In railroading, this is called "pathing" the trains. An essay on the Allegheny crossing of the Pennsylvania Railroad noted, fifty years ago, that a one-minute delay to a train in the overnight passenger fleet would stab the sixth train behind by eight minutes.
Any runway has a finite capacity. The key to understanding this is in understanding time. Only one aircraft is allowed to use the runway at any time. It takes a certain amount of time for a departing aircraft to taxi onto a runway, accelerate to flying speed and lift off. Likewise, it takes a certain amount of time for an arriving aircraft to touch down, slow down and taxi off the runway. The time it takes the typical airliner to do either one -- land or takeoff -- is roughly one minute.
The math is as simple as it is inescapable. Roughly 60 airliners can use a runway in one hour if conditions are absolutely perfect. It is physically impossible to improve that number. However, it can get a lot worse.
That's loosely how that delay to the sixth train happens. But the train engineer often has two things going for him (three, if the controller's post is also saying something about route knowledge) that the pilot doesn't. Perhaps the train has cab signals, which many engineers have described as a great help in order to be able to maintain track speed in rain or fog. And the switch to a different track does not come as a surprise in signalled territory, as there are two or three signal aspects before the train has to change direction. Pilots apparently don't get an "approach limited" followed by a "diverging clear" aspect in advance of the taxiway the ground controller would like the plane to use. (Could that be solved relatively cheaply by putting some additional lights in the center of the runway?) The route knowledge works like this: an experienced engineer has learned that a ten-pound reduction going past the "approach limited" followed by a release as the train goes over the Six Mile Road crossing means the train will negotiate the crossover at the diverging clear at precisely the required speed. (And yes, he or she had better sound the 14-L for that Six Mile Road crossing in time to alert a driver who isn't expecting a railroad crossing just ahead in that rain or fog.) I would think that an experienced pilot would have similar familiarity with the airport: runway 27, five taxiway exits to port, cross runway 21, five more taxiway exits to port, nominal approach under dry conditions means check at the first taxiway past the crossing to turn into the second one.
It helps in understanding all this if you’ll focus on the simple, physical acts required in aviation instead of dwelling on the confusing complexities. Imagine it is foggy at the airport. An aircraft landing on the runway has to find a taxiway to get off the runway just as you must find the entrance to the parking lot when you’re driving down a foggy street. In good visibility, you can see the entrance from a distance and slow down at the last second. In poor visibility (rain, fog, snow) you must slow down to ensure you have enough time to spot the entrance and then make the turn. Every extra second an aircraft spends on the runway -- searching for the taxiway -- decreases the capacity of the runway. There is another aircraft flying towards the same runway and air traffic controllers cannot allow it to land until the first aircraft is clear of the runway.
That, however, is not the main point the controller wishes to make. Rather, he's got to maintain safe following distances (and if the planes are queuing to land, at speeds close to stalling speeds, sometimes over great distances.)
What the accident investigations, the cockpit voice recorders and the air traffic control tapes don’t reveal to you is the enormous pressure the people in aviation work under. Pilots, air traffic controllers and even airline CEOs are under constant pressure to make the airplanes fly and to make sure they fly on time. The pressure to fly in poor weather, to tighten up the spacing between aircraft and to wring every last drop of efficiency out of the system is incredible.In railroading, there's a practice called "riding the yellows" that everybody learns about, we hope not the hard way. A following train might lose less time by treating each approach signal as though the next one will also display approach, and the trick works until the next one is displaying stop, and the marker of the preceding train is just past that stop signal. Tightening the spacing leads to broken equipment and dead people, just as dead as if they fell from the sky.
That gets to the heart of the controller's complaint.
First and foremost, as I hope I have shown you, there is an absolute limit to the number of airplanes any runway can handle, per hour, even in perfect weather. At an absolute minimum that limit should be enforced -- by rule and regulation -- for every commercial airport in the country. Currently it is not and -- unbelievably -- airlines are allowed to schedule more flights than the runways can handle in even perfect weather. It is madness.No, it's not madness. And it's not greed.
The reason is as old as it is simple -- greed. Airlines can make more money selling 70 airplanes worth of tickets per hour than they could if they limited themselves to the 60 airplanes per hour that the runway can handle. In fairness to the airlines, it’s not in their interest to limit themselves. It is easier to sell the tickets and blame the delays on the weather or the “antiquated” air traffic control system. Especially if the flying public doesn’t understand runway capacity limits and therefore fails to notice that the “antiquated” air traffic control system is delivering more airplanes to the runways than the runways can handle.It's really the absence of properly-defined property rights, as the controller next shows.
That second solution strikes me as practicable whether or not Congress acts. Britain's Network Rail has to assign paths to competing train operators in such a way that the trains run to time, or close to time, and it gets so precise that steam tour operators can only run steam from London Kings Cross before six in the morning as the only spare paths later in the morning were for faster trains. A diesel-down steam-up option satisfied those constraints. The fact that several carriers are negotiating for the paths precludes one inventive strategy: no airline is able to contract for exclusive use of arrival, departure, and airspace slots by committing to run fewer planes. In a dire emergency, as the controller notes, that 8.37 Northwest to Fargo may push back at 8.37 but it won't be wheels-up at 8.57.
The government has abdicated its responsibility in this area. The economic portion of the aviation industry was deregulated in 1978. The safety portion -- supposedly -- was not. The Federal Aviation Administration has the legal authority to limit the number of flights into Kennedy, Newark, LaGuardia, Chicago O’Hare and Washington (D.C.) National airports. The FAA should be given the same authority for all commercial airports. And Congress should compel them to use that authority. Currently they do not. The FAA was forced into lifting the slots restrictions at JFK and the result was predictable -- massive delays. The FAA reimposed slot restrictions at Chicago O’Hare (after the last public outcry about delays) and delays went down. They are currently being pressured to relax or lift those restrictions at O’Hare. If they do, you can be assured that increased delays at O’Hare will return.
Congress should pass legislation mandating that each commercial airport’s maximum hourly capacity be established and published. Furthermore, the FAA should impose limitations on the number of flights that can be scheduled at each commercial airport. That number should be less than the maximum capacity, taking into account such factors as typical weather patterns for the airport, routine maintenance and any other factor that typically limits capacity. It is time to recognize the inherent limits imposed upon the National Airspace System by runway capacity.