Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 12
Improvements to the Mode 1 alert boundaries are as follows:
• Envelope Modulation - (see paragraph 1.3.1.7)
• Glideslope Deviation Bias - The “outer” alert boundary (“Sinkrate”) is desensitized when the aircraft is
above the glideslope beam. This prevents unwanted alerts when the aircraft is safely capturing the
glideslope (or repositioning to the centerline) from above the beam.
• Steep Approach Bias - If steep approach is enabled by configurable item then it changes the alert
boundaries to permit safe, but steeper than normal, instrument precision approaches (e.g. MLS or GPS
approaches) without unwanted alerts.
• Flap Override Bias – If flap override is selected by input discrete, it changes the alert boundaries to permit
potentially higher descent rates resulting from flaps not being set to “landing” during approaches. This bias
is smaller than the steep approach bias.
1.3.1.2 Mode 2 - Excessive Terrain Closure Rate
Mode 2 provides alerts when the aircraft is closing with the terrain at an excessive rate. It is not necessary for the aircraft to
be descending in order to produce a Mode 2 alert, level flight (or even a climb) towards obstructing terrain can result in
hazardous terrain closure rate. The terrain closure rate variable is computed within the EGPWS computer by combining radio
altitude and vertical speed in a non-linear complementary filter.
Mode 2 has two sub-modes, referred to as Mode 2A and Mode 2B, the active sub-mode being determined by aircraft
configuration. The Mode 2A alerting envelope is illustrated in Figure 1.3.1.2-1, and the Mode 2B envelope is shown in Figure
1.3.1.2-2.
When TA&D, TCF and Geometric Altitude functions are enabled and of high integrity, certain Mode 2 functions are safely
reduced to the TSO minimums or placed in ILS Mode 2B operation.
2500
2000
1500
1000
500
0
1000080006000400020000
TERRRAIN CLOSURE RATE (FEET/MIN)
RADIO ALTITUDE (FEET)
"PULL UP!"
"
T
E
R
R
A
I
N
T
E
R
R
A
I
N
"
Speed Expansion
"PULL UP!"
"TERRAIN
TERRAIN"
FIGURE 1.3.1.2-1: MODE 2A - EXCESSIVE TERRAIN CLOSURE RATE
Mode 2A is enabled when the conditions for enabling Mode 2B are not satisfied (see below). If the aircraft penetrates the
Mode 2A alerting envelope, the aural message “Terrain Terrain” is generated initially, and alert discretes are output for
driving visual annunciators. If the aircraft continues to penetrate the envelope, then the aural message “Pull Up!” is repeated
continuously until the warning envelope is exited. At this point, an altitude gain function, described in section 6.2.2.2,
activates. The aural message reverts to “Terrain, Terrain...”, but will only be given if the terrain clearance continues to
decrease. The visual alert will remain on until either the aircraft has gained 300 feet of barometric altitude, or 45 seconds has
elapsed, or the altimeter loses track. At that point all visual alerts stop. As shown in Figure 1.3.1.2-1, the upper boundary of
the Mode 2A alert envelope varies as a function of aircraft speed. As airspeed increases from 220 knots to 310 knots, the
boundary expands to provide increased alert times at higher airspeeds.