Technical Articles

A Case for Compatibility with Structural Integrity

3/24/2010

When we ask the question, "How do you determine compatibility between approved parachute components?” what do we mean? I recently had the pleasure of participating in several rigging classes at the Skydiving Expo 2010. There I asked this question. Overall the answers weren't bad. While no one could quote chapter and verse most had the general idea. Some who didn't have a clue fell back to the rhetorical "Form, Fit & Function". Sorry, two of those don't really apply. Form is what you are judged for when doing freestyle, and has nothing to do with rig functionality. Who cares about Form, as long as it works? Fit: Some say if a reserve canopy is too bulky, the container won't open. I say if you can get the canopy in the container it should work. This goes to function which is critical and one of the two key parameters. The other and equally important parameter is Strength. What good is a rig that comes apart? Mike Fury (Glide Path), once said that we should be shedding body parts before our equipment fails. Right on! The correct and complete answer may be found in Advisory Circular 105-2c. This governing regulation should be so simple that the assembling rigger needn’t make any decisions. Currently it is not so.

  Presently, parachute compatibility is effectively governed by the FAA Advisory Circular 105-2c. This AC has the effect of law. It provides guidelines for components from different manufacturers and standards, to be mixed and matched. If it weren’t for AC 105-2c we would not have the latitude to mix and match components and only equipment TSO’d as a complete system could be supplied, certified and used in our sport. As you might imagine there’ve been companies who relished this prospect. It would have meant total market domination for any company that produced containers, and reserve canopies.

For example, a harness from TSO C23b may be assembled, certified and used with a reserve canopy from a different manufacturer, which is certified under TSO C23d - two different manufacturers and two different versions of the TSO. But wait. Why do we have to consider the different Standards? Shouldn’t any component from any standard be compatible? Yes, they should; but they aren’t. Thus the reason for this commentary.

 Here’s some history and some facts: TSO C23b was originally written back in the 1940’s 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 MPH and Certified to 3000 pounds. This category required large block letters decrying “Limited to use in Aircraft under 150MPH”. Ugly! It also had “Standard Category”. This category required no warning labels and had neither weight nor speed limitations, and was tested and certified to 5000 pounds. It’s 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% you would only see a 5% increase in opening force; likewise if you double that given weight you would only see a 10% 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 oft derived from mass or weight we associate the hard opening with primarily weight. Let’s look at the calculations.

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

 Force = Total opening forces.

Cd = Drag Coefficient of canopy

So = Square footage of canopy

Q  = Dynamic Pressure in Pounds per/Sq. Ft. (1/2rho* V^2)

X1 =    Decreasing Load Factor. There are 2 methods for deriving this factor. The Pflanze method and a lookup of the chart included in the reference             manual. The chart is the simplest for personnel parachutes and effectively results in being one tenth of the pound per square foot loading.

Cx =    Shock load Coefficient, derived from testing and includes such things as slider size, brake setting, angle of nose cut, etc. For this exercise we will use a value of 1 as this number ranges from .5 to 1.5 or so. Without a slider it can go as high as 10.

Therefore: Force= Cd*So*Q*X1*Cx

 If we group the Cd*So*Q and calculate them, at first we get a big number ie:

Cd = .8, So = 200Sq. Ft., Q = 33 PSF @ 117MPH, together = .8*200*33 = 5280 pounds. This number is then ameliorated by the X1 decreasing load factor and the Cx Shock load factor. If the Cx is 1 (and we will assume this for this effort) then it has no effect on the outcome.

 The X1 factor is the key because it is based on pounds per square foot loading, times .1. If you try different weight values and reiterate the formula you can see it only changes the X1 factor by fractional amounts and only affects the outcome minimally as described earlier

  A word about the math before we go on, some will say, "This math was developed for round parachutes", intimating that it is not applicable to squares. To them I ask, "What laws of physics is different for round vs. square parachutes?" This math, like all prediction math, gets better as it is honed for a specific canopy. We perfect the Cx factor with experience. In any event it is a proportional representation of the physical components of opening shock. I have personally used this equation for some 20 years in the development of some 50 canopies and find it tracks very well. The French government has used it with great success. It is, after all, used in both the U.S. Air Force and Navy Parachute design guides.

 NAS 804 has the best requirements for structural integrity of any standard written to date. This is because it has a Strength requirement: 3000 pounds for the Low Speed Category and 5000 pounds for the Standard Category. Other standards (AS8015), use a performance requirement (Weight vs. Speed) for structural integrity verification. This would be OK except for one small problem. AC 105 allows for mix and match 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 Cx value. Therefore if a harness is built and tested using a canopy with a low Cx and matched with a canopy (under the provisions of AC 105), with a high Cx, 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 150MPH, AT ANY WEIGHT. Likewise the Standard Category of 5000 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 5000 pound shock load, it would come apart before the harness or canopy. At less than 150 MPH, I don't care if it is Shaq under the canopy - it won't exceed 3000 pounds.

It may be evident to you now that there is big hole in our structural requirements due to the mixing or matching of approved components under the performance standard vs. a structural standard.

 We got ourselves into this mess when people decided to change from a Structural Standard (NAS-804) to a Performance Standard (AS 8015b). Now we have no way to determine compatibility for C23c (AS8015b), but it has a performance standard!

Equipment shouldn't have structural limitations but because we tried to define them we have them. Speed limits would be acceptable but probably unnecessary.

What we currently have in AC-105-2c is probably the best we can do for the time being. Correction will have to go way beyond the scope of AC-105. At best it is currently a Band-Aid. But if we didn’t have AC-105 we would not have any compatibility. While I have no personal or business concern as to whether we have mix and match capability or not, I do think the jumping public and USPA might have something to say?

 I believe it is not possible to have compatibility using Performance Standards alone. That is why we added placcarding for the “Weight tested to” for harnesses and the “Force generated” for canopies to 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 doesn't mean they saw the same opening forces. Different Canopies open with different characteristics. Below please see a sample scenario of 2 systems tested the same structurally. The math is the same as previously discussed. The 2 canopies have very different opening characteristics and they produce very different results when tested at the same levels. When a mix of the 2 systems is applied and subjected to a high stress sport jump the capability of the harness is exceeded. This is unacceptable.

 Hypothetical Comparison of Opening Characteristics of 2 Different Systems Tested to The Same Performance Standard

 Tested to 300# @ 180Kts (207MPH) Cat “B” C23c.

System 1: 200 Sq. Ft. Canopy W/.8 Cd produces a 1304 pounds force on opening at test speeds

System 2: 100 Sq. Ft. Canopy W/.9 Cd produces a 3668 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.

Let’s say I build a harness for System #1 using 1500 lbs capable hardware.

It passes the structural drops, as it only sees 1304 pounds.

Another company builds System #2 and develops a very hot canopy but it tends to open hard.

The jumper wants the hot 100 sq. ft. canopy in his new System #1 rig.

If compatibility is derived from performance standards then these are compatible since

they were both tested to 300# @180KTS.

It is entirely possible that a sport opening under extreme conditions could produce an opening of 1600 pounds, which exceeds the capability of the 1500 lb. hardware.

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C23b doesn't really have any limitations except for speeds below 150 for "Low Speed" category with no weight limit. That standard is good for all skydiving scenarios. 3000# is strong enough for anything and Force is a good base line for compatibility.

 The current Regulation for placcarding (AS8015c) calls for “Average Peak Force”. It should say “Maximum Force”, as does the AC. It (AS8015) needs to be changed.

 C23c is the bastard child and there is no way for a rigger to make the necessary determinations without help from the manufacturer doing some kind of retro placcarding. Fortunately, I believe there are only a small number of systems/components certified under this rendition. The Racer Tandem is the only complete system certified under this standard that I know of. Any component so certified would not be able to be used as there are no guidelines for compatibility. This should be an incentive for the manufacturers to release force data for these systems/components, but it hasn’t been so far. A paragraph about this might be added to AC-105

So, we have defined for you, a mess. How do we fix it going forward? To begin with we must forget about “Performance Standard “ for structural integrity. We can and must continue to use them for functional verification. AS 8015 must be revised to reflect the following: All harnesses should be tested and certified to 5000 pounds. This would not hurt the industry, as I know 3 of the major manufacturers already have this. Canopies may be tested to a Performance Standard with a cap of 5000 pounds generated force. They would be certified to a weight and a deployment speed. These limitations would consider rate of descent as well as opening force, and would be the only limitations placed on the system.

 Testing for compliance to the Structural requirements would be for just the components in the load bearing chain (canopy, risers & harness). It must be done without any attenuation or deployment devices (no bag, no slider). These tests could then be carried out at slower speeds  from more available aircraft. There would be more accessibility for smaller manufacturers and start ups. All force tests must be measured! This method would prove the system integrity in the event of attenuation/deployment device failure (line dump or burbled slider).

 The canopy/system would be tested to the Performance Standard requirements, as we currently do. Remember I said that the canopy should be capped at 5000 lbs. - not required to reach 5000 lbs., mind you, but it must stay together at the level to which it is certified. Additionally, a minimum strength should be set at 3000 pounds (for the canopy).

If the next TSO reflected these considerations, its’ products would have a structural integrity which would require no further consideration by the rigger and the user would have a hard deck for usage. Additionally, there would be no further need to update the compatibility section of AC-105, as compatibility would no longer be an issue and structural integrity would be guaranteed.

 John Sherman

3/19/2010