Unexpected altitude change in RVs and the 737Max8

[vlittle]

Originally Posted by Sam Buchanan

Yep, get the trim switch off the control stick. It really isn't an inconvenience to operate a trim switch on the panel....

I have been flying behind a trim controller of my own design for several years. It has autotrim functions and it predates the Dynon and Garmin offerings.

A couple of years ago, I converted the original assembly language program to 'C' so that it would be easier to add functionality. I also added flaps control at the same time.

Part of this redesign included added safety features. It also involved careful thinking about failure modes, beginning with the pilot and human factors.

The location of the trim switch is critical. Turns out that if you fly HOTAS there are only a couple of choices... stick or throttle. I tried both... and after accidentally hitting the throttle trim switch with my knee, I reverted back to the stick (just a software change).

For the autotrim function, (which is in the news this week). I have 4 ways to disconnect: pull the trim breaker, disconnect the autopilot, touch any trim button, or push a button on the panel. In an emergency, when the pilot's IQ drops, the natural instinct is to manually press one of the trim switches, turning off the autotrim.

On the otherhand, if I have a trim switch failure, I can engage the autotrim system to take over, and fly the aircraft with the autopilot.

So... I can recover from pilot-induced failures, faulty trim switches, or a runaway autotrim system.

Makes one think that a mechanical trim system is much safer (but less convenient).

V

[ColoRv]

I have enough miles under my wings to circumnavigate three times at the equator and have never had an issue with trim on the stick. If I were to build 100 RVs they would all have trim on the stick. Hand on throttle, hand on stick in the pattern...easy no look trim adjustment. I have no desire to go back to Cessna like juggling. If something you?re doing in the cockpit causes you to max your trim out by laying on the stick, I would consider moving whatever it is you can?t reach....so that you can. Just my two coppers...worth what you paid for it.

[SuperCubDriver]

I also did read about trim failures which on some planes are not or difficult to recover.
On my RV-8 I have the Dynon autopilot panel which includes a trim controller throuig which also the autopilot can trim the aircraft. Flying manually it accepts 3 sec continuous trim in each direction then stops, after releasing the trim switch the sequence starts over.
So what happens when the trim controller electronics fails somehow and drives the trim motor in one or the other direction? This is not covered in the manual and may be not probable. But for this very reason I installed a guarded switch which directs the trim commands direct from the controller to the trim motor. Uncovering the guard and switching directs the power via an extra springloaded switch to the trim motor, so no electronics inbetween anymore.
My circuit breakers for the trim autopilot/trim module and servos both have a collar to easily find and pull them when necessary. I also have an abnormal list which covers trim failures. During flight testing I trimmed in both directions at different airspeeds, C/Gs and flap settings and realized that the stick forces can be handled. So I think that trim malfunctions are adequately covered this way.

[gereed75]

I have a 6 with the typical MAC servo electric trim with a stick mounted coolie hat

During phase 1 I did some testing to determine how much trim force would be generated in a runaway trim occurrence. I was very surprised at the speed and magnitude of the force generated by the typical trim system. My conclusion was that the typical trim system is way too powerful - the motor has way more speed and travel than needed with the intended for manual trim tab.

Short of rebuilding the whole system my risk mitigation approach was to bias the trim servo and linkage to limit the nose down trim to the minimum required to balance the elevator at Level flight max cruise. The result is even more excessive up trim capability, but my reasoning is that an up runaway could be more easily controlled with a reduction in throttle in a climb that would bleed airspeed to near stall where stick forces to overcome the nose up trim would eventually be manageable

Secondly I added the safety trim limiting system set on three second trim bursts and slow speed

I live with this risk mitigation approach but if I had to do it over I would install manual elevator trim

I Think that people with a stock system will need some very quick reflexes and a lot of luck to catch a nose down runaway in cruise by pulling a breaker or a switch in time to avoid A very difficult situation

Just my opinion and intended to be good for more discussion

[airguy]

Originally Posted by ColoRv

I have enough miles under my wings to circumnavigate three times at the equator and have never had an issue with trim on the stick. If I were to build 100 RVs they would all have trim on the stick. Hand on throttle, hand on stick in the pattern...easy no look trim adjustment. I have no desire to go back to Cessna like juggling. If something you’re doing in the cockpit causes you to max your trim out by laying on the stick, I would consider moving whatever it is you can’t reach....so that you can. Just my two coppers...worth what you paid for it.

+1, and flaps to boot. I've got trim, PTT, AP disconnect, flaps up and down all on the stick. Never have to pull my hands off stick or throttle for any reason close to the airport. Coming up on 400 hours like that and not one issue in flight so far.

[Caveman]

Originally Posted by ClayR_9A

The best suggestion I saw was to install a "Safety-Trim" which limits any press of the trim button to 3 seconds before you have to release and press it again. I'd like to hear from anyone who has installed one of these because it seems that the limit should be 1 or 2 seconds instead of three. I think it could happen again similarly with a 3 second press of the button unless I slow the trim motor down a lot. (which might actually be a good think to make it more sensitive to fine adjustments)

I've had the TCW safety trim since I built my 7 almost eleven years ago. It works as advertised and has been trouble free. It does have the airspeed switch which slows down the trim travel at high speeds. I have the coolie hats on my grips and like them that way. No inadvertent trim issues or runaways to date. The 3 second time out is spot on for my airplane and the flying I do. I do have the trim breaker placed where I can easily reach it if for some reason the safety trim switch didn't work. During the 730 ish hours I've put on the airplane so far I have never felt that I couldn't physically overcome the trim system long enough to get safely back on the ground by manipulating the throttle to help counter the trim force even if the TCW did fail and I had a runaway. I tested for this in my phase one fly off.

[akschu]

Originally Posted by vlittle

I have been flying behind a trim controller of my own design for several years. It has autotrim functions and it predates the Dynon and Garmin offerings.

A couple of years ago, I converted the original assembly language program to 'C' so that it would be easier to add functionality. I also added flaps control at the same time.

Part of this redesign included added safety features. It also involved careful thinking about failure modes, beginning with the pilot and human factors.

The location of the trim switch is critical. Turns out that if you fly HOTAS there are only a couple of choices... stick or throttle. I tried both... and after accidentally hitting the throttle trim switch with my knee, I reverted back to the stick (just a software change).

For the autotrim function, (which is in the news this week). I have 4 ways to disconnect: pull the trim breaker, disconnect the autopilot, touch any trim button, or push a button on the panel. In an emergency, when the pilot's IQ drops, the natural instinct is to manually press one of the trim switches, turning off the autotrim.

On the otherhand, if I have a trim switch failure, I can engage the autotrim system to take over, and fly the aircraft with the autopilot.

So... I can recover from pilot-induced failures, faulty trim switches, or a runaway autotrim system.

Makes one think that a mechanical trim system is much safer (but less convenient).

V

I have something similar on the breadboard for my project. It's basically an atmel running the aurdino IDE that drives a motor driver to do the polarity switching and PWM if I want to slow it down.

One of my requirements is to run two trim servos in sync and to have a go to neutral button. In order to do this I need to sense the linear resistor on both servos so that I can limit the range of one should the other fail and so that I know when to stop driving the servos when I hit neutral.

In order to get a trim display on my EFIS I incorporated a digital pot (which I can also blink/wave should I detect a fault).

Lastly, I added a rotary encoder with an actual trim wheel, and added a switch with stick/wheel/off settings so that I can disable either the trim wheel or stick buttons in case something goes wrong.

I would have been happy to just use your design as you are far better at electrical engineering than I am, but it didn't really do everything I needed.

Perhaps you would be willing to take a look at my breadboard design when it's done. That would help me a ton.

schu

[smt6]

How is your static system setup, specifically the static port on the back of the autopilot head?

[vlittle]

Originally Posted by akschu

I have something similar on the breadboard for my project. It's basically an atmel running the aurdino IDE that drives a motor driver to do the polarity switching and PWM if I want to slow it down.

One of my requirements is to run two trim servos in sync and to have a go to neutral button. In order to do this I need to sense the linear resistor on both servos so that I can limit the range of one should the other fail and so that I know when to stop driving the servos when I hit neutral.

In order to get a trim display on my EFIS I incorporated a digital pot (which I can also blink/wave should I detect a fault).

Lastly, I added a rotary encoder with an actual trim wheel, and added a switch with stick/wheel/off settings so that I can disable either the trim wheel or stick buttons in case something goes wrong.

I would have been happy to just use your design as you are far better at electrical engineering than I am, but it didn't really do everything I needed.

Perhaps you would be willing to take a look at my breadboard design when it's done. That would help me a ton.

schu

Review this thread Fire me an email. Thanks.
(Sorry for thread drift).

[kenmorris]

I have a new Safety Trim model: ST-2-12V-2SP with Airspeed switch for sale for $250.00