Poorly explained in the docs? May well be. But in substance, on a DFC 90, as with the STEC it replaced, the only time you NEED NAV rather than GNSS is when you fly a coupled ILS as it won’t capture the glideslope in GNSS mode. That said, I know that a number of Cirrus pilots habitually use NAV all the time and it works.
Each to his own…… !
I know it works but I never understood the need for the distinction. Maybe you can only get rid of it with more integrated systems.
Unfortunately a lot of these issues are installation specific.
I have an Aspen +GNS + KFC225 Autopilot. With auto CDI enabled on the GNS, and providing the LOC is tuned in, the GNS will auto switch from GPS to VLOC, in GPSS mode and set the CDI on the Aspen to the final appr course.
But it was beholden on me to get intimate with all of my systems, and in particular figure out what I would loose should any element in the chain fail.
Sure separate bits of knowledge and theories of how it should all work are available, but I cannot realistically expect anybody else to have such an in depth understanding of the whole, unless they regularly flew or trained on the exact same set up. Throw in different software versions of the same unit and it becomes almost impossible.
I do watch the system like a hawk on every approach as it has thrown it’s toys out on a few occasions, some due to me having a senior moment :-), but the key in all those situations is to be ahead sufficiently to know what to expect, and if it is all starting to go pear shaped, as Jason says, disconnect the automation and fly the aircraft.
Imperfect as it may be, self sufficiency is the reality of our world.
E
Jason
On the Cirrus G2 (Avidyne) at least, there’s no real absolute need to distinguish between NAV and GNSS other than the specific case of a coupled ILS when you need to be in NAV mode to capture the glide slope and fly it coupled.
GNSS mode anticipates the turning point and starts the turn early to keep it smooth. NAV turns at the turning point and is more abrupt. In practice, if the turn is less than about 90 deg, it’s smooth either way. If it’s more, GNSS is easier.
I understand you all are taking about G P SS (not GNSS…).
The DFC100 autopilot (which is verry much the same as the DFC90) doesn’t even have the GPSS key anymore. The GPSS key was left on the DFC90 becasue it was meant a pull out push in retrofit to the S-TEC55 and the idea was to give the users pretty much the same user interface.
In reality NAV does the same as GPSS – but it will captuer the loc/gs – and it will also enable the automatic switching of the PFD and the automatic setting of the HSI
This whole NAV/VNAV/LOC/VLOC business is horrible and this mostly totally confusing and confused discussion shows why very type specific training is absolutely necessary.
The terminology varies with aircraft and with the equipment brand. It also varies with the way the avionics shop wired it up; I have seen different bodges in the way say a GNS430 can be wired up to the autopilot which change the way one selects whether one wants it to track the GPS track, or a localiser.
I recall a flight to Exeter in the early days with the TB20, when I was finishing off the IMC Rating with one of those “wot das da HSI doo” instructors. I intercepted the LOC and thought aren’t I a great pilot, but the GS didn’t appear when expected…. it turned out I was tracking the GPS track, not the localiser, and it just so happened that the last leg of our route coincided with the runway track!
11 years later, during a very, ahem, very recent instrument test, I was trying to intercept the inbound leg to the holding fix using the GPS but could not make it intercept. It turned out the “GPS/NAV” switch (the cause of many mysteries in aviation)
was set to NAV, not GPS, because I had earlier flown an ILS and since then had been flying in the HDG mode where it doesn’t matter, because in HDG mode the autopilot is a standalone box and doesn’t care what the GPS is doing (which is itself extremely useful because you can set up the GPS to the OBS mode and get it to display the bearing line of say an ILS).
In this case I switched to HDG very fast and carried on… the heading bug was already set, as it should be in all phases of flight. And anyway I have an “ILS” item in the checklist now, to make sure everything is correctly set to fly an ILS, with the MFD showing the inbound track as a mental guidance.
Traditionally, “NAV” means tracking VOR,LOC,GS and “GPS” means tracking a GPS track, but the more recent avionics jumble this up with stuff like VLOC.
Also, in a “classical” installation, the “NAV” button on the autopilot will make it track both GPS or VOR/LOC sources – because the autopilot doesn’t know where the guidance is coming from. So you have two “NAV” thingies: one (on the autopilot) does all nav sources, while the other (in the panel, or on the GPS or EHSI if appropriately wired) specifically selects the two types of nav sources. Both have the same name: NAV.
What is necessary is some level of understanding of the avionics interconnections… a bit like this
but where are you going to get that? (that diagram is obsolete BTW)
The more controversial point is that not every PPL holder – that can afford to splash out say $600k – will be able to get his/her head around it… On another site, the pilot was (predictably for that site) getting lynched for flying IFR in his mid 70s and it was suggested that this should be banned, but I don’t think it is that simple. It needs a different training approach.
GNSS mode anticipates the turning point and starts the turn early to keep it smooth. NAV turns at the turning point and is more abrupt. In practice, if the turn is less than about 90 deg, it’s smooth either way. If it’s more, GNSS is easier.
Yeah, but exactly why? That is just a totally bizzare way of arranging lateral navigation. Properly integrated avionics should not give you two ways of doing the same thing, with one doing a different quality of intercept.
The only thing which is worse is having an HSI with roll steering (a.k.a. GPSS) where the plane will turn to the next leg of the GPS route but the HSI course pointer doesn’t move. Loads of people have that.
GPSS roll steering in its original meaning gets the data directly from the GPS navigator while “NAV” traditionally “chases” the needle of the CDI. Therefore GPSS is more precise and anticipates the turns as described.
But the DFC90 autopilot does not work this way and flies the GPS data in NAV aswell. I am still trying to understand the exact difference, but Avidyne and other Cirrus pilots told me that there is none and that it flies just as precise in NAV.
But the DFC90 autopilot does not work this way and flies the GPS data in NAV aswell. I am still trying to understand the exact difference, but Avidyne and other Cirrus pilots told me that there is none and that it flies just as precise in NAV.
Yet, that can’t be right, because you have two completely separate nav sources
and each of these can do vertical guidance too, potentially
Do these systems really try to merge the two navigation sources under one heading? That would be bizzare. A pilot has to make the choice consciously – that is how “IFR” works.
There is no way to be tracking anything from a GPS when flying an ILS. What is possible is that when an ILS is detected (e.g. by tuning an ILS frequency on the radio) the system switches automatically from GPS based nav to LOC based NAV (but continues to fly the current heading until LOC intercept, etc) but such a mode change must be clearly annunciated somewhere.
Yes, that’s what I say. The standard ILS procedure is used as an “ovelay” and there’s even a message on the GNS that GPS is “for guidance only”. Once you get close to the localizer the system will automatically go from GPS to VLOC and set the FC on the HSI.
The bizarre thing is that the NAV and GPSS keys essantially do the same thing if you use the autopilot enroute. So it makes no sense to use “GPSS” at all.
I would not expect the DFC90 to be controlled with analog signals – more likely ARINC429 straight from the GPS/PFD box. So the whole NAV/GPSS stuff on the AP front panel is a bit silly. If I have that right, the GPSS button should be placarded INOP 
