to 50 knots, is displayed by lateral and fore-aft direction
indicators, each mounted on a single radius arm and driven
from a dc torquer motor via a 4:1 reduction gear. The
oscillator drives two inverter transistors which provide the
intersection of the two direction indicators is related to the
output transformer primary with an 80 Vac pk-pk square
circular scale to obtain total airspeed.
wave. The ac is next stepped down to required secondary
voltages by the output transformer and rectifier assembly.
a. A three-position flag motor is used to indicate
either status (GOOD) (matt black), status (FAIL) (red
(1) V o l t a g e O u t p u t s . After the power supply has
and black diagonal stripes) or (OVER-RANGE) (O/R). The
started, a relay switches the source of supply for the +10
display surface is illuminated by miniature 5 Vdc red
Vdc from the helicopter 28 Vdc supply to a secondary on
lamps mounted in a recess behind the front cover.
the transformer. This allows greater tolerance of helicopter
Examples of displayed information for various flight
conditions are shown in figure 1-29.
Voltage outputs are as follows:
b . The torquer motor is mounted on a pointer drive
± 1 5 Vdc analog supply.
a s s e m b l y . A double sided printed circuit board attached
± 5 Vdc TTL logic supply.
INJ current microprocessor supply
C o n n e c t i o n s to the LAI are made via a 19-pin socket on
(derived from 5 Vdc supply).
the rear of the instrument.
± 1 5 Vdc LAI supply (current limited to 1.0A).
L A I F u n c t i o n a l D e s c r i p t i o n . A block diagram of
t h e LAI is included as figure 1-30. Positional feedback is
(2) P e r f o r m a n c e . Output voltages and currents are
obtained from a potentiometer which is integral with the
torquer-motor. This feedback signal is compared with the
input signal and fed to a high gain amplifier. The feedback
s i g n a l is phase-shifted and used to provide rate damping.
This rate damping provides optimum response to large
(a) The main (+80 Vdc) supply is monitored for
input signal changes (e.g. at switch on) and also improves
over-current protection are inherent in the boost regulator
a. The forward channel has an over-range (O/R)
(b) This monitoring provides a fail signal to the
i n d i c a t o r when the forward airspeed input to the indicator
is in excess of 50 knots.
(3) P o l a r i t y R e v e r s a l P r o t e c t i o n . Supply
b . The indicator has built-in failure monitor circuits
reversal protection is provided by diode protection on the
which flag a failure if the indicators fail to respond to an
28 Vdc input to the EPU.
i n p u t . This fail signal will be repeated on the EPU, but
will not affect the operation of any other LRU in the
ADS. In addition the indicator monitors the ADS Fail
The LAI (Fig. 1-28) is
s i g n a l and displays a failure to the pilot if the system
h o u s e d in a standard 3 inch instrument case, fitted at the
top center of the pilot's instrument panel. Low airspeed, up