Cabin upholstery in standard category airplanes must be what?

Cabin upholstery in standard category airplanes must be flame resistant due to safety regulations established by aviation authorities. The primary concern is to minimize the risk of a fire within the cabin, which can have catastrophic consequences in the event of an emergency. Flame-resistant materials help to slow down the spread of fire and provide additional time for passengers and crew to evacuate safely. This requirement ensures that the materials used in the cabin environment contribute to the overall safety of the aircraft and protect the occupants in case of a fire.

While other factors such as waterproofing and soundproofing can enhance passenger comfort, they are not mandated in the same manner as flame resistance, making flame resistance a critical safety standard that must be adhered to. Aesthetic considerations are important for the overall look and feel of the cabin but do not override the importance of fire safety measures in aviation.