GNC 355 or GNC 255

[AlphaMike]

I know this has been discussed before but I'm getting close to decision time. I already have a GTN 750 with 2 G5s so I have a primary GPS and VHF nav. This will be for my com 2 and backup nav system using a GI-106. What's your opinion? GPS or ground based nav for backup?

 

[Salty]

Nav gives you redundancy and more options. GPS only gives you redundancy

 

[jrcox19]

Nav gives you redundancy and more options. GPS only gives you redundancy

I'm genuinely curious, what additional options does a second VHF NAV give?

 

[Salty]

I'm genuinely curious, what additional options does a second VHF NAV give?

Oh sorry, didn't consider the GTN has nav. Duh. I'd probably go with GPS in that case.

 

[CA182R]

A few other small benefits with the GNC 355: Second WAAS source for transponder if you don't already have one, update database with GTN 750 cross-fill instead of using micro-USB key. One drawback: a new GPS antenna on the roof.

 

[asicer]

What's your opinion? GPS or ground based nav for backup?

I'd go GNC255 to save $3k and ~1/2inch of rack space. It's rather unlikely GPS #1 and NAV#1 will both fail so why spend so much on #2?

One drawback: a new GPS antenna on the roof.

If the OP really wanted to go that route, a GPS antenna on the dash might be an option.

 

[N1120A]

I think secondary GPS is more important than secondary NAV. An equipment failure, as rare as that is, is more likely than a complete GPS failure.

 

[GRG55]

Why wouldn't you have the second Nav or GPS on your G5?

From Garmin in July 2019:

"...The G5 electronic flight instrument for certified aircraft can soon be paired with dual digital Garmin navigation sources, such as a GTN™ 650/750 or GPS 175/GNC 355/GNX 375, and manually switch between the sources on the G5..."

 

[CRQFlier]

gnc355. if you're protecting against equipment failure, way more options for gps approaches than ils/loc/vor.

Sent from my SM-G960U1 using Tapatalk

 

[MooneyDriver78]

I think secondary GPS is more important than secondary NAV. An equipment failure, as rare as that is, is more likely than a complete GPS failure.

I don’t think it’s unlikely, there are notices of GPS possible outages on occasion on a local level, usually near military areas.

 

[AlphaMike]

Why wouldn't you have the second Nav or GPS on your G5?."

I have a 3rd party autopilot. From what I’ve read you can’t do that if you have an autopilot other than the Garmin.

 

[AlphaMike]

I'd go GNC255 to save $3k and ~1/2inch of rack space. It's rather unlikely GPS #1 and NAV#1 will both fail so why spend so much on #2?

If the OP really wanted to go that route, a GPS antenna on the dash might be an option.

With the setup I have (GTN750) its possible to loose COM1, GPS1 and NAV1 with a failure of just the 750. That's my concern and why I plan to get a good backup. At this point I'm 75% sure Ill get the 355. I just wanted to reach out and make sure I'm not missing something.

 

[asicer]

With the setup I have (GTN750) its possible to loose COM1, GPS1 and NAV1 with a failure of just the 750. That's my concern and why I plan to get a good backup.

Right, with the loss of the GTN you'll either have COM2+GPS2 or COM2+NAV2 remaining. I never said that you should forgo a backup. What I am saying is that getting GPS2 instead of NAV2 isn't worth the extra $3k and ~1/2inch rack space given the odds of a complete GTN failure are very low, in my opinion.

Plus, NAV1+NAV2 will simplify those 30 day VOR checks.

BTW, if you go the NAV2 route you'll have a GPS/FIS-B enabled iPad/aera/GPSMap/etc to help you out, right?

 

[AlphaMike]

That’s a good point about 30 day vor checks. Didn’t really think about that. I have a ton of extra room in the stack so the 1/2” doesn’t concern me. Thanks

 

[N1120A]

I don’t think it’s unlikely, there are notices of GPS possible outages on occasion on a local level, usually near military areas.

Yeah, it's rare that we actually see a major problem with that.

I have a 3rd party autopilot. From what I’ve read you can’t do that if you have an autopilot other than the Garmin.

Can't do what? You can definitely switch between NAV sources.

 

[WannFly]

Can't do what? You can definitely switch between NAV sources.

Quoting @Ryan Klems from another thread

Sure, you can easily connect both a GTN750 and a GNC355 to a GAD29B. You would have to have a garmin autopilot (or no autopilot) to do so however, as dual nav interface is not approved when interfacing with a 3rd party autopilot.

 

[Ryan Klems]

It’s an STC limitation with 3rd party autopilot. The GAD29B only provides heading error and course error, it does not provide the analog signals for nav tracking (L/R, Up/Down, flags), those come from the navigator itself. As such, you’d have to switch those separately from the course error signal, which leads to the possibility of having course error set to one navigator and the Nav signals from another navigator, which would be a bowl of worms, as such, they just prohibit doing it per the STC install manual.