“ Scheduled air carriers are presently required to conduct a proving flight over a proposed route even though it is a minor modification or extension of an existing route already flown by the air carrier. Under certain conditions the safety of the proposed operation is not advanced by such a proving flight and, where this is true, an undue burden is placed upon the air carrier. It appears that the public interest will best be served if a proving flight is not required where it is unnecessary in the interest of safety. ”