Taking Off and Landing in Dense Fog: A Miracle of Technolgy

On Sunday, I flew out of Sea-Tac on a flight that took off around 8 AM.  The region was covered in dense fog and the horizontal visibility at Sea-Tac Airport was reported at .12 miles (630 ft) at 1453 UTC (7:53 AM)--see below for hourly reports.   The sky was obscured, which means the cloud reached the ground.

Here was the situation looking out the window while we were on the taxiway.  We were in cloud and one could barely see objects a few hundred feet away.

We reached the well-lit runway and visibility appeared to be a few hundred feet.

As we accelerated down the runway, the visibility seemed to decline.

But as we lifted off, we got out of the fog in seconds-- it was only a few hundred feet deep--with bright sunshine above it.

 A few minutes later, the sky was blue and fog was far behind.

The visible satellite imagery at 8:30 AM shows the low clouds/fog, with fog over the lower elevations of westerns Washington...although the Strait of Juan de Fuca was mainly clear.  The mountains were in the sun.

The fog was capped by a strong inversion, with temperature warming with height.   Below is the vertical sounding at the NOAA Seattle Sand Point facility, warming about 8C in 1000 meters.  The inversion is caused by sinking (and warming) air aloft, coupled with cooling at the surface or the top of the fog bank.

So how can planes take off and land during such terrible visibility?  In fact, the arrival/departure board at Sea Tac at 7 AM showed most planes leaving on time, but with several delays (but few cancellations) of arrivals.   Thirty years ago, the airport would have been crippled.

So what has changed?  To get the answer, I asked a friend, Wally Powelson, a highly experienced Alaskan Airlines Captain.  He explained to me that there are two main technologies that make such foggy landings/take-offs possible:  Heads Up Guidance and a technology called AutoLand that takes control of the plane during the critical last moments of flight.

Heads Up Guidance (HGS) provides a visual display of what the surface and runways look like, even when visibility is near zero.  If you want to see what it is like, check out the video below.


The AutoLand system takes control of the airplane, using radio beacon signal for horizontal navigation and a highly accurate radar for vertical height.

According to Captain Powelson,  to take off in fog, a runway with centerline illumination, a heads-up guidance system, and roughly 300 ft of horizontal visibility is needed.   When I took off, they just had enough visibility, and the runway was lit like a Christmas tree.

To land, they need at least 500 ft horizontal visibility, the heads-up system, and the AutoLand system.  The pilot MUST see the runway when they get down to 30 feet, if not they have to do a missed approach landing and head back into the air.  I have been on a few AutoLand landings and they were as smooth as silk.

An amazing technology that makes air travel safer and more efficient. And by the way, the fog will soon be gone as winds increase with approaching Pacific storm systems.

from Cliff Mass Weather and Climate Blog https://ift.tt/2CBnqqC