cessna-172-training-supplement

Because the IAUs are not cross-linked, any functions handled by just one IAU will
be lost if its corresponding display fails. If the PFD fails, NAV1, COM1, and GPS1
will be unavailable. If the MFD fails, NAV2, COM2, and GPS2 are unavailable.
Other optional avionics may also become unavailable, depending on the
particular avionics configuration.
Electrical Failure
If the alternator fails, a red “LOW VOLTS” annunciation will appear on the PFD. All
G1000 equipment will initially remain on, powered by the main battery. Follow
the appropriate emergency checklist to verify the failure and attempt to reset
the alternator.
If the “LOW VOLTS” annunciator remains on, the first step in the load shedding
procedure is to switch Avionics Bus 1 off. This will disable optional equipment
and cooling fans, but the PFD, ADC/AHRS, #1 radio, etc. remain powered by
the essential bus. Continue with the checklist and prepare to land as soon as
practical. The main battery will supply power to the main and essential buses
until M BUS VOLTS falls below 20 volts.
Once the main battery is depleted, the standby battery system will supply
power to the essential bus for approximately 30 minutes. The standby
battery does not power the equipment on Avionics Bus 2, including the MFD,
transponder, COM2 / NAV2 radios, and audio panel. COM1 MIC and NAV1 must
be selected on the audio panel before power to Avionics Bus 2 is lost, or the
radios cannot be tuned.
NOTE: If you are not in IMC, turn off Avionics Bus 2 at the end of
the Electrical Load Reduction checklist to further reduce the
draw on the main battery.
Aircraft Systems • 15