Radar Corner

The diagrams below show windshear alert areas and altitudes. For takeoff, the system will begin scanning as soon as the qualifiers are met and continue until reaching an altitude of 1800’ to 2300’ AGL. Warnings are provided for events detected below 1200’ AGL within 3nm of the aircraft and within +/- ½ nm of aircraft track. Cautions are provided for events below 1200’ AGL within 3nm of the aircraft and +/- 35 degrees. Advisories are provided for events below 1500’ AGL within 5nm of the aircraft and +/- 40 degrees.

During landing the advisory and caution regions remain the same but the warning region becomes 1.5nm ahead of the aircraft. At approximately 365’ AGL the warning region decreases with altitude. This is so the system doesn’t provide a warning beyond the touchdown point, causing the aircraft to execute a missed approach and fly into the windshear. Alerts are also inhibited below 50’ AGL and above 100kts because it would be safer at this point to continue the takeoff or landing rather than distract the pilot and/or to abort the takeoff/landing.

Aural advisories are tailored to the phase of flight, caution or warning and by the OEM. Caution aural alerts may either be a chime, “whoop, whoop” tone or “Monitor Radar Display” for both takeoff and landing. The warning aural would consist of “Windshear Ahead, Windshear Ahead” for take-off and “Go Around, Windshear Ahead” for landing. Amber or Red windshear alerts are also provided as lights or integrated onto the primary flight displays on newer aircraft. Note that the alert type (advisory / caution / warning) is based on the range and azimuth to the event, not the strength of the windshear. Once the windshear exceeds a pre-set threshold, an alert will be issued.

Windshear Testing

This installment will conclude by going back in time to June 30th, 1994. As part of the windshear detection certification process, one of the requirements is to make a windshear penetration flight with the FAA on board and correlated by a ground-based radar. The aircraft is a Convair 580, selected because it can accommodate a 30-inch air transport antenna and also because of the instant power available from its turboprop engines. The crew consists of experienced flight test pilots, engineers, FAA inspectors, and airline representatives.

While flying at 1000’ AGL above South Florida, weather begins moving in rapidly from the West and the RDR-4B weather radar detects embedded windshears in the cells.

The event cycled for 25-30 minutes and during that time, six penetrations were completed. Out of the six penetrations, the third was the most dynamic. At the start of this run the aircraft was approximately 10nm from the cell, headed towards the expected windshear location. At 5.1nm from the windshear’s center the advisory ICON displayed with the leading edge at 4.6nm, providing 108 seconds notice.

Due to the windshear’s severity, the flight crew intended to maintain 1000’ AGL and 170kts for as long as possible. When the ICON leading edge reached 3nm, the caution chime illuminated. When the ICON leading edge reached 0.7nm, the “Go Around, Windshear Ahead” warning was given. As the headwind component of the horizontal outflow was encountered the aircraft gained altitude due to the excess lift. As the aircraft passed through the windshear center to the opposite side, the full force of the downdraft and tailwind was experienced. The aircraft plunged from 1,175 feet to 725 feet in 24 seconds, a loss of 450 feet at 1,125 feet/minute with the engines at full power and the gear and flaps up. Approximately 50 very tense seconds later the aircraft penetrated and freed itself from the event.