The rigger, when making the determination as to whether a particular canopy and rig combination is compatible, must consider all of the above areas. If there is any doubt, the rigger should contact the rig manufacturer for guidance.

TSO-C23b was originally written back in the 1940s before the advent of square parachutes. It had two categories under which a parachute system could be certified. The “Low Speed” category was limited to use in aircraft under 150 miles per hour (MPH) and certified to 3,000 pounds. This category required large block letters decrying “limited to use in aircraft under 150 MPH.” It also had a “Standard Category.” This category required no warning labels and had neither weight nor speed limitations and was tested and certified to 5,000 pounds. It is important to note that neither category had a weight limitation.

Weight has only a minimum effect on parachute opening forces. To be exact, if you were to increase a given weight by 50 percent, you would only see a 5 percent increase in opening force; likewise if you double that given weight, you would only see a 10 percent increase in opening force.

This seems counter intuitive until you think about it. Speed is the critical factor that hurts us and our equipment when we have the occasional hard opening. But because speed is often derived from mass or weight, we associate the hard opening with primarily weight. Let us look at the calculations.

The math model for opening forces is described in the “Recovery Systems Design Guide” by Theodore Knacke. The definitions and formula is as follows:

If we group the C_d × S_o × Q and calculate them, at first we get a big number ie:

C_d = .8 S_o = 200 square feet Q = 33 PSF @ 117 MPH together = .8 × 200 × 33 = 5,280 pounds. This number is then ameliorated by the X_1 decreasing load factor and the C_x shock load factor. If C_x is 1 (and we will assume this for this example), then it has no effect on the outcome.

The X_1 factor is the key because it is based on pounds per square foot loading multiplied by 1. If you try different weight values and reiterate the formula, you can see it only changes the X_1 factor by fractional amounts and only affects the outcome minimally as described earlier.

National Aerospace Standards (NAS) 804 has the best requirements for structural integrity of any standard written to date. This is because it has a strength requirement: 3,000 pounds for the Low Speed Category and 5,000 pounds for the Standard Category. Other standards (AS8015) use a performance requirement (weight versus speed) for structural integrity verification. This would be acceptable except for one small problem. AC 105-2, Sport Parachute Jumping, allows for mixing and matching of approved components. This is a problem because different canopies open with different opening characteristics at the same weights and speeds. This is defined and accounted for by the C value. Therefore, if a harness is built and tested using a canopy with a low C and matched with a canopy (under the provisions of AC 105-2) with a high C, the results could be disastrous.

NAS 804 systems need no further consideration other than originally called for. The Low Speed designation is limited to use in aircraft under 150 MPH at any weight. Likewise, the Standard Category of 5,000 pounds has no weight or speed limitations. This is an unlimited category. One reason for this is because of the limited effect of weight on opening forces. Speed is what kills. If a human body were to reach a 5,000 pound shock load, it would come apart before the harness or canopy. At less than 150 MPH, even at a high weight, it will not exceed 3,000 pounds.

It may be evident now that there is a flaw in our structural requirements due to the mixing and matching of approved components under the performance standard versus a structural standard. This came about as a result of the change from a Structural Standard (NAS-804) to a Performance Standard (AS 8015b). Now we have no way to determine compatibility for TSO-C23c (AS8015b).

It may not be possible to have compatibility using performance standards alone. That is why we added placards for the “weight tested to” for harnesses and the “force generated” for canopies to TSO-C23d. There is no way to determine compatibility from one parachute system to another within the same category of the same standard if they are judged using a performance standard. Just because they were tested at the same weight and speed does not mean they saw the same opening forces. Different canopies open with different characteristics. Listed below is a hypothetical comparison of the opening characteristics of two different systems tested to the same performance standard. The math is the same as previously discussed. The two canopies have very different opening characteristics, and they produce very different results when tested at the same levels. When a mix of the two systems is applied and subjected to a high-stress sport jump, the capability of the harness may be exceeded.

Both systems were tested using a 300-pound test dummy at 180 knots (207 MPH) Cat “B” TSO-C23c.

System 1: 200 square foot canopy W/.8 C produces a 1,304 pounds force on opening at test speeds.

System 2: 100 square foot canopy W/.9 C produces a 3,668 pounds force on opening at test speeds.

The ratio of opening force differential is 2.8 to one or System 2 opens with 2.8 times greater force than System 1.