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Sun 'n Fun Tiedown Tests

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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.

Comments (40)

Hint: Your ultralight will be destroyed in a tornado no matter what kind of tie down is used. Also the higher the wing is above the ground (say, like a Husky sitting up on floats) the easier it will get picked up or turned over.

If you notice, low-wing production aircraft on tricycle gear fared just fine as long as they had even flimsy cork-screw tie downs in the soft soil.

Posted by: Mark Fraser | April 13, 2011 7:48 AM    Report this comment

I appreciate your analysis of the tie-downs. It is an important topic especially to those of us with just liability insurance. I own a Cozy MKIV which I built. Hull insurance is overly expensive for my airplane. The advantage to the Rutan-derivative canards is that they park nose down. Wind coming at the airfoils is actually forcing the airplane down rather than providing a lifting force. N9WK

Posted by: william kastenholz | April 21, 2011 5:36 AM    Report this comment

Not addressed and often forgotten is the question of slack. If the aircraft is bouncing about on slack ropes then it will pull *any* tie down out of the ground. Slack can arise two ways: initially or through the knot slipping. Walk around any tie down area and you will see a variety of laughably useless knots. Often a large loop is ready to provide slack when the knot slips.

Posted by: Paul Beardsell | April 21, 2011 5:58 AM    Report this comment

Here's a good article on tie downs, with plans, illustrations and knots: http://www.airventure.org/planning/tying_down.html

Posted by: Eric Miller | April 21, 2011 7:01 AM    Report this comment

replace space with slashes www.airventure.org planning tying_down.html

Posted by: Eric Miller | April 21, 2011 7:03 AM    Report this comment

The Rutan-drived canards are forced down IF they happen to be pointing into the wind. Looks to me that if the nose is pointed the wrong way, they pretty much should behave like a tail-dragger pointed into the the wind! Again, the ultimate outcome of any aircraft largely depends on wind direction and speed.

Posted by: John Austin | April 21, 2011 7:48 AM    Report this comment

Here is a new tie-down on the market. http://tiny.cc/wkww3

Posted by: Jim Shaw | April 21, 2011 7:52 AM    Report this comment

http://tiny.cc/wkww3

Posted by: Jim Shaw | April 21, 2011 7:53 AM    Report this comment

Well, it won't allow a website link to be posted. Just Google "Storm Force Tie Downs". There is an interesting video of the comparisons between The Claw, and this new product.

http://www.stormforcetiedowns.com/Why-Choose-Storm-Force_ep_43.html

Posted by: Jim Shaw | April 21, 2011 7:57 AM    Report this comment

Dog tiedowns are for tethering dogs, not airplanes. First of all the helix is not tight enough, secondly they are not sturdy enough for firm ground insertion and thirdly the ropes should not be tied to the top, but to the helix at the soil level (but this won't work because of #1 above). That said, any tiedown will fail under the right circumstances, but I would guess that the dog tiedowns will go first. How about a real comparative pull test Paul?

Posted by: Rae Willis | April 21, 2011 8:01 AM    Report this comment

Storm Force Tiedowns and Fly Ties are tested by pulling at an angle while The Claw is tested by a vertical pull. I wonder why the test is not done in the same manner? Disclaimer: I own none of the three & have no interest in them.

Posted by: Rae Willis | April 21, 2011 8:14 AM    Report this comment

Any person that has an ounce of common sense should know that taking shelter in a Port-a-Potty isn't a good idea. So I guess instead of getting wet, they wanted to get wet and...well you know the rest.

I wasn't at Sun-n-Fun this year, but I'm kind of glad that I didn't take my plane down there. If the wind can blow over a Caravan, then it probably can blow over anything else. Anyone that didn't get blown over should have probably went and played the lottery the next day...

Posted by: R. Doe | April 21, 2011 9:19 AM    Report this comment

Storm Force tiedowns aren't new. We've tested them, or I should say we tested that type, perhaps under a different brand name. Rae, we pull tested them all comparitively...that's the part where I explained about the pull dynamometer. I've posted a video link above and you can see the tests.

You're not entirely correct about the K-9 augers, as our comparison clearly shows. In the right soil, they provide respectable resistance; sometimes better than The Claw.

The problem with them is that in years past, they used to be cold rolled steel. Very strong. With all the manufacturing in China, they're now much lighter and appear to be stamped or cast pot metal. Hard to tell.

Bottom line: They're weaker and will fatigue fail when driving them in. If you look carefully, I think you can still find the heavier duty augers.

Posted by: Paul Bertorelli | April 21, 2011 9:20 AM    Report this comment

I really wonder how "FLY TIES"did since I bought a set last year for my Montana flying adventures.I know they work well.Unlikely any tie down system could work 100% in the wet turf and wind velocities experienced at Sun & Fun.Charlie Lincoln,Mt

Posted by: Charlie Sherman | April 21, 2011 9:30 AM    Report this comment

Have you tested this type - heavy but low cost & deep penetration: "5000 lbs" (?)

www.milspecanchors.com

MSA Store

MSA Store

"HEAVY DUTY" Screw in Anchor 3/4" x 48" w/6" Helix

Screw Eye Anchor Heavy Duty Construction 3/4" Shaft! Anchors are tested to HUD Regulation 3280 to withstand a total of 4725 lbs

$14.22

“HEAVY DUTY” Screw in Anchor 3/4″ x 48″ w/6″ Helix

Great for soft to moderately Firm soils

Anchors are tested to HUD Regulation 3280 to withstand a total of 4725 lbs

Shaft Diameter: 3/4″ Length: 48″ Long Helix Diameter: 6″ Round Load capacity: Up to 5000 lbs (Dependent on Soil Conditions)

Posted by: Cameron Fraser | April 21, 2011 10:04 AM    Report this comment

Correction: We tested Deal Associates which is similar to Storm Force in appearance, but a different anchor system. We'll obtain one and try it.

In the video, I'm not sure that the test was done objectively. It loaded the Claw from its weakest--an angle rather that straight up.

We shall see. Let me dust off the dynamometer.

Posted by: Paul Bertorelli | April 21, 2011 10:13 AM    Report this comment

Any well-designed tiedown will survive ordinary high winds. I have see aircraft tied down at shows with twine attached to plastic tent stakes. The short pieces of re-bar rented at airshows will not do much good. Disc augers hold poorly at an airshow, because they chew up and loosen the dirt on the way down, and they are hard to install to boot. They are all right when the earth consolidates for a local installation, but I lost a Luscombe when two 6"X48" disc-type screw anchors pulled out of the ground. They were screwed into clay-loam to a depth of 3 feet, but the soil was soft in the spring. A used agricultural disc buried to that depth would have held.

Posted by: Ken Zimmerman | April 21, 2011 10:36 AM    Report this comment

Paul,

On the Storm force video, I see the test of The Claw with a vertical pull and the Storm Force and Fly Ties with an angular pull. Are we looking at the same video?

Cameron,

We are talking about portable tiedowns. Are the milspecanchors removable?

Posted by: Rae Willis | April 21, 2011 10:52 AM    Report this comment

Rae: The Claw is first tested (in the dirt) with an angular pull (pulling away from the camera). It is tested vertical in the asphalt.

Posted by: Rush Strong | April 21, 2011 11:07 AM    Report this comment

There are several videos out there. Look the one linked above. It shows the side pull on the Claw, which the company doesn't recommend.

To be fair, you need to test them both with an angular and a vertical pul and compare the resistance.

Posted by: Paul Bertorelli | April 21, 2011 11:13 AM    Report this comment

Totally of topic, but a long time ago in a galaxy far, far away, I recall how the Navy prepared for incoming hurricanes in FL. The expensive stuff would be flown out. The cheap stuff, like SNJs and T-34Bs would remain. They would be jacked up and the gear retracted, then lowered onto old tires, then tied down to the hefty ramp stanchions. Don't recall ever losing one.

My real point would be, with today's forecasting, and presumably with a bunch of downlinked XM Wx all around, why weren't many of those planes moved elsewhere? 100 miles would have done it.

Posted by: Edd Weninger | April 21, 2011 11:23 AM    Report this comment

Edd - We can forecast hurricanes, and more important, track them. Tornadoes are simply unpredictable - warning times would be measured in seconds or minutes.

Posted by: Rush Strong | April 21, 2011 1:45 PM    Report this comment

Plus, on radar, it didn't look that bad. The day before, the approaching line looked a low worse and the wind really kicked up. But it fizzled, so there was little reason to believe the second storm wouldn't do the same.

Posted by: Paul Bertorelli | April 21, 2011 4:17 PM    Report this comment

Hope this is not a duplicate as I just registered... Paul, I know you may not heard of our tie downs (Abe's Tie Down Systems), but I can hopefully answer some of the above questions regarding tie down systems. You folks are correct in saying that the conditions at the tie down site is what matters, period. All of us manufactures can give you some real numbers, but they may not be realistic. You must compare apples to apples as the video referred to above did. In 2010 we tested 3-versions of our tie down systems with The Claw and the FlyTie. We did the testing on the same day, at the same location and within a 15'-20'circle. This involved 7-different locations. We found that on the average, ours held 34% more weight than #2 (ours),40% more weight than #3 (The Claw), 45% more weight than #4 (ours), and 55% more weight than #5 (The FlyTie). So compare apples to apples and don't park in front of a tornado.

Posted by: Bill Ables | April 21, 2011 4:27 PM    Report this comment

My Kitfox III was tied down in Home Built Camping using the Claw. A plane next to me was totaled and one in front flipped. I believe both used "Flyties" When the storm hit I was in Hangar A. After the storm I walked back to my plane and saw at least a half dozen screw type anchors (LJR?) lying on the ground.

Posted by: Pete Christensen | April 21, 2011 4:51 PM    Report this comment

Bill, if you'll loan me one of these, I'll add to our test list.

Posted by: Paul Bertorelli | April 21, 2011 6:11 PM    Report this comment

any observations on the knots that were used.

Posted by: Mark Swaney | April 21, 2011 6:14 PM    Report this comment

It would appear that around 70 - 90 knots were involved.

Posted by: Rush Strong | April 21, 2011 7:14 PM    Report this comment

Paul, Email me your address and a phone number that I will contact you and I'll get you what you need.

Posted by: Bill Ables | April 21, 2011 7:43 PM    Report this comment

On the MSA anchors, we tested the retrievable type plate augers. They are (a) hard as hell to put it and (b) disturb the soil so much that they don't hold well. They really need to be concreted in.

We did not try the spade-type anchors, but I'll add that to the list. They are only practical in the retrievable form.

Posted by: Paul Bertorelli | April 22, 2011 5:47 AM    Report this comment

I was at S&F last year (thankfully not this year). I used the auger-type (flat blade, not helical wire). I don't know how they would have done this year, but I didn't have to worry about slack in the lines. I used camlock tiedowns like I use to tie my dirt bikes down. You can pull them snug so there isn't any slack to let the plane build up momentum.

Posted by: John Worsley | April 22, 2011 4:50 PM    Report this comment

On a slightly different note, if the tie down does hold, I wonder how many aircraft suffer structural damage anyway, due to the wind bending a spar?

Posted by: Eric Nelson | April 25, 2011 9:37 AM    Report this comment

I met a guy on the way flying home that had visible damage to his wing tip and stab, who was flying his plane home. His damage was caused by a blast from a military jet.

Posted by: Pete Christensen | April 25, 2011 9:54 AM    Report this comment

Paul, can you test the homebuilt tiedowns that were printed in EAA Sport Aviation? These are the three 18" spikes driven into inverse cone with small plate. These are supposed to be extremely effective. I made a set and have high confidence in them.

Posted by: Christian Sturm | April 25, 2011 10:05 AM    Report this comment

The big storm came through LAL on Thursday, March 31, not April 7. I was there, enjoying a nice AOPA luncheon in the Pavilion when it hit. AOPA guy saw it coming and thankfully ordered all of us to take shelter in the room behind the stage.

Posted by: Dan Coffman | April 25, 2011 10:59 AM    Report this comment

At Charleston AFB during Hurricane Hugo (1989) with highest recorded wind gusts of 108 MPH, a static display C-124...

Specs: Wingspan: 174 feet 1 inch Length: 130 feet 5 inches Height: 48 feet 4 inches Weight: 185,000 pounds

...turned 180 degrees while dragging its array of very large concrete anchors.

The total display included a C-47, C-121, and C-124, all of which were tossed about.

Impressive sight.

Posted by: T M | April 25, 2011 11:08 AM    Report this comment

A supercub type display aircraft (that survived)was stauchly tied down at the wings (inc double ropes) but the tail tie-down rope was intentionally left with a foot or more of slack by the experienced Alaskan pilot. He told me they do it that way in AK so that if the wind can lift the tail it will lower the wing angle of incidence and decrease lift. Food for thought.

Posted by: Ron Apfelbaum | April 25, 2011 10:34 PM    Report this comment

Any tie-down even permenant ones can fail. I have seen the pictures of three Caravans tied-down on an Oklahoma airport a storm blew thru. All thre were upside down still tied to the permenant tie-downs now sitting on the ground with a few feet of dirt still attached to the tie-down. The winds pulled tie-downs and dirt right out of the ground.

Posted by: Franklin Berry | April 25, 2011 11:14 PM    Report this comment

2 years ago we had almost that nasty wx pop up at ATW during AirVenture. My plane was tied down with a simple, single 2' piece of rebar, bent to have a top handle, and put in at a 45, as were most of the others. They held great in the northern clay soil, but you do need a good hammer to set.

Posted by: Bob Bittner | April 27, 2011 10:16 PM    Report this comment

An added technique for taildraggers: Raise the tail by putting in on a milk crate or other solid support -- this reduces the wing angle of attack, and greatly reduces the risk of wing damage due to overstress in high winds, as well as reducing the lifting force on the wing tiedowns. This is a routine practice tying down gliders in the mountain West.

Posted by: Daniel Johnson | April 28, 2011 12:19 PM    Report this comment

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