GPSS / roll steering

I don’t see the GPSS-capability…

Rob wrote:

You are using GPSS to fly a RNAV approach finalising in a ILS, at what time and how do transit from GPSS roll steering to ILS. Do you at some stage change to heading mode and arm the autopilot to capture the ILS as you would normally do.

You probably meant using GPSS to fly RNAV arrival (rather than approach) finalising with ILS approach. I don’t know for the other boxes but G1000 does the switch automatically at IAF. Depending on autopilot, you have to do some additional action or not. In case you have GFC700, you don’t have to do anything – the autopilot will just continue to follow the new source (ILS). In case you have KAP140, you’ll get warning of changed source and you have to press “APP” button on autopilot.

Rob wrote:

Garmin 530w looking to upgrade to 750txi
Ki 525

Edit: I saw later on you posted your avionics but the autopilot is missing. BTW I believe 750TXi acts in the same way as G1000 while for 530W I really don’t remember, I has been few years since I flew aircraft with this box.

Emir wrote:

Rob wrote: Garmin 530w looking to upgrade to 750txi Ki 525

Edit: I saw later on you posted your avionics but the autopilot is missing. BTW I believe 750TXi acts in the same way as G1000 while for 530W I really don’t remember, I has been few years since I flew aircraft with this box.

It does, at least in the Garmin’s GTN simulator. :)
@Rob, you can try it yourself…

Rob wrote:

Sorry yes I did mean the arrival. The Auto Pilot is KFC 200 so I think I would need to select APP button

The KFC 200 is an analog autopilot and designed with the assumption that vectors to final is the method of joining an ILS. Many pilots will remain in heading mode and use GPSS to join the final approach course on an ILS and then engage APPR mode. This works sometimes, but not always. As it was explained to me by the King engineer, the KFC 200 calculates the angle of intercept and projects a point of intercept after which HDG is extinguished and APPR is annunciated with course tracking. GS intercept does not arm until after this point. So I recommend that APPR be engaged before the intercept angle to the final approach course is close to zero, which can project an intercept beyond the GS intercept point. So when the GPSS derived heading is 10 to 30 degrees before alignment with the FAC, press the APPR button on the KFC to provide best results.

Jacko wrote:

It looks as if the GTN 650 is anticipating the turn, as it is supposed to do, but just a few seconds too late.

Is this to be expected? I wonder whether it has to do with the Maule-specific installation in which the “roll” servo operates on the rudder, or perhaps the S-TEC AP limiting roll to 90% (?) of a standard rate turn?

The ST901 does not have an adjustment for gain. The amount of overshoot is related to speed and rate of turn that the autopilot will command. Slowing down will help. You can also check the autopilot to determine the maximum rate of turn it will command in an established turn. To do this use heading mode and command a 90 degree left and right turns and record the rate of turn mid turn. It may be different for right and left turns. Also note the rate of turn at 30, 20, and 10 degrees. It should be proportional. The GTN or GNS commands a maximum turn bank equal to standard rate, so if the AP is turning at a rate less than standard rate, you can expect some overshoot on a steep turn of 90 degrees.

Getting back to this topic, why exactly can’t one get roll steering out of a KLN94?

The obvious answer is that it doesn’t emit the data stream, which is ARINC429 label 121.

But the box does output the flight plan, fully documented, via RS232, including the current track and I think the track of the next leg. From that you can compute the required turn geometry, and I bet that is exactly what a WAS navigator does! It doesn’t have access to your heading, TAS, etc. So one could make a roll steering converter which takes in the KLN94 data stream and generates the required ARINC429 commands. Years ago, I had a conversation with DAC (who make these RS boxes) and the guy said it is impossible, because the KLN94 outputs the data at 1Hz which is too slow. This I am sure is nonsense…