Operating Systems

The New Economy


Is it Safe up There?


Suppose you are waiting at an airport to catch a flight. Suddenly you hear the news that a plane just departed and caught fire, crashed and burned possibly with many fatalities. Would you continue on your journey, or cancel?


What if instead of a plane you were about to board a bus when you heard of a bus crash? What about a car? What about a bicycle?


A lot of people stop flying after a plane crash. Death by aircraft is scarier than other forms of transportation. It is also the most uncommon. In reality, death in an air crash is probably more painless than most other forms expiry. Yet, all of us have the quite justifiable terror of being crushed while inside a high-flying, high-speed metal tube.


The Rockaways are a pretty enclave in the suburbs of New York City. Located on a thin strip of land, off the coast of New York it is lined with beaches on all sides and nice fenced houses. Suddenly, it has become the focus of international attention due to the doomed American Airlines Flight 587. The timing of this incident is rather unfortunate, coming so close to the heels of the World Trade Center disaster.


Every air disaster is scrutinized heavily, every piece of evidence, down to the last bolt is carefully examined and many months later the cause sometimes emerges from a million-piece puzzle. It is much too early to say what happened to this aircraft, but it seems to be a mechanical malfunction. AA-587 started its journey at John F Kennedy International Airport at 9:15 am on an Airbus-300. The aircraft was built in 1987 and was received by American Airlines on July 12th 1988.


Runway 31L at JFK points to the northwest. Ahead of AA-587, Japan Airlines 47, a Boeing 747-400 sped off for Tokyo. AA-587 was next, 1 minute 45 seconds later, narrowly violating the 2-minute rule for takeoff spacing. The 15-second shortening of the safety margin made the Airbus hit the “wake turbulence” of the 747 a bit harder.


Wake turbulence is one of the frightening aspects of aircraft induced wind phenomenon that every pilot is briefed upon, over and over gain. “Do not get behind or under a jet, ever”, they are told. Wake turbulence is mainly caused by “wingtip vortices” which are concentrated tornadoes that spread out and under from the wingtips of an aircraft. They spiral downwards slowly and seem to linger forever. The vortices behind a 747 can flip a smaller aircraft upside down in a matter of seconds, if the victim is close enough.


AA-587 hit the vortices of JAL-47 causing it to shudder. What happened next is still unclear. As of today (Thursday Nov.15, things can change tomorrow) the speculative story is that AA-587 turned to the left and headed south, after the shaking. According to the sounds on the cockpit voice recorder, 121 seconds after takeoff, it hit more turbulence, probably from the same JAL-47. Four seconds later, the co-pilot called for maximum power, and two seconds later, a voice said that the plane is out of control. 15 seconds after that, the recording ends. Probably, the shuddering broke loose the tail of the aircraft, something that has never happened in aviation history. The loss of the tail made the plane totally uncontrollable and it went into an uncoordinated spiraling dive. The shaking and swerving shook off one of the engines, and then another and then the plane impacted ground, amongst the serene picture-book houses lining the Rockaway coast.


Why do air crashes happen? The experts talk of human error, maintenance problem, mechanical malfunctions, weather problems and such. Yet, reality is that air crashes are so rare that there is no clear statistical pattern. Every time a plane goes down, serious changes are made in piloting procedures or maintenance procedures or aircraft design, to prevent that particular cause. It works. The following crashes are due to some other completely unforeseen circumstances.


Pilots are trained to divide their attention to many tasks and to avoid focusing on one problem. “If there is something wrong, do not get obsessed by it, keep your attention divided amongst all the other tasks”. Violating this basic maxim caused a terrible tragedy,

in December 29, 1972. At about 11:30 pm, Eastern Airlines 401 was approaching Miami International airport in the dark. The aircraft was a new Lockheed L-1011 Tristar, a tri-engined behemoth that can carry 300 people and hailed as the next generation superliner. At the controls was Captain Stockhill, with over 30,000 hours of flying experience. Miami control tower was babying the flight to the runway. As the plane turned for the final approach tower instructed, “Eastern 401, heavy, continue approach to runway 9L”.


The captain flipped down the landing gear. There was the reassuring thud of the undercarriage extending and locking. The indicator light for the nose gear should have come on but it did not. For the next minute or two, the captain and the co-pilot fiddled with the levers. While everything felt fine, the absence of the light obsessed them. They informed the tower of the decision to abandon landing, perform a “go around” and circle till they resolved the issue. “Eastern 401, heavy, roger” said the tower controller, “pull up, go to two thousand feet….”


The plane was put on autopilot. The captain worked on unscrewing and disassembling the indicator light. The co-pilot climbed down the avionics bay to look at the landing gear. Nothing conclusive was found, and they decided to attempt a landing.


Eastern 401 never landed. While the two were fiddling around, the captain had accidentally pushed on the yoke and the autopilot had kicked out. The plane started descending at the rate of 200 feet a minute, a rate so gradual that it is not noticeable. Below were the Everglades a huge swath of swamp, totally dark. Exactly 10 minutes after the fiddling started, the pilots sat down and looked at the instruments. The altitude was almost zero. The cockpit voice recorder recorded the last words “Hey, what is happening here”. Three seconds later the plane impacted the marsh.


The airline disaster stories are fascinating yet grim. Every one seems to have a completely different set of circumstances. Singapore Airlines (Oct. 2000) lost a Boeing 747 when the captain made a wrong turn on the ground and started the takeoff on a closed runway. Alaska Airlines 261 (Jan. 2000), an MD-11 nose-dived into the ocean off the California coast, as the elevator mounted on the tail stopped working. TWA 800 (July 1996), a 747 exploded off the coast of Long Island amid much controversy. The official cause is a fire in a center fuel tank, however persistent rumors say a missile hit it. ValuJet (May 1996) lost a DC-9 when it blew up because oxygen in the cargo hold caught fire. Air India 349 (Nov. 1996) a 747 collided with a cargo plane in mid-air, near New Delhi. Delta (Aug. 1985) lost an L-1011 as it was slammed into the ground by a microburst (a kind of weather phenomenon with vertical winds).


Statistics on commercial airline accidents do not show much of a pattern. The deaths in American skies were 0 in 1993, 228 in 1994, 152 in 1995 and 270 in 1996. In 1996 there were 21 accidents, of which 3 were fatal. Of the 270 dead that year, 230 were due to TWA-800. In that year aircraft flew a total of 13 million hours and traveled 5 billion miles.


In the same year, general aviation had 1,681 accidents of which 323 were fatal and 578 people died. Of the fatal accidents, the largest number was due to “collision with object or terrain” which generally means gross pilot error (80). The next largest was “non-mechanical engine failure” which generally means running out of gasoline (19), followed by mechanical engine failure (14) and the scary “mid-air collision” (12). The aircraft in this category flew 24 million hours.


If we move on to statistics about cars, the numbers becomes disastrous. In the year 2000 41,821 people died in car accidents and 3,189,000 people were injured. Of the deaths, 16,653 were due to alcohol related impairments and 9,873 due to rollovers. More people have died on American highways, since September 11th, than all those who died in the World Trade Tower tragedy. Of course, more people travel by car and more time is spent in cars, than airplanes and hence exposure is higher.


Airline disasters are here to stay. Pilots go though grueling training and refresher courses with a primary focus—safety. Checklists are prolific to combat any forgetfulness. Maintenance crews face similar drills, and the level of documentation needed for every repair to aircraft is excruciatingly painful. Aircraft designers and manufacturers spend ungodly sums of time and money on safety considerations.


Yet, accidents happen and will continue to happen. Humans and machines can never be made totally infallible. Reaching the extremes of reliability and error free operations have been the goal and will continue to be. In spite of the recent tragedies, we may have already reached close to the pinnacle of achievable safety.


Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha



Partha Dasgupta