Honeywell EGPWS MK V Pilot's Manual page 115

Enhanced ground proximity warning system
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Mark V and Mark VII EGPWS Pilot's Guide
Rev H, August 2011
The Altimeter Monitor function provides the flight crew with
two advisories that inform of improper altimeter setting. The
Below Transition Altitude monitor provides the flight crew
with awareness of an anomaly in the pressure altitude system
while the aircraft is operating below the transition altitude.
The Above Transition Altitude monitor provides the flight
crew with awareness of an anomaly in the pressure altitude
system if the altimeter has not been set to the standard altitude
when the aircraft is operating above the transition altitude.
The annunciations generated from this monitor are classified
as advisory level as crew awareness may be required and crew
response is not necessarily imminent.
The Below Transition Altitude - Altimeter monitor compares
Corrected Altitude from the Air Data Computer (ADC) with
the Global Positioning System (GPS) Altitude from the GPS
receiver. If the difference between the two altitudes exceeds a
computed threshold value an Altimeter Setting aural message
and an optional visual annunciation are generated. Note that
the EGPWS does not perform a cross check of both Air Data
inputs, on some aircraft a function external to the EGPWS
provides an indication to the crew. Therefore the EGPWS
compares Corrected Altitude with GPS Altitude from the
Captain's side data only. The advisory threshold is
dynamically computed based on estimated errors due to non-
standard atmospheric conditions, current GPS accuracy, and
Air Data system errors. Since the threshold is dynamic, the
size of the error that can be detected varies with the current
aircraft state and sensor conditions. The figure below shows
the size of errors that can be detected for typical conditions as
function of the aircraft height above field for different ISA
temperature deviations.
The Altimeter Monitor includes a cross check on the GPS
Altitude to prevent nuisance advisories caused by erroneous
GPS Altitude values. The cross-check compares GPS Altitude
with aircraft altitude computed using radio altitude and the
terrain elevation from the terrain database. This difference
between these altitudes is compared against a dynamically
computed threshold based on the current GPS accuracy, radio
altitude accuracy, and the estimated terrain database accuracy.
If the cross-check fails, then the Below Transition Altitude -
annunciations caused by erroneous GPS Altitude values.
Smart Landing

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