Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 61
6.2.4 Mode 4 -- Unsafe Terrain Clearance
Mode 4 generates three types of voice alerts based on radio altitude, computed airspeed, and aircraft configuration, commonly
referred to as Modes 4A, 4B, and 4C. Modes 4A and 4B static alert envelopes are illustrated in Figures 6.2.4.1-2 through -8.
Dynamic envelopes are not included for these Modes because they do not differ significantly from the static cases. The Mode
4C static envelope is illustrated in Figure 6.2.4.2-2. A dynamic alert envelope for Mode 4C is illustrated in Figure 6.2.4.2-3.
6.2.4.1 Modes 4A and 4B
The MKVI and MKVIII EGPWS can use a normal or alternate airspeed range for Mode 4, which is determined by aircraft
type (see Installation Design Guide). The alternate range is intended for those aircraft that fly more slowly, or have the
capability of flying in the slower alternate range during approach if needed. The Figure 6.2.4.1-1 is a block diagram of Mode
4A and 4B.
MODE 4 LIMIT
RADIO
ALTITUDE
AIRSPEED
IN APPROACH MODE
LANDING FLAPS
LANDING GEAR
MODE 4 TOO LOW TERRAI
N
MODE 4A ALERT
COMPARATOR
X
Y
Y
X
MAX
MAX
Y
X
MODE 4B ALERT
COMPARATOR
X
Y
MAX
MAX
MODE 4
ALERT
LOGIC
[Y < f(X)] = < 4A
ON GROUND
MODE 4 TOO LOW GEAR
MODE 4 TOO LOW FLAPS
[Y < f(X)] = < 4B
MODE 4 INHIBIT
RAD ALT VALID
TAD HIGH
INTEGRITY
FIGURE 6.2.4.1-1: MODE 4A/4B FUNCTIONAL BLOCK DIAGRAM
For ease of use the following list of tables identifies which Mode 4 Types are depicted where:
Figure 6.2.4.1-2 shows the Mode 4A envelope used by Mode 4 Types 6,8
Figure 6.2.4.1-2A shows the Mode 4A envelope used by Mode 4 Type 5
Figure 6.2.4.1-2B shows the Mode 4A envelope used by Mode 4 Type 1
Figure 6.2.4.1-3 shows the Mode 4A envelope used by Mode 4 Types 7,9
Figure 6.2.4.1-4 shows the Mode 4B envelope used by Mode 4 Type 5
Figure 6.2.4.1-4A shows the Mode 4B envelope used by Mode 4 Type 1
Figure 6.2.4.1-5 shows the Mode 4B envelope used by Mode 4 Type 6
Figure 6.2.4.1-6 shows the Mode 4B envelope used by Mode 4 Type 7
Figure 6.2.4.1-7 shows the Mode 4B envelope used by Mode 4 Type 8
Figure 6.2.4.1-8 shows the Mode 4B envelope used by Mode 4 Type 9
Figure 6.2.4.2-2 shows the Mode 4C envelope used by Mode 4 Type 6,7,8,9
Figure 6.2.4.2-3 shows the Mode 4C envelope used by Mode 4 Type 5
Figure 6.2.4.2-3A shows the Mode 4C envelope used by Mode 4 Type 1
The standard upper boundary for Mode 4A (turboprop aircraft) is at 500 feet radio altitude. If the aircraft penetrates this
boundary with the gear still up, the voice message will be “Too Low Gear”. Above 178 knots (Mode 4 Types 5,6,8), or 148