Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 73
Both envelopes allow additional deviation below 150 feet of radio altitude to allow for normal beam variations near the
threshold. This is shown in the envelope of Figure 6.2.5-2 as the sloped portion of the curve, where the equation relating radio
altitude and glideslope deviation required for the soft alert is:
MIN TERRAIN CLEARANCE (FT) =243 71 43−•.()GLIDESLOP
E
DEVN DOTS FLY UP
and in Figure 6.2.5-3 for the hard alert:
MIN TERRAIN CLEARANCE (FT) =293 71 43−•.()GLIDESLOP
E
DEVN DOTS FLY UP
Figure 6.2.5-4 shows the enable/disable conditions for Mode 5. All of the following items must be true for Mode 5 to be
active:
1) Valid radio altitude and glideslope inputs must be present (ILS Tuned and glideslope data valid).
2) An ILS front course has been established. To prevent Mode 5 nuisance alerts due to false fly up lobes during backcourse
approaches an external backcourse Inhibit is provided by a discrete glideslope inhibit input.
3) The system must be either in approach mode (see section 5.13), or landing flaps have been selected to prevent possible
nuisance alerts during takeoff, before the landing gear is retracted
4) Landing gear must be down. At certain airports this gear down requirement is overridden, see the envelope modulation
description in section 6.8. Landing gear status is equated to landing flaps status on fixed gear aircraft.
5) The pilot has not selected glideslope cancel. This is an optional cockpit mounted switch, typically part of the glideslope
Lamp assembly. The glideslope cancel switch is configured to operate as follows: the “Glideslope” alert can be manually
canceled by the crew by momentarily activating the glideslope cancel discrete any time below 2000 feet nominal radio
altitude if the ILS is tuned. The cancel can be reset by ascent above 2000 feet nominal, or descent below 30 feet. The cancel
can also be reset by selection of a non-ILS frequency.
The state of the glideslope cancel selection is always retained during loss of system power.
Numerous complaints of unwanted glideslope alerts while capturing the localizer have been received from operators. These
unwanted alerts typically occur while laterally capturing the localizer below 1000 feet, and during straight and level flight
intercepts of the localizer. In both cases localizer capture is occurring inside the outer marker.
6) The MKVI/MKVIII EGPWS will generally not receive a localizer input. In those cases where it does, it can be used to
solve the lateral capture problem. When above 500 feet AGL, glideslope alerts are only enabled if the Localizer is within ±2
dots. This reduces nuisance alerts when initially capturing ILS. Below 500 feet the Localizer requirement is overridden.
Envelope modulation can raise the 500-foot level such that it is 500 feet below the modulated Mode 5 limit. For installations
without localizer, the glideslope alerts are enabled.
7) To solve the level flight intercept problem, the upper altitude limit for the glideslope alert is modulated with vertical speed.
For normal descent rates above 500 FPM, the upper limit is maintained at the normal 1000-foot level. This is then linearly
reduced to a bottom limit of 500 feet for level flight or climb rates. For a level flight intercept of the localizer no glideslope
alert would be possible until 500 feet AGL was reached. In all cases if altitude rate is not valid then the nominal 1000-foot
AGL Mode 5 enable altitude is used. Note that this change also has the additional benefit of shutting off the glideslope alert
when the pilot corrects his flight path back up towards the glideslope after receiving an alert. In addition the altitude limits
are raised with envelope modulation as detailed above.