Elevator bungee springs

[EricJ]

52 minutes ago, DCarlton said:

A fixed pushrod to the bungee!!!  I couldn’t figure out where the heck the opposite force was absorbed.  I can sleep now but I’ve gotta open my tail now and find that fixed attachment to the airframe.    

Understanding how the tail works on a Mooney is a big benefit to anybody who flies one or owns one or even just has to take care of it on the line.   All owners/pilots would be better off knowing the nuances of how the tail works on their particular airplane, since there are subtle differences between some of the models.   Taking a peak at the trim wheel gear box, torque shaft, jack screw, hinge, hinge stop, and assist bungee springs and pushrod while the airplane is opened up for annual inspection is a good idea for those even minimally mechanically inclined, I think.   Open up the avionics hatch and see how the electric trim servo works, too, if you have one.

There are other small airplanes that don't have elevator trim tabs, but I don't know of any others that implement it the way Mooney does.   It's a pretty unique system, but it also has it's own benefits.   I still find that some very experienced pilots are kind of amazed to learn how it works.

 

 

 

[Mark942]

A little off topic, but should you be able to lift the tail nose cone about a 1/4" during preflight inspection?  I've always been able to do this. Kind of bothers me, but trim and stability during flight is perfect.

Thanks, -mark

[MikeOH]

50 minutes ago, Mark942 said:

A little off topic, but should you be able to lift the tail nose cone about a 1/4" during preflight inspection?  I've always been able to do this. Kind of bothers me, but trim and stability during flight is perfect.

Thanks, -mark

I believe there is a limit; I can't remember exactly what it was, unfortunately.  1/4" seems quite a bit.

Mine has NO play, FWIW.  OTOH, I rented a B model for many years that probably had at 1/8" of play!

[PT20J]

9 hours ago, Mark942 said:

A little off topic, but should you be able to lift the tail nose cone about a 1/4" during preflight inspection?  I've always been able to do this. Kind of bothers me, but trim and stability during flight is perfect.

Thanks, -mark

That’s way too loose. Here are the limits from the M20J S&MM.

[EricJ]

10 hours ago, Mark942 said:

A little off topic, but should you be able to lift the tail nose cone about a 1/4" during preflight inspection?  I've always been able to do this. Kind of bothers me, but trim and stability during flight is perfect.

Thanks, -mark

It'll all be under tension during flight and take the slop out, but it's telling you it needs some attention.

[Mark942]

Thanks guys,

I'm going to ask my AP/IA to take a look at it with me.  When I first got the plane it was the same, and I asked my AP at the time (an old timer) if that was ok, and he said yeah, they're all like that so I never worried about it but always check it as part of my pr-flight walk around to make sure it has not changed.  My current AP/IA has looked at it and said it is alright, but I want it really checked out.

Thanks again guys,

-mark

[cliffy]

9 minutes ago, Mark942 said:

Thanks guys,

I'm going to ask my AP/IA to take a look at it with me.  When I first got the plane it was the same, and I asked my AP at the time (an old timer) if that was ok, and he said yeah, they're all like that so I never worried about it but always check it as part of my pr-flight walk around to make sure it has not changed.  My current AP/IA has looked at it and said it is alright, but I want it really checked out.

Thanks again guys,

-mark

Look at the play in the jackscrew itself AND the play in the steel bracket that attaches the jackscrew to the airframe - it has two bolts crosswise and their holes sometimes wear producing slop. 

[cliffy]

On 10/20/2025 at 9:57 PM, EricJ said:

Understanding how the tail works on a Mooney is a big benefit to anybody who flies one or owns one or even just has to take care of it on the line.   All owners/pilots would be better off knowing the nuances of how the tail works on their particular airplane, since there are subtle differences between some of the models.   Taking a peak at the trim wheel gear box, torque shaft, jack screw, hinge, hinge stop, and assist bungee springs and pushrod while the airplane is opened up for annual inspection is a good idea for those even minimally mechanically inclined, I think.   Open up the avionics hatch and see how the electric trim servo works, too, if you have one.

There are other small airplanes that don't have elevator trim tabs, but I don't know of any others that implement it the way Mooney does.   It's a pretty unique system, but it also has it's own benefits.   I still find that some very experienced pilots are kind of amazed to learn how it works.

 

 

 

The tail trim design was a result of Lockheed during WWII in the XP-90. Much after that the same design was used on the Lockheed U-2 (early models) and the Lockheed Jetstar corporate jet.

But did you know Al Mooney spent many years at Lockheed back then ?  :-)

[Justin Schmidt]

2 hours ago, Mark942 said:

, and he said yeah, they're all like that

Famous words that has been a top killer of people throughout civilization. 

[MikeOH]

4 minutes ago, Justin Schmidt said:

Famous words that has been a top killer of people throughout civilization. 

Yuup!  Right up there with, "Hold my beer, and watch this!"

[Justin Schmidt]

1 minute ago, MikeOH said:

Yuup!  Right up there with, "Hold my beer, and watch this!"

At least that one you know something incredibly stupid BUT fun and hilarious is about to happen 

[EricJ]

15 hours ago, cliffy said:

The tail trim design was a result of Lockheed during WWII in the XP-90. Much after that the same design was used on the Lockheed U-2 (early models) and the Lockheed Jetstar corporate jet.

Yes, that's why I specified for "small airplanes".   I don't know of any other GA airplanes that use that system.  The XP-90 and the M-18 were developed at about the same time, prior to Mooney joining Lockheed, so who knows who influenced who.   The Jetstar design could certainly have been influenced by Al Mooney, though.

15 hours ago, cliffy said:

But did you know Al Mooney spent many years at Lockheed back then ?  :-)

Yes, but that was after development of the M-18 and M-20.

[Vance Harral]

We had our jackscrew assembly rebuilt by LASAR a while back (long before the move to Prineville) due to excessive slop.  In the course of discussions, Dan Reisland explained to us that there is something of an art to choosing the correct internal shims during the rebuild.  Too few and you get more than allowed movement as specified in the maintenance manual.  But too many causes the whole assembly to bind up, making it hard to turn the trim wheel, and causing immediate, excessive wear on the components.  This applies to the whole system, not just the jackscrew assembly.  Don't seek to make it overly tight.  You want "some" slop, just not so much as to exceed limits.

The best practical advice I ever got on this subject was from a mechanic who advised me to surreptitiously lift the tail of every Mooney I came across that I could lay hands on.  The service manual is the gold standard, of course, but there is something to be said for comparing the slop in your airplane against what you find in others.

[47U]

I think this SB (from 1960) is superseded by guidance in mx manuals applicable to long rudder airplanes.  For short rudder airplanes, it may provide some perspective.  

https://mooney.com/wp-content/uploads/2020/12/SBM20-62.pdf

[Mark942]

Thanks again guys,

-mark