Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 71
6.2.5 Mode 5 -- Descent Below Glideslope
Mode 5 provides two levels of alert when the aircraft flight path descends below the glideslope beam on front course ILS
approaches. Figure 6.2.5-1 is a functional block diagram description of Mode 5. A delay of approximately 0.8 seconds is
inserted between the alert output and the enabling logic during an alert condition to help prevent nuisance alerts.
RADIO
ALTITUDE
GLIDESLOPE
DEVIATION
MODE 5 GLIDESLOPE
MODE 5 LOUD VOLUME
GLIDESLOPE PAUSE TIME
(Only applies if Declutter Disabled)
SOFT ALERT/WARNING
COMPARATOR
UPPER LIM
X
Y
Y
X
HARD ALERT/WARNING
COMPARATOR
X
Y
Y
X
GLIDESLOPE
PAUSE TIME
GENERATOR
UPPER
LIMIT
MODE 5 MAX ALT
FIGURE 6.2.5-1 MODE 5 FUNCTIONAL BLOCK DIAGRAM
Logic is provided which suppresses the aural alert after one message has been given. Follow-on alerts are only allowed when
the aircraft descends lower on the glideslope beam by approximately 20%. Note that this is NOT a 20% radio altitude
change, but 20% of the current Mode 5 curve (as if the whole curve was shifted 20% to the right). For example, at 500 feet
the curve is 1.3 dots, so the next alert would occur at 1.56 dots. The caution lamps remain on until the excessive “Fly-Up”
condition has been corrected. Once the aircraft exceeds 2 dots “Fly-Up” below 300 feet the aural alert changes to a loud
double glideslope followed by a 3-second pause. This will be repeated approximately every 5 seconds.
Additionally, Mode 5 “Glideslope” alerts can occur during penetration of the Mode 1 outer envelope while the Mode 1
“Sinkrate” audio is suppressed.
Figure 6.2.5-2 displays the static envelope for the first alert boundary. The dynamic case does not differ significantly from
the static envelope, and therefore is not illustrated. The maximum upper limit of 1000 feet nominal allows capture of the
beam before enabling this Mode. Higher upper limits are used at certain airports to improve alert/warning envelope
protection (via envelope modulation). Deviation boundaries are shown in “dots” below the beam (i.e., Fly Up) where one dot
equals 0.0875 DDM. The first alert activation occurs whenever the aircraft is more than 1.3 dots below the beam and is
called a “soft” glideslope alert because the volume level of the “Glideslope” audio warning is approximately one half (-6 dB)
that of the other alerts.