Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 20
1.3.1.7 Envelope Modulation
The Envelope Modulation feature provides improved alerting protection at some key locations throughout the world, while
improving nuisance margins at others. This is made possible with the use of navigational signals from GPS. All position data
is cross checked to ground based navigational aids, altimeter and heading information, and stored terrain characteristics prior
to being accepted for Envelope Modulation purposes.
Modes 4, 5, and 6 are expanded at some locations to provide alerting protection consistent with normal approaches. Modes 1,
2, and 4 are desensitized at other locations to prevent nuisance warnings 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 tables that store the Envelope Modulation data are maintained in non-volatile memory.
1.3.1.8 Terrain Clearance Floor
The Terrain Clearance Floor (TCF) alert, illustrated in Figure 1.3.1.8-1, adds an additional element of protection to the
standard Ground Proximity Warning modes. It creates an increasing terrain clearance envelope around the intended airport
runway directly related to the distance from the runway. TCF alerts are based on current aircraft location, radio altitude and a
distance based upon nearest runway center point position minus half the runway length. TCF is active during takeoff, cruise
and final approach. This alert mode complements existing Mode 4 protection by providing an alert based on insufficient
terrain clearance even when in landing configuration.
A Runway Field Clearance Floor alert is also provided for runways that are located on top of a hill. This alert is similar to the
TCF alert, but is based on field clearnace (height above runway) instead of radio altitude. More details on this can be found
in section 6.3.1.3 of this document.
TCF Alerts illuminate MKVI/MKVIII EGPWS cockpit lamps and produce the aural message “Too Low Terrain”. This aural
message will occur once when the initial envelope penetration occurs, and one time thereafter for each 20% degradation in
radio altitude. EGPWS caution lamps (or warning lamps if using Lamp Format 1) will remain on until the alert envelope is
exited. Further details can be found in seciton 6.3.1.3 of this document.
"TOO LOW TERRAIN"
400
AGL
700
AGL
4NM 12NM 15NM
RUNWAY
FIGURE 1.3.1.8-1: TERRAIN CLEARANCE FLOOR