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Old 03-20-2019, 04:50 PM
ClayR_9A's Avatar
ClayR_9A ClayR_9A is offline
Join Date: Jan 2005
Location: Dallas, TX
Posts: 173
Exclamation Unexpected altitude change in RVs and the 737Max8

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)
Clay R- RV9A - Flying since 2004 - 425 hours
VAF dues paid through end of 2021.

Last edited by ClayR_9A : 03-20-2019 at 04:51 PM. Reason: added a sentence
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Old 03-20-2019, 06:06 PM
plehrke's Avatar
plehrke plehrke is offline
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,828

Originally Posted by ClayR_9A View Post

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).
RV-6A - flying 970+ hours
Based at 1H0 (Creve Coeur)
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Old 03-20-2019, 06:39 PM
Bavafa Bavafa is offline
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,639

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.
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Old 03-20-2019, 06:41 PM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,851

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.
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Old 03-20-2019, 08:55 PM
sirlegin's Avatar
sirlegin sirlegin is offline
Join Date: Feb 2006
Location: Keller, Texas
Posts: 323

My electric trim is completely away from the stick and for the very reasons you are speaking of. Nice smooth flights here thank you.
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Old 03-20-2019, 09:22 PM
N941WR's Avatar
N941WR N941WR is offline
Join Date: Jan 2005
Location: SC
Posts: 12,887

Simple solution; manual trim and Dynon.
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
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Old 03-21-2019, 02:47 AM
Simon Hitchen's Avatar
Simon Hitchen Simon Hitchen is offline
Join Date: Apr 2006
Posts: 306
Default 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.
Simon Hitchen
Port Perry, ON, Canada
7 Tip Up, Titan XIO-360, Dual P-Mags, Airflow Performance matched Injectors, Sensenich FP Prop, 2 Dynon Skyviews, GPS-175, GTR-200, GTX-327. Day/night VFR/ IFR
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Old 03-21-2019, 05:19 AM
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maniago maniago is offline
Join Date: Aug 2014
Location: Bowie MD
Posts: 890

Originally Posted by N941WR View Post
Simple solution; manual trim and Dynon.
+1 for manual.....
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Old 03-21-2019, 09:02 AM
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Sam Buchanan Sam Buchanan is offline
been here awhile
Join Date: Dec 2004
Location: North Alabama
Posts: 4,462

Originally Posted by sirlegin View Post
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....
Sam Buchanan
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Old 03-21-2019, 12:07 PM
Longmire Longmire is offline
Join Date: Jan 2018
Location: Alpine, WY
Posts: 11

Thank you for such a thought provoking discussion and communicating this to us all.
new RV10 Builder, not ordered yet, not started yet, but soon? Looks like 2020 for the Kit and then the Synergy training

'77 Piper Archer II, N5323F is our airplane now. Great little plane but can't wait till we get our RV10

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