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EFIS questions
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. |
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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.
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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.
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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.
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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.
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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. |
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