Terminal Windshear Systems
Ching-Huei Hsu (Jeff ), a visiting engineer from the CAA in Taipei,
working on the AOAWS MDS display at the Boulder lab.
From an operational perspective, the AOAWS windshear detection component provides two major functions:
- To improve safety by providing tactical decision making information to pilots for windshear events.
- To provide strategic meteorologists at the weather stations for use in decision-making for optimizing terminal efficiency and capacity.
Because airspace is limited, especially during bad weather, it will be important to utilize all available AOAWS information in order to optimize operations.
Maximizing the utility of the AOAWS windshear components is a function of how the system is ultimately used in an operational environment and this is governed by its capabilities and performance.
Convective Windshear and Microburst Products
Microbursts are well recognized as an aviation hazard. Until the U.S. TDWR and Enhanced LLWAS systems were developed and implemented, no explicit microburst alerts were available. Onboard reactive windshear devices now provide windshear alerts to help pilots recognize that they have entered a windshear event, but they are reactive and do not provide an alert that can help pilots avoid windshear before they enter the event. Forward-looking systems are now just coming into operation and these will provide alerts of a minute or two ahead of entering windshear events. The ground-based systems (TDWR and LLWAS), which define microbursts as windshears with peak-to-peak windspeed differences of > 30 kts over distances < 2.5 nm, have microburst probabilities of detection (PODS) > 90% and false alarm rates (FARs) < 10%. Due to the high performance, the U.S. airlines have developed the policy that aircraft operations (approaches or departures) must cease if a "microburst alert" is given by these systems. When the TDWR or LLWAS systems generate a "windshear-with-loss alert", which is defined as a peak-to-peak windspeed difference of 15-29 kts over a distance of < 2.5 nm, it is up to the pilot to determine whether to continue operating.
Convective windshear events and microbursts typically last about ten minutes. This short duration is due to a combination of the true life cycle of the events coupled with the fact that they are usually moving with the precipitation cells. At times when the precipitation cells are moving more slowly (< 20 kts), the windshear events may impact the airport operations for a longer period. The avoidance of the hazard is an obvious safety benefit; however, every delayed departure and missed approach will disrupt the traffic flow and hence, impact capacity (if airspace is limited). The geographical information provided to the air traffic supervisors and managers on the GSD provides an opportunity to see the "bigger picture" and minimize disruptions.