Secondary Nav Preferences: GPS or VOR/ILS?

All,

I'm finally at the stage of building where I get to make fun and expensive decisions about the avionics for my RV-6 project. I plan to install dual 7" Skyview displays and configure the airplane for IFR.

In Canada the regulations require that IFR aircraft have "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."

My intended primary navigation radio is the Garmin 430W. Although this marvelous box has WAAS GPS and VOR/ILS, it doesn't count as duplex navigation capability because of potential single-point failure of the power supply or display. Ergo, some other navigation source is required.

Here is my question: what type of secondary nav capability would provide the greatest IFR utility? Options under consideration are:

1. Buy a used TSO'd approach-certified IFR GPS, such as King KLN-89/90 or Garmin GPS-155. Both can be found for under $2000, providing duplex GPS approach capability; or
2. Buy a Garmin SL-30, providing a back-up VOR/ILS/GS capability. I know that the SL-30 is more expensive, and has a COMM radio, so it's not an apples-to-apples comparison, but the point is...

In our future IFR world, what will provide the best long-term utility: single-GPS and dual VOR/ILS, or dual-GPS and single VOR/ILS?

Opinions RVators?

Rob Erdos
RV-6...in progress
Ottawa, Canada

IMHO, WAAS GPS gives far more utility than VOR/ILS. If GPS goes out, or is jammed, you only need one backup.

Have you considered a GPS-400W?

Sure it's more expensive than an old non-WAAS GPS, but you could save database subscription costs (keep one box up to date, and the other one cycle behind). It also would allow you to maintain a precision approach capability.

I think Garmin gives a pretty good price break to those who buy them in conjunction with a G3X system.

You can provide two power supplies to the 430W for com and nav, with separate fuses or CB's. That's how mine's set up....

Sorry - that doesn't help your duplex nav requirement does it?

Why not a 420W and an SL30? e.g., move the gps and vor into separate boxes.
As a plus, the SL30 VOR is way superior to the 430's. And while the future is gps, there are times when only an ILS will get you in.

The backup nav is only useful after a failure of your primary nav system. In Canada, today, if the primary nav system fails, a VOR/ILS backup is perhaps more useful. In the USA, today, an IFR GPS is probably a more useful backup. The balance in both countries is moving towards GPS as more and more RNAV or LPV approaches come online every year.

A second COM is also potentially useful. I've had two cases where I could not transmit on my GNS-430W COM. Both proved to be high resistance in a connection in the 430 COM power line.

My vote is a second nav source that has a COM. Whether the second nav source is GPS or VOR/ILS is less important than the second COM, in my opinion. A 420W and a SL-30 is a good combination.

Also consider database subscription cost

Also consider the cost of the additional database required for your backup GPS.

Another datapoint, is the potential requirement to have multiple datacards if you are going on a fairly long cross country. The 300XL from Garmin will not store all of USA or North America in its card. You will need several cards if travelling on long trip.

For our case, we chose a GNS480, and looked around real hard for a used SL30. The SL30 also connects digitally to our Dynon HSI.

Both of your options are viable Rob - either a GPS or an =VOR/ILS will work for you - so the bottom line can be reduced to what is most available.

Since i don't know the fine details of the Canadian rules, i don't know if you have to have to be able to PROVE that you have IFR-certified backup Nav capability prior to take-off. If you do, that limits your options for equipment. In all of our airplanes, we have a certified IFR GPS and several backup GPS's, and of which has proven their ability to get s down to non-precision approach minimums with no worry. If for some reason, the GPS signals went away, all of our planes have VOR/ILS capability. If I got in a situation where the "big Box" went up in smoke and took out both the IFR GPS and VOR/ILS, I am quite certain I can rely on our other GPS's to get me to a place with good enough weather - because that is how we fly IFR in our airplanes - RV's have enough range to be able to give us an "out" to VFR almost all the time. If we can't have a solid VFR alternate within 400 miles, we are probably waiting for better weather.

Valuable Suggestions

Thanks Guys!

I agree with Kevin's feedback that dual COMM, while not a regulatory requirement, makes good sense. I should have been clearer in the original post, but that's my intention.

WAAS GPS and VOR/ILS capability are both essential for modern IFR, in my opinion, and there is still a budget to consider. The suggestions of a G420W and SL30 would meet the requirement, as would a G400W, SL30 and another Comm (ideally Dynon's way-cool new radio)...whichever is cheaper.

C'mon Oshkosh! It's shopping time.

Rob