Appendix 3 APPENDIX 3. SEATS AND RESTRAINT SYSTEMS
1. Seating Configuration and Restraint System Safety. Not all seating and restraint system configurations used in jump aircraft provide the same level of safety in the event of an emergency landing. This appendix provides general information concerning the relative safety of commonly used seating configurations and restraint systems. These safety assessments are based on available research data and in-service experience.
2. General Information.
a. Quick Release Track Fittings. Single stud quick release track fittings have been shown to release from the track at dynamic loads much lower than their rated strength. Dual stud quick release fittings did not exhibit this behavior in dynamic tests. Therefore, dual stud quick release fittings of the type shown in Figure 2, Dual Stud Quick Release Track Fitting, provide a much more reliable restraint anchorage than single stud fittings.
b. Lap Belts. Lap belts are only effective if there is a solid support surface behind the occupant, such as a seat back, aircraft sidewall, or bulkhead. Otherwise, a tether restraint that attaches to the parachute harness provides more effective restraint.
c. Restraint for Aft-Facing Parachutists. Research has shown that to restrain aft-facing parachutists, the most effective point to attach a tether restraint to a parachute harness is at the junction of the leg straps, main lift web, and the horizontal back strap. Figure 3, Tether Restraint Usage, illustrates this attachment method, in which the tether loop encircles the junction by passing between the main lift web and the horizontal back strap, and between the upper leg strap and the lower leg strap. One way to achieve this is to route the tether loop under the upper leg strap, then under the main lift web before latching the loop, as depicted in Figure 4, Pass Tether Loop Under Upper Leg Strap, Figure 5, Pass Tether Loop Under Main Lift Web, and Figure 6, Latch Tether Loop Around Parachute Harness. Since these two components of the harness are easily accessible by the wearer, this attachment method should not be prone to misuse. It also provides more effective restraint than attaching at other points on the parachute harness since the restraining force is applied near the seated occupant’s center of gravity (CG).
d. Restraint Belts or Tethers. Past experience and testing have shown the validity of attaching a restraint belt(s) or tether(s) to the parachute harness as part of the overall integrated restraint system. However, most manufactuers have not tested their parachute harness configurations to see if they can accept the load vectors that would be experienced during the actual use of this type of restraint configuration. Because of this, any parachute harness that has been subjected to actual use as part of an integrated restraint system must be removed from service and inspected by the manufacturer or a parachute rigger designated by the manufacturer to determine the continued airworthiness of the parachute harness. If the inspection shows that the harness is Airworthy, it may be returned to service.
Appendix 3
3. Specific Seating/Restraint Configurations.
a. Side-Facing. Conventional side-facing bench seats employing dual point lap belts are a superior means of carrying parachutists in aircraft large enough to accommodate them. They offer the advantages of being simple to use and can be designed to provide significant vertical energy absorption.
b. Rear-Facing Floor Seating.
(1) Restraints are more effective if attached to the floor instead of the sidewall. Only use sidewall attachments if floor attach points are not available.
(2) Effectiveness is increased if overall tether length is kept as short as possible and the tether attachment to the aircraft is aft of the harness attachment point.
(3) Single point, single tether restraints are not recommended.
(4) Dual point, dual tether restraints offer superior restraint compared to single point, single tether restraints. This restraint method consists of two straps, each connecting the parachute harness to the aircraft floor on both sides of the parachutist as shown in Figures 7, Tether Restraint Attachment To Floor For Rear-Facing Floor Seats, Figure 8, Dual Point, Dual Tether Restraint Configuration For Rear-Facing Floor Seats, and Figure 9, Dual Point, Dual Tether Restraint Attachment To Floor For Rear-Facing Straddle.
c. Rear-Facing on Straddle Bench.
(1) Straddle benches can offer more occupant crash protection than floor seating since they can be designed to provide significant vertical energy absorption.
(2) As with floor seating, restraints are more effective if attached to the floor instead of the sidewall.
(3) Restraint effectiveness is improved if the tether strap is attached to the floor such that it is at an approximately 45 degree angle, as shown in Figure 9.
(4) Single point, single tether restraints are not very effective.
(5) Dual point, dual tether restraints offer superior restraint compared to single point, single tether restraints.
Appendix 3 FIGURE 2. DUAL STUD QUICK RELEASE TRACK FITTING
FIGURE 3. TETHER RESTRAINT USAGE
Appendix 3 FIGURE 4. PASS TETHER LOOP UNDER UPPER LEG STRAP
FIGURE 5. PASS TETHER LOOP UNDER MAIN LIFT WEB
Appendix 3 FIGURE 6. LATCH TETHER LOOP AROUND PARACHUTE HARNESS
FIGURE 7. TETHER RESTRAINT ATTACHMENT TO FLOOR FOR REAR-FACING
FLOOR SEATS
Appendix 3 FIGURE 8. DUAL POINT, DUAL TETHER RESTRAINT CONFIGURATION FOR