Clouds, Computers and Composites: The New Crisis in Aviation

By Manuel Garcia, Jr. · 4 Jul 2009

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Picture: littlepois
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The loss of Air France Flight 447, an Airbus A330-200 has raised many doubts among the flying public and even some aviation professionals about the safety of the newest generation of passenger airplanes. These new airliners have composite materials replacing metal for many structural elements and control surfaces, and they are reliant on computer-controlled flight and navigation systems.

The impetus for developing this new generation of airliners is the need to improve fuel economy so as to maintain the profitability of the passenger air transport industry. Between 1986 and 2001, the world price of crude oil remained steady at near $22 a barrel. From 2002 to 2008, the world price of crude oil rose steadily from $25 to $95 a barrel (the prices quoted are rough averages, and in 2007 dollars). Modern airliners that are lighter and stronger than their older-generation all-metal size equivalents can carry more payload with less fuel consumption, and this translates to economic sustainability.

The quest for a more efficient airliner began with the first example of the type, Igor Sikorsky's S-22 of 1913, the Ilya Muromets, a four engine biplane with an enclosed cabin for 16 passengers. Russia's military needs in WW1 swallowed up the commercial potential of the S-22, and the production was shifted to bombers. The post-war rebirth of commercial aviation began with the Farman twin engine biplane transport of 1919, the F-60 Goliath, seating 14 passengers. Since then the quest for "better, faster and cheaper" passenger aviation has never stopped.

In 1972, Airbus introduced its A300, the first twin turbofan widebody air transport. The Boeing Commercial Airplanes company introduced its first widebody twin turbofan airliner, the 767, in 1981. Airbus chooses to be an airplane manufacturer that leads the industry in the application of engineered materials (composites) and computer-controlled aviation. Boeing is an airplane manufacturer that seeks to maintain its reputation for robust, reliable and increasingly efficient designs, which it gained early in its history with airplanes like the revolutionary 247 of 1933, the first truly modern airliner (all-aluminum monoplane of semi-monocoque construction with cantilever wings, wing flaps, retracting landing gear, trim tabs, autopilot, and deicing boots for the wings and tailplane).

Airbus and Boeing are today's main competitors for new airplane orders worldwide. As noted earlier, it is the cost of fuel that drives the economics of commercial air transport, and in turn the replacement of older aircraft with newer models. The competing demands of safety, reliability, strength, carrying capacity, volumetric efficiency, speed and fuel economy drive airplane designers toward a convergence of characteristics, so that today both Airbus and Boeing airliners look, sound and feel largely the same to most passengers.

Each iteration of a manufacturer's model type will have a higher proportion of weight-saving composite material, and a more extensive array of electronic and computer systems. How and where composites and computers are used by Airbus and Boeing may be quite different between their competing models of comparable type, but inevitably both manufacturers increase their use of both composites and computers, to remain competitive. The Airbus A330 and A340 series of airplanes, introduced in 1992 and 1993, and their classmate the Boeing 777, introduced in 1995, will be replaced by the Boeing 787 Dreamliner, set for introduction in 2010, and the Airbus A350, set for introduction in 2013. Both the 787 Dreamliner and the A350 are nearly all-composite airplanes. The 787 Dreamliner is 80% composite by volume, and by weight it is: 50% composite, 20% aluminum, 15% titanium, 10% steel and 5% other. By weight, the A350 is: 53% composite, 19% aluminum and aluminum-lithium, 14% titanium, 6% steel and 8% other.

The challenge facing the civil aviation industries today is to answer the questions raised by the mysterious loss of Air France Flight 447, and to convince the public that any problems that may be uncovered about the use of composites and computers in AF447 will be fully understood and solved before building and flying all-composite airliners with even more complicated computerized control systems.

So, it is no wonder that Boeing Chief Executive Jim McNerney defended electronic flight control technology and the Airbus A330, in an interview prior to the Paris Air Show: "The causes of the [AF447] accident are unknown, and I don't think there is any link with a serious fault with the aircraft...the A330 is a reliable and proven aircraft."

The AF447 crisis in civil aviation may be similar to that of the two de Havilland Comet crashes of 1954. The Comet, introduced in 1949, was the world's first passenger jet transport. During both January and April of 1954, de Havilland Comet airplanes broke apart at altitude while flying in clear weather over water. After the second crash, the fleet was grounded, many pieces were recovered from the seabed to assemble partial reconstructions, and many tests were conducted on another intact airframe. The cause of spontaneous disintegration was eventually found to be metal fatigue in the aluminum alloy used for the skin, by the cumulative effect of many cycles of cabin pressurization and de-pressurization.

The changes in design, materials and manufacturing techniques needed to solve the problems of the de Havilland Comets of 1949-1954 were used to produce an improved Comet, which returned de Havilland to passenger aviation in 1958. However, those same lessons had already been divined by Boeing to produce the 707, their first commercial jet transport, which was also introduced in 1958 and immediately went on to dominate passenger air transport through the 1960s.

If the air transport industries fail to fully resolve the AF447 mystery, then a portion of the public will assign an apprehensive image to the coming generation of composite computer-controlled air transports, a psychology we could think of as "'54 Comet dread," as opposed to "'60s 7-0-7 optimism."

AF447 In The Clouds

In the early pre-dawn hours (~2:15 UTC) of 1 June 2009, Air France Flight 447 from Rio de Janeiro to Paris fell out of the sky into the Atlantic Ocean near the equator about midway between Brazil and Senegal, with the loss of all 228 people aboard. The aircraft was one of the most modern, a dual engine Airbus A330-200.

UTC is Coordinated Universal Time, which replaced Greenwich Mean Time in 1964 and is defined for the time zone straddling much of 0 degrees longitude. There are 24 time zones each generally of 15 degrees longitude, but there are numerous deviations of time zone boundaries.

The accident occurred after the airplane had flown over 90% of its planned northeast-directed transect of 227 km through the width at mid length of a mesoscale convection system (MCS), a cluster of storms 800 km long east to west, and 160 km wide north to south.

The last radio message from the crew of AF447 was a routine notification at 1:33 UTC that the flight at 35,000 feet (10,671 m) along oceanic high altitude route UN873 had reached waypoint INTOL, near the outer boundary of airspace monitored by radar from Brazil. The pilots of AF447 expected to reach waypoint TASIL, near the edge of airspace radar-monitored from Senegal, in 50 minutes (by 2:23 UTC), a distance of 663 km between waypoints. A mid Atlantic gap of at least 500 km exists between the limits of Brazilian and Senegalese air traffic radar surveillance.

Between about 1:46 UTC to 1:56 UTC, AF447 flew through the western fringe of the top of a storm that reached to between 35,000 feet (10.67 km) and 40,000 feet (12.2 km). It seems AF447 had shifted somewhat to the left, or westward, from its planned flightpath in order to avoid the brunt of this storm, and in anticipation of weaving between storm cells ahead. Thunderstorms in the tropics are usually very localized, of short duration, and produce abundant rainfall. They can develop so quickly that a Paris-bound flight 4 hours out from Rio de Janeiro might encounter an active storm cell over a patch of ocean that had been cloudless prior to takeoff. This is why airplane weather radar had been developed, to alert pilots of weather threats ahead, and to guide their weaving between active storm cells when they became unavoidably embedded in weather systems with numerous storms. After crossing about 42 km of clear airspace, AF447 entered the main MCS thunderstorm cluster, at about 1:59 UTC.

A sequence of satellite images of the MCS cluster show the large 800 km by 160 km (roughly) cloud mass with its variegated edge, drifting, evolving and fragmenting during that day. These images show the merged shape seen from above of the laterally spreading "anvil" tops of the many individual storm cells in the cluster. The updrafts in these cells had sufficient energy to push moisture up to between 40,000 feet (12.2 km) to 56,000 feet (17.1 km). Moisture that rises into the base of MCS clouds, perhaps near 3281 feet (1 km) at 20 C (68 F), can be chilled by strong updrafts to arrive at -40 C (-40 F) at the 10.67 km cruising altitude of AF447, and continue rising and chilling to as low a temperature as -80 C (-112 F) at 17 km elevation. This storm cluster was typical, not unusual, for the location and time of year.

AF447 proceeded northeast through the MCS cluster, guided by its weather (moisture, rain, hale) radar along a corridor of mild radar reflectivity (and of anticipated least relative 'storminess'), about equidistant between a strong cell to the west and the strongest cell of the moment, which was about 30 km east. About 8 minutes after entering the MCS system (2:07 UTC), AF447 began penetrating what was probably the most energetic part of the storm cluster along its flightpath.

At 2:10 UTC, the first of a series of automated signals was sent by AF447's onboard computerized maintenance system, via satellite, to Air France computers in Paris logging maintenance information. The series of automated messages had a combined time span of 1 minute and occurred until 2:14 UTC; 5 failure reports and 19 warnings were transmitted. The earliest automated messages reported on the failure of the Pitot Tube sensors, which measure the airspeed of the airliner and provide an estimate of altitude based on the static pressure of the atmosphere. Subsequent messages in the initial burst indicated that the auto-pilot (automatic 'steering') and auto-thrust (automatic 'gas pedal') systems had been disengaged, the collision avoidance system (to detect other nearby airliners) had a fault, that the flight control computers (three for redundancy) had shifted to an "alternate" mode where they made fewer automatic adjustments to the airplane's control surfaces, and placed fewer limits on the range of manual inputs by the pilots that would be implemented as motions of the control surfaces (ailerons, rudder and the many types of flaps). (5)

From 2:11 UTC to 2:14 UTC, messages indicated the failure of the gyroscopes (air data inertial reference system, ADIRU, used to provide the artificial horizon orienting the sense of 'up,' 'down,' and 'level,' essential during nighttime) and resulting faults in the instrument panel displays (screens and electronic images instead of mechanical dial gauges); there was disagreement between systems that interpreted air data (such as for airspeed and angle of attack of the wings into the airflow); that a fault had occurred in the flight control computer system (that transmits commands to the hydraulic actuators that physically move control surfaces); that a fault had occurred in the computer system that captures and processes pressure and electrical outputs from air and motion sensors that supply data; and finally, a "cabin vertical speed warning" indicating a rapid loss of cabin air pressure, due to either a rapid descent or a breaching of the cabin shell.

AF447 may have entered its period of most severe jolting, buffeting and external cooling near 2:07 UTC, when it began crossing the core of the MCS cluster between its most active cells. Some as yet unknown excessive structural strain -- perhaps exacerbated by material embrittlement or loss of plasticity and cohesion due to excessive cooling, such as by micro-strains induced by the expansion of trapped moisture freezing inside composite materials -- may have been delivered by turbulence and initiated the subsequent fragmentation of the aircraft.

Pressure sensor icing sustained during at least the three minutes prior to 2:10 UTC seems to have initiated the cascade of air data (speed, pressure, altitude and attitude) processing and instrumentation failures, and contributed to the growing uncertainty of the decision-making electronic processing for the navigation and flight control systems.

Pilots rank their priorities during flight, especially in emergencies, as: "aviate, navigate, communicate." The pilots of AF447 would be working first to keep their airplane at a proper speed: fast enough to stay aloft at the given elevation and weight of the airplane, and not too fast to damage the structure because of excessive pressure differences produced by airflows near the speed of sound, and by excessive structural stresses induced by the alternating jolts of updrafts and downdrafts in turbulent air spaces. Given that the aircraft remains aloft and is not being rattled to pieces, the next priority is to point it in a safe direction, for example away from active thunderstorm cells, and along the best route to a safe landing. The third priority is to communicate the status of the flight to air traffic controllers, a useful task as long as it is not a distraction from essential aviating.

Troubleshooting a torrent of error messages from a computerized flight control system to then compose a radio report for air traffic controllers is not a sensible allocation of attention during an emergency to control an airliner in a storm. We can understand why the crew of AF447 might not send any radio messages during their 3 minutes (and possibly as much as 11 minutes) of weaving between the storm cells and riding the waves of turbulence, before the first automated alarm of trouble was transmitted at 2:10 UTC. At this point, AF447 had crossed 154 km of the MCS cluster, the last 42 km of which were probably the roughest. During the next 4 minutes, when the automated messages were sent, the flight probably travelled 56 km. At 2:14 UTC, AF447 was about 2 to 3 minutes (28 km to 42 km, at 14 km/minute) from exiting the northern edge of the MCS cloud system, and it sent its last transmission.

AF447 Into The Sea

The search for AF447 began at 2:23 UTC. Brazilian air traffic controllers called their Senegalese counterparts when they failed to receive the expected confirmation that AF447 had announced itself to Senegal by radio, as required upon entry to a new airspace. The Brazilian Air Force dispatched search planes, a Spanish maritime patrol plane searched southwest from the Cape Verde Islands, and the search effort quickly expanded in the following days to include Brazilian naval vessels, cargo ships within the search area, French military planes and ships, and satellites.

Fernando de Noronha is an archipelago of 21 islands 354 km (220 miles) northeast from the eastern tip of Brazil. AF447 flew past (~1:18 UTC) and to the west of Fernando de Noronha on route to waypoint INTOL at 565 km (351 miles) from the coast. At about 2:44 UTC, the pilots of a TAM Airlines flight from Europe to Brazil reported observing "orange dots" on the surface of

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