How to Tie Down Your Airplane
By Dan Grunloh, Editor – Light Plane World, EAA 173888
Damaged at Sun ’n Fun, Eipper GT-400, N80360, won’t be rebuilt, according to the owner.
The storm and tornado that struck the 2011 Sun ’n Fun International Fly-In & Expo demonstrated a strange phenomenon I’ve witnessed in the wake of other storms at other fly-ins in the past. While no plane is going to withstand a close encounter with a tornado, it’s not unusual to see some ultralights or light planes survive while heavier aircraft with the same wing area are totally destroyed. One of the reasons is fairly simple.
Ultralight and light plane pilots typically operate from unimproved strips where built-in tiedowns like those on the FBO ramp aren’t available. Their little planes desperately need tiedowns; pilots already own them and know how to use them. Of course, some of the planes destroyed at Lakeland-Linder wouldn’t have been saved even if they had been tied to bulldozers with log chains.
Many, however, weren’t damaged in place. Their tiedowns failed for various reasons, and some traveled hundreds of feet during their destruction. Photos and videos taken right after the storm don’t show a lot of wings with torn ropes still attached. There may be some, but not many. Instead you’ll see tiedowns pulled out of the ground and still attached to the ropes, and some planes with ropes missing entirely.
More Is Better
Three tiedowns per aircraft should be considered the minimum and suitable only for normal conditions unless the anchors are very strong or it’s a low-wing aircraft. The use of two tiedowns or anchors on each wing plus tiedowns on the nose and tail does more than double the insurance against a failure. The additional anchor points prevent the bouncing and wiggling in a storm that can work the ropes loose and pull the anchors out of soft soil. Furthermore – with two points on each wing – if one fails, the plane is still secure. When there are three tiedowns per plane and one fails on a wing, it’s all over as the other two won’t resist the bouncing and thrashing that can result. Some of those ultralights saved at Sun ’n Fun had a lot of tiedowns.
Securing high-wing taildraggers can be challenging, and the tail rope may be the most important part. It can be hard to keep the tail rope tight. If it becomes loose or the anchor pulls out, the plane can often flip upside down while still tied to the wing ropes. When using two anchors per wing, place one forward of the attachment point and the other aft to help lock the plane in place. Frankly, wheel chocks don’t help that much in a storm.
Choose Your Anchors Carefully
The ubiquitous corkscrew pet or “dog” tiedowns are used more widely than any other type. They’re readily available, economical, and work well in a variety of conditions (excluding loose soil or wet sand like we had at Sun ’n Fun). The quality of this type of anchor varies tremendously. The poorer ones can bend, break, or pull out of the ground. Avoid those that have a prominent crimp on the shaft intended to retain a ring for the dog leash. The cross section is reduced and the metal is work-hardened, creating a weak spot. High-quality steel or titanium anchors of this type can give excellent service especially if multiple anchor points are used.
Next, consider the triple-pin driven anchors like the Claw and others. Three straight pins driven at different angles on each anchor spread the load and make it easier to place the anchor in rocky soils. Many consider this type as the ultimate anchor, but it works poorest in soft or sandy soils. Most were reported to have held at Sun ’n Fun. The almost unbelievable amount of rainfall in the days prior to the tornado limited the effectiveness of all types of anchors. The soil at Lakeland-Linder is a mixture of sand, rock chips, and fill that can be highly variable. An anchor that can’t be driven into the ground in one spot may go in too easily just a few inches away. This trait may explain some of the apparent randomness of the destruction.
In addition, consider using earth anchors of the type that features a round helical plate at the end of a long shaft that is screwed into the earth. They’re terrific in soft substrates but can be very difficult to use in rocky soils. This type of anchor is widely used to brace utility poles, temporary structures, and much more. If you take a look at the tiedowns used by experienced pilots who have flown thousands of miles in rough country, you’ll often see earth anchors. Avoid lightweight versions of this type as they can fail where the round plate is welded to the shaft.
Weights Don’t Work
In an emergency or in desperation, it’s tempting to tie the plane to something heavy. It doesn’t take much math to figure out why this approach often fails. I once made the foolish mistake of tying my ultralight to a pair of 80-pound concrete blocks on a calm day. It worked great until a fairly small dust devil happened to pass by and flipped over my plane, flinging the blocks on the ends of their ropes like they were toys. At Sun ’n Fun, it was reported that a destroyed Kitfox was picked up while tied to a pair of 600-pound weights. That may sound fantastic until you realize how much weight those wings can normally lift when the wind speed is more than twice the minimum flying speed.
Ropes and knots are generally preferred over ratcheted webbing cargo straps because they’re lighter and easier to pack, but it’s important to use knots that won’t loosen or slip. Most pilots use some version of the taut-line hitch which can be seen in this video. We like it because it’s adjustable. You can go around the airplane and snug up all your tiedowns. However, it can loosen in gusty conditions because it works best when under constant tension. The anchor hitch or fisherman’s bend shown here might be better for extreme conditions. Be very careful if you use the ratcheted webbing cargo straps. They’re strong enough and easy to fasten, but the ends are usually open “S” hooks! If your landing gear deflects and allows the wing to dip as much as one inch during a storm, it could slip right off.
Finally, seek help or shelter for your plane when it’s offered. Don’t be a hero or stubbornly insist that your tiedowns will hold anything. As the storm approached the ultralight area at Sun ’n Fun, a group of volunteers hastily moved 21 airplanes into a nearby hangar, thus saving them. At least one pilot declined the offer to move saying that his tiedowns would hold just fine, but he was proved wrong. Some planes were saved because buildings, trailers, or trees nearby apparently diminished the winds just enough, so pick your spot carefully. And always travel with enough tiedowns. The owner of a light plane destroyed at Sun ’n Fun said he searched all over the town of Lakeland but couldn’t find any additional tiedowns. Everyone was sold out. To quote a famous phrase, “For want of a nail, the horse was lost.”