had he offered: “Oh, probably 30 years”. After all, the average automobile and industrial vehicle is pretty much used up at 10 to 20 years. It’s pretty audacious to suggest that HIS product would have a service life twice that of a Cadillac or John Deere. What would Walter have said if you said your crystal ball reports that some of his products will be in service after 50 years?

We’re all aware of some very old machines that have the appearance and performance of a new machine. This observation may be confirmed on any weekend at steam shows, fly-ins, car-shows, tractor shows, etc. Some of these machines are simply well preserved. Others required an investment of thousands of hours labor and thousands of dollars in a restoration effort. It’s easy to visualize a heavily used, 5-year old, poorly maintained machine that’s ugly to look at, unsafe and perhaps unserviceable while a similar, well cared for 50-year old machine might be a pleasure to look at and just as serviceable as the day it left the factory.

It is obvious that suitability of any machine to function for its intended purpose is a trade off between (a) effects of wear-and-tear combined with (b) a willingness of the owner to spend time and dollars to maintain and/or restore the machine. The chronological age of the machine is not a limiting influence on the possibilities for the machine’s longevity. I’ll suggest, therefore, that the term “aging aircraft” has no useful meaning. What we’re really discussing is poorly maintained aircraft.

Folks who claim the greatest understanding of service life put routinely repaired, replaced or overhauled components in a separate basket - isolated from parts having demonstrably longer service lives. This means that engines, batteries, tires, brakes, paint, upholstery are treated differently from wires, wing spars, landing gear struts, etc. It seems obvious that EVERY part of EVERY airplane has a service life. Some parts are quite long lived while others are relatively short. Further, based on differences in environmental and service stresses any part may out-perform its clone on another airplane. From the perspective of inspecting an airplane for continued airworthiness, no component should be favored with a low priority for inspection based exclusively on age of the airplane.

With this alternate perspective, let us consider an article “Wired for Disaster” which appeared in Sport Aviation. The article offers four major ideas pertaining to concerns on wiring. Insulation, corrosion, poor workmanship, and choice of materials. We’re also encouraged to consider a cursory load analysis that suggests installation of a larger generator may be in order.

(1) Corrosion: Yes, surfaces of most metals change with age . . . and here’s the keyword - SURFACE. You can bet that every wire strung from pole-to-pole in our nation’s power grid has some corrosion on the outside as do all the fittings associated with carrying the flow of energy from generator to reading lamp. It’s a certainty that many of these wires and fittings have been exposed to atmosphere in a totally unprotected environment for decades. Yet, we don’t observe cadres of workers running from pole to pole with Scotchbrite pads and wax trying to return all these materials to factory new appearance.

This is because each joint that was properly designed and properly assembled forms a gas-tight connection through which electrons reliably flow. The same conditions apply in airplanes. Properly applied terminals, tightly assembled threaded fasteners can appear pretty bad on the outside while maintaining perfectly satisfactory current carrying performance inside. Corrosion is a concern for joints where the original gas-tightness has been lost. For old aircraft, cars, boats, motorcycles, this phenomenon is most likely to occur at the threaded fasteners that have worked loose (or improperly tightened in the first place).

This is so poorly understood that even hallowed documents like AC43-13 “Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair” offers the following advice: “Bus bars that exhibit corrosion, even in limited amounts, should be disassembled, cleaned and brightened, and reinstalled.” Hmmm . . . here’s a real cash-cow activity for every FBO in the country:

“Gee man, we spotted some corrosion on your bus bar. Wasn’t much but you can’t be too careful. Corrosion is insidious stuff. We disassembled your whole breaker panel, polished all the parts and put it back together. Looks like brand new. We were kinda worried that this project would get out of hand. I figured we’d need at least 24 hours to do it right but Joe got ‘er ship-shape in only 14 hours! He ‘saved’ you a bundle.”

I’m sure we’d all appreciate the practitioner who carefully follows AC43-13 to the letter.

The paragraph should read, “Bus bars that exhibit corrosion should be inspected as follows: (a) electrical connections to a bus bar should be inspected for evidence of discoloration due to heating. (b) Apply recommended assembly torque to threaded fasteners and watch for movement. If the fastener is loose, the joint is suspect and should be disassembled for closer inspection, cleaning and reassembly. Alternatively, (c) use a low resistance ohmmeter or bonding meter to measure the resistance of joint between bus bar and joining conductor. Low current (10A or less) joints that exceed 1 milliohm should be opened for cleaning and reassembly. Finally, (d) joints carrying more than 10A are more accurately evaluated by measuring voltage drop across the joint when the circuit is energized. 10 millivolts is a reasonable upper limit for voltage drop across a bolted connection.

(2) Insulation: One of my favorite rentals was a nicely maintained C-120 with an electrical system added. This airplane featured original cotton-over-rubber wires. I’ve had several occasions to put my hands on those wires. They were smooth, intact and flexible. Further, this airplane was so carefully maintained that you could eat your lunch off the inside of the cowl . . . it was bright and clean. The engine compartment was totally free of oil and accumulated dirt. The rest of the airplane was similarly maintained.

That is the oldest airplane I’ve ever flown fitted with an electrical system. The specter of aged insulation lurking in dark places waiting to ruin my day never entered my mind while enjoying the use of this unique aircraft. Many airplanes in the local rental fleet are products of the PVC insulation era. Still going strong after 40 years. Some of these airplanes (particularly under the cowl) have been updated or repaired in places with Tefzel wire.

Suppose I open the cowl on one of these airplanes and find run of brand new PVC wire? Is there a reasonable cause for concern about this one piece of new PVC wire added to several hundred feet of 30 year old PVC wire? If we understand and respect suggestions offered in articles on aging aircraft, we’ll feel MUCH better if the new-wire smoke is “less hazardous” than the old-wire smoke stored in several hundred of feet of original wire still in service.

(3 & 4) Choice of materials and workmanship: The article featured a number of anecdotes where terminal sizing, choice of tools, carelessness and craftsmanship were found lacking. I’ll suggest this has nothing to do with aircraft age. The same shortcomings are found on machines of all ages and origins. During my first visit to Oshkosh in 1986, I observed airplanes surrounded by trophies, award plaques and blue ribbons that were marvels of mechanical skill and craftsmanship. These airplanes were but a few years old. None the less, some of these prize-winning, show quality machines had examples of wiring technique and materials selection worse than those used to illustrate the article “Wired for Disaster”.

The answer for certified singles is stone-simple: DECERTIFICATION. Our brothers in Canada can de-certify simple, out-of-production aircraft. Once de-certified, the airplane can be owned, operated, maintained and UPDATED as if it were an RV-6 or a Long-Ez. This has a huge influence on the owner-operator’s perceptions of return-on-investment for repairing or updating the airplane. The likelihood of seeing a factory-new looking Tri-Pacer or C-150 is about 100x greater in Canada as compared with the United States.

In conclusion, I’ll suggest that contemporary “aging aircraft” issues are a misuse of aviation industry regulatory and management effort. It’s a non-issue in the OBAM aircraft community. Owner-operators of such aircraft are encouraged and permitted to maintain their airplanes in what ever manner their return-on-investment decisions dictate. There’s no practical reason for year 2000 RV’s not to be flying in the year 2050. There are really nice Thorp T-18’s flying now that are pushing 40 years of age.

Finally, keep in mind that “ageing aircraft” deliberations are being conducted by folks who don’t own, operate or maintain airplanes. They’re crafting a systematic approach to solving a “problem” that arises from regulatory systems already in place. A regulatory approach will produce few (if any) safer airplanes, nor will it encourage rational approach to breathing new life into an old airplane. It’s a certainty that the regulatory approach will increase cost of ownership such that more owner-operators simply give up. An airplane in pieces is often worth more than an airworthy machine. Much of the certified side of general aviation is cannibalizing itself for survival. How long can it last?