When the gusts are strong, and the wind direction is just right, airplanes by nature will try to lift off. Each year numerous airplanes are damaged by windstorms due to negligence and improper tiedowns, according to the FAA. Due to the large number of airplanes in attendance, tiedowns are required at EAA AirVenture.
Airplanes should be tied down with equipment capable of withstanding sufficient tensile forces. The FAA recommends tie-down anchors for single-engine aircraft should each resist 3,000 pounds or 4,000 pounds for multiengine aircraft. The weight alone of a multiengine aircraft will not protect it from windstorm damage.
Several types of anchors are available for tiedowns. EAA does not recommend the "doggie ring" style that can quickly fail. The triangle-shaped top portion straightens under heavy loads and the anchor can easily be pulled from the ground. Screw-type tiedowns are also not recommended. Both the auger and spiral style loosen the surrounding dirt as they go into the ground, weakening their anchorage. Cable anchors (straight stakes with one helix at the end), however, make good tiedowns because of their length.
The best anchors are multi-pin anchors that consist of several stakes entering the ground at different directions (view illustration 1). These anchors can easily be constructed as shown in the August 1993 issue of EAA’s Vintage Airplane magazine.
Effective tie-down anchors can also be made from 1/2-inch diameter steel rods with a minimum length of 18 inches. The stakes should angled away from the airplane and be driven all the way into the ground at an angle of at least 30 degrees. All anchors should be placed outward from the aircraft, not directly underneath the wing (view illustration 2).
Effective tiedowns also depend on the quality of the rope. The best ropes are UV-resistant braided nylon or dacron. Manila ropes are not recommended because they shrink when wet, are subject to mildew and rot, and have considerably less strength. All ropes should be regularly checked for damage due to chafing.
Planes should be tied down only at the tie-down rings. Tying rope to a strut can cause damage and may bend the strut. The tension of the tie-down ropes should allow for 1 inch of movement. Too much slack allows the aircraft to jerk against the ropes and can cause structural damage. The jerking motion can also pull out or damage the tiedowns. Tight tiedowns impose inverted flight stresses that many aircraft are not designed to withstand.
A tie-down rope, however, holds no better than its knot. Antislip knots, such as a bowline or a square knot, are recommended (view illustration 3). For maximum strength, tie ropes through the loop of the anchor and around the stake.
Along with tiedowns, it is a good idea to fully secure your plane whenever it is unattended. Secure all doors and windows to avoid interior damage. Engine openings (intake and exhaust) and pitot-static tubes can be covered to prevent damage and debris from entering.
Locking or restraining flight control surfaces can prevent damaging movement. For airplanes not equipped with integral gust locks, external padded battens (control surface locks) can be positioned or the control stick and rudder pedals inside the cockpit secured.
While it is important an airplane is properly tied down, it may still be in danger if its neighboring planes are not secure. Sharing tiedown information with other pilots will protect their airplane, as well as your own.