What must cabin upholstery materials installed in current standard category airplanes be?

Cabin upholstery materials installed in current standard category airplanes must be at least flame resistant to comply with safety regulations. This requirement is in place to minimize the risk of fire in the aircraft cabin, as materials that are not flame resistant can contribute to the spread of flames, creating a hazardous situation for passengers and crew. Flame resistance is a crucial safety characteristic because, in the event of an onboard fire, materials that can resist ignition and combustion significantly enhance passenger safety and can buy critical time for evacuation. Therefore, designing cabin interiors with flame-resistant materials is a vital aspect of aircraft safety regulations.

While other options like waterproofing, visual appeal, and lightweight characteristics may certainly contribute to the overall design and functionality of cabin materials, they do not address the most critical safety requirement concerning flammability. Thus, flame resistance stands out as the primary concern that must be prioritized in the selection of cabin upholstery materials.