AIRPLANE
OPERATIONS
MANUAL
NAVIGATION AND
COMMUNICATION
REVISION 39
2-18-75
Page
9
Code
01
AOM-145/1114
Prior to an ILS capture, the ability of the HGS to calculate and display
AIII approach guidance is indicated when "A
III" is displayed on the right
half of the HCP Standby display line. This display indicates that the
HGS’ equipments and related sensors are operating normally and in
the correct mode, with the flight deck configuration ready for the
approach. The system conditions that must be met in order to achieve
AIII capability include:
− IRS in NAV mode;
− AHRS 2 in Full Performance mode;
NOTE: AHRS is not applicable in case of dual IRS installation.
− All reversion selections in NORMAL position;
− All NAV receivers tuned to same ILS frequency;
− Basic attitude (pitch, roll, heading) data from dual sources within
defined limits;
− All sensors/input data valid and no HGS BIT faults present.
− Radio altitude greater than 500 ft;
− Glideslope set between -2.50° and -3.00°.
AIII mode becomes available for selection when the above listed
conditions are satisfied and the Approach On Course (AOC) logic law
is met. Approach On Course logic depends on the following
conditions:
− Flight Director Lateral Capture Mode indicates LOC and FD
Vertical Capture mode indicates GS;
− No Localizer and Glideslope deviation for a period of 5 seconds;
− Difference between IRS Magnetic Track and the Selected
Course 1 less than 15°.
When AOC logic is satisfied and all required systems are operating
properly, the HGS will automatically annunciate on the Combiner and
HCP that the AIII mode is available for selection. This is indicated by
"AIII" flashing on the Combiner in the upper left portion and an "A
III"
displayed on the left half of the HCP Standby line.
The AIII mode must be selected before the airplane descends below
500 ft above the runway elevation. If not selected, the “A
III” indication
will be reverted to “IMC”. The flashing "AIII" on the Combiner will
disappear.
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