Mark V and Mark VII EGPWS Pilot's Guide
Another enhancement provided by the internal terrain
database, is the ability to look ahead of the aircraft and detect
terrain or obstacle conflicts with greater alerting time.
This is accomplished (when enabled) based on aircraft
position, flight path angle, track, and speed relative to the
terrain database image forward the aircraft.
Through sophisticated look ahead algorithms, both caution
and warning alerts are generated if terrain or an obstacle
conflict with "ribbons" projected forward of the aircraft (see
following illustration). These ribbons project down, forward,
then up from the aircraft with a width starting at 1/4 nm and
extending out at ±3º laterally, more if turning. The look-down
and up angles are a function of the aircraft flight path angle,
and the look-down distance a function of the aircraft's altitude
with respect to the nearest or destination runway. This
relationship prevents undesired alerts when taking off or
landing. The look-ahead distance is a function of the aircraft's
speed, and distance to the nearest runway.
A terrain conflict intruding into the caution ribbon activates
EGPWS caution lights and the aural message "CAUTION
AHEAD, TERRAIN AHEAD". An obstacle conflict
OBSTACLE" or "OBSTACLE AHEAD, OBSTACLE
AHEAD" message. The caution alert is given typically 60
seconds ahead of the terrain/obstacle conflict and is repeated
every seven seconds as long as the conflict remains within the
caution area. When the warning ribbon is intruded (typically
30 seconds prior to the terrain/obstacle conflict), EGPWS
warning lights activate and the aural message "TERRAIN,
TERRAIN, PULL UP" or "OBSTACLE, OBSTACLE,
PULL UP" is enunciated with "PULL UP" repeating
continuously while the conflict is within the warning area.
Rev H, August 2011