AP DISC/TRIM INT does not inturrupt pitch trim.

[flapman]

In the spirit of AbstractArtist's struggle to master the Skyhawk, I (a long time user of the C172) finally bound my pitch trim and autopilot switches using the A2A Input configurator. I love the realistic trim rate, and proper autopilot disconnect functionality... but the increased realism revealed something I had not seen in at least 5-6 years (since I last flew a real one).

On a C172, the yoke autopilot switch actually performs two functions. It also interrupts electric pitch trim movement, to protect against a pitch trim runaway. In the A2A version, this does not function. I am still able to trim the aircraft with the AP DISC/TRIM INT switch pressed down on my yoke. In the real aircraft, this is one of the required checks that must be performed before using the KAP-140 2 Axis Autopilot.

Checking the pitch trim portion of the autopilot involves 2 tests. The first checks each half of the split trim switch for functionality, making sure trim does not move. I understand that 99% of FS users don't have a split trim switch, and this test can be ignored. The next check is to command both switches in the same direction, verify trim movement, and... while still holding the trim command... pressing and holding the yoke AP disconnect and verify that trim motion stops. This last test is the one which the A2A aircraft fails.

I had never tried it before, as I was under the impression that the yoke AP button was inop. Upon discovering that it is available (and realistic yoke trim switches (although single switch)), I first attempted this test sometime around 2 days ago.
Just an interesting tidbit, and a really minute detail for you to consider.

Thanks,

[Caldemeyn]

Nice one, would like to hear more from others about this

[Oracle427]

It should disconnect the electric trim. I haven't had the chance to test in sim myself as I don't have that button mapped.

[AKar]

FWIW, the trim interrupt is pretty clearly described in KAP 140 installation manual:



172R wiring diagram reflects this configuration as well, with slight alterations depending on the S/N range. Essentially, servo amplifiers are powered from the A/P main circuit breaker, and the A/P disconnect slash trim interrupt switch is directly in series. As long as the button is held down, no trim movement - or any other servo movement for that matter - is possible.

Speaking more generally, typically the autopilot interfacing and trim simulation are not too precisely modeled in FSX/P3D simulations. There are only very few simulations on the platform that get these details at least relatively right, and most appear to rely heavily on external coding of flight controls. For general flying this is typically not a problem, however, limits are often reached if one goes more into it: this kind of tests may fail, pitch channel is controlled exclusively by the trim/THS, incorrect routines, certain sequences are skipped etc. At least some of the above issues are typically present.

-Esa

[Nick - A2A]

Good observation flapman. Yeah, it seems the manual electric trim function in the A2A 172 is actually independent of electrical power too, as discussed in this thread which you may find interesting. In other words, manual electric trim is functionally the same as manual (trim wheel) trim.

Not sure if this is an issue which would be of high priority for future updates or whether it's a fundamental limitation of the FSX/P3D trim system itself and the way it's designed to work with our hardware.

Thanks,
Nick

[AKar]

Good observation flapman. Yeah, it seems the manual electric trim function in the A2A 172 is actually independent of electrical power too, as discussed in this thread which you may find interesting. In other words, manual electric trim is functionally the same as manual (trim wheel) trim.

Not sure if this is an issue which would be of high priority for future updates or whether it's a fundamental limitation of the FSX/P3D trim system itself and the way it's designed to work with our hardware.

I'd vote that different simulation of electrical pitch trim, manual pitch trim, and A/P elevator and trim channels' separation are all an issue on the platform worth studying to overcome one day.

This limitation in FS trim system and autopilot simulation was actually a subject of rather fascinating issue when the good old Project Tupolev began their IL-62 project years back. The aircraft has a mechanical elevator control, a classic aerodynamic trim tab that flies the elevator, and a trimmable horizontal stabilizer. Now that was some pitch control setup to simulate correctly!

If simulating manual trim wheel, how bad would it be if the unsteady (in rate) manual application was modeled? Together with an appropriate sound of hand turning that wheel...

-Esa

[some1 - A2A]

We don't simulate these to the letter because such setup would be impractical in the sim. Even if you disconnect electric trim switches, you should still be able to trim the aircraft using manual trim wheel. We could in theory make the trim switches work as described in the manual and tell people to map another trim axis as a manual backup trim. But, unless you have some kind of motorized trim wheel connected to your PC, your trim axis will not be in sync with the trim in the simulator after you use the electrical trim switches. The moment you touch the wheel, the trim will instantly jump to the new position and probably destabilize the aircraft.

So it's not a limitation of the simulator, but rather of the hardware we use. Same with overpowering autopilot servos, which would require FFB controllers.