Knots Landing

[EchoMax]

I was amused and bemused by the recent photograph of a "tripod" aircraft in a "tail-dragger" posture due to an ice storm.

I guess it's OK to laugh (if it's not on your dime, and no one was hurt). From looking at the photo, I suspect that there was a tie-down failure related to a "slip-knot", a lack of "tensioning", or both.

I have always been mystified by the myriad variety of tie-down knots that I see on the tarmac. When I first started in aviation, I thought to myself that there is surely a "best practices" of knot-tying for tie-down.  Not so.

I see many so-called knots that are actually "slip-knots". I see knots that are more appropriate for monofilament nylon fish-line than for the twisted polypropylene lines that I usually encounter. Most of the aviation tie-down videos that I have seen on YouTube are pathetic, if not ridiculous, although there are exceptions.

I'm not a mathematician, topologist, or Boy Scout, and I am not much of a sailor, but it strikes me as strange that, given our seafaring legacy, pilots do not pay more attention to the simple tie-down knot.

In the case of this aircraft, the damage repair may cost a few AMU. If there had been a prop strike after a thaw, it could have been much worse.  The owner owes thanks to his fellow aviator in kindly restoring the safe posture of the plane.

The best tie-down knot should be:

1. Secure (not a slip knot)

2. Appropriate for the "suture material"

3. Easy to tie

4. Easy to untie

5. "Tensionable"--perhaps most relevant for this event

6. Standardized:  "Let the best knot win!"

 

What tie-down knots are used by the MS community? (I have one that I prefer, but I do not want to prematurely bias this "survey".)

[AH-1 Cobra Pilot]

Taut line hitch.

[Hank]

When I lived on the Oh-WV border, Cessnas were frequently in this position every winter, all it takes is a couple inches of snow. Never heard of any damage to them, even from friends who had to brush snow off the tail and spin the plane around to get sunlight onto the wing surface in order to fly.

As a former Boy Scout, I use taut line hitches for many things, but have stopped using them for airplane tie downs. They kept my tent off of my face well, and they hold tarps and boxes into the back of my truck, but they just don't perform well for tying down airplanes. Not that they come undone, they just don't stay taut . . .

So I have converted to the "it's not a real knot" tiedown method pass along by my CFI--pull the rope through the loop, snug up a foot or so and loop the end through twice with a hard tug after wrap #2. Then repeat to use up left over line, because no one wants 6' of rope flapping in the wind and beating against the wing.

But I'm certainly open to other ways that work! And no, don't bother suggesting ratchet straps--they are too heavy, awkward to use, impossible to stow neatly and I generally hate them.

[PT20J]

1 hour ago, Hank said:

So I have converted to the "it's not a real knot" tiedown method pass along by my CFI--pull the rope through the loop, snug up a foot or so and loop the end through twice with a hard tug after wrap #2. Then repeat to use up left over line, because no one wants 6' of rope flapping in the wind and beating against the wing.

I believe that what you are describing is a midshipman's hitch. That's usually what I use. It's similar to the taut-line hitch but more secure.

https://www.animatedknots.com/midshipmans-hitch-knot

Here's another one that is favored by a large flying club in Palo Alto, CA. But I can never remember how to tie it. 

Skip

[EricJ]

The knot I've seen most referenced in FAA literature is a bowline, you know, where the rabbit comes out of the hole around the tree and back down the hole.   In A&P school we had to practice this particular knot because it can be tested by the DME as part of the practical exam, for the general test iirc.

There's actually an AC on this topic.   It is split into two pdfs, so here's the link to the FAA page with both.   It says to use a bowline, too, or a square knot.

https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentid/22573

Edited January 6, 2021 by EricJ

[Mark89114]

Not sure how any kind of knot would have prevented this inferior aircraft from sitting on its tail?

 

[Hank]

21 minutes ago, Mark89114 said:

Not sure how any kind of knot would have prevented this inferior aircraft from sitting on its tail?

 

It wouldn't have, but it's a good discussion anyway. 

[StevenL757]

53 minutes ago, Mark89114 said:

Not sure how any kind of knot would have prevented this inferior aircraft from sitting on its tail?

I agree.  There are several Cessnas tied down across from my hangar, and this looks more related to the weight of the ice on the tail rather than a failure of a tiedown.

To @EchoMax's point however, many of those folks as well as others on the airport who choose to tie down have resorted to ratchet straps, although they typically don't have as much "give" versus a well-tied knot.

Love the picture tho.

[GeeBee]

I've seen worse on over zeolous soft field technique with 172's and it just ground the tie down eye a little bit.

 

 

[EricJ]

20 minutes ago, StevenL757 said:

To @EchoMax's point however, many of those folks as well as others on the airport who choose to tie down have resorted to ratchet straps, although they typically don't have as much "give" versus a well-tied knot.

When I go to SoDak in the summer I have to leave the airplane outside, and they get sudden, crazy wind storms up there.   I don't trust my own knot-tying capabilities, so I take some ratchet straps and tie it down with those.   On typical ratchet straps the S-hooks on the ends are the usual failure point, as they'll straighten out and release the load under sufficient impulse loads.   I got some of the aviation-specific straps with decent end attachments and they've been good so far.
 

Just now, GeeBee said:

I've seen worse on over zeolous soft field technique with 172's and it just ground the tie down eye a little bit.

When I was learning in Cessnas at a club all of our 150s and the 172 had flat spots on the bottom of the tail tie down eye. 

[PT20J]

43 minutes ago, EricJ said:

I got some of the aviation-specific straps with decent end attachments and they've been good so far.

I think that's the best idea. Where did you get them?

[laytonl]

Midshipman’s hitch for me.  Lee

[BlueDun]

Midshipman's hitch. I end with two half hitches to secure instead of one. This is much more secure and less likely to come apart. Midshipman's is a form of taut line; the second wrap that pops down into the first wrap under friction is called an Awning Hitch.

West Valley is just a modified bowline.

[PT20J]

If you run out of ideas, there are 3900 of them in here...

[EricJ]

16 hours ago, PT20J said:

I think that's the best idea. Where did you get them?

I wish I could remember!   They're like the straps shown here, but were bought as only the straps without the screw anchor system.

https://www.aircraftspruce.com/catalog/pspages/bigscrew.php

Sporty's has some with decent ends as well.

Edited January 7, 2021 by EricJ

[Ibra]

I am sure extending ice flaps would have helped with some nose down pitch? Does strap the yoke fully back rather than leave it forward helps 

 

 

 

[Ibra]

[thinwing]

12 hours ago, PT20J said:

If you run out of ideas, there are 3900 of them in here...

my wife bought this book for me...and it contains every knot known to mankind!!!!

[skydvrboy]

I use a midshipman's hitch like several others. If I have enough extra line I'll tie a second further down. 

If I need tio tie the other end to the ground, I'll use a bowline on that end. 

[BlueDun]

1 hour ago, skydvrboy said:

I use a midshipman's hitch like several others. If I have enough extra line I'll tie a second further down. 

If I need tio tie the other end to the ground, I'll use a bowline on that end. 

Good point. I sometimes do this as well.

[carusoam]

Holy cow...  Ice flap extensions!

Brand Ce has technically figured out how to get more lift in icing conditions.

Crazy ice machine...

-a-

[Guest]

A hangar for me, no tie down required!

Clarence

Edited January 8, 2021 by M20Doc

[Hank]

7 hours ago, M20Doc said:

A hangar for me, no tie down required!

Clarence

You don't ever travel during winter?

[Ibra]

On 1/7/2021 at 6:09 AM, PT20J said:

If you run out of ideas, there are 3900 of them in here...

I got a small booklet with 50 of them, I guess the complexity can go to 3900 easily 

On the back it does not talk about flying though, use at own risk and not "certfied KTS" !

 

[Ibra]

9 hours ago, M20Doc said:

A hangar for me, no tie down required!

Sadly last two years did show that even hangars need to be attached (or aircraft moved)...

Edited January 8, 2021 by Ibra