(a) Period of time No alien crewman shall be permitted to land temporarily in the United States except as provided in this section and sections 1182(d)(3), (5) and 1283 of this title. If an immigration officer finds upon examination that an alien crewman is a nonimmigrant under paragraph (15)(D) of section 1101(a) of this title and is otherwise admissible and has agreed to accept such permit, he may, in his discretion, grant the crewman a conditional permit to land temporarily pursuant to regulations