Although airplanes equipped with FBW systems are reportedly easier to fly, the very efficiency can conceal defects that might be otherwise discovered by hands-on mechanical operations and may allow a plane to be operated under conditions where a human operator would fail.
The accident rates for Boeing and Airbus are similar: however, there have been some unusual Airbus accidents apparently caused by computer malfunctions. One of the first occurred in 1988 shortly after the Airbus was placed in service. During a flyover at a French air show, the computer assumed that the plane was supposed to land since it was close to the ground and the landing gear was down. Although the pilot attempted to accelerate and climb, the computer ignored his input and landed the plane in an adjacent forest killing three passengers. Airbus attempted to blame the accident on pilot error.
Another incongruous accident more recently occurred during the testing of a brand new 472-passenger Airbus A-340-600 being delivered to Etihad Airlines in 2007 at the Toulouse airport. As the flight crew ramped up the four engines to takeoff power with the brakes on, a takeoff warning horn sounded because the computer sensed that the plane was not properly configured for takeoff. When the crew silenced the alarm, the computer apparently decided the plane was flying and trying to land with its brakes on. The computer released the brakes and the plane accelerated into a crash barrier at full power.
The spectacular televised landing of a JetBlue Airbus at the Los Angeles airport in 2005 with its nosewheel locked in place crosswise to the fuselage brought to light at least 67 earlier "nosewheel failures" on a variety of Airbus aircraft that were usually repaired by the replacement or "reprogramming" of the Brake Steering Control Unit computer.
A rudder design implemented by Airbus in 1988 increased the sensitivity of actual rudder movement to the pilot’s movement of the pedals by slightly more than one inch and allowed for a wider degree of rudder travel per pound of force on the pedal. Rudder movement is necessarily restricted at cruising speeds; however, the Airbus computer did not impose a limit at lower speeds, such as during takeoff.
These rudder changes contributed to the crash of American Airlines Flight 587 on November 12, 2001 shortly after takeoff from Kennedy Airport in New York City when the aircraft encountered wake vortices from the preceding aircraft. As the copilot attempted to maintain the Airbus’ steady-state left turn, he sought to correct an unexpected, vortex-caused "overbank" by using the rudder attached to the back of the tail fin. The copilot commanded rapid left-right rudder movements that exceeded the design loads of the vertical stabilizer, and the computer was not programmed to limit the command at low speeds. The all-composite stabilizer was ripped from the fuselage and the aircraft became uncontrollable. Its crash killed nine crew members, 251 passengers and five people on the ground. The relatively intact tail fin was found floating in the waters of Jamaica Bay.
Although several catastrophic Airbus crashes into the ocean with major loss of life have been blamed on pilot error, including the 2000 losses of Kenya Airways Flight 431 and Gulf Air Flight 072, the crash of an Airbus belonging to Air New Zealand on November 27, 2008 into the Mediterranean Sea has raised new questions about Airbus safety. Seven crew members engaged in a test maintenance flight died in the crash, and the tail section was found floating where the plane went down. No official cause for the accident has been reported.
One month previously, an accident aboard Qantas Flight 72 on October 7, 2008 that injured 106 of the 313 passengers was apparently caused by a malfunction of the FBW system. While traveling at 37,000 feet, the computer reported an autopilot irregularity and trouble with the inertial reference system.
After the Airbus A330-300's autopilot was disengaged, the computer caused the aircraft to suddenly pitch down and rapidly descend 650 feet in 20 seconds before the pilots could regain control. Three minutes later, the computer again caused the plane to pitch down and descend 400 feet in 16 seconds. The crew declared a Mayday and made an emergency landing at the Learmonth airport.
Preliminarily, the "likely origin of the event" has been blamed on the failure of an Air Data Inertial Reference Unit that supplied incorrect data to other aircraft systems. The Unit may have falsely reported that the airplane "angle of attack" was very high resulting in the flight control computers commanding the nose-down movements, or the computer may have believed that the plane was going too slow and put it into a dive to increase its speed.
In the late evening hours of May 31, 2009, Air France Flight 447 operating an Airbus A330 carrying 216 passengers from Rio de Janeiro to Paris was four hours into its flight and was above the Atlantic Ocean about 400 miles off the coast of Brazil. Its speed was about 550 mph at an altitude of 35,000 feet when the pilot reported that the plane was approaching a towering thunderstorm containing black, electrically charged clouds. Satellite data showed the thunderstorm was sending 100 mph updraft winds as high as 41,000 feet.
Ten minutes later, Air France received a four-minute series of automatic failure and warning messages from the Airbus’s Aircraft Communication Addressing and Reporting System, relayed by satellite, reporting serious problems aboard the aircraft. The autopilot had disengaged and control had shifted to the pilots. The computerized control system had switched to alternative power, and there was a deterioration of flight control systems. Then came warnings that the systems to monitor air speed, altitude and direction had failed and that there was a failure of wing spoilers and the main flight computer. The final message reported a complete breakdown of the electrical and pressurization systems as the plane apparently fell apart and plunged down almost seven miles. The helpless terror experienced by those aboard the aircraft during the long seconds it took to strike the ocean surface is beyond imagination.
That final message also reported faults with the plane’s Air Data Inertial Reference Unit that, among other things, provides speed warnings. In addition, as a result of earlier incidences involving a loss of airspeed data during the cruise phase of Air France A340s and A330 and recent tests, it had been determined that icing of the external speed monitors known as "Pitot tubes" could be a factor in a loss of speed data at high altitudes.
Although Airbus had issued a recommendation in September 2007 to replace the tubes, replacement was not considered a mandatory safety concern. Air France did not commence the replacement of the airspeed indicators with an improved Pitot tube in its fleet of A330s until April 27, 2009, and it did not get around to the aircraft operated by Flight 447 before it encountered a violent thunderstorm over the Atlantic Ocean in the middle of the night.
Irrespective of the cause of the "inconsistency in measured air speeds," the inability of the flight control computers to accurately calculate speed while flying at a high altitude could have contributed to the disaster. If it was falsely believed that the airplane was going too fast, particularly if the plane had already been slowed down to enter the thunderstorm, the plane could have easily stalled and a recovery in a storm would have been difficult or impossible. Or, if it was falsely believed that the speed was too slow and a stall was imminent, an unnecessary increase in speed could have taken the plane beyond its design capacity.
The plane’s tail fin was found floating in the ocean indicating that the aircraft broke up in midair. Otherwise, the plane would have been torn into small pieces and sunk immediately when it struck the ocean surface. In addition, 44 bodies have been recovered thus far from the ocean surface, some of which were separated by as much as 53 miles, also indicating a midair disintegration of the aircraft.
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