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An Even Strain - With Paired Flying Wires It’s Critical
By Ron Wanttaja (originally published in EAA Sport Aviation, November 2001)
When a homebuilt aircraft loses a wing in flight, owners of that type of plane sit up and take notice. When the investigation determines that neither a material flaw nor workmanship errors caused the accident, it really catches your attention.
In May 2000, the left wing of a Bowers Fly Baby separated during a local flight in Maryland. The pilot did not survive. The National Transportation Safety Board inspector did not find any overt evidence of a cause; a flying wire and a turnbuckle had failed, but no sign of corrosion or prior damage was found.
The apparent cause is a subtle one. What’s more, there are lessons here for the owners of other homebuilts as well. Let’s take a few moments to examine what happened.
A Few Basics
The Fly Baby’s structure and construction were driven by the rules of the first (and so far, only) EAA design contest. They required that the aircraft be simple and inexpensive to build, and have folding wings.
Designer Peter M. Bowers met the requirement with a design that featured a conventional wooden wing whose wire bracing joined at central attachment points. Flight loads were supported by 1/8-inch flying wires (stranded cable, actually) that ran from the wheel axles to the front and rear spars of each wing. Two cables ran in parallel to each of the four wing spars.
Why pairs instead of single cables? At 6 Gs, a single cable would be under about 3,000 pounds of tension. That would require either a 3/16-inch cable or a solid tie rod. Few homebuilders, then or now, had the equipment to install Nicopress fittings on cable that large. Similarly, solid tie rods would require expensive custom work.
So Bowers specified low-cost, easy-to-construct pairs of 1/8-inch cable. The solution was similar to that used by biplanes that ran solid tie rods side-by-side for their wing bracing.
A Question of Tension
There’s one caveat with this dual-cable approach: Whether solid tie rods or stranded cable, the tension of the two parallel elements must be the same. If one of the pair is slack, the tighter cable carries a higher proportion of the load. If it’s too loose, it breaks, and the remaining cable can’t handle the sudden overload.
Let’s return to the Fly Baby accident. Reading the NTSB report, the accident aircraft’s maintenance status sounds a bit unusual. The owner reportedly had purchased the aircraft as a source of parts. He had lent it to the accident pilot, apparently cautioning him that before flying it the aircraft required a condition inspection by a certificated airframe and powerplant (A&P) mechanic. The pilot did not have an A&P perform this inspection, and the aircraft’s previous maintenance history was unknown as the logbooks weren’t located.
The NTSB investigator concluded that the probable cause of this accident was “an overload failure of both flying wires…due to improper balance between the two flying wires, by unknown person(s).”
We’ll never know for sure. But with the absence of any other evidence, and with the aircraft’s uncertain maintenance history, it certainly looks like one of the cables was slack, and the other failed while trying to carry too much of the load.
Fly Baby Rigging
Fly Baby rigging is a bit unusual in one aspect: A single turnbuckle sets the rigging pre-load, while the individual wing turnbuckles are used to equalize the tension. This is a product of the Fly Baby’s folding-wing concept.
Under each wing the four flying wires (two pairs of cable-and-turnbuckle assemblies) join at a single shackle pinned to the wheel hub. Similarly, the wing’s four landing wires (atop the wing) join at a steel terminal. The two terminals slide into slots just forward of the cockpit and connect to either end of the master turnbuckle.
The master turnbuckle controls the overall cable tension. When it’s shortened, the entire system tightens up. This central turnbuckle is backed off and tightened every time the pilot folds and unfolds the wings. However, when a builder first rigs the airplane, he or she must use the turnbuckles on the pairs of cables to equalize the tension on each pair.
In the plans, Bowers gives instructions on how to get approximately equal tension without using a cable tensiometer. Basically, the builder alternately finger-tightens the turnbuckles and then uses a nail for leverage to tighten the turnbuckles two more turns. Canadian Fly Baby owner Paul Bowyer used this system, and then he checked each cable’s tension with a $1,000 cable tensiometer and found them within 13 percent of each other.
While this system has proven adequate for 40 years, low-cost cable tensiometers are now available. I bought a dial-type meter for just $130. A half-hour’s work brought me equal tensions on my cables and a degree of mental comfort.
The lesson should be clear for Fly Baby owners and others who fly aircraft with paired bracing cables: Buy a tensiometer and balance the tension in your cable pairs, and recheck them at every annual condition inspection, if not more frequently.
There are also lessons for homebuilt owners in general, especially for those who buy used homebuilts. We may chafe at the requirement to hire an A&P to perform the annual inspection, but a professional inspection of this aircraft might have prevented tragedy.
Finally, every design has its maintenance quirks. Buy or build, it’s important to do your research. Talk to other builders and other owners. Check out the information available from EAA, from builder’s groups, and from online sources.
If you fly a homebuilt aircraft, your life depends not only on your piloting skill, but also on your ability to ensure your aircraft is properly constructed and maintained. Fly Baby or Lancair, Hummel Bird or RV, mistakes in the shop, cockpit, or hangar can ruin your whole day.
Fly Baby Info
Persons interested in the Fly Baby can find additional information on this and other accidents at www.wanttaja.com/flybaby.