June 1992
NF177
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
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.
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