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Backup Instrument Requirement
I have a 1977 Archer that has had a G5 installed. I was wondering what the requirements are for backup instruments. I'm looking at installing either a second G5 or the new AV-30-C and would like to remove my vacuum system. Where is the required items listed for EFIS backup?

7 Replies
1217 Posts
Per the G5 STC, if you replace the AI and DG with a pair of G5's, for IFR operation, you must retain the original airspeed, altimeter, VSI, and TC, but you can lose the vacuum system. And for interoperability reasons, you might want to stick with Garmin, using a GI275 rather than an AV30C as the second instrument, although dual G5's works just fine.
Not sure if Ron answered your question? The standard requirements for IFR are covered in 14 CFR 135.163 and 135.165.  There is a difference in my mind between "standard" avionics and instrument requirements and "back-up" requirements, of which there are actually very few. It is generally perfectly legal to fly IFR with a single vacuum-driven attitude indicator and directional indicator. No "back-ups" are required. Might not be the smartest thing to do, but it is legal. You do need two radios, but only one has to be a voice transceiver.  Of course, specific airplane certificate and STC requirements are above the minimum 135.163/5 requirements and most modern technically advanced airplanes will have those.
Andy Elliott
1217 Posts
Unless you're operating under Part 135 carrying passengers or cargo for hire, Andy's answer isn't applicable.  The baseline requirements for noncommercial light plane IFR are in 14 CFR 91.205(d), which only requires one comm and one nav radio, and they may be in the same box (i.e., a single nav/comm), even if that may not be a real smart thing to do.  However, there are additional requirements in other regulatory material which come into play when you involve GPS or EFIS in addition to or instead of "traditional" instruments and nav radios.  As it applies to this question, backup instruments are explicitly required when using an EFIS as your primary flight instruments.  The G5 STC linked earlier specifies those requirements when G5's are installed as primary instruments in certified aircraft, and STC's are regulatory.  I haven't looked at the AV30C or GI275 STC's, but I'm sure there are similar requirements.

BTW, the implicitly required "backups" for the vacuum instruments in a classic panel are, for the vacuum AI, the T&B/TC, which must have a different power source (either electric or running off a separate venturi) from the AI, and the mag compass for the DG.  As such, you cannot fly IFR with "only" an AI and DG -- there are backups required (TC and MC) even if we may not think of them as such.

And yes, Andy, I know the rules are different for that E-AB aircraft you fly, but Jesse's Archer is covered by the regular Part 23 certification and 91.205 instrument/equipment requirements.
Thanks for the clarifications, Ron. I seem to have narrowed my aviation "world" way too much. ☹
26 Posts
I can't direct you to a required items list, but I have a C182Q, and when I put in my new auto-pilot (GFC-500), I also put in a dual G5 system, which has two AHRSs.  After this upgrade the only instrument still requiring a vacuum system was my relocated legacy Artificial Horizon.  Then later, I had my engine replaced, and we decided to remove that AH and not install a vacuum system with the new engine.  No avionics changes were needed at that time, because of the dual G5s and the ability for one to backup the other (i.e. two AHRSs).  Hope this helps a little bit. 
My club put a pair of G5s in our Cherokee 180.  They went in the spots for the Attitude Indicator and Heading Indicator.  We removed the vacuum system completely.  Pump came off the engine, and a plate was installed over the hole where the vacuum pump used to be.  We still have the three pitot/static system instruments in their original positions, and the turn coordinator with an electric gyro.  

This is an IFR-legal setup.  No standby vacuum instruments are necessary.  

The Garmin G5s are about the best value in avionics, in my opinion.  When paired with a GTN unit, they make instrument flying much easier than with traditional "steam gauges."  They don't cost a whole lot more than replacing with vacuum instruments.  They are more reliable than vacuum instruments (they can back each other up, and they have their own backup batteries good for hours of use if the electrical system fails).  They let you completely eliminate the troublesome vacuum system from your aircraft.  They aren't as snazzy as a full glass panel solution.  They don't have synthetic terrain or display your charts and checklists.  They also don't set you back 10s of thousands of dollars.  

I recently spoke with a local DPE about how to plan and train for the "partial panel" portion of a practical test for the instrument rating or CFII in our airplane.  He said about the best he could do realistically would be to fail one of the G5s.  The thing is, if you just put whichever G5 is still "functional" into PFD mode, it contains the information from every instrument in your six pack all by itself.  Having a pair of these gives you enough reliability and redundancy to essentially eliminate the idea of "partial panel" approaches in your airplane.