Atmosphere And Weather

Understanding Wind Shear



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"A flight controller watched Delta Air Lines Flight 191 emerge from an intense thunderstorm at a low altitude and radioed the urgent command 'Delta, go around!' to abort its landing seconds before the jumbo jet crashed and burned Friday night, (August 5, 1985.)"

"...investigators uncovered evidence that suggests the crash at Dallas-Ft. Worth International Airport may have been caused by a deadly wind condition.

'Minutes after the Delta L-1011 plunged to the ground, computerized airfield sensors set off a control tower alarm warning of divergent winds,' said spokesman Ira Furman of the (NTSB) safety board.

Furman said that such a divergence is symptomatic of windshear, a sudden and sometimes violent change in wind direction."

Wind shear was relatively unknown to the traveling public, prior to the mid-1980s, when two disastrous airliner crashes caused great loss of life, prompting awareness. The aforementioned at D-FW in 1985 has been staggering to process for those to whom it was a current event.  The 1982 Pan American aborted take-off crash in New Orleans has been no less mystifying. 

According to Merriam-Webster online, to "shear" is "to cut off (something); to subject to a shear force; to cause (as a rock mass) to move along the plane of contact"  The understanding of wind shear requires a basic understanding of physics.  

First, an earthbound human body can move about in space on the Earth's surface, easily in good conditions. A body can move, horizontally and to some degree vertically.  Add an adversity to the conditions, like wind and rain, and the body's ability to move efficiently is reduced.  (One Law of Inertia is clearly visible in this case.  "A body in motion will remain in motion, unless acted upon by an outside force.")

When any body moves through three-dimensional space, that body moves in one direction, while air/atmosphere/wind pushes back in the exact opposite direction.

According to Newton's Third Law of Motion, "For every action there is an equal and opposite re-action."

How does this apply to flight?  

The resistance that pushes down on an object from above, (such as the weight of an ocean of air,) and the resistance of the tandem gravitational pull from below the object must be nullified or minimized in some way in order for that object to break the shackles of its earthbound status.

Rockets release hot gases in one long downward thrust, which pushes it fuselage upward away from the pull of gravity. 

Birds flap their wings up and down in repetition.  This provides both upward and forward thrust in the ratio needed at any given moment to keep the bird aloft.

Airplanes achieve and maintain flight by a synthesis of these two types of flight.

Since airplane wings must of necessity remain stationary, the speed at which air passes the wing from above and from below must be altered in order to provide less resistance above the wing. This is accomplished by forming the wing with a flat underside and a skyward side that rounds up in front in the shape of a sand dune or a rolling hill, then sloping to the back where top and bottom meet in a rounded point. 

This intentional resistance against the air above the wing slows its airspeed above the wing, providing the conditions necessary for lifting the airplane.   When the backward thrust from the engines increases the overall speed of the airplane, the ratio of airspeed below to airspeed above the wings is summarily increased.

Simply put, the flight of an airplane is a fantastic feat in the best of conditions.

When the speed and direction of the wind around the wings of an airplane in flight are changed, even slightly, the crew and passengers could be expected to experience a "bumpy ride."

When the speed and direction of the wind around the wings of an airplane in flight are changed, significantly, the crew and passengers could be expected to alter direction, speed, and altitude, separately or severally to restore parity.

When the speed and direction of the wind around the wings of an airplane in flight are changed, significantly and suddenly, the crew and passengers could be unable to maintain control of the airplane, being forced to experience disastrous results.

Wind shear, which is sometimes called (or is more accurately created by) a microburst of wind from a cloud bank, is just such a significant and sudden change in the immediate vicinity of an in-flight airplane.

The microburst (aka "a blowing of wind that has a relatively-focused diameter and a strong speed/force,") could qualify as a "surgical-strike" of warcraft were it to be used as a stealth weapon by an intelligent mind. The microburst may be said "to subject to a shear (aka maximum) force..." anything in its path.

A more visible illustration of wind shear may be comprehended by simply imagining a surfer, who is practicing his/her technique in a wave pool. The surfer can ride the wave very well as long as there is a wave to ride.  When the jets of water, (which produce the waves,) are turned off as they periodically are, then the wise surfer will ride the last wave in for a landing.

Airplanes ride the wave/current of air very well as long as there is such a wave to ride.   When the airspeed of the airplane is reduced, intentionally on the flight-deck in anticipation of a landing, or is mandated by loss of one or more engines, the wave/current of air which an airplane may ride is reduced as well, which prompts the wise pilot to bring the plane in for as safe a landing as is possible.

Returning to the surfer illustration, imagine some malevolent individual standing on a tall platform above the wave pool, holding a fire engine hose at the ready. A surfer may not see the individual as the water jets shut down and as a landing is in process, but he/she will know that something significant is occurring when blindsided by a high-pressure blast of water that slams both surfer and board into the concrete wall of the pool, causing inestimable damage and/or loss of life.

Change "board" to "airplane" and change "water" to "wind," then the visual and the effects of wind shear are virtually the same.

When the Delta pilot heard, "Delta, go around!" on that Friday in August, 1985, there was no doubt a serious disconnect between desire to comply and ability to comply.

The anticipation/avoidance curve of microburst/wind shear incidents is similar to the anticipation/avoidance curve of the incidents of tornados.

The experience of encountering either would be expected to be similar, since the force of wind in both appear to be similar in force and speed.

What can be done to avoid loss of life, due to wind shear?  

1.  A warning system could be tailored to the recognition of conditions in which microburst/wind shear are favorable.  (This has been in place for tornados for many years.  No doubt the FAA already has some sort of warning system in place by now for wind shear.)

2.  When conditions are favorable for wind shear, then "Safe, rather than sorry" must be standard operating procedures.  

For instance, when Destination A has a cloud bank laden with potential wind shear, then airplanes en-route should be warned more than 30 minutes prior to final approach for the purpose of giving the choice to prepare for wide-bearth circle patterns or beginning to locate an alternate, Destination B, within one to three hours drive-time from Destination A.  Given the alternatives between inconvenience or worse, passengers could be expected to readily choose inconvenience.

Understanding how wind shear disrupts the conditions of safe flight could lead to the reduction of its current level of danger.

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ARTICLE SOURCES AND CITATIONS
  • InfoBoxCallToAction ActionArrow http://articles.chicagotribune.com/1985-08-05/news/8502210018_1_microburst-windshear-sensors
  • InfoBoxCallToAction ActionArrowhttp://www.merriam-webster.com/dictionary/shear
  • InfoBoxCallToAction ActionArrowhttp://teachertech.rice.edu/Participants/louviere/Newton/law3.html