Knowthe ropes so your aircraft won't be gone with the wind
By Jeff PardoPhotography by Mike Fizer
Ifyou've ever been a sailor, gone rockclimbing, or earned a few meritbadges, chances are you've comeacross at least a few dozen kinds of
knots. Even if you haven't done any ofthose things, chances are pretty good that atsome point, you've had to fasten somethingto something else. Keeping an airplane onthe ground after you've walked away fromit certainly comes to mind.
Improper tiedowns can be as potentiallydamaging to your airplane as inattentivetaxiing. Unless you're into chains, or youuse self-ratcheting tiedown straps, youought to know a bit about knots: What kindof lines should you use, and how shouldthey be tied?
Knots can be complicated and, forthose of us who don't have great spatialmemories, challenging. The ideal knot isone that is easy to remember, won'tcome loose, is easy to untie even afterbeing under a heavy load, and-perhapsmost overlooked-reduces a line's break-
ing strength as little as possible. Everyyear, pilots continue to sacrifice aircraftto merciless storms. Although it's goodnews if nobody gets hurt, the bad news isthat this damage occurs after they'velanded and gone home. And much of itcan be prevented.
In many cases, the enemy is a thunder-storm. I hasten to add that if the origin ofan ill wind was a tornado or hurricane, the
AOPA Flight Training. October 2004 55
best thing you could do (if you had enoughtime) is get in the aircraft and fly it out ofthere. If your hangar is built from rein-forced concrete, you're definitely in a veryprivileged minority, although if you have ahangar at all, you're still in the minority.Knowing how to properly tie down an air-plane is the only other defense available tomost of us.
If you own an aircraft, you're probablyalready fairly weather-conscious. But evenfor you and certainly the rest of us, here aresome of the most important things toremember when high winds threaten:. Aside from the tied owns themselves,
right, in about the same vertical/lateralplane as your wing tiedown eyelets. What-ever the case, the angle of all tiedownlines, main wing and tail, should never bein the same direction. In other words, lines
from the wings should angle forward totiedown rings, while the line from the tailshould angle back. If all the lines angle inthe same direction and the airplane movedtoward the ground anchors, all the lineswould then slacken, thus compromisingtheir ability to hold down the airplane.. Never use tent stakes or manila rope.One good rain, and you'll find out thatmud doesn't hold very well. Manila can
lock it under the first set with a sharp tug
(see illustration at right). After the secondset is done, I usually tie an additional half-hitch and snug it up against the second set,as a safety.. The attachment at the other end of the
tiedown line, on the ground, tends to betaken for granted. Check it occasionally;andif something doesn't look right, report it toyour airport manager, or fix it yourself ifyou're able.. After cutting a length of synthetic line, theend strands should be "whipped" or melted
together to prevent fraying.. If your airplane is likely to be endangeredby truly furious winds, consider padded 2-inch by 2-inch spoiler boards bungeed acrossthe top of the wing, about one-quarter chordwidth from the leading edge. See FAA Advi-
sory Circular 2()-35Cfor details.. Again, if you live within a couple of hun-dred miles of the East Coast or the Gulf
Coast, you already know about hurricanes,and the best defense is to retreat.
. Though it isn't a standard practice, sea-plane owners have partially or completelyfilled floats with water, when tied downashore.. For most small airplanes, the minimum
acceptable diameter tiedown line is one-halfinch. 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 Depotor Boatu.S. Some prefer to use chains, andmany prefer the newer adjustable lockingbelts. (In my flyingclub, we use five-eighths-inch nylon double-braided line.)
How strong is strong enough? Severalorganizations, such as the American Boatand Yacht Council, offer guidelines andintroductory information. To sum it up, thereare two types of metrics. One is tensilestrength or static breaking strength, which isdetermined by applying tension to the linebetween two large capstans until it breaks.For our five-eighths-inch nylon line, that's atleast five tons! However, there is another
measure: working load. A safe working load,according to the Cordage Institute, is any-where from one-fifth to as little as one-fifteenth of a line's tensile strength. (Thatmore critical value is applied to lifelines.)For our five-eighths-inch line, for example,
S one estimate for its working load is about~ 2,800 pounds. (Three-strand twisted~ polypropylene line of the same diameter,
j however, has only a third of that.) Such~ safety factors are really based more on engi-
The ideal kno* is easy *0 remember, won'* come
loose, is easy *0 un*ie, and maximizes *he line's
remember to batten down the hatches.
Flight controls should be locked either bythe use of a standard yoke-mounted gustlock inside the cockpit, or with externalpadded battens. Brakes should be on, andthe wheels blocked fore and aft with the
familiar prism-shaped chocks (not two-by-fours). Attaching each pair together fore-and-aft with rods bolted into the ends of
each chock makes for an even tighter"boot" around each wheel. Going beyond
locking all doors and windows, cover allengine intakes and exhausts as well as pitottubes to keep out windborne intruders-insects and debris.
. Use an approved three-point tiedown,ideally installed pointing into the prevail-ing wind. Each tiedown anchor for a sin-gle-engine airplane should be able towithstand a 3,000-pound pull (4,000 for alight twin).. Although vertical tiedown lines offerthe most tenacious anchor against highwinds as well as the lowest impact loadsimposed by gusts, they fall short on for-ward and lateral stability. The recom-mended configuration calls for pavementtiedown points several feet outside andbeyond their corresponding points on theairplane.. Tiedown rings at your airport might nothave been installed "ahead of and behind"
the aircraft. Your configuration might onlyoffer wing tiedowns toward the left and
36 A Good Pilot IS Always Learning!
mildew and rot; it's much less strong thansynthetic line, and it shrinks when wet.That could effectively impose significantinverted loads in which the aircraft would
be pulling on the rope, approachingdesign limits.. Unless your airplane is a bantam weight,tiedown lines should also be good for 3,000
pounds (more on what "good for" actuallymeans, later).. Never tie directly to a strut; they werebuilt for longitudinal strength, and sidewayspulls could bend them.. Don't cut any slack-literally. Oscilla-tions can permit even an anti-slip knot toloosen and come undone. But don't pullthem tight with a draught horse or a blockand tackle, either. (That could also entaillarge inverted loads, although you reallywould need several hundred pounds of lat-eral pull to do that.). The best types of knots to use includethe bowline or a modified double half-
hitch. There are many Web sites on knots;some even have animated instructions for
the topologically challenged. The besttime to learn them is before you desper-
ately need them. My favorite involves twosets of half-hitches, where the first set istied about six inches to one foot from the
airplane's tiedown eyelet and the secondabout the same distance from the first. In
tying the second half-hitch in each set, Ibring the line around over the first and
neering than superstitious caution: As linesage, they experience stretching, chafing,occasional shock loads, UV radiation, and
heat (though for nylon and polyester itwould have to be over 300 degrees Fahren-heit). Most importantly, every time youknot a rope, you can consider its tensilestrength to have been cut in half.
And there's that problem of elongation.Almost any synthetic line approaching itsworking load can easily stretch by at least10 percent. (This happens to be about themaximum allowed by the UIAA, an inter-national mountain climbing federation.)What does that mean for your tiedown? Itmeans that if the wings generate enough
lift, your airplane could leave theground anyway. With vertical tiedown
lines, where it comes down againmight be on top of the chocks;with angled tiedowns, it would
be a little more consistent.
One technique I use isto tie the main tiedowns
first, before I chock
the wheels. (Ourtied owns are
pretty muchalong the
"datum plane" as that of our Skyhawk'sstruts.) Then I loop the tail tiedownthrough and pull back on the loose enduntil the airplane has rolled back a fewinches. Exerting just a 50-pound pull, mov-ing it back 6 inches until the main tiedownsare angled back about 5 degrees furtherrearward would theoretically add about 550pounds of tension (through the magic ofcomponent vectors), but only if the tiedownline were a steel cable.
You'd get even better results by pushingback from any structurally strong point inthe front of the airplane and then quicklyplacing a chock in front of one wheel, tolock in the tension. But the nylon line'sstretching (less than half of 1 percent) stillconfers some extra tautness to the tiedown
lines (well under 100 pounds of pull butenough to get a nice bass fiddle "twang"out of the main tiedown lines).
The lowly tiedown is the last and in mos