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Unexpected altitude change in RVs and the 737Max8

ClayR_9A

Well Known Member
Patron
Last evening, while enjoying glass-smooth air at about 4000' just after sunset, my RV9a unexpectedly pitched down hard. I had just reached over to pull a little cabin heat when it happened.

My first thought was I just hit a massive downdraft - something was pushing my nose down hard. I could pull back to counter it, but had to pull on the stick hard and it wasn't letting up. I pulled throttle back and it eased up a little. My second thought was "how could cabin heat cause this" when I didn't have an autopilot that might be affected by the changing pressure in the cabin.

Then, I tried pitch trim up (electric trim, with a switch on my panel just above the throttle) and it just as quickly corrected itself.... problem solved!

But, it certainly got my heart racing, and also thinking about how the pilots in the recent 737 Max8s that crashed must have felt as something 'external' was pushing their nose to the ground.

I certainly didn't think my right hand finger was pushing the trim button at the same time my left hand was pulling the cabin heat knob, but my right hand was resting on the throttle where it normally does, and I suppose it could have happened as I slightly leaned forward to pull that knob. I don't suspect a faulty Ray Allen switch, but I suppose that could have happened too.

I also took out my original ray-allen trim indicator on my panel when I installed my GRT efis, moving the trim indicator to the EFIS. However, the trim indicator isn't shown when I'm in PFD mode with a split screen with moving map, so I didn't know the trim had moved.

I checked the forums this morning and found several other threads describing similar incidents where people had bumped the trim buttons on their stick, or had malfunctioning switches. They described very similar symptoms as I saw...

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 also thought that perhaps installing a big red "TRIM" light on my panel would alert me whenever the trim motor is activated. That might get annoying though during regular use though and I'm not sure how to wire it.

My third thought is to see if I can program the EFIS to display an error when the trim reaches a certain point away from center. That would have helped me identify that the trim was out of normal range. Has anyone done this on a GRT EFIS?

At the VERY LEAST, I STRONGLY ENCOURAGE EVERYONE with electric trim to practice dealing with unexpected trim by establishing cruise speed at a decent altitude,press the trim button up or down for 4 or more seconds to see what it feels like! See how much back or forward pressure you need on the stick to counter it! I wish I would have done this when people talked about these issues before so I would have been prepared for it when it happened.

In any case, I'm sharing this to hopefully avoid others from experiencing the same thing, and to discuss the best possible options to keep it from happening again... or at least quickly identify the issue when it does.

Please, don't discuss manual vs. electric trim here... I have electric, and I'm not changing it now, although I might put manual in my next build.

Discussions in previous threads:

Thread 1

Thread 2

Thread 3
(and there might be more)
 
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I also took out my original ray-allen trim indicator on my panel when I installed my GRT efis, moving the trim indicator to the EFIS. However, the trim indicator isn't shown when I'm in PFD mode with a split screen with moving map, so I didn't know the trim had moved.

Can you put the original Ray Allen trim indicator back in? I like the simple trim indicator right up front next to my stand alone AoA indicator.

I am a big proponent of safety of flight type indicators are dedicated and/or always visible (not buried down in some menu).
 
A run-away safety trim switch is a great thing to have, I have it now as part of my Garmin GAD 27 but I soon discovered that 2-3 second was not adequate for my normal operation. The flap in a RV14 is so effective and needs lots of nose up trim and now I have increased the time to 6 seconds. Perhaps as I get more comfortable with it, I reduce the duration but it is a nice thing to have.
 
Even though I don't have it, I think the safety trim is a good idea. Also, I'm pretty sure it comes with a speed control built in (runs more slowly at higher airspeeds). I think it also has provisions to reverse the wires, so you can restore in-trim flight manually and carefully.
As someone else (above) does, I have my autopilot and trim CBs side by side, right in front of me, with a painted box around them (my autopilot can also control trim). I also have the original indicator on the panel, but truth be told just one bar is a significant deviation from in-trim, so I think I'd detect the movement before that indicator. So if I have trim runaway (and am smart enough to guess at the problem) I can quickly pull both breakers, then trouble shoot the problem. Depending on where the failure is, I may (or may not) be able to restore in-trim condition using the autopilot trim or the manual trim, with the other off.
In the -10, I'm usually holding the trim switch for more than 3 seconds in the flare, although maybe that's a bad habit. I think 2 sec is too short.
 
My electric trim is completely away from the stick and for the very reasons you are speaking of. Nice smooth flights here thank you.
 
Safety trim

I have the safety trim system in my 7, Dual axis and its great. 3 seconds is more than adequate, I have the trim rate dialled down to half speed above 105 mph which is great for cruise trimming and it’s very accurate. The only issue I have is pitch trimming in the climb following a touch and go or go around, I’m usually at or above 105 quickly and trimming can take a while, two pushes of the button. Otherwise it’s brilliant. Highly recommend the system.
 
My electric trim is completely away from the stick and for the very reasons you are speaking of. Nice smooth flights here thank you.

Yep, get the trim switch off the control stick. It really isn't an inconvenience to operate a trim switch on the panel.... :)
 
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
 
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.
 
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.
 
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
 
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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.
 
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.
 
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
 
How is your static system setup, specifically the static port on the back of the autopilot head?
 
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).
 
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