DA42 Systems – Electrical circuit, recalling an accident
7 busses, 4 batteries, 8 diodes, 10 relays, 5 switches, 2 generators. Electrical instruments, electrically actuated flaps, electrical pump for the landing gear and the de-icing system. No doubt, the DA42 is an electrical plane.
The main battery is permanently tied to the hot battery bus which feeds two consumers: the ELT and the pilot map reading light. This makes possible to use the light even before boarding the plane and ease boarding, but it also makes this light a perfect tool for draining the battery. It played this role in a famous DA42 accident, but more on this later on.
Both left hand (LH) and right hand (RH) main busses are connected to the battery bus via circuit breakers. Both main busses feed the respective engine ECU busses via two diodes, to prevent any back current. When the electric master switch is turned on, the hot battery bus is connected to the battery bus. On both sides, the main bus and engine bus are then fed by the battery. Each engine ECU bus is also fed by an ECU backup battery, to avoid temporary loss of voltage if the alternator or the main battery can not sustain a momentary high load, but more on this later on.
The LH and RH alternators are connected to both their respective main bus (via the alternator relay) and engine ECU bus (directly). As both main busses are inter-connected via the battery bus the load between the alternators can be balanced. As all diesel engines, there is one engine master switch per engine. When it is activated, the ECUs are connected to the engine ECU bus and the alternator is connected to the excitation battery. The same excitation battery feeds both alternators. Each engine is started separately by actuating the starter switch, which is a key with three positions: “off – left – right”, thus making impossible to start both engines simultaneously.
The 7th bus is the avionics bus, which is fed from the RH main bus, via a relay controlled by the avionics switch. This asymmetrical connection might seem strange, but remember that both main busses are connected together via the battery bus, so even if the main battery and the right alternator are lost the avionics bus is still fed by the left alternator.
Now, the famous accident. A DA42 had a flat battery, because someone left the pilot reading map turned on. Remember, this is the lamp that’s directly connected to the battery. The battery was so flat that it was no longer charging at all. The crew started both engines using an external power unit and taxied out. When the pilot retracted the landing gear, the extra load required was more than what the alternator could sustain and for a very short time the output voltage was lower than normal. Both main busses and engine ECU busses on both sides were underpowered, and the ECUs went off, stopping the engines. By that time, there was no ECU backup battery supporting the voltage on the engine ECU busses…
These two extra batteries were introduced after this accident, and the procedures were modified. It is now prohibited to start both engines using external power. The idea is that if the LH engine is started and the battery is loading, the LH alternator and battery should be sufficient to start the RH engine after a couple of minutes… Click here to read more about this accident and the service bulletin which introduced the new batteries and modified the procedure.
To be fair, it took me some a couple of hours and the composition of this post to get a good understanding of the TwinStar’s electrical circuit. Is this a sufficient knowledge ? Would I react quickly enough if faced up with an electrical problem ? Would my reaction be the correct one ? It’s hard to say at this time, and there are so many circuit breakers..
5 Comments, Comment or Ping
Ron
There are a lot of “different” things about the DA42. The elctrical system is certainly one of them. I believe the fuel system is another. For every gallon it runs through the engine, something like 3/4 of it is returned to the tank — this is how the fuel is heated to keep it from freezing.
It’s a nice aircraft, but needlessly complex.
For what it’s worth, the DA40 DiamondStar also has a hot battery bus and the left-side map light is also connected to it. I always warn students to ensure that light is off after shutdown. From a practical standpoint, it’s not usually a problem. Either the flight ends at night, in which case it’s easy to see that the light is still on, or it ends during the day and that map light has probably not been utilized at all. I suppose the biggest risk would be a night flight that ended during the day, or ended on a very brightly lit ramp where it was hard to see that the light was still on.
Mar 23rd, 2009
Vincent
Ron, the fuel return is more something like 1/4, and this is specific to all TDI engines. The DA40 and C172 TDI have the same feature. Because the DA40 has no fuel selector (except for emergency) one must periodically transfer fuel from the right to the left thak. The C172 has a fuel selector, but no “BOTH” position, so it is important to periodically switch tanks, for both fuel temperature and balance concerns.
If this is the price for the easiness and simplicity of single lever operation, I’m ready to accept it.
Mar 23rd, 2009
Ron
Why is “single lever” operation important? Is it really that hard to lean an aircraft engine properly? Even with today’s engine monitors and lean assist features? I’m a fan of manual engine control. This isn’t to say I’ve got anything against electronics or advancement. The sooner we get decent electronic ignition options, the better. I like the EI/mag combo, personally. But anyway, I’ve got about 800 hours in the SR22, and even after all that time, I miss being able to control the RPM manually.
The biggest problem I see with the DA40 is not avionics-related, it’s an airframe thing. It’s the low Va, Vno, and Vfe speeds. The plane cruises at or above Vno until you get up pretty high, and it’s basically always cruising above Va. I like Cirrus because you’re always cruising Vno and usually below Va as well. That means there’s no need to slow down just because it gets a little bumpy. The low V speeds in the DiamondStar also mean you can’t make any high speed descents.
Mar 27th, 2009
Vincent
You raise some good points Ron. I like single lever because it makes flying simpler and simple is better, particularly in case something goes wrong. That being said, the reason why diesel / FADEC engines have more success in Europe than in the US is because of the opportunity to use less expensive Jet-A1 fuel…
The V speeds of the DA40 are not really a problem when it is the TDI version. The picture I used to create the banner of this blog (131kts, already above Vno, descending along a glide-slope) was with power somewhere between 80 and 90%. Even on the glide-slope, it would not get faster than 135. Don’t expect more than 125 in cruise, so “high speed” descents are not really an option.
I never had the chance to fly the AVGAS version of the DA40, but I agree that if it reaches the 145 knots promised in the Diamond’s brochure, slowing it down, of flying bumpy rides is probably a problem… Can you share more about your experience with the AVGAS DA40 ?
Mar 27th, 2009
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