Say you have 2 x GNS430W, 2 x GTN750 or 2 x IFD540.
Is there a switch somewhere which selects which one to use?
If you have a mechanical HSI then you have to switch it somehow – and switch loads of wires (the OBS back to the GPS has to be switched too) so that needs a relay or two.
If you have an EHSI then that will take two GPS sources so the switching is done inside the EHSI. Then you have to lose the traditional NAV/GPS switch which implicitly limits you to one GPS (unless you have yet another GPS1/GPS2 switch as well).
What about the autopilot source? One could drive the autopilot from the EHSI’s FCS outputs and that will take care of it, but most installations connect the autopilot direct to the GPS to get roll steering, so the autopilot needs a switch for that also. And you need to avoid a situation where the EHSI is displaying GPS1 while the autopilot is following GPS2, etc.
I know there are TB owners who have 2 x GNS430 (that was a Socata factory option) but AFAIK their autopilot and their HSI is NAV1/GPS1 only.
I know there are TB owners who have 2 x GNS430 (that was a Socata factory option) but AFAIK their autopilot and their HSI is NAV1/GPS1 only.
This is how I would install it. Keep it simple. If you want redundancy, take the hex key along with which you can remove the units from the panel and swap them in flight… they are held in place with a single screw
There is no smiley emoticon so I’m not sure whether the in-flight box swap is serious or not. I’ve got a dual 430 setup but there is no way you could get them out of the racks in the air, its hard enough on the ground – quite apart from them fouling the full control wheel range.
I have one GPS driving the HSI and the other drives a CDI, I leave them on crossfill so I can get full screen map on 2 and nav data on 1. There is a switch to select which GPS drives the (very simple) A/P.
There is no smiley emoticon so I’m not sure whether the in-flight box swap is serious or not.
Oh, sorry, I forgot … 
But over the years I have seen some awesome in-flight repairs being done. During one of my first IFR lessons, in full IMC, the instructor (and flying school owner) laid on his back head down from his seat and disappeared under the instrument panel to re-attach the antenna cable that had come off the ADF receiver…
Most installation using mechanical HSI/CDI don’t switch the #2 with the autopilot because of the number of wires involved and not having a source annunciation on the HSI. With an EHSI, or PFD, it can generate the autopilot outputs and switches what is driving the autopilot based on what source is the active source on the display. I would not do it with a mechanical HSI. You can still fly the airplane with the number 2 system that has its own dedicated CDI without coupling it to the autopilot. If I were to couple anything, it would be to switch the roll steering output from the two units with a 1 – 2 switch. This would not provide vertical guidance from the #2 nor would it support the autopilot flying the ILS from the number 2.
I have swapped COM boxes in flight. I was on airways when one box failed and the other became unusably noisy. I swapped them, which did indeed involve some fairly audacious cross control flying in an AA5B for a few seconds, and got one working (the other failure was in a corroded aerial). What could have been a serious incident turned trivial because I had the Allan Key with me. I would not now fly without one (though it is more commonly used to remove and rerack an item, rather than swap.)
The G500 has a button to select the primary GPS
You could use one for Nav, one for RNAV approach?
I take the above points on board but they seem to suggest that the “lower” GPS is basically wasted, and does little more than give you
It seems a big waste to do that.
The problem is that – in the general wisdom posted here previously – MFDs are a dead technology, so the traditional “FMS+MFD” functionality (obtained very effectively with e.g. a KLN94+KMD550, or more recently with say a GNS430W+EX600, etc) is to be given up for ever, and anybody doing a serious upgrade should make do with just one big GPS, which right now means a GTN750 and nothing else.
But a GTN750 is not an MFD. It can display everything an MFD can display but it takes several button pushed to move between the display functions. So, if you are to assume that MFDs are indeed dead, you are looking at say a GTN650+GTN750, but if you want the 750 as the MFD, the GTN650 will be doing all the “driving”, which is arguably a waste of the 750 and its $14k raw-box purchase price (=~€20k-25k+ installed in Europe).
But the 650 is too small IMHO. Like the IFD440, it makes a nice touch screen keypad for the “other box” and sure enough that is how it is usually demonstrated at exhibitions
That leads to waiting for the IFD540 and putting in two of them. It looks a technically superior product anyway – from comparing them at shows. Of course one could install 2 x GTN750 but that is more height than most centre stacks can take.
I am also not convinced that MFDs are dead. They probably are dead (no new capability being developed) if you want to show advanced mapping data but such data usually involves annual subs into 4 digits and not many pilots actually spend the money – after the first free year during which they take all the photos I certainly wouldn’t pay it. But the only database an MFD needs for actually flying is the basemap, and e.g. my KMD550 is from 2010 and still fine.
They probably are dead (no new capability being developed) if you want to show advanced mapping data but such data usually involves annual subs into 4 digits and not many pilots actually spend the money – after the first free year during which they take all the photos I certainly wouldn’t pay it. But the only database an MFD needs for actually flying is the basemap, and e.g. my KMD550 is from 2010 and still fine.
Peter, what do you mean by advanced mapping data?
Peter,
Surely you can fly a #2 GPS with a dedicated CDI for any type of approach without an autopilot.
If the GPS is what failed, then the autopilot should still be operational and can easily be controlled with heading, pitch, and altitude hold if you must. Can’t use the full features of the autopilot, but certainly you can use the full features of the GPS/Nav system. If you have roll steering, as it is only a pair of wires, they are easily switched and can be used for all lateral navigation except one of those old fashioned ground based LPV,s, whoops, meant ILS.
