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Type 17 Riser Study

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Objective: To Provide Failure Cause Analysis and route cause definition for the field failures of type 17 risers.

Background: Recent breakage of type 17 risers from several manufacturers has spurred an intense investigation of the problem by the parachute industry. Most notable in this investigation is the report from Troy Loney of Para-Flite as presented to the Fall PIA meeting. Mr. Loneys’ report identified separation of the webbing just above the lower riser ring as one of the failure modes where definition was required. This report deals primarily with the efforts to identify and resolve this mode of failure.

Method: A Dillon 10,000 lb. Dynamometer was configured to accept a 1″ wide riser, attaching both legs to the top, or fixed head, and a RW-7 harness ring to the lower, or movable head. Risers were typically hung on the top head and the mini 3-ring was assembled to the harness ring on the lower head. Loads were applied to failure and the readings recorded in Appendix A.

Discussion: While reviewing the results recorded in Appendix A, it was noted that one brown type 17 riser, ID # 11, failed at 3600 lbs. by breaking the lower riser ring. This ring is specified to have only a 500 lb. tensile strength. This indicated that a strength of 3600 lbs. could be attained with a type 17 riser without webbing failure (see 3Ring Force Reduction Study). When comparing this riser #11 to other risers that ultimately failed with failure mode #1, it was noted that on riser #11 the edges of the webbing rolled under further up the riser, by about a quarter of an inch, than those failing in mode #1. Thus the selvage edge of the riser was protected, and the resultant thickening of the webbing effectively “choked” the scissor action that occurs between the base and middle ring.

Another interesting test result, was that of ID #9 & #10. These sample risers were manufactured of type 25 webbing which has a 4500 lb. rating. These risers failed in failure mode # 2 (“Pulling through of the yellow cable”.) Type 25 webbing is too thick to allow a proper relationship of the harness ring to the lower riser ring. In other words, these risers will probably always be “out of spec.” and not yield suitable force reduction. Tests were made to determine the force required to pull the yellow cable through the grommet. The results of these tests revealed a requirement of 60 lbs. to accomplish this feat.

This 60 lbs. of force, when compared to the total riser loading, will identify the overall mechanical advantage of a particular mini 3-Ring set. In the case of sample #11 a force reduction ratio of better than 60 to 1 is apparent. When compared to sample #9 a ratio of only 30 to 1 is identified. This discovery prompted the preparation of a spreadsheet which would elementally break down the force reductions of the 3-ring system.

This spreadsheet is presented in Appendix B with two scenarios shown. Scenario “I” is the representation of the dimensional values found on sample #11 and scenario “G” represents the findings on sample #9. Additionally, a scale drawing of each of these two scenarios is attached. The values predicted by the spreadsheet tracked favorably to all of the test results.

Conclusions: The primary cause of failure ( Mode #1) is the guillotining of the webbing during the onset of the load on the riser. This phenomenon is prevented by maintaining the webbing roll-under to a point past the engagement of the harness ring. This bulk effectively “Chokes” the guillotine.

The ultimate strength of the mini-3-Ring is approximately 3600 lbs., regardless of what type of webbing is selected.

The use of webbing heavier than type 17, or the use of reinforcement to the type 17 can cause the geometry of the rings to change and lower the ultimate failure level of the product.

Type 17 risers with mini 3-Ring can be manufactured to accept a force of 3600 lbs. without failure.

Appendix “A” Test Results
ID #ColorWeb TypeRing GeometryFailed @Failure ModeManufacturer
3Red17Spec.27001US
5Red17Spec.30001US
1Brown17Spec.30001SR
2Brown17Spec.30001SR
4Black17Spec.26001US
6Black17Spec.26001SR
7Navy17Spec.27001other
8Navy17Spec.23001other
9Black25Long22002GP
10Black25Long18002GP
11Brown17Spec.36004SR
12Brown17Spec.26002SR
Note:
Failure mode #1Cutting of webbing above lower ring
Failure mode #2Pulling through of Yellow cable
Failure mode #3Breaking beside grommets
Failure mode #4Ring Breakage  

Appendix “B” Test Results

Scenario = I
3Ring Force Reduction Study
Magenta = Input Variable
Initial Riser Force =3,600.000Lb.
Rw7 Pulley Reduction =0.500
Resultant Force =1,800.000Lb.
Length ofTotal
Force ArmLoad ArmLoad
Rw3 Nominal Dia. =1.041
Arm Ratio Rw34.4790.1900.851
Force @ Top of RW3 =401.880Lb.
Rw4 Nominal Dia. =0.680
Arm Ratio Rw43.5330.1500.530
Force @ Top of RW4 =113.740Lb.
Loop Reduction Factor =0.50056.870Lb.
Scenario = G
3Ring Force Reduction Study
Magenta = Input Variable
Initial Riser Force =2,200.000Lb.
Rw7 Pulley Reduction =0.500
Resultant Force =1,100.000Lb.
Length ofTotal
Force ArmLoad ArmLoad
Rw3 Nominal Dia. =1.041
Arm Ratio Rw33.7320.2200.821
Force @ Top of RW3 =294.762Lb.
Rw4 Nominal Dia. =0.680
Arm Ratio Rw42.6170.1880.492
Force @ Top of RW4 =112.633Lb.
Loop Reduction Factor =0.50056.316Lb.
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