Mode 4B -Proximity to Terrain with Flaps Up
Modes 4A and 4B are similar, however, mode 4B becomes active when the landing gear has been
extended. At airspeeds greater than 250 knots IAS, if the flaps are not extended, the EGPWS will
illuminate the red PULL UP light and announce "TOO LOW, TERRAIN" every three seconds
between 750 and 170 feet AGL. Below 170 feet AGL the warning message changes to "TOO LOW,
FLAPS". Extending the flaps to the landing configuration will cancel this warning.
In case specific conditions require a flaps retracted landing, or the pilot wishes to practice flaps
retracted landings, mode 4B warnings can be eliminated by pressing the amber TAWS FLAP OVRD
switch on the instrument panel. The switch may be engaged any time the altitude is at least 50 feet
AGL. The yellow TAWS FLAP OVRD ON will illuminate, this will prevent flap warnings or will cancel
flap warnings which have occurred. Illumination of the amber TAWS FLAP OVRD ON portion of the
switch/annunciator indicates that the warning has been disabled.
The flaps override function may be selected off by pressing the switch/annunciator again or by
descending below 50 feet AGL upon landing, when it is automatically reset.
Mode 4C - Proximity to Terrain after Takeoff
This mode alerts pilots to the fact that the terrain is rising at a steeper gradient than the airplane is
climbing. It does this by monitoring and noting a decreasing radio altitude even if the barometric
altitude is increasing. A “minimum terrain clearance floor” is created by the TAWS when a takeoff is
made, or when a go around is made from below 200 feet altitude AGL. This floor is generated
starting at 100 feet AGL during a takeoff and 200 feet AGL during a go around. As the airplane
climbs, the EGPWS monitors the radio altimeter and records the highest point reached above
ground level. If the radio altitude decreases to 75% of that maximum, the red PULL UP light will
illuminate and TOO LOW, TERRAIN will be repeated every three seconds until the airplane climbs
above the altitude at which the alert started.
Above 750 feet AGL the mode becomes inactive until the system detects another takeoff, or a go
around from below 200 feet AGL.
Mode 5 - Descent Below Glide slope
Excessive descent below glide slope on an ILS approach will result in an alert from this mode. For
the mode to become active the following conditions must be met:
1. A valid glide slope signal must be in reception from the NAV receiver selected for the
approach.
2. The landing gear must be down.
3. The airplane must be below 925 feet AGL.
4. The glide slope cancelled function (BELOW G/S//G/S CANCELED) must not have been
selected.
If the above conditions have been met, the voice message "GLIDE SLOPE" will be heard between
925 and 300 feet AGL when the airplane is greater than 1.3 dots below the glide slope. The initial
warning will be at a reduced volume, which is referred to as a “soft warning”. Repetition of the voice
warning varies with radio. As the radio decreases, the warning will be repeated more and more
frequently.
If the airplane deviates more than 2.0 dots below the glide slope, or passes through 300 feet AGL,
the aural warning will increase to the normal warning message level. This is referred to as the “hard
warning”. All of the alerts cancel when the airplane returns to less than 1.0 dots below the glide
slope.
Cessna Citation XLS - Instrumentation & Avionics
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