What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Crazy full-up elevator issue...

I don't have an auto-pilot, but I do have a question.... How can you trim with the AP engaged? It would be like you, the pilot, trimming while the co-pilot is trying to keep the aircraft level.

Exactly. And this is how autopilots have worked for decades (unless they have autotrim capability). The autopilot doesn't have any idea *what* is causing the aerodynamic force that is pushing it off of the setpoint (or outside the deadband). Could be a trim tab, could be the pilot or copilot pushing on the stick, could be a gust of wind, could be a change in power setting...all it "knows" is that the airplane is deviating from the setpoint, and it needs to send the signal to the servo to move the control surface to correct back to the setpoint.
 
I don't have an auto-pilot, but I do have a question.... How can you trim with the AP engaged? It would be like you, the pilot, trimming while the co-pilot is trying to keep the aircraft level.

Not knowing what I am talking about, I might think that disabling the trim power during the time the AP is active (and vise versa) would be no brainer.

If the autopilot doesn't have an auto trim function, the usual method is get the aircraft stabilized in cruise and carefully trimmed. Then the autopilot is engaged and the servo doesn't need to fight an out-of-trim condition. This works fine until a lot of fuel has been burned or large airspeed changes occur which requires retrimming. The autopilot can be briefly disengaged to check/reset trim then re-engaged....or you can hand fly the altitude change or approach.

Large pitch excursions when the auto pilot is released are usually caused by not having the aircraft in proper trim when the autopilot is engaged. We're talking basic airmanship here. :)
 
Last edited:
If the autopilot doesn't have an auto trim function, the usual method is get the aircraft stabilized in cruise and carefully trimmed. Then the autopilot is engaged and the servo doesn't need to fight an out-of-trim condition. This works fine until a lot of fuel has been burned or large airspeed changes occur which requires retrimming. The autopilot can be briefly disengaged to check/reset trim then re-engaged....or you can hand fly the altitude change or approach.

Large pitch excursions when the auto pilot is released are usually caused by not having the aircraft in proper trim when the autopilot is engaged. We're talking basic airmanship here. :)

I have a Dynon SkyView Touch with the Dynon autopilot, but do not have auto-trim (the separate AP panel is needed for that function).

You can keep the AP engaged through cruise, turns, climbs, and descents, and any time the AP senses that the airplane needs to be retrimmed, it prompts the pilot to retrim manually (via electric trim in my case), all while the AP stays engaged. The prompt is a visual alert followed by an aural alert 10 seconds later, e.g., "Trim Nose Down". The prompt goes away once the airplane is back in trim.
 
Last edited:
I have a Dynon SkyView Touch with the Dynon autopilot, but do not have auto-trim (the separate AP panel is needed for that function).

You can keep the AP engaged through cruise, turns, climbs, and descents, and when the AP senses that the airplane needs to be retrimmed, it prompts the pilot to retrim manually, all while the AP stays engaged. The prompt is a visual alert and then an aural alert 10 seconds later, e.g., "Trim Nose Down". The prompt goes away once the airplane is in trim.

I can see this working great with manual (non electric) trim. But with electric trim (operated manually) that can get activated many ways without you knowing, just looks like a failure point that can be avoided with some simple wiring changes to avoid a runaway trim with the auto pilot engaged.
 
Last edited:
I can see this working great with manual trim. But with electric trim that can get activated many ways without you knowing, just looks like a failure point that can be avoided with some simple wiring changes.

I should clarify that I have electric trim. I used the phrase "manual trim" (by the pilot) to differentiate from "auto-trim" (by the system). The system has worked well for 1250 hours so far. I edited my post above.
 
Last edited:
I can see this working great with manual trim. But with electric trim that can get activated many ways without you knowing, just looks like a failure point that can be avoided with some simple wiring changes.

Such as...? A RAC servo with no relay box and no autotrim is about as simple as a circuit could possibly get. Battery-fuse-switch-servo-ground.

There are some additional wires for talkback to an indicator or EFIS, but those are low-voltage data serial lines, not power for the servo.
 
I have a Dynon SkyView Touch with the Dynon autopilot, but do not have auto-trim (the separate AP panel is needed for that function).

You can keep the AP engaged through cruise, turns, climbs, and descents, and any time the AP senses that the airplane needs to be retrimmed, it prompts the pilot to retrim manually (via electric trim in my case), all while the AP stays engaged. The prompt is a visual alert followed by an aural alert 10 seconds later, e.g., "Trim Nose Down". The prompt goes away once the airplane is back in trim.

I get the same thing; the autopilot squawking that the trim needs to be set UP or DOWN. It's fairly tolerant of the airplane being more out of trim than you'd like if you were hand-flying. You can tell by how much you feel like you need to adjust the trim once you turn the autopilot off and take over. But the autopilot is actually not bad and only nags for trim adjustment on fairly rapid climbs or descents. One thing I've realized through all this is that I'd like to have an autopilot with auto-trim. Apparently Dynon doesn't have it, does Garmin? Or if not, who does?
 
One thing I've realized through all this is that I'd like to have an autopilot with auto-trim. Apparently Dynon doesn't have it, does Garmin? Or if not, who does?

Yes, Dynon does offer auto trim. As I said in my post, you need the separate autopilot control panel to get auto trim. You can add it to your existing SkyView system.

Link --> https://dynonavionics.com/autopilot-control-panel.php

autopilot-handv.png

"The Autopilot control panel is also a full featured dual-axis trim controller. It can control your electric pitch and/or roll trim motors, replacing the relay decks normally used.

With Autopilot Auto-Trim, the SV-AP-PANEL can keep the the aircraft in trim while under autopilot control. This eliminates the need for the pilot to manually actuate their trim to follow the Autopilot’s built-in trim advice. "​
 
Last edited:
I should clarify that I have electric trim. I used the phrase "manual trim" (by the pilot) to differentiate from "auto-trim" (by the system). The system has worked well for 1250 hours so far. I edited my post above.

I also went back and made some changes (in bold) to my post to make my concern a little clearer.
 
If the autopilot doesn't have an auto trim function, the usual method is get the aircraft stabilized in cruise and carefully trimmed. Then the autopilot is engaged and the servo doesn't need to fight an out-of-trim condition. This works fine until a lot of fuel has been burned or large airspeed changes occur which requires retrimming. The autopilot can be briefly disengaged to check/reset trim then re-engaged....or you can hand fly the altitude change or approach.

Large pitch excursions when the auto pilot is released are usually caused by not having the aircraft in proper trim when the autopilot is engaged. We're talking basic airmanship here. :)

Then a safe way to wire the trim is to have it active only when the auto pilot is disengaged..... unless you HAVE auto trim.
 
Then a safe way to wire the trim is to have it active only when the auto pilot is disengaged..... unless you HAVE auto trim.

You're just moving the failure point around. You'd have to have a relay in the trim circuit, activated by power to the autopilot servo, and there are ways for *that* to fail, either open or closed. Not to mention that failure of the relay might cause a failure of the autopilot itself (which could be particularly bad if, say, in IMC).

FMEAs are not as simple as just adding more complexity to the system.
 
Yes, Dynon does offer auto trim. As I said in my post, you need the separate autopilot control panel to get auto trim. You can add it to your existing SkyView system.

Link --> https://dynonavionics.com/autopilot-control-panel.php

View attachment 31309

"The Autopilot control panel is also a full featured dual-axis trim controller. It can control your electric pitch and/or roll trim motors, replacing the relay decks normally used.

With Autopilot Auto-Trim, the SV-AP-PANEL can keep the the aircraft in trim while under autopilot control. This eliminates the need for the pilot to manually actuate their trim to follow the Autopilot’s built-in trim advice. "​

Ah, gotcha. Thanks. Now if I could just *get* a Skyview HDX...been trying to buy one since November and they've been out of stock!
 
Mark,
I read the thread but did not see the following asked.
Are you engaging the pitch autopilot during the climb phase after takeoff and asking the autopilot to level the plane at a set altitude, with zero trim adjustment after AP engagement in the climb?

If so, this is the problem. It would correlate with time and distance of repetitive flights. My RV needs a major nose down trim change from climb to cruise with the largest fraction due to speed increase rather than attitude. I have had the AP kick out once or twice due to bad pilotage of not enough trim adjustment for proper AP operation.

I assume you have hand flown the suspect flight path with no autopilot but electrically connected trim with no issues?
 
Mark,
I read the thread but did not see the following asked.
Are you engaging the pitch autopilot during the climb phase after takeoff and asking the autopilot to level the plane at a set altitude, with zero trim adjustment after AP engagement in the climb?

If so, this is the problem. It would correlate with time and distance of repetitive flights. My RV needs a major nose down trim change from climb to cruise with the largest fraction due to speed increase rather than attitude. I have had the AP kick out once or twice due to bad pilotage of not enough trim adjustment for proper AP operation.

I assume you have hand flown the suspect flight path with no autopilot but electrically connected trim with no issues?

Yeah, I know what you're saying. Yes, I engage the autopilot on climb-out as I always do, setting an altitude and rate of climb. The plane is trimmed before engaging the autopilot, and if I'm not properly trimmed or it changes during the climb, the autopilot is very "vocal" about setting it correctly. As I've said it shows either "UP" or "DOWN" on the display along with an associated arrow, and (what I've forgotten to mention before) it "bucks" several times. It's impossible to miss this bucking...it really gets your attention! And it's never bucked before this runaway trim issue, so now I'm not so sure about the "autopilot kicking out" theory now.
 
You're just moving the failure point around. You'd have to have a relay in the trim circuit, activated by power to the autopilot servo, and there are ways for *that* to fail, either open or closed. Not to mention that failure of the relay might cause a failure of the autopilot itself (which could be particularly bad if, say, in IMC).

FMEAs are not as simple as just adding more complexity to the system.

Are you over thinking this, or is it really that complicated. If you have one wire that disconnects the AP, and one wire that removes power to the trim, then one simple toggle switch on the panel would give you one or the other and you would never have a runaway trim that was hidden by the AP.

Just the fact that you chose electric flaps and electric trim also added failure points to your aircraft.
 
Are you over thinking this, or is it really that complicated. If you have one wire that disconnects the AP, and one wire that removes power to the trim, then one simple toggle switch on the panel would give you one or the other and you would never have a runaway trim that was hidden by the AP.

Just the fact that you chose electric flaps and electric trim also added failure points to your aircraft.

ETA: I don't think I'm overthinking it by positing a relay. You don't want to simply switch power between AP and trim, because pulling power from the AP to use trim means (at least for Dynon) that the AP servos are no longer visible to the network and the EFIS, and I don't believe that reapplying power to them will make them operable again without rebooting the EFIS. And you D/C the A/P from the stick via shorting a line to ground, but it's hard to see how that would help if you just want a toggle switch.

And that toggle switch can fail. Maybe it fails so that you can't use the trim at all. Or it fails such that it fails to lock out the trim when the AP is engaged. Or it fails so that neither one can be used. Etc.

This is how "normal failures" occur...people start adding components that they think increases safety, but in fact they *decrease* it. Safety Hazard Analysis can be tricky. :)

FWIW, I've not had (nor heard of from any of the half dozen fellow RV pilots I fly with locally) a runaway trim on an electric RAC trim servo. I *have* had a mechanical system fail (Piper Cherokee) in the past.
 
Last edited:
Yeah, I know what you're saying. Yes, I engage the autopilot on climb-out as I always do, setting an altitude and rate of climb. The plane is trimmed before engaging the autopilot, and if I'm not properly trimmed or it changes during the climb, the autopilot is very "vocal" about setting it correctly. As I've said it shows either "UP" or "DOWN" on the display along with an associated arrow, and (what I've forgotten to mention before) it "bucks" several times. It's impossible to miss this bucking...it really gets your attention! And it's never bucked before this runaway trim issue, so now I'm not so sure about the "autopilot kicking out" theory now.

This is like pulling teeth...

Again, CAN YOU POST THE SCHEMATICS. And do you have any sort of datalog for these flights?

And most importantly, what is the *exact* configuration of the vehicle before and during these events? A/P on *in what mode*, when was the trim adjusted, what was the rate of climb or airspeed selected, etc., etc.

Here we are several pages in and you're just now telling us about some "bucking" of the system.

Have you tuned the autopilot parameters according to the autopilot manual?
 
Back
Top