Rails in the Sky
In the terminal area, Air Traffic Control (ATC) is charged with the responsibility of maintaining a separation between aircraft of at least three miles horizontally or one thousand feet vertically. Of equal elementary importance is that aircraft approaching to land are constrained to fly along a "common path," like a railroad track through an invisible tunnel in the sky, sloping downhill for 12 miles or more. No altitude separation.
Airplanes coming from different directions are angled in one-by-one through the "approach gate" -- hah! a term appropriated from railroad interlockings -- whence they descend along the "glide-slope" toward the "touch-down zone" on the runway.
Whitey Miller sat hunched over his radarscope. On the radio, he spoke with a flat voice, clipping his words short, all business, economical. Off the radio, Miller might have been somewhat less sparing in his verbal expressions. I seldom heard him crack a joke, however. That set Ron Miller apart from the other controllers more than anything else. During occasional conversations -- and with effort -- I learned that his two children were the same ages as mine. On his day off, he would take them kite flying on Far Rockaway.
"When one airplane is entering the approach gate," Miller told me, "the next one must not be any closer than 3 miles." He pointed at a pair of blips an inch apart. They were silently marching along a line toward the center of the screen. He explained that the same separation must prevail all the way along the common path. "As this guy touches down," he said, "the plane following behind must still be at least 3 miles out." Miller handed me his empty coffee mug.
"Regular?" I asked. He nodded unsmiling.
Ideally, all planes would approach at the same speed, and controllers like Whitey Miller could simply vector them to the beginning of the common path exactly 3 miles apart. That was seldom the case in the late fifties. Airliners flew at whatever speed their pilots chose. When I returned to my seat beside Miller, a case in point was developing.
"This guy is a Goony doin' maybe 120," said Miller pointing at the screen. He keyed his mike and issued a clearance for the DC-3 to turn onto final. A speaker on the panel had been turned on for my benefit. The pilot of the DC-3 acknowledged the call with a cheery "Roger."
Miller spoke over his shoulder without removing his gaze from the screen. "I got a 707 in trail, pushing 180. Watch what I do to him."
The difference in speed was obvious even to me. Miller had to give the slower plane a head start. He called the jet and asked its pilot to take a heading at an angle away from Idlewild.
"The best I can do is try to get the 707 to arrive at a point exactly 3 miles out from the runway just as the DC-3 is landing." Miller made two more radio calls. Gradually the faster blip stepped along a curving path back toward the airport. The DC-3 disappeared off the scope. Outside in the dark, its wheels would be squealing onto the pavement. Sure enough, the 707's blip was at that moment 3 miles from the airport. I shook my head then checked my watch.
Exactly one minute later the jet landed. For the case at hand, the runway capacity appeared to be one plane per minute. I made an entry an my notebook. When there was a break in radio chatter, I offered my observation to Miller.
"Hell, if all we got were 180-knot jets," he said, "we could do 60 landings per hour all day long."
Later that evening, I was sitting with another controller who was handling departures. Whitey Miller called my name and gestured for me to rejoin him.
"Got a good one for you here," he said hurriedly. He pointed at the flight progress strip on the bottom of his approach stack. A soft-lead pencil mark said "LE," Lockheed Electra. The next one up said "D-18."
"Beechcraft," Miller explained over his shoulder. "Slower than shit." He brought the Electra straight from the stack to the approach gate and told him to contact the tower at the outer marker. The speaker on the shelf crackled an acknowledgement. A blip set off from a cluster of fluorescent smears and began its march.
Miller cleared the D-18 to descend on the same heading. As the slower plane arrived at the approach gate, the Electra was already safely 3 miles ahead and opening up more distance along the radio-guided rails with each sweep of the radar.
"How long is the common path, Whitey?" I asked.
I began a quick calculation then looked back at the radarscope. Miller's Electra had covered the remaining 9 miles in 3 minutes and vanished from the screen. The D-18, chugging on down the common path at 120 knots, was only halfway there -- 6 miles to go. I glanced up, surprised to see Vince Bonelli standing behind us in the dim light, coffee mug in hand.
Whitey Miller lit a cigarette. He tossed the two flight strips into a wire basket. I clocked the Beechcraft's inter-arrival time as 3 minutes -- equivalent to only 20 planes per hour.
"Something interesting, Vince," I said. "A 50% variation in approach speed makes a 3-to-1 difference in runway capacity."
Bonelli shrugged then winked. "You think Whitey here might make junior controller someday?" he asked. He clapped Miller on the back and strolled on down the row of radar positions.
The need to enforce speed controls on airplanes was obvious -- for both safety and efficiency. Why, I wondered, hadn't that elementary procedure been appropriated from railroading along with the approach gate? I decided to take up my calculations with Henderson and the others back at NAFEC. I decided something else, too.
In this or any other incarnation, I would never want to be an air traffic controller.
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