Chapter 4
Lottery Aloft

My first assignment at NAFEC was the independent technical review of the "Dual ILS" (instrument landing system) scheduled for installation at the then-busiest airport in the world, Chicago Midway.

The idea was simple enough and enjoyed Henderson's full support: put in a parallel runway, to double the capacity for accepting arriving traffic -- to clear out the stacks.  The FAA had previously engaged statisticians to analyze field data from the single landing system already in operation at Midway.  For months, observers were paid to stand out in the Illinois wheat fields during the worst meteorological conditions.  They stared up into the dank sky and strained to hear each Chicago-bound airliner as it approached the airport.  Thus did the researchers acquire statistical data from lateral measurements at stations along the approach course.

As one might expect, electronically guided airliners, groping their way blindly through the clouds toward the runway, do not always fly exactly down the centerline defined by the radio beam ("localizer," it's called).  Some of the data showed a large steering error, particularly at observation points most distant from the touch-down zone.  If two such guidance systems were to operate side-by-side, those errors could result in fatal conflicts.

The official analysis, which we were asked to review, concluded that the Dual ILS would be safe -- that, with a planned runway separation of 3,600 feet, "there would be no more than one conflict every 40 years."

John McLaughlin, one of our mathematicians looked over the analysis, giving special attention to the statistical assumptions.  One was crucial -- that the field data fit a "normal distribution."  It permitted simple mathematical interpretation of the data.  Accordingly, all the other results in the FAA's report ramified from that assumption, including the conclusion of 40 years between collisions.  And it was wrong!  McLaughlin rendered the assumption invalid one afternoon by making a pass over the field data using a desk calculator, performing a "chi-squared analysis."

"It doesn't necessarily mean the conclusion is flawed," said McLaughlin.  "Statistically, mid-air conflicts might not happen for, say, 50 years or even more.  The data just does not yield to 'closed form' methodologies."

That was where I came in.

When I was an undergraduate at UCLA in the early fifties, I worked with Dan Gerlough of the Institute for Transportation and Traffic Engineering.  In one project sponsored by the State of California, we postulated a hypothetical vehicular metering systems for freeway on-ramps.  You know them today as one-car-per-green signals ("metering lights," on the morning radio).  The idea was counter-intuitive if not radical.

It raised the question, How do you speed up traffic by slowing some of it down?  Thanks to the SWAC computer at UCLA and what is called Monte Carlo analysis, Gerlough and I were able to make a convincing case.  The rest, as they say, is history.

That first week at NAFEC, I told McLaughlin about the Monte Carlo method.  He and I wrote a computer program that accepted the raw field data in the form of spatial "histograms."  The program applied them as numerical templates for a large quantity of simulated aircraft approaches.  Each arrival time and steering error was perturbed randomly within the constraints of the "model" -- a lottery aloft.

My contribution was limited to writing a random number generator.  The word "software," by the way, was not yet in wide use, even among computer folk.  Hard to imagine there would be much challenge to create -- or use for -- a program that produces numbers which have no relationship to anything.  Still, my "roulette" algorithm was picked up as the first of its kind in a paper published in the proceedings of the fledgling Association for Computing Machinery.

The only computer available to us was the RW-300, a first-generation machine with a magnetic drum memory.  Slow as it was, we could nevertheless simulate the approach-to-landing of airliners at the rate of fifteen pairs per second -- 2,000 times faster than real life.

McLaughlin and I finished checking out the program late one evening.  Our plan was to let the thing run through the night.  That would correspond to a dozen years of rush-hour operations at Midway.  We would review the results in the morning.  A few more days of continuous computing would give us 40 years of simulated operations.  I shut off the lights in the computer room and made for the door.

McLaughlin was waiting for me in the hallway.  We stood for a moment looking through the observation window into the computer room.  Digital technology was as much a marvel to us as to any of the high-schoolers who often came to our laboratory on field trips to gaze through that window and observe "giant brains" at work, tirelessly executing -- oh, my -- thousands of instructions every second.  The lights flickered eerily on the computer panel.  John and I turned to leave.

"What's that?" he asked suddenly.

The Flexowriter had come on.  It had been powered up by our program and began typing out a message.  With McLaughlin at my heels, I dashed back into the room and tore the paper off the platen.  We already had our first collision.

Statistical anomaly?  Not quite.  The next day there was a small pile of paper on the floor behind the Flexowriter.

Soon, I became absorbed in the design of the computer-driven displays for the man-machine simulator.  McLaughlin wrote the Dual ILS report and took most of the heat from our client.  Weeks later, the review, which declared that a runway separation of 3,600 feet was not enough, put NAFEC into an uproar.  McLaughlin told me that at the final show-down meeting, one of the FAA's section managers, a former tower chief named Henderson, actually said these chilling words: "We asked you to confirm our results, not challenge them."

So much for the canons of evidence and proposition.

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