Mark V and Mark VII EGPWS Pilot’s Guide
System Description 060-4241-000
26 Rev H, August 2011
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
fun
ction 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
1/2 Runway Length
Envelope Bias Factor
4 NM
12 NM
15 NM
TCF Alert Envelope
700'
30'
400'
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