Ask A Rigger—Kill-Line Pilot Chutes
By Derek Vanboeshoten
The kill-line pilot chute is a critical part of your parachute system. It is a high-wear part that requires periodic inspection and maintenance to ensure that it will continue to work properly.
Before inspecting your pilot chute, you first must understand how it should function. Your pilot chute needs to deploy your parachute when you’re traveling at anything from a very slow speed (think balloon or helicopter jumps) to well over 120 mph, and it should not deploy unintentionally. On every deployment, the pilot chute pulls the pin out of your closing loop, overcomes the friction of the main deployment bag and the container’s main tray and lifts the main deployment bag out of the container. Then it pulls the lines free from their stows. Finally, it rapidly collapses as the canopy leaves the bag (to reduce drag).
Pilot chutes drop onto the ground with each landing. And who hasn’t walked back to the packing area to the calls of “dragging!” (and discovered that the pilot chute has trailed behind on the ground the whole way)? A pilot chute holds up to this abuse amazingly well, and you won’t notice a difference from one deployment to the next, unless it has worn so severely as to cause a malfunction.
If the pilot chute deploys your main bag too quickly, it can cause bag strip, which is when the canopy comes out of the deployment bag before line stretch. Bag strip can result in an opening hard enough to damage the canopy, the lines or the risers. Sometimes it can cause injury or even death. Even if the bag stays closed until line stretch, you can experience a high snatch-force (where the canopy is traveling too slowly when it comes out of the deployment bag and then re-accelerates to your speed).
If the pilot chute doesn’t slow your canopy enough, the deployment bag can rotate during deployment, causing an off-heading opening or line twists. Eventually, it won’t have enough drag to lift the deployment bag out of the main pack tray.
Bill Booth, founder of United Parachute Technologies and the inventor of the hand-deploy pilot chute, recommends that your pilot chute slow your main deployment bag down 50 feet per second, plus or minus 20 feet per second. You can use outside video of your deployment sequence and frames per second to determine how much your pilot chute is slowing your main deployment bag.
Let’s do some math ...
There are several variables for getting the time to line stretch within the 30-to-70-feet-per-second range:
1| The speed of jumper when they deploy the pilot chute. Slower speeds mean less drag produced by the pilot chute, and higher speeds mean more drag produced by the pilot chute.
2| Tightness of the main closing loop. Too tight and not enough drag can cause a pilot-chute-in-tow malfunction.
3| The amount of friction between the main deployment bag and the main packing container. For the same container and deployment bag, a smaller canopy reduces this friction and a larger canopy increases it.
4| The tightness of the line stows. Tighter stows slow the time to line stretch.
5| The weight of the main canopy, lines and deployment bag. A heavier main canopy will take longer to get to line stretch than a lighter main canopy.
6| The size of the pilot chute. Larger pilot chutes create more drag than smaller pilot chutes (all other things being equal).
7| Pilot chute material. Zero-porosity pilot chutes produce more drag than F-111 pilot chutes (all other things being equal). F-111 pilot chutes are usually larger than zero-porosity pilot chutes, negating the difference in drag.
8| Pilot-chute condition. As pilot chutes wear, they produce less drag.
Inspection and Tips
Kill-line pilot chutes wear at several points. The following is what to inspect and some suggestions for increasing the life span of your kill-line pilot chute.
The tapes on the mesh should be sewn on the bias or 45-degrees to the lines in the mesh. This limits the amount the mesh stretches under tension. If the tapes are sewn parallel to the lines in the mesh, the pilot chute is built incorrectly. This issue has become rare.
Pilot chutes are almost always constructed with a Spectra kill line. The Spectra kill line shrinks from the heat generated from sliding through the bridle (friction) as the pilot chute collapses and the bridle stretches over time, affecting the original calibration. Because the change is so gradual it often goes unnoticed until you begin to experience hesitations on deployment.
To check for correct kill-line length, cock the pilot chute and—this is important—hold tension on the entire bridle between the deployment bag to the base of the pilot chute. Compare the kill line to the one or two white ribbons (limit tapes) that run from the apex (the top of the pilot chute) to the bridle. The kill line should be the same length or slightly longer than the limit tape or tapes. If it isn’t, then the kill line is too short and won’t allow the pilot chute to completely inflate.
Your rigger should be able to replace the kill line, or if the kill line was manufactured with enough excess line, your rigger can simply extend the kill line. (I use 500-pound Vectran for my kill lines. This removes the issue of the kill line shrinking a little bit every deployment.)
Check the stitching that holds your handle to the pilot chute. If the handle rips off, you will have a total malfunction. Check the bridle, especially where the pin attaches to the bridle. If any of the stitches have come out, have your rigger repair it. If the pin is ripped off the bridle, you would have a pilot-chute-in-tow malfunction.
As you fly your canopy, the deployment bag is free to spin around the bridle. Eventually the bridle will wear out and break at the grommet. Have your rigger use waxed nylon cord to tack both sides of the rapide link to the reinforced part of the deployment bag to prevent the bridle from rotating in the grommet. Some manufactures sew the pilot chute bridle to the deployment bag, eliminating this problem. The disadvantage of a sewn-in pilot chute is they are difficult to replace, usually a job for a rigger. Another design is a confluence wrap of material that is difficult to pull through the grommet on the deployment bag and holds the bag from sliding down the bridle
If your pilot chute uses a zinc-plated rapide link inside the deployment bag, have your rigger replace it with a stainless-steel rapide link. The stainless link won’t have the problem of the zinc plating flaking off or corroding, which wears the bridle.
Whether your pilot chute is a kill-line model or not, the zero-porosity or F-111 fabric wears just like the fabric that your parachute is made from. The porosity of the fabric degrades with use, decreasing the drag of the pilot chute.
A patch on a pilot chute is difficult to sew and could cause it to spin on deployment, creating line twists. As the mesh wears, holes will develop. Once the mesh tears, the hole will become larger quickly. Replace a pilot chute with holes in the fabric or mesh.
Checking your pilot chute every time you do your 30-day 3-ring maintenance (hint-hint) isn’t difficult and could prevent a malfunction. If you are not confident inspecting your pilot chute, have your rigger help you. An ounce of prevention is worth a pound of cure.
Derek Vanboeshoten | D-18847; AFF, Static-Line and Tandem Examiner; FAA Master Rigger