• Passenger jets flight plans are programmed before pilots arrive in the cockpit

  • But it's not unusual for pilots to change course to avoid things like bad weather

  • They do that by punching commands into the plane's flight management system

  • Authorities aren't sure exactly when Flight 370's change of course was entered




(CNN) -- You may think that pilots fly passenger jets by steering a joystick. In reality, a lot of the turns the aircraft makes are predetermined.


The disclosure that the first major change of course by missing Malaysia Airlines Flight 370 was most likely programmed through a computer in the cockpit has raised questions about how modern planes navigate.


Here's a quick primer on how it works.


Before takeoff


An airline dispatcher creates the flight plan. When the pilots arrive at the plane, their route is already set and programmed into the plane's flight management system.





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Malaysia Airlines Chief Executive Ahmad Jauhari Yahya says that as far as he is concerned, the missing Boeing 777-200 was programmed to fly to Beijing, its intended destination. "That's the standard procedure," he said Tuesday.


After the pilots arrive


Changing the flight path is simple, according to Mark Weiss, a former Boeing 777 pilot. He says a pilot can punch a few commands into the airplane's flight management system, which operates a little like a car's GPS.


A U.S. official told CNN that somebody programmed Flight 370 to fly off course. But it's unclear if that happened during the flight or before takeoff.


"We don't know when specifically it was entered," said Mary Schiavo, a CNN aviation analyst and former inspector general for the U.S. Department of Transportation.


In flight


Pilots often have reason to alter their flight path when they're in the air, Weiss says: "That's a very typical scenario, you may have weather in your path, you may have oncoming traffic in your path." To change the course, they type a new navigational waypoint, a kind of virtual checkpoint in the sky, into the system.


Reports, not yet confirmed by authorities, have suggested that Flight 370 followed navigational waypoints after it turned off its original route less than an hour into its journey on March 8.


Ability required


Programming waypoints into the flight management system of a Boeing 777-200 is "a task that would have been beyond the abilities of anyone but a professional pilot," according to Robert Goyer, the editor-in-chief of Flyingmagazine and a commercial jet rated pilot.


Once the pilots are in the aircraft, "anything is possible," said Ahmad Jauhari.


Notifications


The investigation into what happened to Flight 370 has focused attention on how much data airborne planes send to the ground. The information that the flight path was reprogrammed appears likely to have been transmitted by the passenger jet's Aircraft Communications Addressing and Reporting System (ACARS). That system communicates data -- such as engine reports, maintenance requirements and weather conditions -- to the ground.


The last transmission from Flight 370's ACARS was sent at 1:07 a.m., authorities say. The next update, due at 1:37 a.m, never came. A law enforcement official told CNN that the plane's programmed change in direction was entered at least 12 minutes before the plane's verbal sign off with air traffic controllers at 1:19 a.m. -- that tallies with the time of the last ACARS transmission.


READ: Experts answer #370Qs tweets


READ: Will pilot's simulator give up clues?



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