SEA-TAC AIRPORT, Wash. - When the fog rolls in to Sea-Tac airport, procedures and lighting for pilots changes. Red illuminated stop lines go up along all taxiways that would lead across runway 16L and 34R. Pilots know never to cross a red line.

Instead, they go up to the end of the runway, where another stop line is controlled by the tower to allow planes out onto the runway for takeoff. As each plane crosses over, radar sensors automatically set the line to red again until the next plane is cleared. This system even has a name: SMGCS (pronounced like smigs) for Surface Movement & Guidance Control System.

That's just one piece of technology the airport uses on the ground to avoid collisions and keep the airport operating safely. In poor visibility, lights can be set to higher and higher intensities as the fog grows denser. There are five different levels. Other lights embedded in the pavement of taxiways and runways become increasingly important. Approach lights are shut down along with the center runway to prevent any pilot confusion and traffic is focused on the outer runways.

Inside the cockpit of Alaska Airlines 737s, there's more technology. After another long string of dense fog effectively closed Sea-Tac in the 1980s, Alaska began working with Boeing and other companies develop a HUD or heads up display for civilian use.

A HUD allows pilots to look through a flip down piece of glass that displays critical flight information like altitude and heading and a virtual view of the runway itself until the plane gets down to just 30 feet above the concrete. Usually at that point the pilot can see the physical runway and land.

HUD allows the pilot to keep looking forward through the windshield without having to divert his eyes at a critical moment. At 30 feet, if the runway is still invisible, the plane must go around and the crew hope for better weather or divert to another airport. That rarely happens anymore for airlines equipped with HUDs and the right training in all but the densest fog.

While the pilot in the left seat uses the HUD, the pilot in the right seat is monitoring navigational and other instruments in the cockpit. An auto-land system also plays a role in many of these landings, but can be landed manually.

Alaska says it maintained its flight schedule in and out of Sea-Tac during the fog. And while other airlines have adopted much of this technology, some have not, and like that stretch of fog in the 1980s, are delayed or grounded until the fog lifts.

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