060-4241-000 • Rev. D - March 2000
MK V & MK VII EGPWS Pilot Guide
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 hard surface
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.
1/2 Runway Length
Envelope Bias Factor
(Minimum Elevation Number)
TCF Alert Envelope