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EFIS questions

svyolo

Well Known Member
I am about to start building a QB kit in about 2 months, and have to start making choices. I have a couple of questions about EFIS's, no brand in particular.

Do EFIS's have their own on/off switch, or do you have to wire in a separate power switch for them (beyond an avionics master switch).

Do EFIS's need to power up and align before engine start, or can you do it after? How long does it take for the AHRS's to "align".

Thanks in advance for any responses.
 
I am about to start building a QB kit in about 2 months, and have to start making choices. I have a couple of questions about EFIS's, no brand in particular.

Do EFIS's have their own on/off switch, or do you have to wire in a separate power switch for them (beyond an avionics master switch).

Do EFIS's need to power up and align before engine start, or can you do it after? How long does it take for the AHRS's to "align".

Thanks in advance for any responses.

1. No, you don?t need to make any avionics choices now (but it?s okay to dream!). Avionics and EFIS units in particular are evolving rapidly. By the time you?re ready to buy, it?s highly likely the best choices will have changed.
2. Some units have an on/off switch, some don?t (the logic being you don?t want to accidentally power them off. Use a pullable circuit breaker for emergency shut off.)
3. Some AHRS?s align in 30 sec, some 2 minutes. IMHO the slower ones are also more robust, so some informed decision making is needed here. Many builders install back up batteries; if installed, they can be used for boot up (and keeping things running) prior to engine start. Or boot up after start, while you get the ATIS.
 
Thanks.

Does engine vibration adversely affect the alignment? i.e. do you need to align before engine start?

I am hoping to leave the powered off for start, then power up/align.

Yeah, I am not planning on buying until very late in the build. But I am building my electrical system soon, and trying to sort out switches, relays, fuses, etc.
 
You can boot up after engine start. I do. You do need to wait until preliminary boot up is done before moving the airplane.
 
If you want to monitor the engine immediately after start, then you will need analog gauges or an electronic monitor (possibly the EFIS) powered prior to start. I have had both in my plane, I prefer the latter, with my backup battery providing power to my EFIS. However, prefiously, when the EFIS was powered up after engine start, vibration did not affect alignment.
 
Good info. Thanks. I assumed you couldn't move the airplane until you got an alignment, but wasn't sure about engine start. I am a little surprised how long alignment takes. I would have assumed the solid state gyros would get a platform quicker than old INS's.
 
If going all glass, including engine monitoring, I would highly recommend planning for some form of an essential power bus that will keep the EFIS alive during an engine start. This can be a very simple or a more elaborate design, depending on your needs.

In our case the essential bus is a group of circuit breakers fed from the main bus via a low loss Schottky diode pair, with a small sealed battery to keep the essential bus alive during brown-outs or longer outages of the main bus. It's dead-simple, requires no user input, and works a charm for keeping the equipment happy through the engine start sequence.
 
In the design SteinAir did for me and others the Master switch will turn on your EFIS (running from the main battery) and then there is an Avionics switch to turn on everything else after engine start. There is also a switch to turn on the EFIS from just the backup battery.
 
Some thoughts:
- I have not seen any system that has any special proceedure for ?alignment?. Just add power. It is not like the old analog SINS spinning gyro monsters from the 1960.
- Some EFIS are sensitive to voltage droop on engine start, some are not. Dynon has informed me that the SkyView can be on during any starting sequence. That said, I do not have the same faith in the expensive GTN-650 box so I keep both sides of the panel off during engine start.
- Many will scream that not having oil pressure readout immediately on engine start is a cardinal sin. I do not share this concern. I start the engine with both sides of the panel off, then power them up after start. The few seconds of no oil pressure readout is about equivalent to any other plane where it takes a few seconds for the pilot to remember to look at the oil pressure reading. As already discussed, you can bring up one of the EFIS displays on a back up battery if this worries you. For the first few months I did testing to make sure the SkyVIew worked through the engine start sequence - it did.
- If you have a 1960 Cessna power design (one battery, one Master solenoid and one avionics master switch), I offer that you have a day VFR airplane. If you like to have a robust power distribution design to support IFR flight, PM me your email address and I?ll send some ideas.

Carl
 
Technology has come a long way in the last five years.

My original Crossbow AHRS used to take several minutes before you could move. When I had a failure and replaced it with another product, this took about a minute. I upgraded my panel this past year and the newest ones are almost instant on.

Many of the newer EFIS require that the back up power circuit be on all the time for a keep alive for the clock. Mine will automatically turn off when powered by the backup circuit after a set period of time and the primary power circuit is off.

My PFD and MFD are powered on when I turn on the master battery switch before engine start. I?ve never experienced an issue during engine start. The rest of my Avionics are powered from an Avionics master that I switch after engine start.

While is ok to start thinking about Avionics now, don?t make any purchases until about 6 moths before you need them. Technology is changing rapidly and you don?t know when a particular vendor is going to do a platform upgrade. Many attempt to be backwards compatible, but at some point that just isn?t possible. The older units just don?t have the processor and graphics power to run the newer software.
 
I misspoke in post #4. My EFIS is off during engine start, but my EIS (engine instruments - GRT puts them in a separate box) is on (GRT says this is okay), and a bright red light is also on until there is oil pressure.
 
I power on my dual Dynon 1000T EFIS prior to startup. Both screens have backup batteries to deal with low voltage during engine start. Dynon confirms this is no problem for their system (as do other MFGs). I can power one or both screens on without the master but rarely find a need to do so.

The AD-AHRS receive power from the EFIS in this system so, by the time it is booted up, the system is ready to roll.

I have no analog gauges to monitor oil pressure or anything else during engine start. (KIS)
 
I power on my dual Dynon 1000T EFIS prior to startup. Both screens have backup batteries to deal with low voltage during engine start.

Same and zero problems in 500+ hours. Started with just one screen but went to both after seeing what a non-issue it was.
 
While testing, I would power cycle my Dynons in flight and they would align just fine.
 
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KISS.

No avionics master switch. Master switch ON provides power to everything (I do have the E-buss architecture a la Aeroelectrics).

I just leave everything on and start the engine (since I need the EMS portion of the EFIS visible, at least). EFIS stays on through the resulting voltage sag, because it has a backup battery. Other devices like GNS430W, etc., will reboot. No big deal. (ETA: if you're worried about it, which you shouldn't be, you can always turn radios and such off with their Power/volume knobs at shutdown, then turn them on after engine start).

Way simpler, fewer failure points, and I get engine monitoring during the start like you should.
 
I start with everything on and taxi when I'm ready. The brownout on start and alignment while I taxi have not made any difference that I can tell.
 
Any modern decent quality EFIS "should" be able to cope with being on during engine start, though some manufacturers seem to be a bit paranoid in this area despite saying everything fully complies with DO-160 etc.

In my case the engine is started with compressed air, so having avionics on during start via the master switch is a no brainer, but even if I had electric start I would do it the same way, as I'm firmly in the camp which believes having engine instrumentation during engine start is highly desirable.

My EFIS also has a lightning fast alignment and GPS acquisition, and will be aligned before I've finished the before start procedures. The latest generation are getting very good in this respect.
 
"In my case the engine is started with compressed air," So either you are flying an AIR HOG. or an MP14 ?
 
Does every GRT have 2 power inputs?

My Sport SX200 does and it makes this dead simple.

The diode action is internal to the GRT EFIS. Hook up power pin the normal way and a second battery on a switch to the other power input. The EFIS takes the higher voltage without a brownout/restart. Turn off the small battery after engine start. Same overall as post #7, save for a diode and resistor run to the backup battery from the main bus to charge it while it is normally unused in flight.

By small second battery I mean 12V SLA 800mAh
 
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"In my case the engine is started with compressed air," So either you are flying an AIR HOG. or an MP14 ?
M14P.

All the battery does for start is open a solenoid valve (and powers the ignition coils). Puts a quick end to any "don't start with your avionics on" arguments! :D
 
It?s funny to hear all of the different opinions. Basically, there is really only a few wrong ways to do this, but many ways that work fine. It?s really a matter of what you are comfortable with. The whole electrical system, imho, doesn?t even need to be considered until you are almost done with the finishing kit. I run conduits and don?t install a single wire until the airframe is complete. If I can run a wire in a finished airframe, I know I can replace/service that wire in the future. You have no idea how many planes I have worked on that have wires and components installed in a way that they cannot be serviced. There is always a chance that a wire may need to be replaced or removed down the road. This goes for components as well. If you can install them in a finished airframe, you can pull them out as well, and there is almost a guarantee that they will eventually need to come out.

I am personally in the camp of booting up the EFIS before engine start so I can see engine data, and so the alignment process (which is mostly imperceptible with most modern EFISes) is complete. I really don?t even consider the alignment process unless I?m in a plane with a GRT EFIS. I am a big fan of a backup battery, the Dynon battery in a Dynon installation and the TCW in other installations. I often install a second alternator because the customer wants that peace of mind, but I rarely install a second bus or second big battery. Most systems I install draw a total of less than 10 amps with lights off in flight, and in the case of an alternator failure that can be cut in half fairly easily.

Again, there are a lot of ways to do things and very few of them are wrong. As always, there are risks in every situation, so I try to find a way to eliminate as much risk as possible with the most bang for my buck and the least amount of weight without going to extremes. Planes have been amazingly reliable for lots of years. Yes, the modern electronic systems are more reliable on battery power than systems used to be, but I think the ratio of electrical system problems is hugely lower than the number of vacuum system failures. We are replacing and removing vacuum systems almost weekly in our shop, and very seldom have to replace a master solenoid, master switch, EFIS screen (because of failure), etc.

I think you are doing the right thing to ask the question, but once you read all of the opinions, do what makes you feel comfortable. If I were building your system, I would do it the way you want it, but I would offer suggestions based on my experience during the process.
 
I just bought an MGL iEfis Lite for my project. The airframe is complete. The iEfis lite is just a couple of years old, but at 2017 OSH the company announced a NEW efis! So every 2 years new stuff comes out. So all the guys who are telling you not to decide anything right now are dead on right. I know it is great fun to start planning your panel, but you are trying to hit a moving target. The stuff just keeps getting better, lighter and more capable. Also, look at the vendors who were there 10 yrs ago who are gone now. Another thing to consider. So get riveting and just keep up to date on what's coming out.
 
Thanks for all the great responses from everyone. The only EFIS decision I knew to do was not buy till I need it next month. Same with ADS-B. Just trying to sort out my electric system during start. I had heard lots of different opinions.

Thanks again.
 
CARL;
Thanks for the offer. Simple day VFR, single EFIS and an IPAD. I speak fluent electrical.

Jesse;
Thanks for emphasizing not running wires in an unfinished airplane. I might have. Definitely don't want to build a wiring harness, and build an airplane around it. I haven't done that myself, but have worked on way too many things where that was the norm.
 
I just bought an MGL iEfis Lite for my project. The airframe is complete. The iEfis lite is just a couple of years old, but at 2017 OSH the company announced a NEW efis! So every 2 years new stuff comes out. So all the guys who are telling you not to decide anything right now are dead on right. I know it is great fun to start planning your panel, but you are trying to hit a moving target. The stuff just keeps getting better, lighter and more capable. Also, look at the vendors who were there 10 yrs ago who are gone now. Another thing to consider. So get riveting and just keep up to date on what's coming out.

There is valuable advice in your post - something I try and tell everybody that asks - wait until you can wait no more before buying anything electronic.
As manufacturer I would love to just be able to design an EFIS and be done with it - but alas - that cannot happen. Things change fast - as manufacturer you have no choice - you have to go with the times or you will be dead in the not too distant future.
And yes - thanks for deciding on ours - much appreciated :)

Rainier
CEO MGL Avionics
 
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