“We hope that people will be much better off arriving to their destination compared to if they have to take connecting flights,” Dr. Postnova said in a phone interview. “We anticipate that with our optimized schedule, the direct flights will be better in terms of jet lag, well-being and how you feel on board.”

The pilots will be closely monitored, too, with electroencephalogram, or EEG, tracking of their alertness, and tests on their level of melatonin, a hormone linked to regulation of the body’s circadian rhythms.

The data will be used to devise what schedule of work and rest will suit the pilots operating the extra-long routes.

With airlines such as Qantas and Qatar Airways already operating 17-hour flights, Kit Darby, an aviation consultant, said pilots were likely to adapt quickly to even-longer flights.

“If pilots are regularly doing 16- or 17-hour flights, it is not a dramatic difference,” said Mr. Darby, a former United Airlines captain and Boeing flight instructor. “They will feel it, but it is manageable.”

Longer nonstop routes have an appeal for both airlines and passengers, Mr. Darby said.

“For airlines, the most efficient way of flying an airplane is to fill it up and fly it as far as it will go,” he said. “And from a passenger point of view, it is great to go directly where you want to go to without problems such as missed connections.”

Aviation fanatics who want to get their hands on a ticket for the New York or London to Sydney test flights will be disappointed, however. Qantas said it would use mostly its own employees to make up the passengers and crew, and no seats would be sold.