AS350 B3e course · Power and systems
Electrical system
Starter-generator, battery and bus
The essentials
The Astar has a direct-current (DC) electrical system. One starter-generator on the engine starts it, then becomes the generator. A battery starts the engine and backs up the system; an external power socket saves the battery on the ground.
If the generator fails, the battery carries the load — for a limited time. Shedding non-essential loads stretches that time.
Where a nickel-cadmium battery is fitted, a battery temperature warning watches for thermal runaway.
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Starter, then generator
The same machine does both jobs. During start it draws heavy current from the battery or external power to turn the gas generator. Once the engine is running it's switched to generate, charging the battery and powering the aircraft.
That's why a weak battery affects the start more than anything else: slow cranking means low airflow and a hotter start.
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Generation and distribution
In principle, a DC system like the Astar's works the same way on any helicopter.
- The generator output is held at system voltage by a regulator, which also protects against over-voltage and disconnects a faulty generator.
- Power is distributed through bus bars, each feeding a group of circuits through its own circuit breakers.
- The battery connects to the system through a relay, so it can charge in flight and take over when the generator drops off.
- A loadmeter or ammeter shows how much the generator is supplying.
The buses on your aircraft, and what's on each, are in the RFM's systems section.
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Essential and non-essential loads
Buses are arranged so that what the flight needs — engine control and indication, warnings, basic flight instruments, a radio — can be kept while everything else is dropped.
Some aircraft shed non-essential buses automatically when the generator fails; on others the pilot does it with switches. Either way, know which group a given instrument, light or radio sits in before you need to choose.
When the generator fails
A generator warning means the battery is now carrying everything. Its capacity is finite and it falls faster in the cold and as the battery ages.
Turning off what you don't need — some lights, avionics, heating fans — buys time. Which loads to shed, and what remains powered, is in the RFM's systems section and emergency procedures.
Generator load is engine load
Electrical power isn't free: every amp the generator delivers is shaft power taken from the engine.
In 2013 the FAA issued AD 2013-01-05 for AS350B3 and EC130B4 helicopters fitted with a particular 200-amp starter-generator (an option, not the standard fit). Its power extraction exceeded what the engine maker allowed, reducing the engine's surge margin — with a risk of engine failure. The AD cut its maximum continuous load to 180 A and added a placard below the VEMD.
Generator limits are engine limits too. Respect the placard.
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External power
An external power socket lets a ground power unit carry the electrical load and the start, which spares the battery and gives a faster, cooler start in the cold.
It only helps if the unit is good. A ground supply that sags during the start gives exactly the slow, hot start a weak battery does. Voltage, polarity and the rules for connecting and disconnecting are in the RFM and the operator's procedures; a ground crew who knows them is part of the system.
Battery temperature
Nickel-cadmium batteries can enter thermal runaway: heat lowers internal resistance, which raises charging current, which makes more heat. A battery temperature warning, where fitted, catches it early.
Lead-acid batteries are less prone to runaway but lose capacity sharply in the cold. Battery chemistry varies between aircraft and modifications. Know which yours has, and what its warning asks of you.
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What powers the engine control?
The FADEC is an electronic box. Ask what powers it if the aircraft's electrical system fails, and how long it would keep working.
The answer for your engine is in the RFM's systems description. It's worth knowing before you are low, slow and dark with a generator light on.
What happens to the electrical system if the generator fails?
The battery takes over. Shedding loads extends the time it lasts.
Why does a weak battery matter most during start?
Turning the gas generator slowly gives too little air for the fuel, and TOT rises.
Why did an AD limit one AS350B3 starter-generator's continuous load?
Every amp is engine power. Too much extraction left the engine too close to surge.
A breaker trips, you reset it once, and it trips again. What is it telling you?
A second trip points to a real fault, possibly overheating wiring. Leave it out unless the RFM says otherwise.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 4, Helicopter Components, Sections, and Systems
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Airworthiness Directive 2013-01-05 (AS350B3 / EC130B4 starter generator) · FAA · 78 FR 7641, 4 February 2013
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 8, Ground Procedures and Flight Preparations
- Arriel 2D — Airbus H125 engine · Safran Helicopter Engines
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.