DC Power System
28 volt DC power is supplied by three TR units, which are energized from the AC transfer
busses. The battery provides DC power to loads required to be operative when no other
source is available. On the ground, an amber ELEC light comes on to indicate that a fault
exists in DC power system or standby power system. The ELEC light is inhibited in flight.
Transformer Rectifier Units
The TRs convert 115 volt AC to 28 volt DC, and are identified as TR1, TR2, and TR3.
TR1 receives AC power from transfer bus 1. TR2 receives AC power from transfer bus 2.
TR3 normally receives AC power from transfer bus 2 and has a backup source of AC power
from transfer bus 1. Any two TRs are capable of supplying the total connected load.
Under normal conditions, DC bus 1, DC bus 2, and the DC standby bus are connected via
the cross bus tie relay. In this condition, TR1 and TR2 are each powering DC bus 1, DC
bus 2, and the DC standby bus. TR3 powers the battery bus and serves as a backup power
source for TR1 and TR2.
The cross bus tie relay automatically opens, isolating DC bus 1 from DC bus 2, under the
following conditions:
This isolates the DC busses during approach to prevent a single failure from affecting both
navigation receivers and flight control computers
In–flight, an amber TR UNIT light illuminates if TR1, or TR2 and TR3 has failed. On the
ground, any TR fault causes the light to illuminate.
- At glide slope capture during a flight director or autopilot ILS approach.
- Bus transfer switch positioned to OFF.