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While en route to the airport of intended landing, reduce the
electrical load as much as practical. Turn off all unnecessary
electrical items such as duplicate radios, non-essential
lighting, etc. If unable to turn off radios, lights, etc. manually,
consider pulling circuit breakers to isolate those pieces of
equipment from the electrical system. Maximum time of
useful voltage may be between 30 and 40 minutes and is
influenced by many factors, which degrade the useful time.
Loss of Alternator/Generator for Electronic Flight
Instrumentation
With the increase in electrical components being installed
in modern technically advanced aircraft, the power supply
and the charging system need increased attention and
understanding. Traditional round dial aircraft do not rely
as heavily on electrical power for the primary six-pack
instrumentation. Modern electronic flight displays utilize the
electrical system to power the AHRS, ADC, engine indicating
system (EIS), etc. A loss of an alternator or generator was
considered an abnormality in traditionally equipped aircraft;
11-6
Figure 11-6. Note the double rocker switch and the standby battery
switch in this aircraft. The standby battery must be armed to work
correctly; arming should be done prior to departure.
however, a failure of this magnitude is considered an
emergency in technically advanced aircraft.
Due to the increased demand for electrical power, it is
necessary for manufacturers to install a standby battery in
conjunction with the primary battery. The standby battery is
held in reserve and kept charged in case of a failure of the
charging system and a subsequent exhaustion of the main
battery. The standby battery is brought online when the main
battery voltage is depleted to a specific value, approximately
19 volts. Generally, the standby battery switch must be in the
ARM position for this to occur but pilots should refer to the
aircraft flight manual for specifics on an aircraft’s electrical
system. The standby battery powers the essential bus and
allows the primary flight display (PFD) to be utilized.
The essential bus usually powers the following
components:
1. AHRS (Attitude and Heading Reference System)
2. ADC (Air Data Computer)
3. PFD (Primary Flight Display)
4. Navigation Radio #1
5. Communication Radio #1
6. Standby Indicator Light
Techniques for Electrical Usage
Standby Battery
One technique for conserving the main battery charge is
to fly the aircraft to the airport of intended landing while
using the standby battery. A two-position battery master/
alternator rocker switch is installed on most aircraft with
electronic flight displays, which can be utilized to isolate
the main battery from the electrical system. By switching the
MASTER side off, the battery is taken offline and the standby
battery comes online to power the essential bus. However,
the standby battery switch must be in the ARM position for
this to occur. [Figure 11-6] Utilization of the standby battery
first reserves the main battery for use when approaching to
land. With this technique, electrical power may be available
for the use of flaps, gear, lights, etc. Do not rely on any power
to be available after the standby battery has exhausted itself.
Once the charging system has failed, flight with a powered
electrical system is not guaranteed.
Operating on the Main Battery
While en route to the airport of intended landing, reduce the
electrical load as much as practical. Turn off all unnecessary
electrical items such as duplicate radios, non-essential
lighting, etc. If unable to turn off radios, lights, etc., manually,
consider pulling circuit breakers to isolate those pieces of
equipment from the electrical system. Keep in mind that
once the standby battery has exhausted its charge, the flight
deck may become very dark depending on what time of
day the failure occurs. The priority during this emergency
situation is landing the aircraft as soon as possible without
jeopardizing safety.
A standby attitude indicator, altimeter, airspeed indicator
(ASI) and magnetic compass are installed in each aircraft
for use when the PFD instrumentation is unavailable.
[Figure 11-7] These would be the only instruments left
available to the pilot. Navigation would be limited to pilotage
and dead reckoning unless a hand-held transceiver with a
GPS/navigation function is onboard.
Once an alternator failure has been detected, the pilot must
reduce the electrical load on the battery and land as soon as
practical. Depending upon the electrical load and condition
　
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