The Paraglider Emergency Reserve Deployment System Controversy
By Betty Pfeiffer
Most of the paragliding world believes that the diaper (i.e. four flap, cloverleaf) deployment systems are the best for paragliding. In examining similar deployment problems faced in the military, aerospace, skydiving and hang gliding applications, we have strongly favored deployment bag systems.
The purpose of this article is to examine the pros and cons of diaper verses deployment bags for paragliding use. This article will not address ballistic deployment systems, specific parachute designs or construction techniques. We will assume that the emergency reserve is a round style parachute.
Each air sport seems to go through its own set of growing pains. With time and experience a consensus develops as to the best way to proceed.
The evolution of paragliding is no different. When it comes to life saving systems it is best to try to expedite this process as much as possible. By considering the problems and solutions experienced in similar air sports, paragliding can speed up its evolution of safety systems.
The emergency reserve parachute deployment design problems unique to paragliding applications include:
Designing a system that is capable of both high speed and slow speed deployments without damaging the parachute.
Designing a system that will deploy easily when needed but avoids the possibility of accidental deployments. This is a function of handle placement and parachute container design. It will not be addressed in this article.
Designing a system that minimizes the risk of out of sequence deployments.
Designing a system that minimizes the risk of entanglement problems.
Designing a system that will optimize the chance of getting the parachute into the clear air
Designing a system that minimizes the snatch force (force related to acceleration of the deployment system related to the velocity of the load) so as not to produce a catastrophic harness failure or injury to the pilot.
Designing a system that will get the parachute out with the least amount of altitude loss.
Before we can discuss deployment systems it is important to understand some basic parachute dynamics. These dynamic conditions are true no matter what the application:
The suspended weight, bridle, lines and canopy are stretched out in clear air
Air enters the canopy through the air channel (center of the parachute)
Parachute inflates from the top down (similar to blowing up a balloon)
As the suspended weight decelerates the load is transferred through all the lines symmetrically
The parachute has a rebound action (partial deflation) before it fully inflates
4. A properly designed parachute wants to stabilize with the suspended weight directly below it
Paragliding Emergency Reserve Deployment Systems
Currently there are 2 major categories of deployment systems used for Paraglider emergency reserve parachutes. These are the diaper (i.e. cloverleaf, four-flap) systems and the deployment bag systems. Some manufacturers have combined elements of both deployment systems. Some manufacturers have incorporated pilot chutes in their deployment systems. For our purposes we will focus only on the main categories.
The diaper deployment system typically has four flaps that overlap in the middle. The bottom flap has a bungee or cord loop that is threaded through the grommet on the edge of each flap. The diaper is held closed by a loop made with the lines or bridle of the parachute.
Most Paraglider diaper systems use a flat pack when folding the canopy. Essentially the parachute is stretched out on a packing table then flaked and folded in thirds. Starting with the top of the parachute they are Z-folded into the center square of the diaper. Some manufacturers have the lines stowed inside the diaper, others have the lines stowed into the parachute containers and some have a seperate line pocket or rubber band line system attached to the outside of the diaper.
No matter what line stow configuation is used, once the closing stow is released from the loop, the entire diaper opens and the total contents is exposed to the air.
The diaper system emerged in the hang gliding world in the 70s. Most parachute manufacturers quickly went to a deployment bag arrangement. Embury Sky Systems used a version that had a skirt restraint system with their Rapid Deployment Parachute (RDP). The skirt restraint system held the canopy closed until the vent cap pulled the parachute to full stretch. This was important in helping to stage the deployment sequence.
The diaper systems that we are seeing with paragliders have no such skirt restraint systems.
Diaper deployment systems advantages are:
Diaper deployment systems dis-advantages are:
For example, if the parachute experiences partial canopy opening before achieving full line stretch the result can be line-overs, inversions, partial inversions or entanglement between parachute parts. All of these malfunctions can cause damage to the parachute or lines. All of these malfunctions will require more altitude loss and a higher rate of descent while the parachute organizes itself.
2. The distance you can throw the diaper with the parachute still enclosed is determined by how much free bridle or line is stowed outside the diaper. For example, if you leave arms length outside the diaper, your parachute will always be exposed to the air at that distance. If you store a long amount of lines in your parachute container, as soon as you pull your deployment handle and the container opens, all the line can drop out. Here again you risk entanglement problems.
3. A parachute packed in flat diaper systems can start to escape through the corner section before the lines have come to full extension. Many manufacturers have addressed this problem by sewing up the corners of the diaper.
4. Lines stowed inside the diaper can get wedged between the flaps and go through the bungee thus locking the diaper and not allowing it to open properly.
5. The lines can wrap around the cloth and have a negative effect on the deployment.
6. The diaper , once opened, will not contain the mass of the parachute and therefore can not help pull the parachute to full extension.
Deployment bag systems
The typical deployment bag has two compartments separated by a short flap. A larger flap closes off both compartments and contains most of the lines and sometimes even a small part of the bridle.
When packing a parachute in a typical deployment bag, the parachute is stretched out, flaked and folded in the traditional manner. Starting at the apex (top) the parachute is S-folded (snaked) into the large compartment. This compartment is closed off with a short flap. Rubber bands that are threaded through the grommets on the flap are held shut with loops of line from the parachute. The remainder of the parachute line is S- folded into the line compartment. The final flap closes off both the parachute and line compartments with a rubber band loop arrangement locked in place with more parachute line or the parachute bridle depending on the harness. Note: Rubber bands used for this are stretchy and break below 25 lbs. Sky diving rubber bands are not appropriate for this application.
Deployment bags have been used in many different parachute applications since the 1940s. In the late 1970s hang glider reserve manufacturers favored the deployment bags. The parachute was locked into the deployment bags with lines stowed through rubber bands. The rest of the lines were stowed in rubber bands on the outside of the deployment bags. This design evolved to enclose the lines in a separate line pocket.
To extract the parachute from the deployment bag the pilot throws the deployment bag into clear air. When the bridle reaches full stretch it releases the line compartment. As the lines reach full stretch the canopy compartment opens, the canopy snakes out of the deployment bag skirt first. As the skirt is exposed to the air, air is directed into the air channel and the parachute is ready to inflate.
Advantages of the deployment bag:
In his book Parachute Recovery Systems Design Manual, (page 62), T. W. Knacke describes a good parachute deployment system as one that "minimizes the parachute snatch force by controlling incrementally the deployment of the parachute, and by keeping the parachute canopy closed until line stretch occurs." He continues to discuss the importance of keeping tension on all parts of the deploying parachute. "Fluttering causes entanglement, canopy damage, line-overs and canopy inversions." A good sequenced deployment "Minimizes opening time and opening-force scatter caused by irregularities and delayed action during parachute deployment and inflation .Parachute and riser should be stored in a textile envelope for protection during deployment and to ensure a controlled deployment the deployment bag should have separate compartments for the canopy, suspension lines and, if required, for the riser." Deployment bags do all of this.
Disadvantages of the deployment bag
1. If the deployment bag is the old style with the opening at the bottom and if the rubber bands are not properly maintained the rubber bands can break pre-maturely and allow the parachute to fall out of the deployment bag before you have full line extension.
If all this is true why is the rest of the Paragliding world supporting diapers?
Diaper supporters have voiced the opinion that in slow speed situations it is important to have as much parachute exposed to the air as quickly as possible. The idea is that the more parachute you have out in the air the faster you will decelerate.
The reality is that it does not matter how much parachute is in the air, until you load that parachute it is not doing you a bit of good. With a deployment bag, the pilot has a good opportunity to throw the parachute to full line stretch. The stronger the throw the further the deployment bag will travel and the quicker the parachute will snake out. The diaper does not have this advantage. Even with a good strong throw, once the diaper opens the parachute does not have to reach full line stretch before the canopy is exposed to air. Here again you risk malfunctions. Here again you will lose more altitude waiting for a parachute to organize itself than you do with a controlled deployment forced by using a staged deployment bag. Remember parachute material is very fragile. During the process of organizing itself, a parachute can be damaged. Nylon rubbing on nylon causes friction, which can cause heat and damage.
Another argument offered by diaper supporters is that parachutes using a diaper open faster than those packed in a deployment bag. A 10-year study was conducted by the Army at Fort Bragg focusing on the use of anti-inversion netting. A by-product of this study was information related to the importance of controlling the skirt of the parachute during the inflation process. It was concluded that controlled deployments, such as those produced with the use of a deployment bag, increase the reliability of parachute openings. Uncontrolled deployments, such as those produced with Diaper systems, increase the chance of inconsistent deployment times. The diaper opening time can be up to .1 second faster than a deployment bag or 1 or more seconds slower. The deployment bag is consistent and that is the key.
The final issue we need to address is that relating to the speed you are falling when you make the decision to deploy. At higher speeds both deployment systems seem to work. At slower speeds the chance of entanglement or out of sequence deployments are greatly increased by exposing the parachute to the airflow as with a diaper system. You need only view real life paragliding emergency deployments on a video frame by frame to understand how close many pilots have come to very unhappy endings.
To date Paraglider pilots have been very lucky with their use of emergency reserve parachutes. We must be careful not to rely on luck and continue to learn as much about our reserve parachute system as possible. Unfortunately too many pilots have the attitude that an emergency situation will never happen to them. Too many pilots do not anticipate what can happen because it never has happened before.
In closing I would like to say a word about DHV certification. As a paraglider pilot you need to rely on the word of the experts for many aspects of your equipment. You also need to understand the strengths and limitations of your equipment. Often we feel safer with a product that has been "certified". When it comes to Paraglider deployment systems and parachutes i.e. safety systems designed to save your life, you need to be proactive and learn how your equipment was tested. You then must decide if the test really was measuring what you need to know about your parachute and deployment system.
Emergency reserve parachute manufacturers, High Energy Sports and Free Flight Enterprises, both believe that the best way to see how a parachute behaves in the air is to test it out of an airplane using objective instrumentation to determine the rate of descent and flight characteristics. The best way to determine the strength and stability is to throw it out of an airplane with a torso dummy (weight) thus initiating a cross stream deployment. By increasing the speed of the plane you can increase the load placed on different components of the parachute. This is how both companies test their hang gliding and paragliding reserve parachutes.
Some important areas we did not discuss in this article but certainly deserve mention include harness and deployment system handle access and compatibility, bridle routing on the paraglider harnesses, structural integrity of paraglider harnesses to withstand high speed parachute deployments, and parachute construction and performance. As a pro-active pilot you need to triple check your system and ask questions about anything that does not make sense. The manufacturer should be able to easily explain to you why particular design decisions were made.
Special thanks to Gary Douris, Bill Gargano, Chuck Embury, Jody Lucas, the SFBay area PG pilots and everyone who has helped bring this issue to the forefront.