The APU is a source of bleed air and AC electrics for the aircraft, this gives independence during turnarounds, electrical backup in the event of engine failure and provides air conditioning & pressurisation during an engine bleeds off take-off.
It's electrical power source is the battery, many series -500 aircraft have an extra, dedicated APU battery to preserve main battery usage.
AC voltage and frequency may be read on the AC voltmeter and frequency meter for standby power, ground power, generator No. 1, APU generator, generator No. 2 and the static inverter.
Frequency is indicated only when the generator is electrically excited.
The voltage regulator automatically controls the generator output voltage.
Current readings for the two engine IDGs and the APU generator may be read on the AC ammeter.
The TEST position is used by maintenance and connects the voltage and frequency meter to the power systems test module for selection of additional reading points.
An electronic flight instrument system (EFIS) is a flight deck instrument display system that displays flight data electronically rather than electromechanically.
An EFIS normally consists of a primary flight display (PFD), multi-function display (MFD), and an engine indicating and crew alerting system (EICAS) display.
Early EFIS models used cathode ray tube (CRT) displays, but liquid crystal displays (LCD) are now more common. The complex electromechanical attitude director indicator (ADI) and horizontal situation indicator (HSI) were the first candidates for replacement by EFIS.
Now, however, few flight deck instruments cannot be replaced by an electronic display.
Fuel Pump Panel
The fuel panel is used to move fuel across different tanks and to make the fuel available to engines. It is also used to balance the aircraft moving the fuel in the right position.
different indicator analise pump pressure
Hydraulic pump Panel
The 737-500 had system A powered by the two Engine Driven Pumps (EDP's) and system B powered by the two Electric Motor Driven Pumps (EMDP's).
There is also a ground interconnect switch to allow system A to be powered when the engines are shut down.
From the 737-300 onwards each hydraulic system had both an EDP and an EMDP for greater redundancy in the event of an engine or generator failure.