Cessna 172R Pilot Operating Handbook page 259

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CESSNA
SECTION 9 - SUPPLEMENTS
MODEL 172R
SUPPLEMENT 2 - FAA APPROVED
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DENT BUTTON (IDT) - When depressed, selects special
identifier pulse to be transmitted with transponder reply to
effect immediate identification of the airplane on the ground
controller's
display. ("R"
will
illuminate steadily for
approximately 18 seconds. Button illumination is controlled by
the avionic light dimming rheostat.
ALTITUDE DISPLAY - Displays the pressure altitude on the
left side of the display. The display is in hundreds of feet.
"FL" is annunciated to indicate Flight Level altitude.
Flight
Level is a term to indicate that the altitude is not true altitude,
but barometric altitude which
is not corrected for
local
pressure.
For Example, "FL-040" corresponds to an altitude
of 4000 feet, meaning sea level pressure of 29.92 inches
of
mercury.
The Flight Level altitude is only displayed when the altitude
reporting is enabled, i.e. in Altitude mode. If an invalid code
from the altimeter is detected dashes will appear in the
altitude window. Altitude reporting is disabled if the altitude
window is blank or has dashes.
Figure 1. Bendix/King KT 76C Transponder with Blind Encoder
(Sheet 1 of 2)
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FL
IDT
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