Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 101
Latitude and longitude are continuously monitored for the airport locations. Additional data processing is not required until
the aircraft approaches one of the Envelope Modulation areas. Once a latitude/longitude-defined area is penetrated, the other
data inputs are checked for “normal” conditions before any alert/warning envelopes are modulated.
There are currently four types of Envelope Modulation required for the approaches causing problems for GPWS:
1. Bias the Mode 1 alert/warning boundaries for “Sinkrate” and “Pull Up” to the right to allow greater altitude rates before
an alert or warning is generated.
2. Lower the maximum upper limit for Mode 2A and Mode 2B. This limits the maximum radio altitude, or the minimum
terrain clearance required to generate an alert or warning.
3. Lower the maximum upper limit for Mode 4 to allow less minimum terrain clearance before an alert is generated.
4. Expand the maximum Mode 5 radio altitude level where an alert can begin. This will allow “Glideslope” alerts for
higher radio altitudes. The gear down requirement to enable this mode is also overridden during warning expansion, to
allow gear up alerts. The maximum radio altitude for Mode 6 “Minimums” alert is expanded at the same time to the
same Mode 5 maximum, as well as removal of its gear down requirement.
The actual data for each of the established areas is in tables stored in the EGPWC non-volatile memory. This data can be for
either a “snapshot” area or an Envelope Modulation area. In fact, these areas can actually overlap since the Envelope
Modulation is not performed until the “snapshot” conditions have been verified. Every “snapshot” area has an associated
Envelope Modulation area, but not every Envelope Modulation area has an associated snapshot area. This is because some
locations use glideslope instead of the snapshot feature as a crosscheck on Geometric Altitude data. All of the data extracted
for each location is used to form a unique key, which establishes the aircraft position, orientation and altitude.
Stored data for latitude, longitude, terrain elevation, expected elevation tolerance, minimum expected radio altitude, heading
(track) and maximum allowable time to reach the Envelope Modulation area are compared to real time computed values for
these parameters in order to set snapshot latch. This latch is intentionally stored in volatile RAM memory and cleared during
power loss recovery. The associated signal validities are used to establish signal integrity prior to setting the snapshot latch.
The maximum time term is used to clear the snapshot latch once this time has expired unless the Envelope Modulation
conditions are satisfied first.
Logic is required to satisfy one or more of the Envelope Modulation keys. In each case, if the key is required, the associated
conditions are monitored.
Please note that other than Localizer, all Envelope Modulation required inputs are also required for basic GPWS so
they will be available. However, Localizer, which is not required for basic GPWS functionality, may not necessarily be
available. If the Navigation Input category does not select an ID containing Localizer, then the Envelope Modulation
cases requiring Localizer will not be available.
The following is a summary of the Envelope Modulation and snapshot keys:
TABLE 6.8-1: ENVELOPE MODULATION KEYS
Selected Key Description
Envelope Modulation Area Requires valid latitude and longitude to be with defined area
G/S Selected Requires valid glideslope within +/- 2 dots
Loc Selected Requires valid localizer within +/- 2 dots
Hdg Selected Requires valid heading within +/- 30 deg of selected value
Crs Selected Requires valid runway course within +/- 10 deg of selected value
If the aircraft installation does not provide runway course (or selected heading)
then this key is not required.
Min Altitude Selected Requires valid Geometric Altitude to be greater than selected value
Snapshot Selected Requires snapshot detected