Hi all,
my KFC 150 AP doesn’t, since a few flights, start to descend when the GS is active; I’m in ‚normal’ ILS (not RNAV) mode, annunciatior tells me APP and CPLD. It used to start descent at the FAF, but not anymore….
Any ideas what could be wrong ?
I believe the Autopilot needs an ILS active signal to follow the GS.
This comes directly from the NAV radio.
Could be a bad wire or a defect radio.
What NAV radio does provide the ILS signal?
Maybe try reseating the radio, or measure out the Active Signal wire.
The NAV radio is a GNS530. But are you sure – I don’t have a WAAS device (yet, should be coming in a week or so) – so it’s a purely analog problem
sounds plausible thanks
To test this you will need a nav test box, which can emit LOC / GS. There are some cheap ones about, under 1k, but the good one is an IFR4000 which is about 5-6k I think depending on the version. I have a hacked one which does LOC or GS. Then, set an ILS frequency on the radio and probe the ILS ENERGISE wire on the autopilot connector while generating the GS signal. I think most “classic” radios don’t need a LOC to detect the GS, although an EHSI will suppress the GS indication if there is no LOC (which is proper and correct).
Troubleshooting this kind of thing can be a real bugger, due to poor access, as well as often poor access to a hangar where such work can be done.
I’ll start with the assumption of a loose wire, the plane is in 100hr service, maybe they find it. If not I’ll have it checked in Straubing, they have all test equipment ever made by mankind…
Peter wrote:
To test this you will need a nav test box, which can emit LOC / GS. There are some cheap ones about, under 1k, but the good one is an IFR4000 which is about 5-6k I think depending on the version. I have a hacked one which does LOC or GS. Then, set an ILS frequency on the radio and probe the ILS ENERGISE wire on the autopilot connector while generating the GS signal
ILS Energise isn’t dependent on the correct ILS LOC or GS signals being received. It is a switched ground which is present when the Nav receiver is tuned to an ILS frequency. A more likely defect if the autopilot is going into APP mode and capturing the LOC but not the GS, is that the autopilot not seeing the Nav receiver GS VALID flag signal.
I suspect the OP has long ago gone off to give some money to Straubing 
Yes indeed I forgot the ILS Energise works off simply setting a valid ILS frequency on the radio. There is a whole lot of wires involved with the GS; in my Sandel installation where two ILS radios (“NAV1/NAV2”) are connected to the EHSI, I have a 6 pole relay switching all the required wires, and any of the (5, IIRC) signals being disconnected will stop the GS working.
No, not yet I haven’t. But that’s because we had a little trouble with one of the cylinders which delayed the 100hr check. Let’s be honest: from all your remarks alone, good as they are, it becomes obvious that this is best handled by professionals.
