The Asiana Airlines Boeing 777 at San Fransisco. via
Airmanship has changed since the days when barnstorming pilots and wing walkers ruled the skies. Modern cockpits are highly automated, and knowing how to fly in tandem with an onboard computer is vital. But just because commercial jets are highly automated doesn’t mean planes can fly themselves; the crash at San Francisco Airport a few weeks ago should remind us of that. The pilot is always in control, but how he interacts with his computers varies depending on the plane he’s flying. Different manufacturers, like Boeing and Airbus, approach automation differently.
Let’s take two common commercial jets: the Airbus A320 and the Boeing 777 (this was the type of plane that crashed in San Francisco). Both these jets use a fly-by-wire control system, which means the pilot commands the airplane through the wired computer system, e.g. electronically, rather than mechanically. Fly-by-wire isn’t a new technology. The first vehicle to use entirely fly-by-wire control was the Lunar Landing Research Vehicle Apollo astronauts used to learn how to land on the Moon. Using a side stick, astronauts input commands into the computer, which would in turn fire thrusters to keep the LLRV flying slowly and level to a soft touchdown.
In a commercial jet, the fly-by wire system works the same way. It translates all of the pilot’s control inputs into electronic signals that are passed to the flight controls. When a pilot pulls back on his control yoke, he isn’t physically lowering the rear elevator to raise the plane’s nose. Rather, his force on the yoke sends an electric signal to the elevator telling it to lower, which in turn raises the nose. This electric system has a lot benefits. By replacing hydraulic systems or mechanized cables with electronics, the result is a lighter system that's easier to service and repair. It also lightens the pilot’s load. Instead of physically holding his airplane at the right level throughout a flight, he can set the computer to manage the task, freeing him up to keep an eye on how all the systems are working and deal with any problems.
Fly-by-wire also puts the computer in a position to monitor the pilot’s inputs and make sure the plane stays within its safe limits, what’s called the flight envelope. Both the A320 and the 777 have envelope protection as a function of their onboard computers. The computers monitor the flight to prevent potentially hazardous situations like stalls, excessive bank, and dangerous over-speeds. Basically anything that would compromise the structural stability of airworthiness of an airplane. But the two systems aren’t the same.
A Boeing 777-200 series cockpit. via
Boeing, founded in 1916, has been building airplanes almost since the Wright brothers pioneered heavier-than-air flight. And while the company has built a startling variety of aircraft, including the WWII bombers like the B-29 and mockups of the futuristic Dyna-Soar (the program was cancelled before it ever flew), it didn’t introduce a full fly-by-wire system until it launched the 777 in 1993.
Perhaps it’s the company’s longevity in the field of airplane manufacturing (or perhaps I’m reading into the company’s history too much) that led Boeing to retain the classic cockpit layout. For all its computerization, the 777’s still uses a control column, a wheel, and rudder pedals, all of which work just like they do on more conventional aircraft. The idea is to build a highly computerized cockpit around a pilot’s previous experiences with smaller and older planes that have conventional control systems like mechanized pulleys to move the rudder when he pulls back on the stick.
In keeping with this classic cockpit and the pilot’s ease of control, it doesn’t take a lot for a pilot to override the 777 computer and fly outside the envelope protection parameters in an emergency. He just needs to apply more force than the electronic system on the actuators. And when a 777 does fly into dangerous territory–too slow, for example–it will tell the pilot.
An Airbus A320 cockpit, via Wikimedia
Airbus takes a different approach. Founded in 1970, the French manufacturer debuted the A320, the world’s first commercially viable fly-by-wire aircraft 11 years later in 1981. And its cockpit doesn’t retain the vestiges of aviation from decades past. There is no centre stick or wheel in the cockpit, just a side stick by each pilot. Moving the side stick gives the computer a rate command. The manufacturer's philosophy is to have the flight computer take a heavy hand in managing the airplane. While Boeing aircraft can fly outside their limits, an Airbus can’t. The computer is designed to keep the plane flying within its structural and aerodynamic limits.
For both Boeing and Airbus, problems can arise with these highly sophisticated flight control systems do something the pilot isn’t expecting. And here’s where the whole knowing-how-to-fly-in-tandem-with-your-computer thing comes back: a pilot has to know how to command his computer, know what actions it might when sensing a problem, and know when a warning is erroneous. Otherwise, he might find himself in a bad situation.
In 2009, an Airbus operated by Air France disappeared into the Atlantic. A brief loss of airspeed indication triggered a warning from the computer, and rather than keeping the obviously airborne jet flying steady, one of the pilots throttled his engines to full speed and sought to climb. Raising the nose put the plane in an aerodynamic stall and it fell from the sky. In 1991, a Scandinavian Airlines pilot flying a DC-9 out of Stockholm throttled his engines back to clear a surge–ice from the wings had broken off and been sucked into the planes rear-mounted engines. What the pilot didn’t know was that the autothrottle had an automatic thrust restoration feature that prevented the engines from losing thrust during takeoff. Both engines continued surging until they tore themselves apart.
In the case of the 777 crash at San Francisco, the problem (according to preliminary reports) was a loss of airspeed. At the time the autothrottle was engaged, another decades-old system that should keep the airspeed constant, but whether it was in the correct mode is still unclear. The 777 has five operating modes, and it’s possible the pilot set the autothrottle in one mode and thought he was in another. It might be the only explanation for why all four pilots in the cockpit failed to notice a loss of airspeed. Because planes can’t land themselves.