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DFC90 - Sudden "Autopilot Disconnect" in Cruise

Peter, they simply openend the inspection cover on the legt wind (the roll trim cartridge, that's what Cirrus calls it because it is more than just a servo, is behind the left aileron). Then they attached a voltmeter to the servo and let the A/P run from full left roll to full right. As far as i saw it was always "0" volts in the center and -3.5 to 3.5 at full deflection ( but really i have no idea about electrics/electronics). I will check the service manual tonight and maybe send you sone pages!

No, the voltage issue with an autopilot servo is start voltage.

EGTK Oxford

Achim, that's actually one of the things that are very upsetting about GA: The quality of the parts stands in No relation whatsoever to their quality. Beeing a Mercedes (truck and unimog) dealer in my other job i am constantly reminded about how terrible a lot of those airplane parts are. At the same time there is such a fuss all the time about papers and certification ...

What bother's me most is the quality of parts like these servos (i've seen them). Actually i have seen nicer stuff in model airplanes... And there WOULD be alternatives, i am sure. There's certainly higher quality electric motors on the market.

Jason, then it was the other way round... 3.5 to 0... but i really don't know much (yet!) about electronics

A servo full of black dust after 50hrs suggests crap AP software which is driving it back/forth all the time, and probably very fast so you get constant commutator arcing.

Yes, the analog S-TEC autopilot...

I also don't understand where the 3.5V is being measured. On the ground, there should be nothing going into the servo. And in the air, with the AP enabled, the servo control voltage will be according to the control requirements.

The DFC90 installation manual describes the test.

Aileron Servo Motor Voltage Test Note: This test is conducted using the S-TEC Autopilot. If the DFC90 Autopilot has already been installed, see the end of Table 19, Troubleshooting Table in Section 4 for an alternative test method. This test is to evaluate the minimum voltage required to move the Aileron Trim Motor. 1. Gain access to the Aileron Roll Servo 2. Connect a Volt Meter between the motor drive lines (for Cirrus SR2X : Pins 1 and 2 at the P304 Disconnect. Note: If P304 is not accessible using an alternate location is acceptable. If using an S-TEC breakout box, measure the voltage between Pins 1 and 19 on connector P551.) 3. Turn on Autopilot power 4. Sync Heading Bug to the current heading 5. Engage the Autopilot in Heading Mode 6. Vary Heading Bug on the PFD in 1 degree increments and record the DC Voltage where the Roll Motor starts to move. Voltage: ___________ Note: Installers should inspect the flight control system for excessive friction and/or roll motor problem if greater than 3 VDC is measured.

No, the voltage issue with an autopilot servo is start voltage.

Meaning, exactly what???

Is there a test mode in the DFC90 which slowly ramps up the control voltage and you make a note of when the motor starts turning?

Otherwise, there is no way to check this in situ, unless you unplug the servo and connect a tester of some sort into the cable.

It sounds like a rather dodgy design, to be sensitive to this kind of thing.

The DFC90 installation manual describes the test.

This is not going to get fixed on a forum, for sure. What a weird system.

Shoreham EGKA, United Kingdom

No you test the servo itself. Nothing to do with the autopilot - it is a servo issue. You have to remember the DFC-90 is using the old servos.

EGTK Oxford

For an already installed DFC90, Avidyne show the following test, i.e. disconnect the servo and just drive it from a power supply and measure the minimum voltage that makes the servo turn. It should be less than 3V for the roll servo and less than 5V for the pitch trim servo:

It sounds like a rather dodgy design, to be sensitive to this kind of thing.

It's simply a health test of the servo and friction in the aileron system, actually a good test. It doesn't mean the AP will not work but this is a clear limit given in the installation manual and when troubleshooting, it should be addressed.

What a weird system.

By far the best AP on the market for retrofit. Nothing weird about it.

Achim, yes, that's the test that was performed!

Peter, unlike you i am not an engineer, although a pretty technical person. Is it possible i hear some prejudice in your postings about the DFC90? :-))

I think: of all the GA autopilots the DFC90 and the GFC700 are by far the ones that are most precise and most fun to fly with. Actually the precisness of the system is almost scary. Before i had the first failure i flew an ILS in gusty conditions and the A/P flew the plane as if it was mechanically mounted to a rail.

And I agree - it is a great autopilot. My GFC700 is better but only due to some additional features and using brushless digital servos.

EGTK Oxford
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