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Caution alert or a Windshear Warning alert each with distinctive
aural and visual indications to the flight crew.
EGPWS windshear is provided for certain (not all) aircraft types
and is a function of certain additionally required input signals
and enabled internal detection algorithms. These are
established during the initial installation and addressed in the
appropriate Airplane Flight Manual (AFM) or EGPWS Airplane
Flight Manual Supplement (AFMS).
Windshear Caution alerts are given if an increasing headwind
(or decreasing tailwind) and/or a severe updraft exceed a
defined threshold. These are characteristic of conditions
preceding an encounter with a microburst.
A Windshear Caution (if enabled) results in illumination of
amber Windshear Caution lights and may (if separately enabled)
also be accompanied by the aural message
“CAUTION, WINDSHEAR”. The lights remain on for as long
as the aircraft is exposed to conditions in excess of the caution
alert threshold. The Windshear Caution envelope is illustrated
in the figure below.
The Windshear Caution alerting can be disabled by EGPWS
program pin selection so that only Windshear Warning alerts
are provided.
MODE 7
Windshear
Alerting
Windshear
Caution
060-4241-000 • Rev. E - December 2003
24 MK V & MK VII EGPWS Pilot Guide
Windshear
Caution
Windshear Warning alerts are given if a decreasing headwind
(or increasing tailwind) and/or a severe downdraft exceed a
defined threshold. These are characteristic of conditions
within or exiting an encounter with a microburst.
Windshear Warning results in illumination of red Windshear
Warning lights and an aural siren followed by the message
“WINDSHEAR, WINDSHEAR, WINDSHEAR”. The lights
remain on for as long as the aircraft is exposed to conditions in
excess of the warning alert threshold. The aural message will
not repeat unless another separate windshear event is
encountered. The threshold is adjusted as a function of
available climb performance, flight path angle, airspeeds
significantly different from normal approach speeds, and
unusual fluctuations in Static Air Temperature (typically
associated with the leading edge of a microburst). The
Windshear Warning envelope is illustrated in the figure shown
on page 23.
Mode 7 Windshear alerting is active under the following
conditions:
• During takeoff; from rotation until an altitude of 1500 feet
AGL is reached,
• During approach; From an altitude of 1500 feet down to
10 feet AGL,
• During a missed approach; until an altitude of 1500 feet
AGL is reached.
Windshear
Warning
MODE 7
Continued
060-4241-000 • Rev. E - December 2003
MK V & MK VII EGPWS Pilot Guide 25
Due to terrain features at or near certain specific airports
around the world, normal operations have resulted in nuisance
or missed alerts at these locations in the past. With the
introduction of accurate position information and a terrain and
airport database, it is possible to identify these areas and adjust
the normal alerting process to compensate for the condition.
The EGPWS Envelope Modulation feature provides improved
alert protection and expanded alerting margins at identified key
locations throughout the world. This feature is automatic and
requires no flight crew action.
Modes 4, 5, and 6 are expanded at certain locations to provide
alerting protection consistent with normal approaches. Modes
1, 2, and 4 are desensitized at other locations to prevent
nuisance alerts that result from unusual terrain or approach
procedures. In all cases, very specific information is used to
correlate the aircraft position and phase of flight prior to
modulating the envelopes.
The Terrain Clearance Floor (TCF) function (enabled with
TAD) enhances the basic GPWS Modes by alerting the pilot
of descent below a defined “Terrain Clearance Floor”
regardless of the aircraft configuration. The TCF alert is a
function of the aircraft’s Radio Altitude and distance
(calculated from latitude/longitude position) relative to the
center of the nearest runway in the database (all runways
greater than 3500 feet in length). The TCF envelope is defined
for all runways as illustrated below and extends to
infinity, or until it meets the envelope of another runway. The
envelope bias factor is typically 1/2 to 2 nm and varies as a
function of position accuracy.
Terrain
Clearance
Floor
1/2 Runway Length
(Minimum Elevation Number)
Envelope Bias Factor
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