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This visual indication plus the aural warning provide the pilot
with excellent traffic awareness that augments see and avoid
practices.
Terrain Alerting Systems
Ground Proximity Warning System (GPWS)
An early application of technology to reduce CFIT was the
GPWS. In airline use since the early 1970s, GPWS uses the
radio altimeter, speed, and barometric altitude to determine the
aircraft’s position relative to the ground. The system uses this
information in determining aircraft clearance above the Earth
and provides limited predictability about aircraft position
relative to rising terrain. It does this based upon algorithms
within the system and developed by the manufacturer for
different airplanes or helicopters. However, in mountainous
areas the system is unable to provide predictive information
due to the unusual slope encountered.
This inability to provide predictive information was evidenced
in 1999 when a DH-7 crashed in South America. The crew
had a GPWS onboard, but the sudden rise of the terrain
rendered it ineffective; the crew continued unintentionally
into a mountain with steep terrain. Another incident involved
Secretary of Commerce Brown who, along with all on board,
was lost when the crew flew over rapidly rising terrain where
the GPWS capability is offset by terrain gradient. However,
the GPWS is tied into and considers landing gear status, flap
position, and ILS glide slope deviation to detect unsafe aircraft
operation with respect to terrain, excessive descent rate,
excessive closure rate to terrain, unsafe terrain clearance while
not in a landing configuration, excessive deviation below an
ILS glide slope. It also provides advisory callouts.
Generally, the GPWS is tied into the hot bus bar of the electrical
system to prevent inadvertent switch off. This was demonstrated
in an accident involving a large four-engine turboprop airplane.
While on final for landing with the landing gear inadvertently
up, the crew failed to heed the GPWS warning as the aircraft
crossed a large berm close to the threshold. In fact, the crew
attempted without success to shut the system down and attributed
the signal to a malfunction. Only after the mishap did the crew
realize the importance of the GPWS warning.
Terrain Awareness and Warning System (TAWS)
A TAWS uses GPS positioning and a database of terrain and
obstructions to provide true predictability of the upcoming
terrain and obstacles. The warnings it provides pilots are
both aural and visual, instructing the pilot to take specific
action. Because TAWS relies on GPS and a database of
terrain/obstacle information, predictability is based upon
aircraft location and projected location. The system is time
based and therefore compensates for the performance of the
aircraft and its speed. [Figure 3-59]
Head-Up Display (HUD)
The HUD is a display system that provides a projection of
navigation and air data (airspeed in relation to approach
reference speed, altitude, left/right and up/down glide slope)
on a transparent screen between the pilot and the windshield.
The concept of a HUD is to diminish the shift between
looking at the instrument panel and outside. Virtually any
information desired can be displayed on the HUD if it is
available in the aircraft’s flight computer. The display for
the HUD can be projected on a separate panel near the
windscreen or as shown in Figure 3-60 on an eye piece. Other
information may be displayed, including a runway target in
relation to the nose of the aircraft, which allows the pilot to
see the information necessary to make the approach while
also being able to see out the windshield.
Required Navigation Instrument System
Inspection
Systems Preflight Procedures
Inspecting the instrument system requires a relatively small
part of the total time required for preflight activities, but its
importance cannot be overemphasized. Before any flight
involving aircraft control by instrument reference, the pilot
should check all instruments and their sources of power
for proper operation. NOTE: The following procedures are
appropriate for conventional aircraft instrument systems.
Aircraft equipped with electronic instrument systems utilize
different procedures.
3-35
Figure 3-59. A six-frame sequence illustrating the manner in which TAWS operates. A TAWS installation is aircraft specific and provides
warnings and cautions based upon time to potential impact with terrain rather than distance. The TAWS is illustrated in an upper left
window while aircrew view is provided out of the windscreen. illustrates the aircraft in relation to the outside terrain while and
　
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