Answer :
While flight deck door policies can vary, they are generally kept locked for safety and security during flights. Solar sail crafts can indeed be propelled by solar wind particles, aligning with the statement that it is true. The 'fasten seatbelt' sign is a common safety measure during flights. Therefore, the above statement is true.
The question regarding whether the flight deck door can remain locked from when the airplane moves for departure until the end of the flight segment does not have a strict true or false answer as it may vary by airline policy and regulatory requirements. However, for the purpose of safety and security, flight deck doors are typically expected to remain locked during flights as per regulations established after incidents such as the 9/11 attacks.
In relation to the true or false statement about the possibility of propelling a solar sail craft using just particles within the solar wind, the answer is true. Solar sails are designed to be propelled by the pressure of sunlight and the solar wind, without the need for conventional fuel.
The "fasten seatbelt" sign being turned on by the captain is an indication for passengers to remain seated due to possible turbulence or other safety considerations. It is common for flight attendants to notify passengers when this sign is illuminated for their safety.
