Sun 'n Fun Tiedown Tests
Writes reader Mike Gillis: "I think a story could be done on tie downs. I noticed in a video at Sun 'n Fun a row of planes blown away, yet two remained tied down. They had three-legged yellow tie downs. Quite a testament to their holding power. So many secured planes were damaged by planes that got loose."
Amen, brother. Another reader asked what lessons could be learned from the big blow at Lakeland on April 7. The only take–it-to-the-bank lesson is don't park your airplane in front of a tornado. Beyond that, the answer is it depends on soil, rain, wind, rope, proximity and just plain luck. But there are some takeaways. I spent a day following the storm surveying the damage and talking to owners to find out which tiedown systems worked and which didn't. Conclusion: None of the commercial kits that I found worked 100 percent, nor, as far as I can tell, did any of them fail 100 percent.
The big variables—which are largely uncontrollable—are intensity of the local wind, soil conditions and moisture and how the airplane is oriented relative to wind direction. When tying down, you never know where the blow will come from so take your best guess. Further, the wind could be 80 knots at one point, but 30 knots 200 feet away.
The "yellow three-legged" tiedown Gillis refers to is The Claw, the most popular commercial system by far. It consists of three aluminum legs joined at the center and anchored with three long spikes driven through eyelets in an inverted cone. For many owners, it worked a charm. "I'd use them again in a heartbeat," said Dan Mercurio of Van Bortel Aircraft. "They held great." Mercurio had a Cessna 182 secured outside the main hangar area. On the other hand, in the American Legend booth, Dave Graham's experience wasn't so positive. One of Legend's Cubs blew into the tent after the Claws securing it pulled out and one of the legs snapped. (Here's a video of the dynamics.) In that test, the pull is at an angle. This product holds much better if the pull is vertical, so put it directly under the tiedown ring.
Throughout my conversations with people on the field, this it-worked-great or it-pulled-out pattern was repeated. But my impression is that most resisted the blow.
Bill Alexander, of Hunting Solutions, which makes The Claw, says that nothing could have held in the worst of those winds. I'd amend that by saying nothing commercially available. By the way, Hunting Solutions was replacing broken Claws free of charge, which is part of their guarantee. I stood in their booth for 10 minutes and heard a continual stream of testimonials.
In our reviews of tiedowns for Aviation Consumer, we consistently rated The Claw as a top choice, and nothing I saw at Lakeland changes that view. Alexander says that tests he did at Lakeland prior to releasing the product eight years ago showed pull resistance of 750 pounds in that type of soil. Our tests, done with a pull dynamometer, never equaled that in Florida or California soil, but The Claw still beat just about everything else. Soil conditions and moisture will change resistance results by orders of magnitude and over a distance of 10 feet, never mind the distance between tiedown rings.
In the Rans booth, Michele Miller told me she watched a couple of the popular K-9 auger anchors "pop out the ground like corks out of a bottle," sending the airplane cartwheeling toward her and the Piper display. On the other hand, right next door, Liberty Aerospace's K-9 augers held just fine. (Rans are taildraggers, Libertys are tricycle-gear airplanes and that makes a difference, since taildraggers start with a high positive angle of attack.) One hundred feet away, Diamond's airplanes didn't budge. I'll get to why in a minute. But first, note that in our tests, the cheap K-9 augers didn't do badly, sometimes exceeding the resistance of The Claw, depending on soil conditions. The Lakeland failure patterns seemed to confirm these findings.
A note on rope: I didn't see any rope breaks, although there probably were some. I didn't see any tiedown ring failures either, but there could have been some. As for rope, many people use the popular 1/4-inch yellow polypropylene rope, one of the worst choices. Its tensile strength is around 950 pounds, but when you knot any rope, you weaken it significantly and stranded polyprop tends to fracture fibers at the knots, losing a significant amount of strength. The better choice, 1/2-inch dacron, isn't expensive and has a tensile strength of 4500 pounds, so it can give up some knot weakening and not lose much. We never broke a strand of this stuff in our testing, but we almost busted the dynamometer trying.
In certified airplanes, tiedown rings are supposed to be sized to resist a 65-knot wind load from any direction. The load on the rings varies by airplane type and weight, but it can easily reach a ton or more. The last big blow I investigated was Hurricane Charley in 2004 at Punta Gorda, Florida, where 150 airplanes were destroyed in sustained winds of 130 MPH. By sustained, we're talking two or three minutes.
The morning after the storm, I walked some tiedown rows that had been swept clear of airplanes. The common mistake owners made was not replacing old, weathered ropes with fresh line and doubling it up. I found only one tiedown where the wing rings had pulled out. And then only one of them failed. I found plenty of frayed, broken ropes and smashed airplanes downwind. In comparison, Lakeland was a picnic.
At Punta Gorda, the tiedown survivors had doubled up lines through the rings—dacron or nylon—and chocks tied to the wheels. That last item is important, because you don't want the airplane weathervaning; it needs to stay put. One airplane that did weathervane had its tailfeathers torn off. One enterprising owner had used something you don't see often—wing snakes. These are small firehoses filled with sand and placed on top of the wing as lift spoilers. They worked beautifully and although the airplane might have been totaled by flying debris, the owner could at least say he didn't contribute to the clouds of missiles raking the airport.
I've seen some comments that most of the airplanes damaged or destroyed were LSAs. I don't have good data on that, but I saw heavier airplanes upended, including a Caravan, a SuperCub and what looked like an Arrow. The problem with LSAs is this: Because they can generate a lot of lift but they support it with less structure, they just aren't as strong as conventional airplanes. For example, Explorer Troop 491's Kitfox was well secured and we were told it was tied to 650-pound blocks. It appears to have been simply shredded in place, then blown downwind and balled up. I doubt if anything other than a stout hangar would have saved that airplane.
Last, Diamond. Its airplanes stayed put because in addition to being heavier than LSAs, they were secured with custom tiedowns made from a 3/4-inch steel rod 18 inches long driven into the ground at about a 45-degree angle. These are long enough to penetrate beneath the top layer of soggy soil. In the JAARs display area, Darryl Neidlinger showed me a variation on this theme: Three 24-inch 3/4-inch rebars driven into an inverted cone pattern at each tiedown ring and joined to a center point with chains. If that rig comes lose, we're well into subsonic wind velocities. But don't buy the argument that you can't secure an airplane against any wind short of an F3 tornado. A properly tied down airplane can survive well into the triple digits.
Would you bother with this extra effort? Depends on how much you value your airplane and what responsibility you feel toward your neighbors in keeping your airplane from becoming their problem. I actually feel that's the larger issue here. I don't want my airplane getting away and hurting someone or doing damage because I cheaped-out on the tiedowns or just assumed the insurance would take care of it all. I'm going to get some rebar and weld up a couple of these custom pins to see how they work. I'll let you know.
Oh, one last lesson from Lakeland. If one of these storms blows up, don't take shelter in a Port-A-Potty. Yes, that's right. Some folks did, and you know what happened. A couple turned over with the door down and that's just too revolting to detail. So I won't.
See the Aviation Consumer tie down test video here.