DA-42 NG: Electrical System
by Ghost Aviator
On a normal piston trainer the electrics run the radios and the lights, and a total failure is a nuisance. On the DA-42 NG the electrics also keep the engine computers alive — so here, electrical health is engine health. That single fact is the theme of this chapter. Learn the voltage, the sources, and what keeps the engines running when the alternators quit.
4.1 A 28-volt DC system in four parts
The aeroplane runs a 28-volt direct-current system. It's easiest to remember in four blocks: generation (the alternators), storage (the batteries), distribution (the busbars) and consumers (everything that draws power).
4.2 Generation — the alternators
Power comes from two 70-ampere alternators, one belt-driven on each engine. The left alternator feeds the left main busbar and the right feeds the right, each through its own relay and a 60-ampere protective breaker. The two main busbars tie to a central battery busbar through a 90-ampere breaker. Each alternator has its own control unit that regulates output voltage and warns you, with an ALTN FAIL caution, if it goes over- or under-voltage or fails outright.
4.3 Storage — three kinds of battery
There are three distinct battery sources, and knowing what each does is exam gold:
| Battery | Job |
|---|---|
| Main battery (24 V, 13.6 Ah) | Starts the engines and powers the aircraft before the alternators come online. Connected through the ELECT. MASTER switch. |
| ECU backup batteries (12 V each, in series, per engine) | Keep the engine computers alive if the main battery fails — good for about 30 minutes of engine running in a complete electrical failure. |
| Emergency dry battery (non-rechargeable) | Powers the standby attitude instrument and flood light for at least 1.5 hours when the EMERGENCY switch is ON — independent of everything else. |
4.4 Distribution — the busbars
You don't need to memorise the wiring diagram, but you should know the busbars and what hangs off each:
- Hot Battery Bus — wired straight to the main battery and never switched off; feeds a map/reading light and the ELT unit.
- Battery Bus — connected to the main battery through the battery relay (the ELECT. MASTER switch); feeds the main busbars and heavy current to both starters.
- ECU Bus (left/right) — supplies the engine computers, with the ECU backup batteries tied on for resilience.
- Main Bus (left/right) — feeds most consumers; the right main bus also feeds the avionics.
- Avionic Bus — connected to the right main bus through the AVIONIC MASTER switch; feeds the radios, transponder and autopilot.
4.5 Monitoring and warnings
A voltmeter shows system voltage (alternator voltage normally, battery voltage if the alternators are gone) and an ammeter shows how much current each alternator is supplying. The alerts you must recognise:
| Message | Meaning |
|---|---|
| L/R VOLTS LOW | That bus is below 25 volts. |
| L/R ALTN FAIL | That alternator has failed. |
| L/R ALTN AMPS | That alternator's load is above 70 amps. |
Reinforce Chapter 4: DA-42 NG: Electrical System
Test your knowledge and practice actual exam questions for Technical Specific.