Final answer:
The HGU-55 series helmet visor does not have a viscosity as it is not a fluid. The percentage might relate to light transmission, which should be confirmed with manufacturer specifications.
Step-by-step explanation:
The lightweight visor assembly that is used on the HGU-55 series helmet does not have a viscosity, as viscosity is a property of fluids that measures their resistance to deformation at a given rate. Instead, visors are usually made from high-impact materials with specific optical qualities. The percentage you're referring to might be related to the light transmission or tint of the visor, rather than viscosity. The HGU-55 helmets are often equipped with visors that have varying degrees of tint to protect the wearer's eyes from sunlight and glare.
For accurate details about the specific visor assemblies in use and their properties such as light transmission percentage or impact resistance, consulting the manufacturer's specifications or technical manual of the HGU-55 helmet would be recommended. It's important when dealing with such equipment to refer to authoritative sources to ensure the safety and effectiveness of the gear used in aviation and similar fields where precision is crucial.