From the Comet and Airbus to the Spaceplane: The Future of Commercial Aviation

First flown in 1949 and introduced into passenger service in1951, the Comet was the first pressurized, jet-propelled commercial aircraft. Powered by four "Ghost" turbojet engines, the Comet was found to be fuel efficient above 30,000 feet and flew at almost 500 miles per hour, far faster than the most advanced piston-powered airplanes in service at the time.

England’s de Havilland Company rapidly gained a significant advantage in the commercial aircraft market, carrying more than 30,000 passengers and receiving orders for 30 Comets in the first year; however, serious problems with the innovative design quickly developed. Two crashes in the first year in Italy and Pakistan were likely caused by a defective wing profile design that resulted in a loss of lift during steep takeoffs.

A series of catastrophic crashes followed. In 1953, structural failure of the airframe beginning with the stabilizer caused a Comet to crash shortly after takeoff in India. The Comet was equipped with fully powered flight controls that were criticized because they resulted in a loss of "feel" and may have caused excessive stress on the flight control surfaces. Later in 1953, another Comet exploded in midair during a storm over India with the loss of all passengers and crew. The following year, in 1954, two more Comets experienced midair explosive decompression and fell into the Mediterranean killing everyone aboard.

Prime Minister Winston Churchill grounded the fleet saying, "The cost of solving the Comet mystery must be reckoned neither in money nor in manpower." The Comet airframes were subjected to extensive testing that ultimately identified the most likely cause to be metal fatigue caused by stress and strain on the aircraft skin caused by repeated cycles of pressurization.

The first series of Comets were scrapped and modifications were made to the second series; however, the fleet remained grounded until the fourth series was introduced in 1958. Although the plane became the first jet used for transatlantic service, de Havilland had already lost its competitive advantage to Boeing, Douglas and other U.S. manufacturers, who profited from the Comet experience. The last Comet was delivered in 1964, and even the government-owned British Overseas Airways Corporation began to fly American aircraft.

Commencing in the mid-1960s, a consortium of European aircraft firms began to collaborate in an attempt to break the lock held by American manufacturers on the commercial aircraft market by agreeing to collectively manufacture a low-cost "airbus" to transport smaller numbers of passengers over shorter distances. Underwritten by the governments of England, France and Germany, the Airbus was intended to be the first mass-produced "fly-by-wire" (FBW) airliner.

Although pilot control of commercial aircraft had progressed beyond the direct use of cables and pulleys to move aircraft control surfaces by relying on hydraulics and electrical assistance, the introduction of electronic control of commercial aircraft increasingly shifted responsibility from human pilots to computers.

First developed by NASA to augment control of the space shuttle and high-performance military combat planes, FBW technology is similar in some respects to the anti-lock braking systems (ABS) on modern motor vehicles that prevents wheels from locking when the brakes are applied and which automatically controls the allocation of braking between the front and rear brakes. Relying upon sensors on each wheel, the hydraulic pressure to each can be increased or decreased up to 20 times per second, far beyond the abilities of any human driver. However, under conditions other than smooth dry pavements, such as deep snow and gravel, ABS can be far less effective than an experienced operator. Additionally, drivers of ABS equipped vehicles tend to overcome the safety benefit by driving more aggressively.

Airplanes that are flown by "wire" still have a stick, rudders, throttles and brake pedals; however, these controls are only connected to sensors that provide "input" to computers that pass along the information to other computers located at or near the control surfaces, engines or wheels to actuate the desired mechanical response. A software program takes the pilot’s input into consideration; however, it is the computer that controls the aircraft. Relying upon the entire range of sensors, the computer can make as many as 40 adjustments per second.

FBW control over the aircraft presents a new set of problems that can have an effect on aircraft safety. Since the pilot can no longer "feel" the control surface response through the mechanical system, there is a risk that the surfaces can be over stressed due to excessive movement, or that the computer may erroneously decide that the pilot is wrong and that it knows best what is better for flight safety.

Aircraft designers decide the limits of the planes’ performance and program the computers to prevent the pilots from exceeding these limits. The Airbus is designed with very hard limits, while Boeing takes a softer approach. According to the Seattle Post-Intelligencer, John Cashman, Boeing’s director of flight-crew operations, said, "It’s not a lack of trust in technology. We certainly don’t have the feeling that we do not want to rely on technology. But the pilot in control of the aircraft should have the ultimate authority." Cashman also believes that hard limits reduce a plane’s absolute capability. For example a Boeing 747 tumbled out of control over the Pacific Ocean in 1985 and the pilots were able to recover by subjecting the plane to four times the force of gravity. The stress caused by emergency maneuvering of an Airbus is limited to 2.5 times the force of gravity.

Both Boeing and Airbus depend upon FBW technology in aircraft design; however, there are fundamental differences. Basically, a pilot can override the computer in a Boeing aircraft, while Airbus pilots are not allowed to second guess the flight control computer. Boeing pilots also receive greater visual feedback from control surfaces by relying upon a conventional control yoke, while Airbus pilots use a small joystick.

A Boeing pilot can turn the airplane upside down, release the controls and the plane will right itself. If an Airbus pilot wants to lose lift and stall to avoid a midair crash and the computer decides that acceleration and a climb is better, the pilot simply hangs on for the ride. Only if all electronic systems fail does the Airbus default into a "manual backup" mode allowing limited use of basic mechanical systems while the pilots attempt to determine the cause of the electrical and computer failure.

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