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Bill Booth talks about pilot chutes

Exerts from an email conversation between Dave, one of the Perris Organizers, and Bill Booth


Reply from Bill Booth: 
"What happened to you is getting more and more common. Although I hold the patent on the hand deploy pilot chute, I never charged anyone a royality to use it, and therefore never published construction details. I'm afraid this has led to some poorly made pilot chutes as people have copied, but not copied EXACTLY, my original design. As a matter of fact, last Sunday, a jumper came up to me, said that he was having "pilot chute hesitations" on his Vector, and asked me to have a look. His problem, it turned out, was the Booth,-Bill-after-trandem-photoby-NancyDwyer.jpgsame one I've seen countless times before...His pilot chute was made by "God knows who", and made incorrectly. 

I don't know about you, but one of the the scariest malfunctions I can think of, is a streamered main pilot chute that has enough drag to open your main container, but not enough drag to lift out the bag. What do you do? If you just lie there and wait, the ground may "rise up to smite thee". If you pull your reserve, your main is going to simultaneously deploy, and main/reserve entanglements are rarely much fun either. What to do...Easy, don't jump an incorrectly manufactured or adjusted pilot chute. 

So, how can you tell if your hand deploy pilot chute is OK? First, some basic definitions. 1. Apex - The center of the fabric part of your pilot chute. 2. Skirt - Where the mesh and fabric meet. 3. Base - The center of the mesh part of your pilot chute. 4. Centerline - One or two pieces of tape, of fixed length, that lead from the apex to the base. 5. Bridle - A piece of tape, doubled in the case of a collapsable pilot chute, leading from the base to the deployment bag. 6. Kill line - A single piece of line, on a collapsable pilot chute only, that runs from the apex, through the center of the pilot chute, and down through the bridle to the pilot chute attachment point at the bag, or in some designs, to the apex of the canopy. 7. Support tape - 4 pieces of thin tape, sewn to the mesh, leading from the base to the skirt. 8. Bias - Simply put, the direction the mesh stretches the most (a diagonal line, at 45 degrees to the little squares that make up most mesh used in hand deploy pilot chutes). Sorry for all that defining, but if you don't understand those terms, you won't understand what comes next. 

OK, now the easy part. If you want your pilot chute to always function properly, simply make sure, in the inflated state, that no part of the skirt is above the apex. In other words, make sure neither your centerline nor your kill line is too short. I pulled the apex on my hand deployed pilot chute for two reasons. 1. It makes them open faster. 2. It yields 11% more drag. However, IF THE APEX IS PULLED DOWN BELOW ANY PART OF THE SKIRT, THE PILOT CHUTE WILL NOT FUNCTION PROPERLY. 

How do I check that? First cock you pilot chute like you would during packing. Now hold your pilot chute UPSIDE DOWN by the bridle at the base. Simultaneously pull downward on the apex (handle) and each support tape where it touches the skirt. The apex should be equal to, preferable slightly "below", but never "above" the skirt. (Please remember, the terms "above" and "below", in quotes, refer only to the "upside down" pilot chute you are holding in your hands for this test.) Now look how your support tapes are sewn to the mesh. If they are sewn "on the bias" your pilot chute is properly constructed. If they are not sewn on the bias it means that the mesh halfway between each support tape IS on the bias and will stretch more than enough to allow the skirt to get way "below" the apex. Try it. It's like a round parachute with several different line lengths. It simply doesn't work very well. This extremely common construction error might not let your pilot chute fully inflate, or in extreme cases, inflate at all. If your pilot chute is "borderline" when it is new, then things will get worse and worse as it ages. 

Now to "adjusting" a correctly manufactured pilot chute. Kill lines are usually made out of Spectra (Microline). Friction generated during the collapse sequence causes heat, and Spectra SHRINKS when heated. This means that your centerline could eventually get short enough to prevent your pilot chute from inflating correctly. Use what you have learned above to recognize this situation, and correct it. 

This is just a BASIC primer on pilot chute construction, and does not address several other important design considerations such as fabric and mesh choices, and how pilot chute size vs. the weight of your main canopy affects seperation velocity, snatch force, opening shock, and malfunction rate. More about these another time.Bill Booth"

Dave replied: 
"Wow, excellent advice. Tried the test and the apex is nowhere near the skirt. It's inches above. The end of the hackey handle doesnt even reach the level of the skirt. 

The support tape goes in the same direction as the lines of the mesh, not 45 degrees to it. You're saying this is improperly made? Do all "real" parachute manufacturers install the support tape 45 degrees to the mesh? Do I have some sort of homemade pilot chute? There doesn't seem to be any kind of tag or marking to tell me who made it. 

Watching the pilot chute as i pull it down through the air and let it inflate, I can CLEARLY see that the apex falls below the skirt. Until my incident, I wouldn't have know this is unsafe. 

The rigger I bought my rig from gave me a one year warranty on everything. If he hadn't been evicted from the DZ and his house, I'd get my money back. I am quite shocked he sold me potentially (or definitely) unsafe equipment. The good news is he's gone now and I don't have to deal with him anymore. 

Thanks for the info! I'm sure I'm not the only person relying on posts like this to keep me safe in the future. Keep it up! Dave" 

Bill's reply: 
"Your pilot chute is "unsafe" if it doesn't deploy your main correctly. What is "correctly"? You want your pilot chute to open your container and the pull your bag away from your container quickly, but not too quickly. If your pilot chute accelerates your bag away from you too slowly you have the potential for line twists because of the extra time your unsymmetrical deployment bag is exposed to the slipstream, or a bag lock caused by your suspension lines actually "blowing up" above your bag and entangling with it. Not to mention taking more altitude than necessary to deploy. If your pilot chute accelerates your bag too quickly, you have the potential for inertial line dump, high snatch force (when the canopy hits the end of the lines and is decelerated back to your speed), slider rebound (where the slider bounces off the slider stops and your canopy starts its opening sequence with the slider a few inches down the lines. OUCH), and a generally disorganized opening, increasing your chance of malfunction. 

The ideal separation velocity, that is the speed which the canopy is traveling away from you at line stretch, is 50 feet per second. An acceptable range is 30 to 70 FPS. If a pilot chute causes separation velocity outside that range, I do not considerate it "safe". How can you tell what your separation velocity is? You can look at a video of one of your deployments and count video frames at 30 per second. An acceptable time from container opening to line stretch is 0.45 to 0.75 seconds, or about 13 to 22 frames. Any faster or slower and you are "asking for it". 

Separation velocity for any particular system depends on the size, material, and construction details of the pilot chute vs. the weight of the bagged canopy. You of course also have to factor in deployment speed, what your lines are made of, and how you stow them. A very large pilot chute, constructed as yours is, might be perfectly "safe" with a light canopy. The construction details I gave you make the most efficient use of materials, and yield the most consistent results over the most number of jumps. Of course, if you pull the apex down far enough, no pilot chute, no matter how large, will do its job. That's what the "kill line" is supposed to do, isn't it...pull the apex down so far that the pilot chute totally collapses? Bill Booth"

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