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Electrical system

Overview

Most Twotter systems run on 28V DC, supplied by the main battery, a GPU cart, or the starter-generators in the engines. AC power for the remaining systems is supplied by one of two inverters.

Tip

The Inverter switch is above and behind the captain, not covered by any of the camera views.

Components

The electrical system is modelled in quite some detail using the modern system Asobo introduced. It has been upgraded to v2.3 for SU6. Custom code is used to augment it where necessary.

Buses

One might call the left DC bus the "main" bus. It's powered by the battery, the GPU, the left engine's starter-generator and/or (if Bus Tie is in the NORM position) the right engine's starter-generator, and it's the bus the voltmeter queries.

Opening the Bus Tie separates the left and right side DC buses, the right side will go dark unless the right engine is running and its generator is producing power.

The battery bus is hot (always on). The visible effect: the entrance lights stay powered with the DC Master switch in the Off position.

The starters draw directly from the battery bus because they need a ridiculous amount of power.

I added a separate avionics bus which isn't documented in the POH, but the whole avionics stack is out of scope of the POH, and it seems plausible that it would have been added as part of the avionics installation. The separate avionics bus exists mostly to support the optional avionics master switch (I repurposed the Flight Data Recorder switch for this).

Batteries

In the Config tab of the EFB app, you can switch the default 24-volt NiCd main battery (40 Ah) for a sealed lead-acid pack (36 Ah). All things being equal, you would always pick the NiCD pack, because it's stiffer, which means less sag during starter operation, especially in cold temperatures. But IRL, all things are not equal: the SLA pack is cheaper and easier to operate. The NiCD pack is expensive and needs regular maintenance, like topping up the electrolyte, running capacity checks and reconditioning, and it can suffer from thermal runaway.

The difference in maintenance overhead might be reflected in the wear/tear/damage system at some point. Currently, the choice is mainly for roleplayers: pick the SLA pack if you want the budget experience and don't expect to operate at sub-zero temperatures.

Note

A magical device keeps the pack topped up unless you select the Realistic battery capacity option.

Battery temperature is modeled, thermal runaway isn't (yet). The NiCd heats and cools roughly twice as fast as the lead-acid pack.

The TEST button on the battery temperature gauge drives the needle to the top of the scale and lights the 150 °F lamp.

To improve the reliability during engine starts in cold weather, a small auxiliary battery provides power exclusively to engine control relays and ignition. I think the idea is to keep the engine alive even if the main battery sags too deep. In the mod, the auxiliary battery powers the igniters during engine starts and in manual mode.

The emergency lights are powered by an independent battery pack that is trickle-charged while the bus is running (the trickle charging is not implemented, the battery is just magically charged like all other batteries when a flight starts). The emergency lighs are guaranteed to stay up for at least one hour.

Note

The batteries as Asobo models them are quite stiff and behave in an idealized way. Some custom code helps them to behave, erm, less idealized.

The starter-generators

A starter-generator acts either as starter or as generator, never as both at the same time. While cranking, it is a motor drawing several hundred amps from the battery bus. When the engine can sustain itself, the field is re-excited and device starts feeding power back.

You cannot cross-start one engine using just the generator of the other engine. In the real world, physics make this impossible. In the mod, I enforce this by taking the opposite side generator offline while the starter is cranking.

The GCU

The Generator Control Unit (GCU) connects a generator to its DC bus when it provides power and the Generator switch is in the ON position.

If the Generators can trip option is enabled, generators have a small chance to trip when they come on line. When this happens, the Generator annunciator will stay lit until the pilot puts the respective Generator switch to the RESET position for a moment.

Note

If you take a long time to start up, possibly with heavy electrical consumers running, you should consider starting only one engine, and then take a few minutes to recharge the battery from that generator. You should not attempt an engine start if the battery voltage is below 20V before you engage the starter.

Or you can call the GPU cart.

GPU cart

Setting the DC SOURCE switch to EXT or clicking the GPU button on the Systems tab of the EFB app summons a GPU cart. It's despawned automatically when you get ready to move.

Note

Many thanks to Bagolu for fitting the model to the Twotter!

Inverters and the AC buses

A switch behind and above the Captain's right shoulder selects either of two 400-Hz inverters providing 115V and 26V AC for the instruments that need them. The usual procedure is to switch to the other inverter before the first flight of the day to balance the lifetime load.

AC-powered devices:

  • both attitude indicators

  • the Captain's slaved HSI gyro

  • the First Officer's directional gyro

  • the fuel quantity indicators

  • torque, oil pressure and fuel flow gauges

The gauges

The voltmeter displays the voltage of the left DC bus.

The loadmeter changes scale based on the selected source. In the center position, it shows the battery load on a 100A scale. When a generator is selected, the scale changes to 200A. Since the starter draw exceeds the scale, the loadmeter is disconnected while the starters are running.

Known Issues

  • I could not figure out how to let two power sources share the load on one bus. The solver always prefers one generator and puts the complete load on it, while the other generator idles. If you have advice, please share it (pun intended). Until this is fixed, the loadmeter will lie to you.

  • All batteries are fully charged at the beginning of a flight. There doesn't seem to be a good way to persist their charged capacity.

  • Since the circuit breakers are not animated in the visual model, I didn't bother to model them. I might, if it makes sense at some point in the future.

L:vars

L:var R/W Meaning
DHC6_GEN_STATE_1 R Generator 1 state — off line, on line, tripped
DHC6_GEN_STATE_2 R Generator 2 state
DHC6_GEN_RESET_REQUEST_1 R/W Request a GCU reset on generator 1, equivalent to holding the switch to RESET
DHC6_GEN_RESET_REQUEST_2 R/W Same, generator 2
DHC6_BATT_TERM_V R Battery terminal voltage, sag included — dips during the crank
DHC6_BATT_TEMP R Battery temperature

Reviewed 2026-08-26.