Wind Shear

Defense against this force of nature has several routes, most notably predictive and reactive


Reactive systems

Pilots need 10 to 40 seconds of warning to avoid wind shear. Fewer than 10 seconds is not enough time to react, while more than 40 seconds can be too long. Atmospheric conditions can change in that time. Three technologies are currently in use to provide flight crews a one-half to five-mile advance warning of wind shear.

NEXTGEN onboard Doppler radar sends a burst of energy ahead of the aircraft to seek targets such as moisture particles. The returning signal represents the movement which is translated into wind speed. Doppler radar works better than other systems in rain but less well in dry conditions. When the transmitted signal is directed toward the ground it does return ground reflection or “clutter.” However, researchers are progressing in efforts to eliminate this interference. NASA’s Langley Research Center has developed signal-processing algorithms and hardware for enhancing the wind shear application.

Honeywell has introduced IntuVue which is a general weather detection system based on flight path data with constant updates of the latest hazard assessments provided on the display screen. The system also improves cockpit efficiency by virtually eliminating ground clutter via an internal terrain database which also automatically corrects for the earth’s curvature, to ensure the clearest, most accurate view of the weather ahead.

As the first system certified to the FAA’s Minimum Operational Performance Standard (MOPS) for enhanced turbulence detection, pilots will be informed of air turbulence and have the ability to make safer, more informed route decisions. This advanced protection has demonstrated a more than 45 percent reduction in turbulence related incidents.

Wind shear detection and alerting systems compare inertial accelerations with air data information to provide a wind shear encountered warning. By utilizing angle of attack and flight path angle instead of pitch angle in the calculation, the system will be responsive to vertical as well as horizontal shear conditions. An enhanced version of the system is also compensated for roll with the angle being derived from the rate of change of heading. The alerting system is capable of providing visual and aural warnings for a variety of wind shear conditions, such as, head shear, tail shear, head shear followed by tail shear and wind shear trend. The warning system is also capable of providing a wind shear warning which is a function of the radio altitude of the aircraft.

Enhanced ground proximity warning systems (EGPWS) frequently have the ability to sense and alert flight crews to a wind shear event. Knowing local terrain information coupled with radio altitude and vertical navigation enabled rapid and accurate recognition of vertical micro-bursts. Like most systems incorporating digital technology, built-in diagnostics will alert flight crews and maintenance personnel in the event of system degradation or failure.

Advisory Circular 120-41 is available at the FAA.gov web site and provides criteria for the design of wind shear detection systems which includes wind tables and relative elevations.

In short, the one major difference between reactive and predictive wind shear systems: the normal procedure for a reactive alert is an execution of an escape maneuver while an advance warning will enable the crew to initiate an orderly change of the flight path.

Technology does provide several means for contending with microburst but the most basic rule for wind shear avoidance is “Takeoffs are optional. Landings are mandatory.”

Jim Sparks has been in aviation for 30 years and is a licensed A&P. His career began in general aviation as a mechanic, electrician, and avionics technician. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. You can reach him at sparks-jim@sbcglobal.net.

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