understanding. Traditional round dial aircraft do not rely  as heavily on electrical power for the primary six-pack  instrumentation. Modern EFDs utilize the electrical system  to power the Attitude Heading Reference System (AHRS),  air data computer (ADC), engine indicating system (EIS),  etc. A loss of an alternator or generator was considered an  abnormality in traditionally-equipped aircraft; 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 (AFM) for specifics on an aircraft’s  electrical system. The standby battery powers the essential  bus and allows the PFD to be utilized.  The essential bus usually powers the following components: 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 EFDs, 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. 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  of the battery, there may be sufficient power available for  45 minutes of flight—or for only a matter of minutes. Pilots  should also know which systems on the aircraft are electric and  A stand by 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 global positioning  system (GPS)/navigation function is onboard. While  en route to the airport of intended landing, reduce the  electrical load as much as practical. Turn off all unnecessary  electri cal items, such as duplicate radios, non-essential  lightin g, etc. If unable to turn off radios, lights, etc., manually,  consider pulling circuit breakers to isolate those pieces of  equip ment 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.  Techniques for Electrical Usage Standby Battery Operating on the Main Battery 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 Figure 11-6. The standby battery must be armed to work 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. Double rocker switch  11-6