"Tie-down Techniques"

The weakest link in the tie-down can be the knot you tie, and it is therefore important to understand a little about various knots and how to tie them properly. The knot should neither slip nor loosen, and it should be easy to undo.

A knot can fail in three ways: it can come undone through vibration and general movement when there is little load on it, it can pull out when load is applied, or it can break under load. Any break usually occurs where the rope enters the knot.

The ultimate strength of a knot is a matter of design and some knots are naturally stronger than others. Security, on the other hand, can often be improved by the manner in which the knot is finished off. But making a knot more secure may also make it more difficult to undo when the time comes, so there is no point in making a knot as secure as possible  only as secure as necessary.

The US FAA Advisory Circular on aircraft tie-downs recommends the bowline and the square knot (or reef knot).

Research suggests that a reef knot is not suitable for aircraft tie-downs. It is an excellent general-purpose knot for tying two pieces of string or twine (of equal thickness) together, but it is not a long-term or secure knot, and it is used mainly in bandaging, tying parcels, etc.

For a more secure method of bending two ropes together, use a sheetbend.

Sheetbend 

SheetBend The sheetbend is the most commonly accepted knot for joining two ropes together and probably the best, particularly if the ropes are of different sizes.  The thicker rope of the two is used to form a bight, and the thinner rope is passed up through the bight, around the back of the bight, and then tucked under itself.  

The knot should be tied with the ends of the ropes coming off the same side of the knot. However it can easily be accidentally tied with the ends coming off opposite sides of the bend, when it is known as the left-handed sheetbend. This version is to be avoided, as it is less secure.

Bowline  

The bowline is closely related to the sheetbend.  The bowline is the most useful and one of the simplest ways of putting a fixed loop in the end of a rope. It is easy to tie and untie, it doesn't slip or jam, and it has a high breaking strength.  

It is perhaps the best way to secure a rope to a tie-down ring. It's also very good for attaching the tie-down rope to the anchors in the ground.  

For added security, you can finish the knot with a stop knot such as a figure of eight knot to remove any possibility of the bowline slipping.

Form a small loop (the direction is important), and pass the free end of the knot up through the loop, around behind the standing part of the rope, and back down through the loop.

A chant used by many to remember this knot is "The rabbit comes out of the hole, round the tree, and back down the hole again", where the hole is the small loop, and the rabbit is the running end of the rope.

In the same way that a left-handed sheetbend has the running end of the rope coming out of the wrong side of the knot, a cowboy bowline is a bowline that also has the running end of the rope coming out of the wrong side of the knot.  It suffers the same problems as the left-handed sheetbend, and is to be avoided.

To quickly identify if you have tied a normal bowline or cowboy bowline, check to see that the running end exits the knot on the inside of the loop.

Figure of Eight  

Single 

 

The single figure of eight is a useful 'stop' knot to temporarily bulk out the end of a rope or cord. The finished knot looks like its name. It is useful to temporarily stop the ends of a rope fraying, before it is whipped.  

 

Double

The double figure of eight knot builds a non-slip loop at the end of a rope. It is popular with rock climbers (as it is safer than a bowline) who tie their belay rope to their carabineer or harness.

Tie a single figure eight knot near the end of the rope, loop the end of the rope around the carabineer or harness straps and retrace the figure eight.

 

Round Turn and Two Half Hitches  

A round turn and two half hitches is used to secure a rope to a pole or ring, or to start or finish a lashing. It does not jam.  

It is a good knot for securing a rope to the tiedown ring, and it is commonly used by many pilots. While it is easy to tie, it can be more difficult to untie, especially when the rope is wet.  

Pass the running end of the rope over the pole or through the ring twice. Then pass the running end over the standing part of rope, and tuck it back up and under itself, forming a half hitch. Repeat this for a second half hitch.

Conclusion

To become proficient in the art of tying knots requires practice, but it is a useful skill. Although there are many hundreds of possible knots, most of us need only be familiar with a few of the most commonly used ones.

There are many books available on the subject. Learning to tie knots using illustrations can be difficult. Fortunately in this age of technology there is a better way. There are a number of web sites (particularly those of boy scout troops) that have animated diagrams to assist in learning to tie a range of useful knots.

(Try www.troop9.org  and click on Knots)

Make sure that you protect your aircraft adequately with suitable tie-down rope (and a properly spliced loop in the end of the rope makes tying down easier), and that you have the requisite knotting skill to make sure the tie-down does its job.

 

Some Knotting Terms

A bend is used to join two ropes.
A hitch is used to tie to an object.
The bight is the curvature of a rope when its direction is changed from that of a straight line, to the maximum of a full circle. Any point within this curvature is said to be in the bight.
The strength of a knot is the force required to break a rope containing the knot.
The security of a knot is related to the force required to make the knot slip or capsize to an unwanted form.
Whipping is a series of turns of sail twine or similar, forming a lashing at the end of a rope to prevent fraying.

(Article reproduced from Vector)