National Aviation Preparedness Plan Act of 2021 This bill directs the Department of Transportation (DOT) to develop a national aviation preparedness plan for communicable disease outbreaks, including to require that frontline at-risk employees are equipped with appropriate personal protective equipment to reduce the likelihood of exposure to a communicable disease that has the potential to cause an epidemic or pandemic of infectious disease that would constitute a public health emergency of international concern (i.e., a covered communicable disease). In developing the plan, DOT must consult with aviation industry and labor stakeholders such as air carriers. The Government Accountability Office must conduct a study assessing the plan, including (1) whether the plan meets the obligations of the United States under international conventions and treaties, and (2) the extent to which the United States aviation system is prepared to respond to an outbreak of a covered communicable disease.