I’ve been trying to figure out how to use the vertical navigation feature of the G1000 to fly a conventional (NDB, VOR, LOC…) non-precision approach as a Continuous Descent Final Approach with vertical guidance. I’ve found a way which works but it’s clumsy and cumbersome.
In your typical non-precision approach in the G1000 database there will be a waypoint for the threshold. The obvious way to get vertical guidance would be to set the threshold crossing altitude as the VNAV altitude for that waypoint and then set the CDFA glidepath angle as the VNAV flight path angle. However the G1000 doesn’t let me set a VNAV altitude (or flight path angle) for that waypoint.
If instead of using the approach in the database, I define my own “approach procedure” in the flight plan by including a waypoint at the FAF and another one at the threshold, then I can set the VNAV altitude and flight path angle and I get the guidance as expected.
There are several drawbacks with defining my own “procedure”. The major one is that the G1000 doesn’t let me manually enter the built-in waypoints of the approach — particularly not the one at the threshold. Instead I have to define a user waypoint for the threshold and possibly also for the FAF unless it coincides with an NDB, VOR or an existing “regular” waypoint (which usually is the case). Defining a user waypoint from coordinates is obviously something you want to avoid doing in flight (not to mention that Jeppesen charts don’t give you the coordinates — you need the AIP which you typically don’t carry with you) so you have to know in advance what approaches you are likely to use. Also, I won’t get any initial, intermediate or missed approach guidance (unless I manually enter them into the flight plan as well).
Another drawback is too large cross-track error at full scale CDI deflection. When using an approach in the database, the G1000 will narrow the error at full scale deflection as you approach the runway — 0.3 NM at the FAF progressively going down to 350 ft at the threshold. With the homemade approach you will only automatically get 1.0 NM and you can manually set no lower than 0.3 NM.
Using VNAV on a non-precision approach is such an obvious thing that you want to do and the G1000 has all the necessary functionality, so I feel that there must be a better way.
You want Baro-VNAV. It is available as a feature on the G1000 high end systems, I think the King Air retrofit includes it. It is also available as part of the VNV feature, but this only works up to the fix prior to the FAF (usually the IF). Doing it your way would be illegal in the US.
What would be illegal in the US?
What I’m after here is the vertical guidance. As long as you adhere to the particular minimum, crossing and check altitudes of the procedure, I don’t see why using VNAV guidance from the G1000 would be any worse than working it out in your head.
If you are thinking of the lateral navigation, you can still use the conventional navaids rather than the GPS. In Europe it would certainly be legal to use the GPS for lateral navigation as long as you continuously check indications from the relevant conventional navaids — and that’s what I would do if I was flying an NDB approach. I’m not advocating flying NDB or VOR approaches using GPS only (unless they are authorised as overlay RNAV approaches).
The main safety risk as I see it is that you have to set the whole thing up yourself with all the possibilities of mistakes that entails — particularly when defining the threshold waypoint. Of course if you properly cross check bearings and altitudes you should spot any problems as you fly but it would be a risk anyway. So this is not something I would use in the spur of the moment.
I wonder why the G1000 can’t do this, or why it needs baro-VNAV for it. After all, you can calculate the VS manually and set it in the autopilot. And higher end systems have IAS hold.
However I recall reading something (to do with MH370 and the 777 FMS) saying that even high end systems will do only programmed descents, or only programmed climbs, and only down to / up to a preset figure. So a “final altitude” always needs to be set.
As I said, it can. It just doesn’t let you do it in a convenient manner…
If I understand your post correctly, it won’ do it to waypoints which are part of an instrument approach in the database. So you have to fool it by creating an overlay approach using user waypoints – yes?
In that case you also need to manually adjust the lateral sensitivity, to 0.3nm FS once past the FAF.
Yes, indeed I can adjust the lateral sensitivity. But I have to do it manually and I can’t get the narrowing to 350 ft at the threshold as I do with the database approaches.
AFAIK, manually setting it to 0.3nm FS sensitivity is the same thing, in terms of lateral guidance accuracy in the final stage of the descent, as the automatic sensitivity increase from 3nm to 1nm to 0.3nm which you get on a database GPS/LNAV approach.
The caveat is that if you fly the whole T-shaped approach with 0.3nm FS manually set, you will likely get some under-damped turn behaviour (e.g. overshoot and oscillation, though it should damp out eventually) on the 90 degree turn. The automatic sensitivity increase which you get on database approaches avoids this issue, by flying the 90 degree turn with a very low proportional gain (unless you have roll steering).
When I do this kind of thing (usually, flying VOR/NDB approaches using the GPS in OBS mode) I have the KLN94 set to 1nm for the entire flight (I do this before takeoff; the 5nm setting is just too crude) and I set 0.3nm just after establishing on/near the final approach track.
Then, the tracking takes me bang down the middle of the runway, every time. And this is the old KLN94…
LPV is different because you don’t have a constant lateral sensitivity anymore. You have a sort of ILS-like lateral “cone” for the “LOC”, and another one for the “GS”. But that is irrelevant because obviously your system can fly the LPV GS so VNAV is not used.
Have you tried the following (in the G1000 sim):
Press PROC and load your NDB (or whatever) approach and/or activate it.
Press FPL on the MFD and move down to the MAP
There should be a column with the desired altitude at that point that you can maybe set/override.
If you manage to do that you will essentially use the VNAV feature of the G1000 and get a TOD with BOD at the MAP… and you should get vertical guidance. Have not tried to do that myself, but I do use the VNAV feature of the G1000 to either set the desired altitude for a point on my flightplan, or to be at procedure altitude at the IAF (those are already coded in the PROC).
Aviathor, VNAV or approach vertical navigation won’t work on a NPA after the FAF unless it is coded as LNAV+V or one of the approaches with actual official vertical guidance ie VNAV/LNAV.
I think the only way to achieve what you want is with user waypoints and VNAV. Not that I am advocating you do it. The reason why it is not convenient to do it is because they don’t want you doing it.
