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June 1992

The Hazard Index: Langley's 'F-factor'

Cockpit display tells pilots degree of danger posed by windshear ahead of plane.

For the information provided by windshear sensors to be meaningful, a hazard index has been developed by NASA's Langley Research Center. This index indicates the level of danger posed by windspeed changes that have been detected. The index is displayed in the cockpit as an "F-factor" number, which, simply put, is a measure of the loss in rate-of-climb capability that would result from flying into a windshear. The higher the F factor, the greater the hazard. A valuable feature of this system is the ability to preset a hazard threshold to automatically alert the crew of a microburst.

How danger is measured

Cockpit information display

This display in the research cockpit of Langley's 737 aircraft is flashing the location and intensity of two microbursts two to three miles ahead. The plane's position is at the bottom of the cone. One of the microbursts measures 0.09, just shy of what is considered a hazard (0>105). The other, at 0.14, is a confirmed hazard.

The hazard index, which works with both airborne and ground-based sensors, determines the danger level of a microburst by synthesizing information about both the aircraft and atmospheric conditions at, and ahead of, the plane. Taken into account is a wealth of data, such as aircraft speed, engine thrust, drag, weight, altitude, flight-path angle and horizontal and vertical wind velocity. Hazard criteria are based on Langley's analysis, flight tests, comparisons with windshear accident reconstructions and case studies of inadvertent encounters with severe windshear. As a result, hazardous windshear is considered to exist when the average F-factor exceeds 0.105 over a one-kilometer segment along an aircraft's flight path. This is equivalent to the loss of about 1,500 feet-per-minute climb capability over nearly 15 seconds of flight time.

Onboard windshear warning systems graphic