Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 69
0 0.4 0.8 1.2 1.6 2 2.4
(Thousands)RADIO ALTITUDE (FT)
M
IN
T
E
R
R
A
IN
C
LE
A
R
A
N
C
E
(
F
T
)
(T
housan
ds)
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
100
FT
667
FT
1333
FT
500 FT (<= 190 KTS)
(>= 250 KTS)
30 FT
ALERT AREA
EXPANDED
ALERT AREA
1000 FT
FIGURE 6.2.4.2-3A: MODE 4C STATIC ALERT ENVELOPE (TURBOFAN – MODE 4 TYPE 1)
Mode 4C is based on a minimum terrain clearance, or floor, that increases with radio altitude during takeoff. A value equal to
75% of the current radio altitude is accumulated in a long-term filter that is only allowed to increase in value. If the radio
altitude should later decrease, the filter will store its maximum attained value. Further decrease of radio altitude below the
stored filter value with gear or flaps up will result in the warning “Too Low Terrain”.
A simplified example will illustrate this operation. First assume the radio altitude increases rapidly from zero feet to 400 feet.
The filter will begin charging to 75% of 400 feet, or 300 feet. In 20 seconds, the filter will have charged up to approximately
220 feet. Now if the radio altitude decreases so that 75% of this value results in something less than 220 feet (i.e.,
approximately 295 feet or less), the filter remains at a value of 220 feet. Further reductions in radio altitude below 220 feet
will result in the “Too Low Terrain” warning.
This warning is provided to prevent inadvertent controlled flight into the ground during takeoff climb into terrain that
produces insufficient closure rate for a Mode 2 alert. After takeoff, and change over from takeoff to cruise or approach, the
Mode 4A and 4B will provide this protection.
A ratchet function is applied to the Mode 4C voice warning which is equivalent to the ratcheting voice message described
above. Once the message is given, the envelope is biased down by 20% and further alerts are held off until this additional
20% radio altitude is lost. The caution/warning lamps are not affected and will remain on until the terrain clearance problem
is rectified.
Figures 6.2.4.2-4 (turboprop) and 6.2.4.2-4A (turbofan) shows the effective Mode 4C alert protection for a typical takeoff
scenario. This takeoff profile matches the scenario described for the dynamic envelopes of Mode 3 (Figure 6.2.3-3). The
vertical axis indicates terrain clearance at the point of warning and the horizontal axis indicates time after takeoff. The
envelope is cut off at the bottom as described above, and is cut off at the top due to floor limiting at the corresponding Mode
4A boundary. The aircraft flight profile for this scenario is included for reference.