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At The End Of Your Rope

Every year, pilots continue to allow their aircraft to suffer at the nonexistent mercy of storms.Every year, pilots continue to allow their aircraft to suffer at the nonexistent mercy of storms. Although it’s good news if nobody gets hurt, the bad news (and what makes this regrettable) is that this damage occurs after they’ve landed and gone home!

SEEING THE THREAT
Figure 1 In many cases the enemy is a thunderstorm, and the next link in the accident chain is an improper tie-down. I hasten to add that if the origin of an ill wind was a tornado or hurricane, the best thing you could do (if you had enough time) is to get in the aircraft and high-tail it out of there. If your hangar is built from reinforced concrete, you’re definitely in a very privileged minority... although if you have a hangar at all, you’re still in the minority. Knowing how to properly tie down an airplane is the only other defense available to most of us, and even a thunderstorm can generate winds faster than the liftoff speed for small airplanes.

LEARNING THE ROPES
If you own an aircraft, you’re probably already fairly weather-conscious, but when high winds threaten there are some important things to remember...

LOCATION, LOCATION, LOCATION Figure 2

  • Use an approved three-point tie-down, usually installed pointing into the prevailing wind. Each tie-down anchor for a single-engine airplane should be able to withstand a 3000-pound pull (4000 for a light twin).

  • The recommended configuration actually calls for pavement tie-down points several feet outside of and beyond their corresponding points on the airplane. This figure, from the FAA’s Advisory Circular 20-35C would give the main tie-down lines an angle of over 50 degrees from the vertical! ...and greatly increase stability.

    Figure 3

  • Playing The Hand You're Dealt: Tie-down rings at your airport might not have been installed “ahead of and behind” the aircraft. Your configuration might only offer wing tie-downs toward the left and right, in about the same vertical/lateral plane as your wing tie-down eyelets. Whatever the case, the angle of all tie-down lines, main wing and tail, should never be in the same direction, like in this photo -- regardless of the painted yellow lines. Clearly if this airplane were to move (rearward in this case) towards the ground anchors, all the lines would then slacken, thus compromising their ability to hold the airplane down. Of course, if tying down properly puts you in the taxiway, perhaps this is a better option.
ROPE
  • The minimum acceptable diameter: For most small airplanes, nylon tie-down line is one-half inch. Five-eighths is much better, and three quarters should be good for light twins. There are also other options besides a length of nylon braid-on-braid from Home Depot or BoatU.S. The use of chains is preferred by some, and many prefer the newer adjustable locking belts. (In my flying club, we use 5/8” nylon double-braided line.)

  • The end strands of synthetic line, if cut, should be “whipped” or melted together to prevent fraying.

  • Never use tent stakes or manila rope. One good rain, and you’d find out that mud doesn’t hold very well. Manila can mildew and rot, it's weaker than synthetic line, and it shrinks when wet. That shrinkage could actually damage your aircraft, and you might not know until you're in the air.

  • Lines should be good for 3000 pounds (more on what “good for” actually means, later).
KNOTS & HARD POINTS Figure 4
  • Don’t cut any slack (literally). Oscillations can permit even an anti-slip knot to loosen and come undone. But don’t pull them tight with a block and tackle, either. (That could also entail large inverted loads, although you really would need several hundred pounds of lateral pull to do that.)

    Figure 5

  • The best types of knots to use include the bowline (left) or a version of the double half-hitch (right). Most of the folks in my flying club use a modified version of the latter, for which, unfortunately, I could not find a figure. (The one shown could slip; ours won’t.) There are many web sites on knots; some even have animated instructions for the topologically challenged. (The best time to learn them is before you desperately need them.

  • Never tie directly to a strut; they were built for specific loads, and that isn't one of them. You could damage the structure.

  • The attachment to the ground, at the other end of the tie-down line, tends to be taken for granted. Check it occasionally, and if something doesn’t look right, report it to your airport manager or fix it yourself, if you’re able.
GAIL FORCE DEFENSE
  • Consider padded 2” x 2” spoiler boards bungeed across the top of the wing, about one quarter chord width from the leading edge. (See the previously mentioned AC 20-35C for details.)

  • The best hurricane defense is to retreat. Your plane won't get beat up if it's not there. (As for the folks in tornado alley, well... how fast can you preflight?)

  • Seaplane owners have partially filled floats with water or even filled them completely, when tied down ashore.
FINER POINTS
How strong is strong enough? To sum it up, there are two types of metrics. One is “tensile strength” or “static breaking strength”, which is determined by tensioning the line between two large capstans until it breaks. For 5/8” nylon line, that’s at least five tons! However, there is another measure: “working load”. A safe working load, according to the Cordage Institute, is anywhere from one fifth to as little as one-fifteenth of a line’s tensile strength. (That more critical value is applied to lifelines.) For our 5/8” line for example, one estimate for its working load is about 2800 pounds. (Three-strand twisted polypropylene line of the same diameter however, has only a third of that.)

These safety factors are based more on engineering than superstitious caution: As lines age, they deteriorate from wear and exposure. Every time you knot a rope, you can consider that you've cut its tensile strength in half. And there’s that problem of elongation. Almost any synthetic line approaching its working load can easily stretch by at least 10%. Translation: If the wings generate enough lift, your airplane could leave the ground anyway.

THE NOT SO FINER POINTS -- Trigonometry and Vector Analysis 101
One trick I use is to tie the main tie-downs first, before I chock the wheels. (In some of our tie-down spots, the tie-downs align pretty much along the same fuselage station as that of our Skyhawks’ struts.) Then I loop the tail tie-down through and pull back on the loose end until the airplane has rolled back a few inches. Exerting just a 50-pound pull, moves it back six inches and sets the main tie-downs about five degrees back from vertical. That would theoretically add about 550 pounds of tension (through the magic of component vectors)... but only if the tie-down line were a steel cable. But the nylon lines stretch (less than half of one percent), which still confers some extra tautness to the tie-down lines (well under a hundred pounds of pull, but enough to get a dull “twang”).

BOTTOM LINE: The airplane you fly was a huge investment, to you or someone. Simple care and proper maintenance doesn't end when the wheels stop rolling, or even when you've gone home. Airplanes want to fly. Given proper motivation, they will do it without you, and... well... that would be bad.

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