Deciding what's essential

[sbalmos]

I'm mapping out probable electrical block diagrams based on an EFII Bus Manager and a VPX. I'm finding myself, classically, overthinking what in my equipment list is truly "essential", and should be hanging off of the Bus Manager's essential bus, rather than the VPX. Here's my overall list of electrical loads:

EFII dual ECU
Fuel pumps
GDU 460 PFD
GDU 460 MFD
EIS
Heated pitot
Audio panel
GTN 650
GTR 200 COM 2
GTX23ES transponder
GDL39R ADSB In
Autopilot servos and control head
Cabin lighting
instrument lighting
Nav lighting
Strobe lighting
Landing lights

Now I'm considering everything essential if I have a bus failure in night flight, or more likely, IMC. Here's everything I can identify already, which makes for a large list:

EFII ECUs
Fuel pumps
GDU 460 PFD
EIS
GTN 650 (com and approach position source)
Transponder
Nav lights
Strobe lights
Landing lights

I'm wavering on the pitot heat. I'm assuming if the audio panel has no power, it goes into COM 1 cut through. Not having the VPX means no flaps probably, not sure about trim, I guess that depends on how that's wired.

Am I missing anything? Seems like a boat of essentials. Honestly not much left for the VPX to control.

 

[Taildrgr]

Here's my simple essential buss for an alternator/VPX failure:

Skyview PFD and MFD - each have a backup battery so no additional setup needed
GTN 650 - Com1 and Nav has a dual pole switch to connect direct to battery
Pitot Heat - switch to connect direct to battery

Logic:
Com1 can talk to anyone, bypasses audio panel
Nav to find nearest airport and fly an approach if necessary
Pitot heat if in clouds
Skyview backup battery good for 1 hour

All lights will be off to conserve battery power
No transponder - will be talking to ATC anyway or if VPX is working, I can power it on manually
fuel pump - uses a lot of power and a risk I was willing to take


This picture shows my emergency section of the panel with the autopilot breaker, Nav/Com and Pitot switches

 

[Mike H]

The audio panel does require power, however it will default to com 1 when it loses power or is turned off.

You're essential buss powered components should be very minimal because you are running EFII electronic fuel injection. Obviously the fuel pumps and ECUs are absolutely essential, and will require suficcient battery reserve capacity to operate the engine until you can safely land. How many other components can be powered off of the essential buss depend on the amperage draws of each component and the battery capacity available. You do not want to try and power pitot heat or lighting off of the essential buss, these are the biggest loads and will drain the batteries very quickly. Depending on what back up equipment you have, you may not want to power the PFD or the GTN if you have a good hand held comm with external antenna connection and maybe an Ipad that is connected to an ADAHRS that can display attitude and GPS navigation information. You may want to consider checking the current draw of the GDU 460 PFD and GEA 24 engine monitor module, I don't think they draw very much current and if they were on the essential bus they would give you basic flight and engine instruments as well as VFR GPS which is more than enough capability to get you back on the ground.

 

[Mike H]

You may want to consider an accessory case driven back up alternator that will feed the main bus and prevent you from having to load shed and use the essential buss unless there were some major wiring issues.

 

[krw5927]

My thoughts:

EFII ECUs - Definitely
Fuel pumps - Definitely
GDU 460 PFD - Yes (although if you have a backup battery you might want to devise a way to cut the e-bus power to the PFD so that you can save main battery capacity to keep the propeller spinning)
EIS - Unsure - is there an absolute NEED?
GTN 650 (com and approach position source) - Yes
Transponder - No. If you need to be found by ATC you can do that with just a com
Nav lights - No. Declare an emergency and ATC can provide separation
Strobe lights - Same as nav lights
Landing lights - Yes

And I agree with another poster above. You have an electrically-dependent airplane, and you mentioned IMC. A backup alternator on the vacuum pad could turn that really bad day into a non-event.

 

[airguy]

And I agree with another poster above. You have an electrically-dependent airplane, and you mentioned IMC. A backup alternator on the vacuum pad could turn that really bad day into a non-event.

Agreed - I've eliminated the engine-driven fuel pump and went with dual electrics and a full-glass panel, so I'm electron-dependent. I put in a B&C 35 on the vacuum pad and have the two alternators separately monitored, and use the Skyview with its internal battery backup for the PFD. Either alternator is capable of pulling my full electrical load in flight and if they both die the battery will give me more time on the fuel pumps than the Skyview battery will give on the display. If that's still not enough time, then God just meant me to come home that day I guess.

Any situation that requires you to switch to your essential bus or shed load should be considered "emergency operation", and lights are not essential, certainly not nav/strobe/position lights, a good argument might be made for landing lights but if you are low on battery power to begin the approach and hit the lights then, you could get a voltage sag that takes out the fuel pumps and now you've got a real problem. I would take the dark approach over that scenario personally. One call to ATC advising electrical emergency and you don't need the transponder anymore, it can go away, they can still track you with your primary return.

 

[sbalmos]

Thanks, so far. Kurt and Mike, I do have planned for a 30a vacuum pad backup alternator, which further begs the question of what is *absolutely essential*, vs being load-shed by the VPX if the primary alt fails. More selfishly, I'm liking the idea of less hanging off the essential bus - less circuits that have to have real circuit breakers.

As for lights, transponder, and EIS, I was basing this off of the IFR required equipment list. But the more I read, I agree about the lights and transponder. I (should) be already talking with ATC anyway. This initially took me off guard when I was talking this over with my friend last night, and he mentioned his Diamond sheds everything except for landing lights also. But it makes sense now. In a essential bus situation, you've / should've already declared an electrical emergency, and with any declared emergency, the rulebook gets shredded by the prop.

 

[airguy]

For that matter, comms are optional after you've declared the emergency with ATC. Advise them of electrical emergency, state your intended flight path to the nearest airport, and advise you will be NORDO arrival, shutting down now, see you on the ground.

 

[NovaBandit]

For that matter, comms are optional after you've declared the emergency with ATC. Advise them of electrical emergency, state your intended flight path to the nearest airport, and advise you will be NORDO arrival, shutting down now, see you on the ground.

Agreed.

I don't have my 650 on the endurance bus either. If something happens, I just need to get to VMC and on the ground, so I have the bare minimum on the E-bus.

GDU465 (With XM so I know where the VMC is, and a GPS puck, so I know how to get there)
ADAHRS (which also runs the temp probe and magnetometer)
EIS (so I can monitor engine, and what my voltages are dropping to)
GMC305 AP panel (to give me a flight director, takes some load off the pilot in an emergency)

Also, one of my two electronic ignitions are backed up by a separate backup battery.

The above should keep the dirty side down, and show me how to get to a runway in VMC should the worst happen. I can talk to ATC after the fact.