There is a little mystery in this – because most autopilots in common use, including my late-1990s KFC225, were finalised in documentation terms before GPS approaches became operationally significant.
Currently, when flying a GPS/LNAV approach (that’s the sort which does actually exist in Europe and, hey, even in the UK) I load the approach, select the IAF, set a DCT to that IAF (this is GPS dependent but never mind) and the whole approach is flown in the autopilot’s NAV mode.
At 30nm radius to the airport, the GPS automatically switches its sensitivity from 5nm FSD (full scale deflection) to 1nm FSD. Actually I always set the GPS manually to 1nm for all flights, so that’s incidental too.
When the GPS turns onto the final approach track, it winds up its sensitivity gradually from 1nm FSD to 0.3nm FSD. This tightens up the lateral guidance and you end up flying bang on down the runway centreline.
But I never pressed the autopilot’s APR button.
Is that right?
The APR button is used for flying an ILS. You fly in HDG and ALT (possibly -VS) mode, and press it when about to intercept the localiser. It
The last point might apply to GPS/LNAV approaches too. Does it? I have never tried it.
Why don’t you go over to Calais and fly the LPV approach and find out!….I have a GNS430W with a KAP150 A/P and I would imagine that arming the APR mode would work exactly the same as an ILS (as you describe)….but I’d like to know for sure before I go there for my IR flight test in March!
My autopilot (Avidyne DFC90) arms the LPV/GPS approach automatically. All I need to do is load and activate the approach.
I am talking about GPS/LNAV, not GPS/LPV.
Also I am sure the DFC90 does something different…
The GFC-700 does not arm it automatically. If it is not armed by pressing APR (from memory), it will follow the flight plan and fly a lateral path comprising the approach but it won’t change the sensitivity and won’t capture any vertical guidance.
So you are talking about an approach with only lateral guidance? The DFC90 would either be in GPSS mode or in NAV mode which is the same in this situation. The GPS would have the approach loaded and automatically switch to the higher levels of accuracy. It would fly the approach normally as the GPS navigator sequences it.
In reality this means nothing. The GPS is always accurate, it would just crap out if RAIM/EGNOS tell it that the legally required accuracy cannot be obtained. I think this whole sensitivity thing has no meaning for for digital autopilots using ARINC429. My DFC90 is not controlled by CDI voltage unless there is an analog source (such as KX-155) but that would have to go through a glass cockpit (Aspen/Avidyne) which translates it to digital data.
Is there a digital sensitivity mode flag that controls how intercepts are done and I’m not aware of it?
So you are talking about an approach with only lateral guidance?
Yes – that is what “GPS/LNAV” is, AIUI.
The others (relevant to GA) are -
GPS/LNAV+V
This is GPS/LNAV but with the advisory “glideslope”
GPS/LPV
The full “ILS” lookalike
Also I was after only older autopilots: “because most autopilots in common use, including my late-1990s KFC225, were finalised in documentation terms before GPS approaches became operationally significant.”
The KFC225 autopilot does know when it is tracking GPS (as opposed to VOR or LOC) and I suspect the APR button does nothing at all if the GPS signal is active.
I am pretty sure one can (slightly bizzarely) improve VOR tracking by pressing APR (I recall somebody saying it works) which would tend to confirm the above, and I guess it is true for all the older (i.e. not DFC90 and not GFC700) autopilots.
It is years since I tried intercepting a VOR radial with the autopilot… it is probably exactly the same as intercepting a LOC, and APR will have the same effect. I just can’t remember…
Don’t forget LNAV/VNAV Peter in your list. The VNAV there is GPS derived and official unlike LNAV+V.
But I never pressed the autopilot’s APR button. Is that right?
Peter, I think when flying on GPS the APR button substitutes the 30NM to go. So you can already arm the GPS approach when 40NM out.
Although I only have the KLN90B, it is I think similar to the KLN94.
The KLN 90B will automatically change to the Approach Arm mode whenever you get within 30 NM of the airport.
(You can also select this mode anytime by pushing the GPS APR switch light.)
As the aircraft approaches the FAF, the GPS will automatically change to the Approach Active mode and the CDI scale will be down to + 0.3
NM if several conditions are met. (Integrity of unit at the FAF and MAP, RAIM is currently available, the aircraft is headed towards the FAF, the FAF is the active waypoint, and LEG
mode is selected.)
Peter,
A few corrections. A WAAS GPS flies an LNAV+V approach in the same manner as any GPS approach with vertical guidance, be it LPV or LNAV/VNAV or LNAV+V. The main difference is that with the +V, the glideslope is advisory and the MDA must not be penetrated as one may on an APV (LPV or LNAV/VNAV). IOW one is not permitted to treat the MDA as a DA. Airlines and approved operators may, but not private fliers. There is no assurance that the path between the point at which the +V GS reaches the MDA to the runway is in fact clear of obstacles. A second major difference is the annunciation of the approach reflects the most accurate service available on the procedure (LPV then LNAV/VNAV then LNAV+V then LNAV). All downgrades are to LNAV without any vertical guidance.
For the WAAS GPS, enroute FSD for the CDI is +/- 2 NM, not 5. Inside of 30 NM, this is reduced to +/- 1 NM. At the FAF, the CDI is the lessor of +/- 0.3NM or +/- 2 degrees. Most pilots aren’t aware of this angular CDI that shows on LNAV+V and presume it is the pre-WAAS fixed value of +/- 0.3 NM.
With a proper installation involving a WAAS GPS and KFC225, if one is flying an RNAV approach that has LNAV minimums and LNAV+V is available, and the full procedure is being flown including all 90 degree turns and or holds from the IAF, the following is how it is typically accomplished.
While navigating to the IAF, the GNS/GTN is set to GPS and the autopilot is set to NAV. In this configuration, the KFC225 uses its built in roll steering to fly the entire procedure including any curved paths and turns. Prior to reaching the IAF, the approach is selected: Airport, procedure, IAF and loaded. When the aircraft is cleared for the approach, the pilot either activates the approach or selects the fix in the procedure they are assigned to begin the approach at and issues a direct-to on the GPS. The pilot remains in NAV mode and either ALT mode or VS with a pre-selected descent altitude. When the aircraft reaches the last fix (usually the IF) that begins the leg to the FAF (IOW when the FAF becomes the active fix), the GPS will prompt the pilot to enable the GPS autopilot outputs. The pilot performs the enable and then presses APR on the autopilot. Next they must remain awake while the autopilot intercepts the GS or +V (same thing) and descends to the DA or a little above the MDA (+V). In the case of a DA (LPV or LNAV/VNAV) the pilot makes the decision and initiates a go around or continues to the runway. In the case of an MDA (+V), the pilot either levels off at the MDA without descending below it or visually sights the runway and avoids any obstacles.
