Thumb Test?

[steve1 5]

I've heard some jumpers talking about checking older canopy material by using the thumb test. I assume this means trying to push your thumb through the material. I'm just wondering how much pressure you should use. Does this mean pushing as hard as you can? I mean I've got some really powerful thumbs even if they are kind of stubby, and I'd hate to damage a canopy by testing it the wrong way....Steve1

[councilman24 37]

This was something used by riggers and others before we "borrowed" the tensile testing clamps from the hot air balloon crowd. Yes, you can damage good material. It's a feel thing and no good way to describe. You have to try it out with some good fabric, see what you can do, and moderate how much pressure you use. No it isn't pushing has hard as you can. If I don't have my tools with me and I want to see if something is really dead, I still do this from time to time. If the fabric has really failed you can easily poke your thumb through. In fact you can pinch it between your thumb and forefinger and pull it apart like Kleenex.

Now go here http://www.pia.com/piapubs/TSDocuments/ts-108.pdf for a description of the REPLACEMENT for the thumb test. Two modified locking pliers and a spring scale are needed. The procedure calls for 40lb pull. This is debated among canopy and fabric manufacturers. Some believe that 40 is too high and recommend 35lb. This is what I'm using most of the time for general tests not related to a service bulletin.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[caldwellbuilder 0]

I continue to use the thumb test. It is good to have

an old sun damaged canopy to practice on. I am

working on a prototype of a new test instrument that

will be much faster than the spring and clamp system

that can inadvertantly damage fabric. The thumb

test is not a maximum push test. You can practice

using a torque wrench on a nut and pushing on the

wrench with your thumb till it clicks at 40 lbs. I like

using the thumb test during inspections because it

is fast, accurate, and I end up testing every color

on multicolored canopies. I test components like

the freebag and PC. I once got a freebag and PC

that had been out in the field for over a month. If

I had used the spring scale and clamp test, it would

have passed, but because I thumb tested it , I found

the side that had been facing the sun--it failed!

So, test and test often. I also rejected a main

canopy because of weak ribs. Yes , you must crawl

up inside the canopy and some of those ribs would

tear like tissue, while the adjacent top and bottom

skin fabric was ok. Once again I say test and test

often.

[rigging65 0]

Material that is damaged (sunburnt, chemically damaged, just old) takes very little pressure to make fail using the thumb test. Pretty much every field canopy I've ever had fail while testing did so with minimal pressure...If you have to strain that hard, the material is still good.

The 40 lb. pull test will actually damage the material if its done incorrectly...

"...and once you had tasted flight, you will walk the earth with your eyes turned skyward.
For there you have been, and there you long to return..."