And even then, flying an ILS to minima is a piece of cake.
Peter wrote:
Everybody else posting on this topic (myself included) here and elsewhere has been speculating as to what Garmin use GPS for.
I can ask my mechanic (he owns C172 equipped with 2 x G500 TXi, GTN750 Xi, GTN650 Xi, G5 and GFC500) whether he can simply dial ILS frequency on GTN, chose it as NAV source and invoke coupled approach. In that case GFC500 doesn’t have a clue on airport database stored data and should act only on ILS signal (plus air data).
Airborne_Again wrote:
In fact, my experience of both GPS boxes and A/Ps is much the opposite and clearly lots of people on EuroGA have the same experience.
Same here, having in mind components of A/P including servos – there’s much higher probability of A/P failure than of GPS box. Plus you can have two GPS boxes that can be used as NAV/APP sources for A/P.
Emir wrote:
In that case GFC500 doesn’t have a clue on airport database stored data and should act only on ILS signal (plus air data).
It does. The GTN boxes have digital connections to the G5 (and thus the GFC500, since the logic is in the G5) that feed all the data no matter what navigation source is selected.
Airborne_Again wrote:
It does. The GTN boxes have digital connections to the G5 (and thus the GFC500, since the logic is in the G5) that feed all the data no matter what navigation source is selected.
But let’s say I don’t set anything in GTN (no flight plan, no approach chosen) and I just dial in ILS frequency. Do you say that based on this (and recognizing navaid ID and current GPS position) GTN will assume that I’m preparing for approach to particular airport and turn on GPS aid for that approach?
Posts moved to the thread which is on this topic.
Do you say that based on this (and recognizing navaid ID and current GPS position) GTN will assume that I’m preparing for approach to particular airport and turn on GPS aid for that approach?
I think that would be a piece of mind bending logic… for a start it would have to require a current database in the GPS! No such thing is documented, and a lot of people have struggled with such assumptions e.g. none of the flight tracking software can tell where you are landing.
I reckon, and knowing nuffink more than anybody else, but knowing a bit about control algorithms, Garmin use GPS data to improve the control loop – at least for LOC tracking, and maybe even for GS tracking using the rate of GPS altitude (the latter would be tricky because the GS is not always 3 deg, but even a derivative of the VS would be a useful input). The GFC500 knows the LOC bearing (because you have set that up) and if it gets the GPS track then, together with the LOC deviation which it also has once LOC-established, it can see how well it is doing. It would be a useful input into the control loop.
And none of that needs any kind of database in the GPS.
Airborne_Again wrote:
Even with a GPS failure, the GFC500 will work in HDG and V/S modes. The likelihood of a GPS failure happening at the same time that you have to fly an IAP to near minima is even more remote
It should be really no issue, flying with auto-pilot an ILS with HDG & VSI modes to minima should be really non-event and it keeps “hand flying currency and good situaltional awarness” (you fly a target HDG & VSI/ASI and confirm on HSI neddles, anything else is just unstable and 100% behind the aircraft rather than driving it)
Emir wrote:
But let’s say I don’t set anything in GTN (no flight plan, no approach chosen) and I just dial in ILS frequency. Do you say that based on this (and recognizing navaid ID and current GPS position) GTN will assume that I’m preparing for approach to particular airport and turn on GPS aid for that approach?
No, but it sends things like GPS position, groundspeed and – most importantly – the actual track. I wasn’t suggesting the GFC500 used the navigator database.
This has just been posted on a US Beech forum, by a well known pilot who definitely knows about our thread here 
It illustrates the vulnerability:
He then got a bit of a roasting for implying that he can’t hand fly an ILS…
Airborne_Again wrote:
No, but it sends things like GPS position, groundspeed and – most importantly – the actual track. I wasn’t suggesting the GFC500 used the navigator database.
Clear, so you basically assume the sam what @Peter wrote above and you’re probably both right. But the question is if A/P uses navaid for basic approach and GPS data for improving the precision why it can’t continue approach in basic mode when this GPS aid is turned off. I guess it’s what GFC600 (and GFC700) does.
Emir wrote:
Clear, so you basically assume the sam what @Peter wrote above and you’re probably both right. But the question is if A/P uses navaid for basic approach and GPS data for improving the precision why it can’t continue approach in basic mode when this GPS aid is turned off. I guess it’s what GFC600 (and GFC700) does.
As far I understood some of the materials, because in that case basic mode is more expensive to implement and has to be more rigorously tested on each airframe, which will make it even more expensive.
