--by Jennifer Caron, FAA Safety Briefing
Have you heard of the domino effect? It occurs when you make a change to just one thing in an integrated system, and that one change touches and affects every aspect of the entire system, setting off a chain reaction much like a cascade of falling dominos.
Aircraft seat belts are an example of one domino that can affect the entire chain of components in an aircraft seat assembly. If you are repairing aircraft seat belts, you must use materials and data specified for the particular seat assembly subject to the installation.
Re-webbing a seat belt on a dynamic seating system poses unique challenges. Historically, seat certification and subsequent modifications have only involved static structural requirements. However, in the late 1980s, the FAA issued a series of regulations aimed at improving the survivability of aircraft crashes. This effort resulted in stringent design and performance requirements for seat belts used in dynamic seating systems. These dynamic seat regulations, found in 14 CFR parts 23, 25, 27, and 29 (commonly referred to as 2X.562) introduced new dynamic loading and occupant injury criteria for dynamic aircraft seating. With this new regulation, parts of the seats, such as TSO seat belts, were no longer approved separately. In a nutshell, 2X.562 created an integrated seating system evaluation method and imposed requirements on the seating system as a whole.
If you are repairing aircraft seat belts, you must use materials and data specified for the particular seat assembly subject to the installation.
Dynamic seats require demonstration of protection for the pilot and/or passenger (such as head injury criterion, lumbar load, femur load, and restraint loading) when the seats are subjected to dynamic emergency landing conditions (vertical and horizontal deceleration). Manufacturers must assess the performance of the seats under those conditions, and the required dynamic tests for the seating system as a whole include the seat restraints.
Any changes to the restraint webbing material, manufacturing process, stitching, or hardware modifications may affect the entire seating system, including the dynamic loading of the structure, and the loads transferred to the pilot or passenger.
For example, changing the weave pattern of identical webbing material may alter the stiffness, strength and dynamic performance of the seating system, which in turn may impact compliance with protection requirements such as head injury criteria, occupant restraint load limits, and retention of the pilot or passenger.
The bottom line: For proper installation of a seat belt on a dynamic seat assembly, you have to ensure that the proper testing and/or analysis show that the specific requirements for that dynamic seat assembly have been met, and support the repair data you used.
As with any domino effect, repairing or rewebbing aircraft seat belts can affect the entire chain of components in an aircraft seat assembly. Always ensure that any repair of seat belts uses the specified materials and data for that particular seat assembly receiving the installation.
Learn More
- Advisory Circular 21-25B, Approval of Modified Seating Systems Initially Approved Under a Technical Standard Order (TSO)
- Information for Operators (InFO) 17004 on Seat Belt Repairs and Alterations
- NTSB Safety Alert to Check Your Seatbelt Restraints
Jennifer Caron is an assistant editor for FAA Safety Briefing. She is a certified technical writer-editor in aviation safety and flight standards.