OBS resolver and LPV approaches

[VinceCB]

Hi all,

Long time lurker, first time poster as I just bought my Mooney this week.
I will be replacing a KX-170B nav/com with Garmin's newish GNC355 waas/com. The CDI currently installed is an older design, I believe a KI-209, but the aircraft is currently getting its annual so I'm not 100% sure. The avionics shop that will (probably) do the installation told me:
"One thing to note, if it matters to you, your existing indicator does not have an OBS Resolver compatible with the GPS. That will limit you to non-precision GPS approaches."
I assume they are referring to LPV approaches here since the box doesn't have a nav radio and there is nothing else connected to the CDI (though LPVs technically aren't precision approaches?). Does anyone here know more about this? I thought no OBS resolver simply meant you couldn't use the OBS knob in OBS mode, though you could still input a radial in via the touch interface. Do LPV approaches really require an OBS resolver? If so, I may just be poorer than I thought

Thanks,
Vince

Edited September 17, 2020 by vince_cb

[carusoam]

Welcome aboard Vince!

Congrats on your acquisition...

Check in with @Alan Fox he had a box full of the proper displays that you are talking about...
 

Alan deals in pre-flown and overstock devices....

Best regards,

-a-

[kortopates]

3 minutes ago, vince_cb said:

Hi all,

Long time lurker, first time poster as I just bought my Mooney this week.
I will be replacing a KX-170B nav/com with Garmin's newish GNC355 waas/com. The CDI currently installed is an older design, I believe a KI-209, but the aircraft in currently getting its annual so I'm not 100% sure. The avionics shop that will (probably) do the installation told me:
"One thing to note, if it matters to you, your existing indicator does not have an OBS Resolver compatible with the GPS. That will limit you to non-precision GPS approaches."
I assume they are referring to LPV approaches here since the box doesn't have a nav radio and there is nothing else connected to the CDI (though LPVs technically aren't precision approaches?). Does anyone here know more about this? I thought no OBS resolver simply meant you couldn't use the OBS know in OBS mode, though you could still input a radial in via the touch interface. Do LPV approaches really require an OBS resolver? If so, I may just be poorer than I thought

Thanks,
Vince

What you need is a OBS unit that is compatible with your GPS. The KI-209 is not compatible, but a KI-209A is compatible. I am a bit confused by your avionics guy saying it would limit you for non-precision approaches like LNAV or LP - but it may be saying you could use the CDI on the GPS screen in lieu of a external OBS but I thought such a configuration is  not approved for IFR use without a compatible OBS. But I am not an avionics guy.

Prior to glass one would use an external GPS OBS such as the Garmin GI-106A, like your old KI-209. But these days that makes no sense to spend $ on  and when we have much better options with something like a Garmin G5 HSI or something better like a GI-275 before going bigger glass.

[Alan Fox]

The KI 209 IS compatible , This unit will drive a Composite , or resolver type indicator.....  That being said , I have tagged KI209a indicators , and NEW Ki 206 indicators , The 206 is a resolver type indicator , They are 1400 new with paperwork....    

[carusoam]

MS is running at full speed tonight!  

Alan, got any of the Big G units in case Vince (The OP) prefers to match manufacturers together...  re the GNC355...

Best regards,

-a-

[Alan Fox]

Nope.

[VinceCB]

Thanks everybody. I'll double check with the avionics guy as to what he means. If he is correct, I'll be in touch with you Alan.

Much appreciated!

[VinceCB]

For the sake of completeness in case anybody reads this thread in the future...
Turns out I challenged the shop and they got back to me saying the person who wrote me was new and didn't quite understand what they were told. In fact, you can fly GPS approaches including LPVs without the OBS resolver. The only thing you lose is OBS mode. This will be acceptable for me until I swap my DG out for an HSI in a couple of years.
Thanks to all contributed!

[hammdo]

I have the GNC 355 install going on now. Got the VLOC/GPS switch and RS08 relay so it would work with the 209 and the 155. Chief Aircraft set me up with what was needed. Dana was super helpful...  This way I'd still have LPV and ILS/LOC.. 

 

-Don 

[kortopates]

2 hours ago, vince_cb said:

The only thing you lose is OBS mode. This will be acceptable for me until I swap my DG out for an HSI in a couple of years.

I know it doesn't seem like much, but if you're going to use this to earn your instrument rating its pretty important. If you're already instrument rated then you already know what you'll be missing and can fall back to VOR nav when ever you might need it - such as "intercept the 340 radial or bearing to XYZ" - which of course is pretty rare but does happen.

[VinceCB]

You can use the 355 to do this without OBS mode connected to the CDI. In this case, you would do “direct XYZ”. Then, direct again and an OBS button will appear where you can select radial 340.

Edited September 20, 2020 by vince_cb

[Ragsf15e]

On 9/19/2020 at 7:53 PM, kortopates said:

I know it doesn't seem like much, but if you're going to use this to earn your instrument rating its pretty important. If you're already instrument rated then you already know what you'll be missing and can fall back to VOR nav when ever you might need it - such as "intercept the 340 radial or bearing to XYZ" - which of course is pretty rare but does happen.

I agree with you that it’s easier to use a dedicated obs knob on the cdi, but I’m pretty sure I can select obs on my 430w and then select the actual course on the 430 as well.  I haven’t used that, so I’ll have a look, but I’m thinking he’ll still have that functionality on his 355.  I don’t think it would work with a vor signal, but with his 355 that isn’t a problem.

[midlifeflyer]

On 9/16/2020 at 9:10 PM, vince_cb said:

though LPVs technically aren't precision approaches?)

"Technically" true but operationally insignificant in the US except for selection of alternate airports on an IFR flight plan. 

[midlifeflyer]

On 9/16/2020 at 9:28 PM, kortopates said:

thought such a configuration is  not approved for IFR use without a compatible OBS.

I tried to read up on this when a friend of mine acquired an airplane with an IFR GPS which was not connected to the OBS at all. I'm still confused but it seems that, from the FAA's perspective, so long as a CDI is available within certain visual angles, it can be the one in the unit.  OTOH, you will find the AFMS limitations for some units specifically require an external CDI. So I think the answer is, "it depends."

[jamesm]

I have installed a GNC-355 and GNC-255  that I am able to toggle between the two on my G5'S. Both GNC units have lateral CDI on the navigator itself.  Having two deadicated CDI's probably saves you money wise but not panel space wise. If you were to go with a single CDI driven by two separate navigation sources. Especially if one  of the nav units is not Garmin,it gets expensive. you get to  buy a real expensive set of relays, annunciator, switch setup $1200 + I think.

With the two G5's and GAD29B and two Garmin nav source units driving The CDI, takes the need for switch relay annunciator unit. I think have ran out of room behind the panel.my creativity in finding space behind the panel to mount all these extra boxes may have ran out.

James

'67C