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Part Four: Section 2
HTAWS Operation
Section 2: HTAWS Operation
HTAWS Alerting
HTAWS uses information provided from the GPS
receiver to provide a horizontal position and altitude.
GPS altitude is derived from satellite measurements.
GPS altitude is converted to a mean sea level (MSL)-
based altitude (GPS-MSL altitude) and is used to
determine HTAWS alerts. GPS-MSL altitude accuracy
is affected by factors such as satellite geometry, but it is
not subject to variations in pressure and temperature
that normally affect pressure altitude devices. GPS-
MSL altitude does not require local altimeter settings to
determine MSL altitude. Therefore, GPS altitude pro-
vides a highly accurate and reliable MSL altitude source
to calculate terrain and obstacle alerts.
HTAWS utilizes terrain and obstacle databases that
are referenced to mean sea level (MSL). Using the GPS
position and GPS-MSL altitude, HTAWS displays a 2-D
picture of the surrounding terrain and obstacles relative
to the position and altitude of the aircraft. The GPS
position and GPS-MSL altitude are used to calculate
and “predict” the aircraft’s flight path in relation to
the surrounding terrain and obstacles. In this manner,
HTAWS can provide advanced alerts of predicted
dangerous terrain conditions. Detailed alert modes are
described later in this section.
Baro-Corrected Altitude
Baro-corrected altitude (or indicated altitude) is de-
rived by adjusting the altimeter setting for local atmo-
spheric conditions. The most accurate baro-corrected
altitude can be achieved by frequently updating the
altimeter setting to the nearest reporting station along
the flight path. However, because actual atmosphere
conditions seldom match the standard conditions de-
fined by the International Standard Atmosphere (ISA)
model (where pressure, temperature, and lapse rates
have fixed values), it is common for the baro-corrected
altitude (as read from the altimeter) to differ from the
GPS-MSL altitude. This variation results in the aircraft’s
true altitude differing from the baro-corrected altitude.
Power Up
During power-up of the unit, the terrain/obstacle
database versions are displayed along with a disclaimer
to the pilot. At the same time, HTAWS self-test begins.
HTAWS gives the following aural messages upon test
completion:
• “
HTAWS System Test, OK”
, if the system passes the test
• “
HTAWS System Failure”
, if the system fails the test
A test failure is also annunciated visually for
HTAWS, as shown in the HTAWS Alert Summary table.
HTAWS Page
The HTAWS Page is in the NAV group of pages (see
the Pilot’s Guide for detailed information regarding
page groups and pages).
HTAWS Page
190-00356-30 Rev H
