back twice, thus turning the cut edge inside the second fold to prevent fraying and to reinforce the hem. The hem may also include a piece of reinforcing tape, plain or tubular webbing for adding strength. All seams

Figure 10-35.-Bound seam.

Figure 10-36.-Edge finishing stitching.

and stitches pictured are used in the survival equipment shop for modification and repair work on the parachute canopy, pack, seat, and back pads, or for making covers and bags for aircraft, shop equipment, and tools.

Appropriate Spacing of Machine-sewn Seams

The following rules will help you to space correctly more than one line of stitching and to place a seam the correct distance from the edge:

1. Sew regular binding tape one-sixteenth of an inch from the selvage edge of the tape.

2. Sew heavier tapes from one-sixteenth to one-eighth inch from the edge. Sew the raw ends of the tapes one-fourth inch from the raw edge.

3. Sew horsehide and thin leathers oneeighth inch from the edge in patching, trimming, etc.

4. Sew the raw edges of 8 to 15 ounce duck one-half inch from the edge.

5. Fold the material no less than one-half inch for reinforcement.

6. Sew the folded edges of 8 to 15 ounce duck one-eighth inch from the folded edge.

7. Make the second and succeeding rows of stitches one-fourth inch apart. In heavier material, it is sometimes desirable to separate the rows as much as three-eighths of an inch.

8. Heavy duck, heavy fabric, or the heavier leathers may be sewn approximately one-fourth inch from folded edges for best results, while the raw edges of such heavy fabrics need at least onehalf inch to three-fourths inch seams for security.

9. Sew light nylon or aircraft fabric onesixteenth inch from the folded edge. Raw edges of these light materials are seldom sewn together, except as the first step of another seam.

10. When you are sewing a row of stitches and the thread breaks, start sewing again one-half inch behind the break, and sew on top of the existing stitches. This is called backstitching.

The stitches that form the various classes of seams should be tight, even, and well-set into the material. An understanding of how the machine functions to form the stitch and feed the material provides the basis for you to sew high-quality seams consistently.

KNOTS

A boy scout's first achievement is to learn to tie knots. Knots are necessary to many activities-camping, boating, mountain climbing, and parachute rigging. Different knots serve different purposes. For instance, a hangman would be out of a job if he forgot how to make a slip knot. A doctor would have trouble sewing his patient up after an appendectomy if he didn't make a proper surgeon's knot. Also, you could never pack a parachute correctly without a working knowledge of several different types of knots.

Some specialists have a tendency not to be as concerned about knots as they are about other items involved in servicing parachutes. Think of knots as the treads (and the depth of the treads) on your automobile tires. Sure, the tire can perform without treads. But, if the tires are going to grip the road surface and stop the automobile in the shortest time and distance possible, they require good tread depth. Tire treads are designed to meet many performance requirements. The same principles apply to knots used in parachute rigging.

Make sure that all knots and tackings are changed as often as possible. Their "tread" deteriorates and becomes loose. Parachutes are designed to perform under the most unpredictable situations, at speeds and configurations too great to imagine. One poorly made knot and tacking could cause burned suspension lines, excessive opening shock, or oscillation-all of which could result in the failure of the parachute. Remember, no matter how small the task, treat each area of the parachute with the greatest care and concern.

The type of knot used in assembling component parts of parachutes depends on the purpose for which the knot is intended, the strength required, and the kind of thread, rope, or cord that is to be used. Remember, knots, hitches, and turns decrease the tensile strength of rope, cord, or thread, as shown in figure 10-37. Some knots are tied for the purpose of breaking during parachute deployment, and other knots are tied so as not to break. This is why it is so important that only the specified knots be used for a particular job. The following text discusses the knots you will have to tie as you go about your job of servicing parachute assemblies.

CLASS OF STITCHING.- You form class EF (edge finishing) stitching by using the edge of a single ply of material to make the hem. The EFb-4 stitching, as shown in figure 10-36, is made by folding the edge