Audiovisual records of a Project Mercury pilot’s activities during an orbital flight indicate that his visor was left open during reentry and descent to the sea surface, phases of flight during which cabin pressure loss was to be mitigated by suit pressurization; however, the suit could not have been pressurized with the visor open. Thus, for a presently unknown reason, a critical safety step—sealing the visor and making a pressure suit integrity test before reentry—was overlooked in this flight. Later, Space Shuttle flights were carried out with visors unsealed for much of the launch and landing phases, with the false assumption that they could be closed if the crew cabin were to lose cabin pressure rapidly. The lessons are clear: first, spaceflight crews should be trained to seal visors for the entire launch and landing phases; and second, procedure checklists will always be important to crewed flight, in both public and private spaceflight.
Smith, Cameron M. and Tresch, Trent
"Incidence of an Astronaut Not Sealing the Pressure Garment Visor on Reentry,"
Journal of Human Performance in Extreme Environments: Vol. 15
Available at: https://docs.lib.purdue.edu/jhpee/vol15/iss1/1