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the sensors that feed them have also undergone significant
change. Traditional gyroscopic flight instruments have
been replaced by Attitude and Heading Reference Systems
(AHRS) improving reliability and thereby reducing cost and
maintenance.
The function of an AHRS is the same as gyroscopic systems;
that is, to determine which way is level and which way is north.
By knowing the initial heading the AHRS can determine both
the attitude and magnetic heading of the aircraft.
The genesis of this system was initiated by the development
of the ring-LASAR gyroscope developed by Kearfott located
in Little Falls, New Jersey. [Figure 3-36] Their development
of the Ring-LASAR gyroscope in the 1960s/1970s was
in support of Department of Defense (DOD) programs to
include cruise missile technology. With the precision of
these gyroscopes, it became readily apparent that they could
be leveraged for multiple tasks and functions. Gyroscopic
miniaturization has become so common that solid-state
gyroscopes are found in products from robotics to toys.
Because the AHRS system replaces separate gyroscopes,
such as those associated with an attitude indicator, magnetic
heading indicator and turn indicator these individual systems
are no longer needed. As with many systems today, AHRS
itself had matured with time. Early AHRS systems used
expensive inertial sensors and flux valves. However, today the
AHRS for aviation and general aviation in particular are small
solid-state systems integrating a variety of technology such
as low cost inertial sensors, rate gyros, and magnetometers,
and have capability for satellite signal reception.
Air Data Computer (ADC)
An Air Data Computer (ADC) [Figure 3-37] is an aircraft
computer that receives and processes pitot pressure, static
pressure, and temperature to calculate very precise altitude,
IAS, TAS, and air temperature. The ADC outputs this
information in a digital format that can be used by a variety
of aircraft systems including an EFIS. Modern ADCs
are small solid-state units. Increasingly, aircraft systems
such as autopilots, pressurization, and FMS utilize ADC
information for normal operations. NOTE: In most modern
general aviation systems, both the AHRS and ADC are
integrated within the electronic displays themselves thereby
reducing the number of units, reducing weight, and providing
simplification for installation resulting in reduced costs.
Analog Pictorial Displays
Horizontal Situation Indicator (HSI)
The HSI is a direction indicator that uses the output from
a flux valve to drive the dial, which acts as the compass
card. This instrument, shown in Figure 3-38, combines the
magnetic compass with navigation signals and a glide slope.
This gives the pilot an indication of the location of the aircraft
with relationship to the chosen course.
3-23
Figure 3-38. Horizontal Situation Indicator (HSI).
Figure 3-37. Air Data Computer (Collins).
In Figure 3-38, the aircraft heading displayed on the rotating
azimuth card under the upper lubber line is North or 360°.
The course-indicating arrowhead shown is set to 020; the
tail indicates the reciprocal, 200°. The course deviation bar
operates with a VOR/Localizer (VOR/LOC) navigation
receiver to indicate left or right deviations from the course
selected with the course-indicating arrow, operating in the
same manner that the angular movement of a conventional
VOR/LOC needle indicates deviation from course.
The desired course is selected by rotating the courseindicating
arrow in relation to the azimuth card by means
of the course select knob. This gives the pilot a pictorial
presentation: the fixed aircraft symbol and course deviation
bar display the aircraft relative to the selected course, as
though the pilot were above the aircraft looking down.
The TO/FROM indicator is a triangular pointer. When the
indicator points to the head of the course arrow, it shows
that the course selected, if properly intercepted and flown,
takes the aircraft to the selected facility. When the indicator
points to the tail of the course arrow, it shows that the course
selected, if properly intercepted and flown, takes the aircraft
directly away from the selected facility.
The glide slope deviation pointer indicates the relation of
the aircraft to the glide slope. When the pointer is below the
center position, the aircraft is above the glide slope, and an
increased rate of descent is required. In most installations,
the azimuth card is a remote indicating compass driven by
a fluxgate; however, in few installations where a fluxgate is
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