Understanding Airborne Windshear Detection Systems, Part Two
Part One of this series looked at the history behind the discovery and development of windshear detection systems. Part Two will look at automatic activation methods, the types of alerts generated, and how they are tailored for takeoff and landing. While activation methods will vary by aircraft type, the hope is this article will provide an understanding of basic concepts.
Automatic Windshear Activation
Windshear detection systems are automatic, performing detection scans and providing ICON and/or aural alerts to the crew when a windshear is detected. Automatic activation occurs using qualifiers. While these vary by OEM, some of the normal qualifiers for “takeoff” are N1 thrust, engine running and transponder on, groundspeed greater than 25 knots, etc. For “landing” the system activates at a pre-determined altitude, usually 2,300’ AGL or 1,800’ AGL, so that alerts can be provided beginning at 1,500’ AGL. Automatic scanning begins when these qualifiers are met regardless of whether the radar is selected on or not. Each OEM has their own methodology for how activation and deactivation should work, so pilots should consult their AFM to determine the implementation for their aircraft.
The type of alert issued after windshear detection depends on the OEM configuration and windshear location, with aural advisories tailored for takeoff and landing configurations. The system can provide three types of alerts: advisories, cautions, and warnings. Advisories consist of an ICON without an aural alert. Cautions and warnings consist of an ICON and an aural alert tailored for the takeoff or landing phase.
Typical windshear ICONs are shown on the displays below. These ICONs for advisories, cautions, and warnings indicate the range and azimuth to the event. The yellow bars on each side of the ICON are called “searchlight” or “flashlight” beams, and extend to the end of the selected range scale. This makes their azimuth position more apparent if a longer-range scale is selected. Windshear detection applies to 5nm ahead of the aircraft, so the ICON could be difficult to identify on a longer range scale with weather returns present.
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.
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 advance 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.
During Honeywell radar training events, there have been several occasions when pilots have commented that they received a windshear alert and executed a go around, but that it wasn’t that bad and they think they could have landed safely. What they don’t realize is that by executing the go around they increased the aircraft’s energy state, increasing power and cleaning the aircraft up. But more importantly they gained altitude and encountered the windshear above the radial outflow (headwind/tailwind) and only encountered the downdraft in the core. Had they continued the landing the experience would have been entirely different. It’s much better to start out at 1200 feet and lose 450’ of altitude than lose 450’ of altitude starting at 300 feet.
Also, notice in the diagram where the reactive windshear alert occurred. The aircraft was already in the windshear and experiencing a loss of altitude. While radar requires moisture to detect windshears, reactive systems do not, making the two systems very complimentary. Reactive systems can detect extremely dry microbursts devoid of enough moisture for a radar to detect.
Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached at Stephen.Hammack@Honeywell.com.