Weather Blog

How can planes land at Sea-Tac in the dense fog? Holograms!

How can planes land at Sea-Tac in the dense fog? Holograms! »Play Video

It is a marvel of human technology that in fog so dense that driving just 35 mph on a local Seattle street still has you straining to see that traffic light just a half-block away, a plane traveling well over 100 mph on final approach can not only find, but land with pinpoint accuracy on an invisible airport runway -- and still find its way back to the terminal.

In years past, the answer was just to divert inbound planes somewhere where it wasn't foggy and not allow outbound planes to leave until the fog lifted. But no more. Now Sea-Tac Airport, as many others across the globe, has an Instrument Landing System that helps transmit signals to help incoming planes find their way.

But technology has advanced even further to where airlines can take that data and translate it into giving pilots a virtual hologram of Sea-Tac Airport and surrounding area. For example, Seattle-based Alaska Airlines' entire fleet of Boeing 737s has both of what they call "autoland" and head-up guidance (HGS) technology.

Autoland is a system that fully automates the landing phase of an aircraft’s flight, with the pilots supervising the process. It was designed to make landing possible when visibility is too poor for pilots to land the aircraft visually, according to Alaska Airlines. The "HGS" converts signals from an airport’s Instrument Landing System into a virtual image of the runway displayed on a glass screen that hangs down in front of the captain.

Here is a video showing this technology from Rockwell Collins:

Alaska says they are the only airline that gets credit for using both technologies in conjunction for low visibility landings, and it allows their planes to land when it's so foggy the "decision height" is only 30 feet above the runway  -- the height at which the pilot must be able to see the runway in order to continue landing or carry out a missed approach and divert to another airport. The airline says they are also the only airline flying Boeing 737s in the United States that can land at Sea-Tac with a 30-foot decision height.

That technology came into big use in January during a similar two-week long stretch of fog in Seattle. Alaska says 352 flights landed just fine in the "pea soup" -- critical because Seattle is their main hub and widespread delays cause a ripple effect across the nation.

"A large percentage of those flights would not have been able to land at Sea-Tac without this equipment," Alaska 737 Fleet Captain Brian Holm said. "We would have been fogged out of Seattle for most of that entire 13-day period."

And looking at the fog this week shows similar successes. At 10 a.m. Tuesday, Sea-Tac Airport reported just 700-1,000 feet visibility down the runway and just 100 feet of vertical visibility (as in, the cloud base was just 100 feet off the ground), yet there was quite a bit of green, on-time arrivals at the airport, not just for Alaska but across the board.

So even though driving is quite treacherous around the region this week, for Sea-Tac, it's business as usual.