OK; that's what I heard too.
I actually don't believe that explanation because the KLN94 outputs ~1 packet per second which, perhaps with some crude interpolation to generate intermediate output values, would do just fine.
I am holding back on the GPS upgrade because the MFD (KMD550) is not fully compatible with any Garmin GPS, so it would become a major refit, of an aircraft which currently does 99.x% of European IFR requirements (noting that LPV is in its infancy here).
Peter,
Yes, I am referring to the ARINC 429 bank angle command to the autopilot. I understand that the KLN94 has an analog roll steering signal that the KFC225 can use, but I don't know how well it works. When DAC was designing their GPSS product they attempted to use the analog KLN94 roll steering signal but found it totally unsatisfactory and gave up. When you upgrade your GPS, you will want to hook up the ARINC 429 interface as it performs great and the auto-slewing is at best a poor substitute as it still uses the CDI deviations in addition to the auto slew OBS to guide the autopilot. Wind correction is as close to immediate as one can get using roll steering, not to mention being able to fly curved courses such as holds and procedure turns which are not defined by the CDI deviations.
NCYankee,
I take it you are referring to the ARINC (serial data) roll steering input of the KFC225 (GNSx30/GTNx50 scenario) and not its analog roll steering (KLN94 scenario)?
I never did connect up that analog roll steering... those wires involve access to the back of the TB20 centre stack, which is just too much hassle for the potentially ("potentially", which nobody I know can vouch for first-hand) better turn performance. Plus, there are claims from Honeywell that an extra relay is needed, to invalidate the analog rollsteer signals, when some external mode is active.
With the Sandel EHSI I get an auto-slewing course pointer so already get automatic turns at each waypoint; the only drawback is that the turn is initiated too early.
Peter
Peter,
The KFC225 uses the GPS CDI source signal in Nav mode to determine if it will use roll steering (GPS) or CDI steering when roll steering data is available. IOW, if GPS CDI source is false and roll steering is true, it will use CDI steering. If GPS CDI source is true and roll steering is false, then CDI steering is used. If both GPS CDI source and Roll Steering are true, then roll steering is used.
It also affects the lateral sensitivity when CDI steering is used (VOR - low sensitivity and GPS - high sensitivity). Vertical GS/GP guidance is only provided when the Nav CDI source is VLOC and the other conditions of an ILS are met.
Thanks for a great explanation, John.
I never knew the KFC225 even knows whether the nav source is NAV or GPS...
I wonder why it need to know it? Is it to modify intercept behaviour?
The APR button only (I thought) increases the lateral sensitivity.
Peter,
The reason the KFC225 and GNS430W/530W (and GTN) work the way they do where you are prompted to engage approach mode by the GNS430W/530W has to do with the addition of vertical guidance on RNAV (GPS) approaches and how it is implemented. The KFC225 does not support vertical guidance when the navigation source is GPS, it only supports it when the navigation source is an ILS. To fake the KFC225 out on a RNAV (GPS) with vertical guidance, the GNS430W/530W uses a discrete circuit to drive the KFC225 source identification (GPS or VLOC). It fakes it to VLOC, rather than GPS. At the same time, the signal "ILS or GPS approach" is substituted for the normal "ILS energize" and is active on any ILS or GPS approach. At the point in the approach where the ILS or GPS GS/GP normally becomes active, the GNS430W/530W displays a message and the pilot acknowledges it with a PROC "enable autopilot outputs" choice. Immediately after this point, the pilot is expected to engage APR mode on the KFC225. If "prompt" is selected in the configuration, the auto ILS switch function is disabled and the pilot can't enable the feature.
Why did Garmin do this? They did it at the insistence of the FAA to obtain certification for using the KFC225 with the GNS430W/530W. Here is the issue that affected the operation on RNAV (GPS) approaches. With a GPS approach, the navigation data source is GPS. If you engage the autopilot in approach mode, when the GNS430W/530W fakes out the autopilot, the autopilot detects a navigation source change to VLOC and disconnects. This can be overcome, by selecting approach mode a second time, but this was deemed unacceptable by the FAA and the scheme I described was accepted by the FAA. The consequence is that you have to manually assert the autopilot signals via the prompt response on the GNS430W/530W and that auto ILS switch is disabled. I personally don't like anything that auto switches and prefer to make and verify my own choices, so I don't find the solution onerous.
Regards,
John Collins
It does that no problem.
A typical one is an ILS-Y for Annecy LFLP, which we would load in the FMS as a Salev 5Y with an AT transition, then an ILS-Y for runway 04
It does it all by GPS and changes to the ILS at the appropriate time.
I think it's more to do with the quality of the databases and the FMS
we would fly in HDG mode until cleared for the approach
If the whole thing isn't seamless it puts a discontinuity in the flight plan which means it doesn't automatically sequence past that point unless you intervene..
A variation of this might be flying a procedural ILS approach, where you set the fix as a DCT and fly the outbound track in the GPS OBS mode (in NAV), but then you want to intercept the localiser on the way in. The "NAV/GPS" switch has to take place after the outboung leg has been flown, and after you switch to HDG. Again, I can't see any way of doing this other than manually. Or is there a GPS with an overlay of the ILS approach which will automatically switch to LOC for the inbound leg? That would be really impressive.
Falcon has it, we would fly in HDG mode until cleared for the approach. If you have programmed the FMS to intercept the localiser at 50 miles it will do it. But the change to green needles will not happen if there are further waypoints which are not part of the actual approach.
However, the normal plan you would load would be off an arrival/transition/approach all from the database of the FMS. The system expects that, and the start of the actual published approach is one of the rules it uses.There is an option to put vectors as the transition which covers that eventuality.
If the whole thing isn't seamless it puts a discontinuity in the flight plan which means it doesn't automatically sequence past that point unless you intervene..
I am sure that some systems are even more integrated than ours, but the point I was making is that the automation needs to be monitored very carefully, even in a system that has been made for the aircraft. I think everyone has been through a "what's it doing now?" moment.
