They told me that while the GPSS button means that the roll steering signal goes straight from the GPS to the A/P – with NAV it uses the data from the PFD, which is the same data just coming from the PFD … but I am still investigating this :-)
The G1000/GFC700 is a fully integrated system. There is no concept of GPSS. The concepts are of NAV or APR and flying a full procedure for an ILS is seamless.
GPSS is a glue technology and not fully integrated with legacy autopilots. GPSS is GPS Steering. A more correct definition is roll steering using a GPS for the path control. Roll steering has been available on the larger FMS based systems for some time, but hasn’t made its way into the low end of GA until the advent of GPSS adapters. The legacy autopilots use a form of CDI steering for the Nav and Apr modes. IOW, a reference course is modified by CDI deviations left and right of course. This is a relatively crude method of navigating using the autopilot, with some autopilots doing a better job than others.
Roll steering is based on the concept that the FMS or GPS can have a flightplan stored in it, can determine its position (plus other data such as current track and groundspeed) relative to the flightplan, and provide a bank angle to the autopilot to correct the current flight path to match the desired flight path. Roll steering can fly a curved path, CDI tracking can only fly a straight path. Roll steering can be used to anticipate an upcoming course change and steer the aircraft to lead the turn. Generally CDI tracking is limited to tracking to the turn point, passing thru it, and then taking up the new course (although with advanced features such as auto-slew, some of this can be improved on).
Unfortunately, most legacy autopilots don’t support a roll steering input. The later ones such as the KFC225 and Stec 55X integrate a GPSS input, but did not anticipate vertical guidance on GPS approaches. For autopilots that don’t have a roll steering input, the GPSS adapter was invented to convert the GPS roll steering bank angle to a heading error signal. This works because an autopilot responds to the heading error signal (the electrical difference between the heading bug position and the current heading) by commanding a bank angle that is proportional to the heading error signal. IOW, a 5 degree heading error signal causes the autopilot when in heading mode to correct towards the desired heading by a 5 degree bank angle. For a 10 degree heading error, 10 degree bank angle, and so on, up to the autopilot limit (typically 25 degrees or 90% of standard rate turn). The GPSS converter converts the output of the GPS roll steering signal (a bank angle) to a heading error of the corresponding amount. Of course to get the autopilot to follow the GPS roll steering output via the GPSS converter, the autopilot must be in heading mode. Since the GS requires the autopilot to be in APR mode rather than heading mode to track a GS, a change to APR mode from HDG mode is required when on the course to the FAF. Complicating this is the fact that with GPSS, its signal is based on GPS navigation with most installations requiring that GPS be selected as the CDI source (not all require this). The complication is that the CDI source needs to be switched to the Localizer (VLOC) for an ILS. Auto VLOC switching on an ILS approach is a pilot selectable feature on most later GPS’s units (except for a GNS430W/530W or GTN when used in combination with the KFC225).
Flying an RNAV procedure with vertical guidance is somewhat less complex as one doesn’t have to worry about CDI selection changes, but since legacy autopilots don’t directly integrate vertical guidance from the GPS on such an approach, the autopilot has to be faked into believing that an ILS is taking place. This means that one must still switch from HDG/GPSS mode to APR mode, at least if one wants to use a GS tracking feature.
For me, here in the US, I shun flying an ILS except for practice. The LPV is much easier and typically has the same 200 and 1/2 minimums. I haven’t flown an ILS in actual conditions in over 6 years.
Here in the US, the insurance companies generally require that a Cirrus Pilot obtain type specific training from a Cirrus Standardized Instructor Pilot (CSIP) and using the Cirrus training. The training is quite extensive if followed. The Cirrus with the Perspective G1000 system is a very capable system but it takes extensive study to master all of it. Cirrus pilots should receive periodic refresher training. I would not be surprised if the age of the pilot was a factor in this accident.
Yes, but the DFC90 is not one of these legacy autopilots and works different than let’s say an S-TEC55. If you replace the S-TEC55 in an Avidyne equipped Cirrus with the DFC90 you get almost the same level of integration as with the Perspective/GFC700. And it will fly the same anticipated turns in NAV mode! To fly an ILS with vectors you siply use HDG+NAV.
But in a standard approach when you put the ILS at the end of the GPS flightplan in the GNS430 you have to fly this GPS flightplan in NAV mode, because the switch from GPS to VLOC will not happen if you are in GPSS mode.
Well, sure the training with a CSIP can be very good – but in my case i discovered that the CSIP had no idea ( i mean no idea) about how the DFC90 A/P works. And THAT CSIPs was one of the best known around. The problem is that “CSIP” is just a lable, but – like always – it really depends on the person if you will learn much. In my case I learned much more from experienced SR22 owners than from the CSIP.
Also my insurance did not require the standardized Cirrus training.
But in a standard approach when you put the ILS at the end of the GPS flightplan in the GNS430 you have to fly this GPS flightplan in NAV mode, because the switch from GPS to VLOC will not happen if you are in GPSS mode.
GPSS has no effect at all on whether the GPS will switch to from GPS to VLOC, In order for the GNS430/530/430W/530W series GPS units to auto switch from GPS mode to VLOC, several conditions must be met:
There is also a difference in the way that the GNS430W/530W verses the legacy units work with respect to providing roll steering output while flying an ILS. In the W versions, roll steering guidance ceases being output by the GPS at the FAF to the MAP. In the legacy units, roll steering is provided between the FAF and the MAP. Both versions will provide roll steering for a VOR approach.
in my case i discovered that the CSIP had no idea ( i mean no idea) about how the DFC90 A/P works.
Unfortunately I don’t think you will get a concise explanation of the functionality until you find somebody who knows the underlying architecture of your system (what is feeding what, etc).
The only way I could explain to you how my stuff works (and my stuff is much older than your stuff) would be by drawing a diagram of the various boxes, how they join up, what the NAV/GPS switch does, etc.
The beauty of the old 1990s avionics from Honeywell (which I got in the new TB20 in 2002) is that everything worked exactly right, there was just one way to make it fly a GPS track, exactly one way to make it track an ILS, etc. There were 1 or 2 small gotchas but that was all.
Then the manufacturers started screwing in big pieces of eye candy, with variously contrived autopilot connections to ancient autopilots (or current autopilots designed to be sold into the ancient aircraft retrofit market) and almost nobody (who flies) really understands the underlying connections. The installers should understand it but nearly all of them are not pilots. and none of them run training courses for pilots. I have never flown with a pilot flying with any of the current stuff who understands it fully. For all I know that is true for jet pilots too but it doesn’t matter so much there because their stuff is much better integrated, and they have to do type rating courses which cover the standard operating modes (GPS enroute, radar vectored ILS, etc). AF447 was one of the more wild exceptions.
For the wider community, the most unfortunate aspect of this SR22 event was that it was N-reg and flown by an EU based owner who – I would guess – does not hold the “supreme quality” gold plated European IR with its ~£200 annual revalidation instructor fee. As April 2015 is not here yet, he doesn’t have to, but that won’t stop some people saying he was flying on substandard papers. To be honest, in his mid 70s he will be highly unlikely to be doing the whole IR again anyway and – like a few older pilots I know – he will prob99 chuck in IFR and possibly all of flying when 2015 comes.
NCYankee,
I know all that, but: in the Cirrus SR22 with GNS430/DFC90 the switching to VLOC will not happen out of GPSS, only out of NAV mode. Ask ANY pilot with that plane/configuration and you will hear the same. Also for a VOR approach you MUST press the APPR key, while you do not for the ILS.If you are on the intermediate leg of a standard ILS with GPSS the auto swicth will not work, and Avidyne confirmed that to me and also that it will only automatically transition to VLOC frm NAV or HDG+NAV.
Alexisvc,
I believe what you are talking about is the DFC90 function to auto switch from Nav mode to APR mode verses not doing this from the DFC90 GPSS mode. This is not VLOC auto switching, which is strictly internal to the GNS430.
NCYankee,
yes, you are right. Seems like it still confuses me every now and then …
