First, the pilots have their heads measured and scanned. In the Marine Corps, those scans are used to build a portable helmet liner that goes between the F-35B pilot’s head and the hard-shell helmet itself.

“This styrofoam cap fits into any size helmet, enabling a pilot to take the helmet liner from squadron to squadron for use throughout his or her career,” a Marine Corps press release said.

Once the scanning is complete, the helmet optics have to be perfectly aligned with the pilot’s eyes. Technicians use a pupilometer, which measures the distance between the pilot’s pupils within two millimeters of its center.

Next, aircrew flight equipment technicians fit the pilots with oxygen masks and identify any leaks around the mask that could prevent proper oxygen flow.

“The next part is to ensure there is a proper distance between the mask and the visor,” Arteaga said. “We have to make sure when the pilot moves or talks, the mask doesn’t hit the visor. If it’s too close, it will bend the visor and distort the display image.”

That’s not the end of it though. After the initial fit, the helmet is inspected every 105 days and has a 120-day fit check to make sure it’s still snug.