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WestAir

Uncontrolled pitch?

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Yesterday I dusted off my 737-800 and took it for an LAX-JFK cross country. Climbing out of LAX through about 6,000 feet on LNAV/VNAV/CMD1 I encountered a problem.

 

Now, normally I fly with my CH Products Yoke. When I turn on the AP, I usually still have the thrust levers on my Yoke set to TO power, with the FMC set to override my yoke throttle settings. Like usual, I'll idle my physical throttles because I know it won't affect the sim. On this particular flight, the INSTANT I pulled back my yoke throttle levers, the aircraft (still on LNAV/VNAV/CMD) pitched full aft.

 

The plane pitched full aft, went about 80 degrees nose high, stalled, then did a sort of nose-up flat stall all the way into the dirt. By the time I managed to hop back into cockpit view, click the AP disengage (it stayed on even though the A/C stalled), and push down with full throttle I hit some homes in Hollywood. The question I have is has anyone of you ever encountered an erroneous pitch with the NGX, AP engaged? I've had the plane since it came out and never encountered it before, so i was wondering if it could be attributed to my Yoke or some other addon. Any thoughts on what happened? I don't want to try a VATSIM flight and end up in some homes. I'd much rather identify the cause.

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Now, normally I fly with my CH Products Yoke.

 

There's a black trim wheel at the base of the CH Yoke, where the yoke meets the body.  Is this an assigned trim axis?  If so, my bet is that, when you changed the throttle position, it "woke up" some of the other signals coming from the yoke, and that trim wheel was one of them.

 

I'd suggest disabling the elevator trim axis and instead switch to trim up / trim down commands (buttons).

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There's a black trim wheel at the base of the CH Yoke, where the yoke meets the body. Is this an assigned trim axis? If so, my bet is that, when you changed the throttle position, it "woke up" some of the other signals coming from the yoke, and that trim wheel was one of them.

 

I'd suggest disabling the elevator trim axis and instead switch to trim up / trim down commands (buttons).

I've had my CH yoke apart to correct a problem I was having with severe spikes. (It was due to a loose push-on wire terminal on one of the potentiometers.) The trim wheel on a CH yoke is not an axis. When you turn it, it physically moves the mounting for the pitch potentiometer where it connects to the linkage to the shaft, which changes the center point of the potentiometer's motion.

 

The OP's issue sounds like the throttle axis assignment somehow was being crossed with the pitch axis assignment. Possibly a hardware problem, or more likely a temporary software glitch in the processing of the controller inputs to the computer.

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The trim wheel on a CH yoke is not an axis. When you turn it, it physically moves the mounting for the pitch potentiometer where it connects to the linkage to the shaft, which changes the center point of the potentiometer's motion.

 

Nifty - thanks for that explanation!

 

Last time I messed around with one was a good 13ish years ago, and I was even less hardware-savvy then than I am now.

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