A General Introduction to Aircraft Emergency Deceleration Parachutes, and Deep Stall/Spin Recovery Parachute Systems
Manley C. Butler, Jr.
Butler Parachute Systems, Inc.
This short document provides a basic introduction to the terminology and techniques of emergency deceleration parachutes and deep stall/spin recovery parachute systems. Because of the proprietary nature of our designs, only a limited description is provided for any particular product.
Further details are available from the author at Butler Parachute Systems, Inc.
The terms "Rocket Deployment" and/or "Parachute Deployment" are used to denote the initiation of the system to deploy the parachute and recover the aircraft.
The term "Jettison" is used to denote the intentional separation of the parachute from the aircraft following a successful recovery.
The term "Integrated Stinger Mount" in this discussion refers to certain BPS spin recovery products wherein all of the arming, latching, deployment and jettison functions are accomplished within the stinger at the tail of the airplane. Further, all of the load-bearing components are contained within the stinger assembly mounted on the tail of the aircraft. These functions are controlled from the cockpit by mechanical or electrical means. The system installed on the AASI JetCruzer 450 is an example of this technique with a mechanical control system. The system installed on the Swearingen SJ-30 is an example of this technique with an electrical control system.
The term "Lightweight/Remote" in this discussion refers to the BPS technique of mounting the load bearing components of the arming, latching, deployment and jettison devices in the cockpit mounted control quadrant and mounting only the fairlead, the parachute canopy and the rocket motor on the tail of the aircraft. This method reduces the total weight of the system and greatly reduces the inertial effects of the system on the aircraft.
Aircraft emergency deceleration parachute systems and deep stall/spin recovery parachutes are very similar in design and execution; however, they serve distinctly different purposes as indicated by their names. For example, a deep stall system will generally have a smaller diameter chute but a much longer riser than the spin recovery system; whereas, an emergency deceleration parachute system will have a fairly small, but very strong, canopy to withstand the high dynamic pressure at deployment.
All three types must meet the following minimum requirements:
Develop sufficient force to decelerate the aircraft at the required rate, or to break the stall and counter the yawing moment of the aircraft despite operating within the wake of the aircraft in most cases.
Have a reliable deployment system to ensure that the parachute is initially deployed into clean air or areas of the wake with the highest available.
Provide a means to latch and unlatch the parachute riser from the aircraft during times when deployment and inflation of the recovery parachute could endanger the aircraft (take-off and landing, for example).
Provide a convenient and safe means to preflight the entire system, either visually or by some other means (continuity checks on electrical components, for example).
Provide a means to completely disarm the system to prevent accidental deployments on the ground or in the air.
Provide an easy means to operate the system with the minimum possibility of improper operation inadvertently defeating the system. For example, the control system must provide interlocks or guards to prevent activation of the jettison switch before the deployment switch.
Present the minimum feasible hazard to the aircrew and maintenance personnel.
Be readily refurbished and returned to service following a deployment.
Over the years, there have been many different types of parachute jettison methods developed for use with these systems. The basic types are:
In general these systems are suitable only for smaller aircraft (<5,000 lb.) because of the requirement to control a relatively large load (the parachute drag forces) with a relatively smaller force (the pilot's strength via linkages, cables, etc.). Many of the mechanical systems are complex devices with dozens of pieces. This complexity leads to higher manufacturing costs, higher maintenance costs and reduced reliability. The benefit of a purely mechanical jettison mechanism is that is can generally be "re-cocked" and reused without replacing any parts (except for some systems that may have shear pins).
In a few cases, solenoids and linear actuators replace some of the linkages in the purely mechanical system. This method is not commonly used due to the force limitations of these devices.
Electro-Pyrotechnic Riser Severance
In these systems, an electrically initiated cutter is used to sever the riser directly. This is a very simple and reliable method of jettisoning the parachute, but it does destroy the riser (or at least part of it). The relatively high operational cost is offset by the ease of installation. The cutters required tend to be fairly large, but can usually be refurbished and reloaded. A variation of this method is used by BPS in our "electrical interface" systems, but the riser itself is not severed so the size/cost of the cutters and cycle cost is reduced.
Mechanical-Pyrotechnic Riser Severance
This is a very simple and reliable method using the same basic method as pyro-electrical but with a mechanically initiated cutter to sever the riser. A cable (inside a flexible housing) can be used to fire the cutters, but friction build-up must be minimized in order to keep the actuation forces reasonable. A variation of this method is used by BPS in our "mechanical interface" systems, but the riser itself is not severed so the cutter size/cost and cycle cost is reduced.
Mechanical-Hydraulic Riser Release
This is a relatively complex means to release the riser and has only been used (to my knowledge) on relatively large business jets (>40,000 lb. gross). This method can provide very high mechanical forces for operation of the mechanism. However, there must be sufficient reserve hydraulic capacity to operate the system even in the event of an engine or pump failure. In my opinion, this is not a very practical means to accomplish the functions.
There are four basic methods of deploying an aircraft deceleration, or deep stall/spin recovery recovery parachute system and some variations that mix features of these basic variations. A description of the basic methods and a discussion of the relative benefits follows:
Spring Loaded Pilot Chute Deployment
This is the oldest, lightest and cheapest of the common methods.
This is the least desirable deployment method because the pilot chute is ejected directly into the turbulent flow behind the aircraft.
The pilot chute is often ineffective, causing slow deployment of the recovery chute (and occasional total failures).
This method has fallen out of favor with all major manufacturers of spin recovery systems.
Butler Parachute Systems, Inc. has not and will not design and build this type system.
Force Deployed Pilot Chute
This method commonly uses an explosively ejected weight or "slug" and is a significant improvement on the basic spring loaded pilot chute deployed system.
Although the pilot chute is placed into clean air initially, it will quickly swing back into the wake as it inflates, and thus deployment of the recovery chute still depends on the unpredictable aerodynamic force from the pilot chute.
In small systems (for light aircraft), the slug will typically weigh about one pound and will have a muzzle velocity of over 200 ft./sec., which generates a substantial reaction force that must be absorbed by the structure.
Some small systems have also used a drogue slug to deploy the recovery chute itself both with and without the use of a pilot chute as an additional deployment aide. If the size of the system lends itself to this variation, it does provide an improvement in overall performance.
In either variation, the kinetic energy of the parachute system (relative to the aircraft) peaks at drogue ejection and then falls off rapidly. However, the energy of the slug is still sufficient to ensure the deployment of the parachute.
Mortar Deployed Recovery Parachute
This method provides a substantial improvement in the total system performance over the two mentioned above.
The recovery chute (packed in its deployment bag) is explosively ejected from a mortar tube, like an artillery shell fired from a cannon barrel.
This method ensures that the recovery chute is placed into clean air prior to beginning its inflation.
Because the entire mass of the packed parachute canopy (perhaps ranging from 5 pounds for a lightweight 12' chute up to 50 pounds or more for large systems) is ejected by the mortar at a muzzle velocity of 100 to 200 ft./sec., the resulting reaction force can be extremely high. The structural reinforcement required to absorb this reaction force is a significant disadvantage of this method.
The parachute canopy and riser must be pressure packed to a very high density (typically 50 to 55 lb./ft.3) in order to avoid expending large amounts of energy in the useless compression of the parachute canopy during initial acceleration.
The shape of the packed parachute is restricted to cylindrical shapes in order to fit into the mortar.
The kinetic energy of the deploying parachute (relative to the aircraft) reaches its peak as the deployment bag clears the muzzle, then rapidly and continually decreases as the chute travels further from the aircraft.
The last portion of the deployment is typically somewhat sloppy unless the muzzle velocity is very high or unless (as in some cases) ballast has been added to the deployment bag to ensure that it has sufficient inertia to continue to deploy the parachute canopy as it nears line stretch.
The cost of the mortar tube itself, the relatively large and expensive pyrotechnic charges, and the required structural modifications to the aircraft make the mortar deployed system by far the most expensive system to buy, operate and maintain.
Rocket Deployed Recovery Parachute
This method ensures a positive deployment of the parachute into clean air.
The rocket is typically attached to the deployment bag through a steel cable bridle (for heat resistance in the rocket plume) and an incremental tear-strip bridle (to mitigate the high differential velocity at initial pickup).
The rocket is typically sized to provide an average acceleration of 6 to 10 gs (based on the mass of the deployed chute/riser).
The rocket will burn long enough to continue beyond line stretch even at the coldest temperatures and lowest speeds to be encountered.
The kinetic energy of the deploying parachute (relative to the aircraft) rises quickly to a moderate level that it maintains throughout the deployment process.
The parachute canopy will have a positive extraction force applied to the apex throughout the deployment.
The constant extraction force on the apex of the canopy leads to much more consistent inflation and opening characteristics when compared to any of the other techniques.
A further advantage is that the parachute canopy can be stowed into nearly any shape imaginable as long as the deployment path is clear, smooth and of sufficient cross-sectional area to permit free passage of the deploying chute.
OTHER system components
The system components described here are typical of the Butler Parachute Systems, Inc. lightweight/remote systems that have been successfully installed on six different aircraft to date.
The first order of business when designing the system is to size the parachute canopy by determining the drag force and moment arm required to break the stall (or decelerate the aircraft). An engineering estimate must be made by flight dynamics staff of the aircraft manufacturer to account for the reduced dynamic pressure in the wake of the aircraft and the parachute sizing will be adjusted accordingly. The recovery parachute will typically be a ringslot or ribbon parachute designed and built for the program. For light aircraft (<5,000 lb.) the parachute will be typically be 8 to 10' nominal diameter with a drag coefficient of 0.50 to 0.55 normally used for parachute sizing calculations.
NOTE: In general, no allowance is made by BPS for dynamic analysis of the aircraft during the stall/spin event and recovery. Parachute canopy sizing (and/or drag force) and riser length must be specified by the customer.
Parachute Deployment Device and Stowage Compartment
The canopy will be packed into and deployed with a deployment bag that will control the canopy, suspension lines and riser during the extraction and deployment. The deployment bag will in turn be stowed inside the stinger tube or a separate compartment.
The total combined riser and suspension line length from the aircraft attachment point to the skirt of the parachute will be determined during the final design process. The riser will be generally be constructed from nylon webbing of the appropriate rated strength. The reference riser/line length will be measured from the fairlead to the canopy skirt; BPS will require input from customer personnel during the design of the parachute size and riser length.
Fairlead and Fairlead Mounting
The fairlead is used to allow the parachute to pull on the riser in any direction without subjecting the riser to sharp corners. It is strong enough to carry the loads from the riser into the aircraft structure. The fairlead mounting brackets typically must be designed and fabricated by the customer and must be capable of withstanding the parachute's normal (bending) loads (through the fairlead) in any direction. The bolt pattern for mounting the fairlead can be varied somewhat as required.
Extraction Rocket Motor
The parachute will be deployed using a small electrically or mechanically initiated (dual percussion primers) rocket motor. The rocket will be tethered to the parachute deployment bag through a steel cable (for heat resistance in the plume) and the extraction bridle. An incremental tear-strip type bridle will be used. The deployment rocket will be activated through a cable control from the cockpit.
Rocket Launch Tube
The rocket launch tube will house the rocket motor so that the initial rocket trajectory is controlled and the plume is prevented from impinging on other components that might be damaged. The launch tube can be attached to any convenient part of the aircraft structure.
This short document has provided only a basic introduction to the terminology and techniques of emergency deceleration parachutes and deep stall/spin recovery parachute systems.
Because these systems are obviously critical to flight safety, this information is provided only as a general overview of the technology and requirements.
Should you have a requirement for such a system, please contact the author at Butler Parachute Systems, Inc.