Back up GPS or 2nd Nav?

[WDD]

Every plane I've flown in for the past 8 years has had 2 Nav radios, but only 1 GPS (either a GNS 430 or a GTN 650). So even though 95% of flying is with GPS, planned / thought out or not we've seemed to have arrived at a common panel design that has no back up to what we use most of the time and a back up/2nd Nav that has probably never been used.

There are many combinations of possible failures. (Loss of GPS, loss of an entire unit, loss of a VOR, etc.) But what is actually the most probable failure, and what would the most pragmatic back up be?

Assume that our primary navigator is a unit that has both GPS navigation as well as a NAV radio (such as the GTN 650).

Assume also that we're trying to give our back up solution a bit of thought vs bludgeoning this to death with a ton on money on twin GTN 750's.

Should we just stay with today's convention and continue with a 2nd Nav radio, or skip the 2nd Nav radio and add a back up GPS navigator? (like the GNC 355)?

 

[kyleb]

My backup nav is an iphone with WingX.

 

[Ryan Klems]

Personally, I think the likelihood of having _both_ a GPS outage event and a complete failure of the primary navigator such that you have no navigation options left, is very low. A lot of the installs I've been doing lately are something like GNX375 for WAAS + ADSB, GNC255/215 for COM+VHF nav and GTR225/205 for 2nd COM (or GNC355 for WAAS+COM, GNC255/215 for VHF Nav+COM and a GTX 345 for ADSB). Unless you were going to be installing 2 GTNs, I don't see the point of a 2nd VHF navigator.

 

[WDD]

Sounds like your latest installs are then just a primary GPS navigator and a primary Nav radio, with no 2nd Nav radios or 2nd GPS units. Nice data point you have there.

 

[Ryan Klems]

I think it also depends on your planned usage. In my T210 I have a GTN 750/650Xi combo, but in the panel I’m doing for my 172, I’m doing a GNX375/GNC255/GNC225. Different usage profiles.

 

[Clip4]

Sure, if you want to spend $10k to install a back up RNAV that will fail along with the VOR/ILS receivers when you have an electrical failure to some how feel secure.

 

[WDD]

Sure, if you want to spend $10k to install a back up RNAV that will fail along with the VOR/ILS receivers when you have an electrical failure to some how feel secure.

So would a back up / independent power source be more likely to be needed vs a 2nd nav radio or 2nd gps navigator?

I think it also depends on your planned usage. In my T210 I have a GTN 750/650Xi combo, but in the panel I’m doing for my 172, I’m doing a GNX375/GNC255/GNC225. Different usage profiles.

Planned usage would be mostly flying IFR x country in a skylane.

 

[Clip4]

So would a back up / independent power source be more likely to be needed vs a 2nd nav radio or 2nd gps navigator?



Planned usage would be mostly flying IFR x country in a skylane.

Cirrus thinks so.

 

[WDD]

I did not know that.

 

[MauleSkinner]

Even though your primary. gps and Nav radios are in the same box, think of them separately for a moment…

1. The box itself goes dark, so you lose both GPS and NAV. A separate GPS or a separate NAV would work.

2. GPS antenna goes bad, so you lose GPS but still have NAV. A separate GPS would still probably be wired into the same bad antenna, so you’d likely lose both GPSs.

3. Same for a bad NAV antenna…GPS would work fine.

4. Loss of avionics bus or total electrical failure. No navigation capability.

In all of the above scenarios, it really wouldn’t matter what your backup was, you’d end up in the same navigation mode.

Obviously the above isn’t an exhaustive list of malfunctions, only the malfunctions I’ve had. But if you think about other malfunctions from the same perspective, you might come to a conclusion that’s workable for you.

 

[WDD]

What's in my mind is about 3 years ago flying VFR with flight following in a Tiger with a GNS 430. The screen went dark because of a malfunctioning auto display dimming piece of hardware. I could get it back for a bit by covering up the light sensor. If it was IMC and I had to fly an an approach, etc. it would have been a problem.

Obviously the above isn’t an exhaustive list of malfunctions, only the malfunctions I’ve had. But if you think about other malfunctions from the same perspective, you might come to a conclusion that’s workable for you.

Sounds like you've had a few interesting times.

But you've hit the nail on the head. What malfunction(s) is most likely? Bad screen killing the GTN 650xi? Bad cable killing all GPS? Panel power failure?

 

[MauleSkinner]

What's in my mind is about 3 years ago flying VFR with flight following in a Tiger with a GNS 430. The screen went dark because of a malfunctioning auto display dimming piece of hardware. I could get it back for a bit by covering up the light sensor. If it was IMC and I had he rest of your panelto fly an an approach, etc. it would have been a problem.

Sounds like a piece of duct tape would’ve taken care of you.
Obviously your scenario above wouldn’t affect your secondary Nav source, so whatever you’d prefer to fly with would be your best backup.

But you've hit the nail on the head. What malfunction(s) is most likely? Bad screen killing the GTN 650xi? Bad cable killing all GPS? Panel power failure?

I’m really bad at statistics…and I’ve seen some really long odds come in for malfunctions. But it seems to me that how your navigation sources are interconnected to the rest of your panel (can you bring up your secondary nav on the primary display and/or your primary nav on your backup display? Do you have an autopilot, and what can it follow?) is probably a bigger consideration in my mind.

 

[Jim K]

My second nav/comm (kx155) has been having issues, so this is top of mind. I want some kind of backup navigation. I don't care a lot if its GPS or vhf, my main concern is being able to fly an ils or lpv approach. The viable options are a gnc215 for about $5k or a gnc 355, which is about $7k. I suspect the installation would be a little cheaper for the 215, but not by much. I'm thinking that if I'm going to spend 10k either way, I'd be willing to spend the extra 2 to get the GPS. That also adds a lifetime of database subscriptions, but I think the marginal cost isn't that much when bundled with the subscription I already have.

In the end, I have other places I'd rather put the 10k, so I'm investing some effort into cleaning contacts and adjusting the mounting on the antique King. If that fixes my issues, I think my next upgrade will be a standby alternator. In 800 hours, I've had three alternator failures, zero GPS outages, and zero primary navigator failures.

 

[chemgeek]

In 30ish years of post-GPS flying, I've had a bucketload of VOR/LOC/ILS failures, and only one (partial) panel GPS/NAV/COM failure. And I carry a second (bracket mounted portable GPS), third (EFB GPS map), and fourth (phone) emergency GPS units, so a second panel mount GPS seems redundant. I keep a second NAV/COM/LOC/ILS unit in my plane for a backup that is independent of and orthogonal to my GPS nav and com. But the GPS (a 430W) has always functioned sufficiently well enough to complete an IFR flight. NAV/COM/LOC/ILS #2 doesn't get used much.

 

[John Collins]

Even though your primary. gps and Nav radios are in the same box, think of them separately for a moment…

1. The box itself goes dark, so you lose both GPS and NAV. A separate GPS or a separate NAV would work.

2. GPS antenna goes bad, so you lose GPS but still have NAV. A separate GPS would still probably be wired into the same bad antenna, so you’d likely lose both GPSs.

3. Same for a bad NAV antenna…GPS would work fine.

4. Loss of avionics bus or total electrical failure. No navigation capability.

In all of the above scenarios, it really wouldn’t matter what your backup was, you’d end up in the same navigation mode.

Obviously the above isn’t an exhaustive list of malfunctions, only the malfunctions I’ve had. But if you think about other malfunctions from the same perspective, you might come to a conclusion that’s workable for you.

Number 2. It is extremely rare that a GPS shares the antenna between two systems. I don't think any of the popular Garmin or Avidyne systems allow this. However, there are instances where a GPS antenna will break into oscillation and in effect become a local GPS jammer knocking out all GPS in the aircraft, including portables. Turning off the offending system can restore GPS for the still working systems.

 

[William Pete Hodges]

I currently have 2 navcoms and 2 GPS units. The 2 GPS units have different added functions but I went this way so my wife could use one GPS while I navigate using the other. I like having ground based options in case the GPS system becomes unavailable but I confess I don't use VOR that much anymore.

 

[MauleSkinner]

Number 2. It is extremely rare that a GPS shares the antenna between two systems. I don't think any of the popular Garmin or Avidyne systems allow this. However, there are instances where a GPS antenna will break into oscillation and in effect become a local GPS jammer nocking out all GPS in the aircraft, including portables. Turning off the offending system can restore GPS for the still working systems.

Good to know…My understanding when it happened to me was that it was a single antenna, but it also want one of the popular Garmin or Avidyne systems.

 

[Pinecone]

On my traveling plane (Mooney 252) I am going with a 750Xi/650Xi combo. Decision was driven by the low prices savings of replacing existing 650 with lower level unit. I would be fine with a 750 and one of the GNC-3xx boxes for second nav, but happier with the 750/650 It gives full redundancy, and this plane is flown IFR a lot.

My VFR only plane has a King 155 and a GTX-335 for ADSB out. I am planning on going to a GNC-355 for comm and GPS, and swapping the GTX-345 from my Mooney to add ADSB In. I can live with GPS only, as it is Day VFR only.

 

[WDD]

Your Mooney is hard core IFR.

 

[Skyrys62]

In the end, I have other places I'd rather put the 10k, so I'm investing some effort into cleaning contacts and adjusting the mounting on the antique King. If that fixes my issues, I think my next upgrade will be a standby alternator. In 800 hours, I've had three alternator failures, zero GPS outages, and zero primary navigator failures.

Used kx155's are around two grand, so you could buy one, then
$8000 worth of alternators for the luggage compartment.



Or $1000 worth of alternators and $7000 worth of beer... either way.

 

[Jim K]

Used kx155's are around two grand, so you could buy one, then
$8000 worth of alternators for the luggage compartment.

View attachment 128977

Or $1000 worth of alternators and $7000 worth of beer... either way.

Great news...i fixed it! So $10,000 worth of beer!

 

[Pinecone]

Your Mooney is hard core IFR.

That is the idea.

Also remote Storm Scope and SXM for weather and music. Also GFC-500 autopilot. G-5 for backup. And 7" G3X for the right seat.

 

[WDD]

That is the idea.

Also remote Storm Scope and SXM for weather and music. Also GFC-500 autopilot. G-5 for backup. And 7" G3X for the right seat.

Do post a few pictures if you ever get a chance. Would like to see this panel.

 

[Pinecone]

Do post a few pictures if you ever get a chance. Would like to see this panel.

Here you go. That was yesterday, first time I saw everything in place and powered up.

 

[WDD]

First class panel there. Who did your work?

 

[TCABM]

Here you go. That was yesterday, first time I saw everything in place and powered up.

Saw that on MS, I think, and concurred with your assessment that there’s no room left to add anything else, so mission accomplished.

 

[Pinecone]

First class panel there. Who did your work?

Smart Avionics at N71. Super people.

 

[Pinecone]

Saw that on MS, I think, and concurred with your assessment that there’s no room left to add anything else, so mission accomplished.

Looking at some other panels in one of the threads on MS, I think the solution would be a long body. The panel is taller so more room.

 

[AlphaMike]

I faced this same decision a couple years ago when my old KX170 started having issues. I already had a GTN 750 and the kx170 was my com 2 and backup nav. I ended up getting a GNC355 as backup gps and com 2. They work great together for longer IFR trips. Flying IFR I usually leave the GNC on the route page and the GTN on the maps page. It’s nice having the route displayed with all the distances and times right there and the maps page is pretty much always on the GTN. If I’m just out flying around VFR I leave the traffic page up on the GNC and again the map page on the GTN. It’s super convenient and easy to use. They cross feed both ways so when you get a reroute or direct to a fix I can quickly make the route changes on the GNC and the changes automatically go to the GTN. They are completely separate other than that. They both have their own antennas too. (I think they have to )

 

[AlphaMike]

Old setup vs new.

 

[Pinecone]

I faced this same decision a couple years ago when my old KX170 started having issues. I already had a GTN 750 and the kx170 was my com 2 and backup nav. I ended up getting a GNC355 as backup gps and com 2. They work great together for longer IFR trips. Flying IFR I usually leave the GNC on the route page and the GTN on the maps page. It’s nice having the route displayed with all the distances and times right there and the maps page is pretty much always on the GTN. If I’m just out flying around VFR I leave the traffic page up on the GNC and again the map page on the GTN. It’s super convenient and easy to use. They cross feed both ways so when you get a reroute or direct to a fix I can quickly make the route changes on the GNC and the changes automatically go to the GTN. They are completely separate other than that. They both have their own antennas too. (I think they have to )

Good setup.

Interestingly, the GNC-355 is the same size as a King 155, and uses the same mounting holes. Coincidence???