Happy Landings!

By Dr. Sherwood B. Idso

Most landings are happy landings. But now and then, when an airplane approaches its point of contact with the runway, an accident does occur. Statistics tell us, however, that such occurrences are rare—much rarer than automobile accidents.

Some of the most disturbing of these accidents are associated with a particular form of bad weather that scientists are only now beginning to fully appreciate. Whenever thunderstorms are in the vicinity, air that comes in contact with the rain and hail falling from them is cooled. As you probably know, warm air rises and cool air sinks; so this cooler column of air begins to collapse toward the ground. And the cooler the air is relative to the air surrounding it, the faster it falls.

This sinking air associated with rainstorms and hailstorms is called a downdraft. Generally, it produces wind speeds near the ground of only forty miles per hour or less. Recently, however, it has been discovered that some downdrafts descend with much greater speeds, and that they produce an outward burst of very damaging winds when they hit the ground. These downdrafts are now being called downbursts and some of the very destructive ones super downbursts.

It doesn’t take much imagination to figure out what would happen to an airplane if it flew into one of these downbursts while attempting to land or take off. It would simply be pushed down to the ground and crash. But flying right under a downburst is highly improbable. It is much less likely than being hit by lightning, for instance. What is more apt to happen is that the airplane might fly into the “cold-air outflow” or gust front from one of these downbursts. This is the blast of horizontal wind that results when the downburst smashes into the ground and is deflected sideways. When it suddenly occurs in the same direction as that in which the airplane is flying, the aircraft will experience a serious loss of airspeed. This in turn reduces the lift on the wings of the plane, and it will fall toward the earth. If the plane is already close to the ground—as it is during takeoff and landing—it may hit the ground with disastrous consequences.

But don’t airports have weather stations to detect such things as downbursts and their associated cold-air outflows and gust fronts? Yes, and no. They do have towers with equipment for measuring most weather elements, but they do not have enough of them.

First there is the “surface pressure jump detector.” This is a device that measures small, abrupt changes in air pressure that are caused by the passage of the relatively heavy air of a cold-air outflow from a downburst. Large groups of these detectors have been set up around Chicago’s O’Hare International Airport and at Dulles International Airport near Washington, D.C. Stretching for several miles around the airports, these pressure jump detectors can identify almost any advancing gust front that could interfere with aircraft takeoffs and landings well before it approaches the runways of these airports.

Then there are several new types of radars that have been developed to detect these weather disturbances. “Doppler acoustic-microwave radar” provides wind speed and wind direction information from the earth’s surface up to several hundred yards in the air. “Monostatic sounders” likewise detect disturbances above them for several hundred yards. Still other “Doppler radars” detect the velocities of rain and ice particles within storms, and “infrared Doppler lasers” can actually detect the speeds of dust particles in essentially clear air.

Operated in a coordinated manner, these new weather-sensing devices should provide the protection we need to eliminate practically all takeoff and landing accidents related to natural disasters. Such efforts are another good example of science at work in the service of man. We may expect them to bring many happy landings in the future.

1 An example of how the cold-air outflow from a thundersorm may advance several miles from the point of its creation. (Diagram detail courtesy of the National Oceanic and Atmospheric Administration—NOAA.)

2 The experimental array of pressure jump sensors at Dulles International Airport. The ones in red have been activated by passage of the cold-air outflow that created the gust front. (Artwork courtesy of NOAA.)

3 A close-up view of one of the field measurement stations of the gust front detection system. (Photo courtesy of NOAA.)

4 A radar display of rainfall intensity with a color code at the right. The greater the number corresponding to a given color, the greater the amount of rain and hail falling. (Photo courtesy of the National Severe Storms Laboratory.)

5 A Doppler radar display of wind velocity for the same area covered by the rainfall map in photo 4. Again, the color scale at the right gives the wind speeds in meters per second. (Photo courtesy of the National Severe Storms Laboratory.)