Minimum Instrumentation, VFR

[Freemasm]

Forgive my laziness, I'm wicked low on time.

I'm xitioning from certified world and thinking ahead in my build. The FARs that everyone learned for VFR flight required Altimeter, AS, and a magnetic compass. I assumed this was any aircraft. I notice a lot of panel plans and functioning glass panel aircraft without one/all of these individual instruments. Easy to argue the glass will accommodate the AS and altimeter reqmnts. The mag compass, not so much. Can't believe these are day VFR aircraft only. I have no desire to argue the merits.

So much conflicting What do the applicable FARs state? It seems to have been written so that to cover gyro failures; keeping a level of redundancy; vac gyros backed up by electric instrument or a systemless compass. Glass only is acceptable? IFR redundancy is another level, of course. Seems contradictory, especially regarding the compass. My apologies if this has been covered.

[Mel]

Originally Posted by Freemasm

Forgive my laziness, I'm wicked low on time.
I'm xitioning from certified world and thinking ahead in my build. The FARs that everyone learned for VFR flight required Altimeter, AS, and a magnetic compass. I assumed this was any aircraft. I notice a lot of panel plans and functioning glass panel aircraft without one/all of these. Can't believe these are day VFR aircraft only. I have no desire to argue the merits.
So much conflicting What do the applicable FARs state? My apologies if this has been covered.

§91.205 is pretty clear on requirements. §91.205 does not apply to Experimental Amateur-Built Aircraft flown day/VFR. For Night and/or IFR operations, the requirements are the same for EAB and certified aircraft.

You may be confused by the term "Magnetic Compass". That term is not used in the FARs. The proper term is "Magnetic Direction Indicator".

[Freemasm]

@Mel.

So all of these IFR machines I've seen without a whiskey compass (magnetic direction indicator) are legal?

MTF arguments aside; single glass, dual glass but single electrical system architecture, etc. seems contradictory to the FAR original implied intent regarding system redundancy. Sorry. Trying to learn and minimize future expensive rework/mistakes.

[Auburntsts]

Originally Posted by Freemasm

@Mel.

So all of these IFR machines I've seen without a whiskey compass (magnetic direction indicator) are legal?

MTF arguments aside; single glass, dual glass but single electrical system architecture, etc. seems contradictory to the FAR original implied intent regarding system redundancy. Sorry. Trying to learn and minimize future expensive rework/mistakes.

A magnetometer coupled with a display meets the FAR requirement for a magnetic direction indicator hence no need for a whiskey compass. Not aware of any FAR requirement for redundancy in avionics or instruments. Not saying redundancy isn’t a good thing, just that there’s no regulatory requirement that I know of, at least for part 91, E-AB Ops.

[Scott Hersha]

A magnetic direction indicator doesn’t exclusively mean a “whiskey compass”. An electronic magnetometer fills this requirement, and does so much more precisely than the old fashioned wet compass.

[RV7A Flyer]

Originally Posted by Freemasm

@Mel.

So all of these IFR machines I've seen without a whiskey compass (magnetic direction indicator) are legal?

Ahyup. Been hashed out about a zillion times here.

MTF arguments aside; single glass, dual glass but single electrical system architecture, etc. seems contradictory to the FAR original implied intent regarding system redundancy. Sorry. Trying to learn and minimize future expensive rework/mistakes.

Good design principle, but not required. "Original implied intent" ain't got nothin' to do with it.

[BobTurner]

Originally Posted by Auburntsts

Not aware of any FAR requirement for redundancy in avionics or instruments. Not saying redundancy isn’t a good thing, just that there’s no regulatory requirement that I know of, at least for part 91, E-AB Ops.

+1. For aircraft with Standard A/W certificates, most of the IFR redundancy requirements are buried in their type certificate requirements.
Strictly speaking, the FARs limit EAB aircraft to day, VFR only, and don't specify ANY required instruments! The Phase 2 operating limitations you will get when your aircraft gets its A/W certificate will usually, these days, grant you a waiver of those FARs and allow you to fly at night and/or IFR, but only if you also equip per FAR 91.205. And, yes, there are no rules that speak to redundancy. It's left to each operator to determine the level of risk (with regard to backups) that they're comfortable with. I think most, but not all, people who fly IFR also install back up equipment.

[David Z]

There is a Canadian requirement for redundancy. Not that it's applicable to aircraft outside of Canada, but some food for thought.

CAR 605.18
...
(j) sufficient radio navigation equipment to permit the pilot, in the event of the failure at any stage of the flight of any item of that equipment, including any associated flight instrument display,

(i) to proceed to the destination aerodrome or proceed to another aerodrome that is suitable for landing, and
(ii) where the aircraft is operated in IMC, to complete an instrument approach and, if necessary, conduct a missed approach procedure.

In practice it means two seperate navigation sources AND two screens to display the navigation information.

[Freemasm]

I had made a lot of assumptions based on limited experience/observation. Originally a mag compass for DVFR then an added vacuum DG for IFR. Vacuum AI and an electric TC. When I installed an earlier model Aspen years back, you had to keep the vacuum AI. I assumed dual source redundancy was a requirement. It just makes sense to me.

The Garmin literature I've read appears to be the same mindset with Boxes cross checking themselves and each other. Two ADAHRSs (one or two can be a G5's) even for their suggested VFR config. Yes, I's aware they're trying to sell equipment. If there is a Garmin (or other) requirement for electrical architecture redundancy, I haven't read it yet. Maybe they're assuming the device BU batteries fill this need. The more I dig, the more questions that pop into my brain. Sorry.

[Auburntsts]

Originally Posted by Freemasm

I had made a lot of assumptions based on limited experience/observation. Originally a mag compass for DVFR then an added vacuum DG for IFR. Vacuum AI and an electric TC. When I installed an earlier model Aspen years back, you had to keep the vacuum AI. I assumed dual source redundancy was a requirement. It just makes sense to me.

The Garmin literature I've read appears to be the same mindset with Boxes cross checking themselves and each other. Two ADAHRSs (one or two can be a G5's) even for their suggested VFR config. Yes, I's aware they're trying to sell equipment. If there is a Garmin (or other) requirement for electrical architecture redundancy, I haven't read it yet. Maybe they're assuming the device BU batteries fill this need. The more I dig, the more questions that pop into my brain. Sorry.

No need to apologize. You ask good questions. Bear in mind that legal and safe aren’t necessarily synonymous. In the end, it’s about how much risk are you willing to assume? Failure tolerant electrical architectures, redundant systems, and training all go a long ways to reducing risk, but risk mitigation does come with a cost. The trick is finding the happy median and one size does not fit all.