Final answer:
To secure the nape strap of an HGU-55 pilot helmet, a double overhand knot, also known as a safety knot, is used because it provides a strong and reliable fastening.
Step-by-step explanation:
The HGU-55 is a type of pilot helmet used by aviators. When adjusting the helmet to ensure a proper fit, the nape strap is an important component which secures the helmet to the wearer's head. After obtaining the correct fit, the standard knot used to tie off the nape strap is a double overhand knot, also known as a safety knot. This knot is preferred as it provides a secure fastening that is less likely to slip under vibration or movement, which is critical during flight operations.