Heathrow’s schedule is so full that if the runways stand empty for even a moment, millions of dollars’ worth of landing time will be lost.
“Once it’s gone, it’s gone, and you’ll have a delay through the rest of the day that you cannot recover from very easily,” says Martin Alder, a former air traffic controller and pilot who recently retired from flying British Airways 747s and A320s in and out of Heathrow. “The only way it can work to the levels it does is uniformity.”
The stacks also give controllers a choice of airplanes. A big 747 trails powerful tornado-like vortices from its wings, so a smaller Airbus A321 cannot follow as closely. Faster airplanes catch up to slower ones. Airliners flying against the wind touch down at a lower speed so they can turn off the runway faster. Controllers deal a landing lineup out of the stacks based on all these factors, packing airplanes as close as safely possible.
The dealing is done deep within a dull concrete building a few miles north of Heathrow, in a room the size of a small gymnasium, called the London Terminal Control Centre. Controllers at screens along both sides of the room watch over some of the most congested airspace on the globe, handling all airplanes below 24,500 feet flying to and from Heathrow and domestic airports Gatwick, Stansted, Luton, and London City. Today Andrew Garrett has one of the hottest seats in the room. He is the Heathrow director, guiding much of that airport’s incoming traffic.
Right now, his green-glowing screen shows problems: The northern stack, which collects airplanes from Central Europe, has turned into a snarl as pilots wheel around thunderstorms.
The landing rate has dropped from the usual goal of 45 airplanes an hour to about 35, troubling Garrett like too few RBIs would a cleanup hitter. Instead of three miles apart, airplanes entering the stack are about four miles apart, which to Garrett means wasted time. It also means airplanes showing up from around the world stack up faster; monitors in the center of the room show how many minutes each one is behind its estimated time of arrival.
He spots an Alitalia flight from Milan and sees a chance to shave off some time. It’s bound for a southern stack, but he intercepts it. Telling the pilot to turn right, Garrett weaves the Airbus A321 between two other aircraft sliding across his screen toward the runway. That puts one more airplane on the pavement this hour.
Up in the Heathrow control tower, controllers—some in their 20s and wearing T-shirts—sort everything out onto the pavement. The tower takes over flights four miles from the airport. By that point, the airplanes are on final approach, as set up by London Terminal Control Centre, so the tower controllers shouldn’t have that much to do with the aircraft aside from instructing inbound flights to adjust speed to maintain minimum spacing and keeping an eye on potential missed approaches.
Two ground controllers—airport traffic cops—weave 747s between and among A330s. Multi-lateration radar triangulates on each airplane’s transponder signal, showing each as a radar dot while still on the ground—even behind buildings. It helps controllers work them out of dead-end cul-de-sacs amid terminals built for small airplanes like Hawker-Siddeley Tridents but quickly clogged by modern airliners twice as wide.
A jumbo pulling out may gridlock other airplanes for five minutes or more, and start blocking taxiways. Controllers know almost every patch of spare tarmac where they can put airplanes to sit and wait, but, says Phil Layton, Heathrow’s air traffic control manager, “There’s less and less places for us to hide them.”
A fourth terminal was built on the far side of the southern runway when space ran out in the airport center; to reach it, airplanes must scoot across the runway between the takeoffs and landings of others.
Moving airplanes gets easier at night, when controllers use a touch screen to turn on lights in the runways. Controllers tell pilots to “follow the greens,” illuminating a line of green lights along the right path. Red lights across the taxiway mean “stop.”
The time between each takeoff is known as a “Heathrow minute,” and in the tower, the goal is to shave it to the bare minimum. Jetliners take off as close as a minute apart, as long as they follow different routes so there is no risk of one catching the other. Controllers shrink the minute even further by craning their necks and watching for an airplane’s wheels to lift off the pavement. That’s when the next aircraft goes.
“At Heathrow, we call that a minute,” says Paul Hooper, a tower supervisor. Then he grins. “Depending on the airplane, it’s probably a bit less than a minute.”
It is the most critical timeline at Heathrow. There is a constant campaign to keep pilots from dawdling on the runway; if each airplane throughout the day wastes a few seconds, it adds up to a few airplanes getting stuck when the nighttime flight curfew shuts down nearly all operations. Stephen Mathewson, a former engineering professor who got bored by academic life and joined airport owner BAA as an internal consultant, went to great lengths to measure how fast pilots get on and off the runway. He posted traffic police at the end of the pavement with radar guns to clock the speed of each airplane. Later he turned to ground radar to track each pilot’s habits.
Then he crunched the numbers and showed each airline how its pilots were doing. Soon, slowpokes picked up their pace. Virgin Atlantic worked timing into its simulator training. Pilots set their brakes to make better use of high-speed runway exits, which are angled so airplanes can turn off at 50 mph. That adjustment shunts them out of the way faster—arrivals average 50 seconds on the runway. No one wants an airplane poking along, sitting in the way, or braking too late, missing its turn, trundling to the next exit—a time-waster.