Elevator bungee springs

[cruiserflyer]

Hello all. I am finally an owner. I recently purchased M20F 3288F, a 1967 model. I love the aircraft and ferried it back from Washington state with my A&P/CFI. Two issues that are demanding attention are an oil leak which is "lubricating" the nose gear, and what appears to be weak elevator bungee springs. I will be at the hangar with my A&P this Thursday to start addressing these issues. I did buy the service manuals (but I left them in the aircraft an hour away) but I'd like to know if this is something that can be solved by opening up the bungees and replacing springs. Both the A&P who last looked at the aircraft and my A&P think this needs to be addressed so I'm 100% on board, just trying to get a lead on part sourcing. Any guidance would be helpful. Can the bungee assemblies be opened up and, what I presume are springs, be replaced?

Thanks!
Paul

 

[47U]

19 minutes ago, cruiserflyer said:

I did buy the service manuals (but I left them in the aircraft an hour away)

There are copies of the service manuals in the downloads section…

[N201MKTurbo]

How did he determine the springs were weak? I don’t think there is a way to know that from the handling of the plane. Considering there is one for each elevator, which one is weak? The left or right?

The springs can break. If you remove the bungee, they are easy to disassemble. If you do disassemble them, clean, repaint and grease them. They probably haven’t been serviced since they left the factory.

[cruiserflyer]

I think just by feel, that there does not appear to be any resistance or spring pushback. I was honestly deferring to them, but I didn’t feel any slack or troubling feeling in the controls. From pushing gently up and down on the elevator the bolt moves all the way out and in. Since I am deferring to them I took it on faith that it must be wrong, but maybe it’s fine. My A&P has owned about a dozen Mooney’s so I just listen to them. The pics are of me pushing and pulling.

[N201MKTurbo]

There were a few different elevator arrangements in the Mooneys. Some of them have a definite force while sitting on the ground. The arrangement you have does not. It will just center around the trimmed position. The bungees put a spring bias to the elevator depending on trim position. The bungees are responsible for the elevator not lining up with horizontal stabilizer in cruise. The bungees put the elevators in the optimal position for minimum drag, which is not lined up.

[Ragsf15e]

4 hours ago, cruiserflyer said:

I think just by feel, that there does not appear to be any resistance or spring pushback. I was honestly deferring to them, but I didn’t feel any slack or troubling feeling in the controls. From pushing gently up and down on the elevator the bolt moves all the way out and in. Since I am deferring to them I took it on faith that it must be wrong, but maybe it’s fine. My A&P has owned about a dozen Mooney’s so I just listen to them. The pics are of me pushing and pulling.

I had an F, now i have a K, the elevator arrangement is completely different.  I’m saying this because maybe your mechanic had different models and the elevator was set up differently.  My F looked just like yours, it should roughly center the elevator to the horizontal stab on the ground.  If it’s not broke, maybe it doesn’t need fixed?  If they really want to take it apart, maybe the goal should be to clean it up?

Heres the real point… is there somewhere in the maintenance manual telling you (or your mechanic) how much spring force there should be and what the limit is?  I don’t think there is, but you can look.  And if they still think it’s not right, maybe ask why they think that?  

[Ragsf15e]

6 hours ago, cruiserflyer said:

Hello all. I am finally an owner. I recently purchased M20F 3288F, a 1967 model. I love the aircraft and ferried it back from Washington state with my A&P/CFI. Two issues that are demanding attention are an oil leak which is "lubricating" the nose gear, and what appears to be weak elevator bungee springs. I will be at the hangar with my A&P this Thursday to start addressing these issues. I did buy the service manuals (but I left them in the aircraft an hour away) but I'd like to know if this is something that can be solved by opening up the bungees and replacing springs. Both the A&P who last looked at the aircraft and my A&P think this needs to be addressed so I'm 100% on board, just trying to get a lead on part sourcing. Any guidance would be helpful. Can the bungee assemblies be opened up and, what I presume are springs, be replaced?

Thanks!
Paul

 

To your second question, they all leak on the nose a little.  A tiny bit of oil makes a big mess.  How bad is it?  It might be worth searching for but you might decide to live with it.  Also, there are three tubes coming out of the cowling near the nose tire.  The case vent, a drain for the mechanical fuel pump and the sniffle valve drain.  There’s a drain for the electric fuel pump and the gascolator farther aft (under the pilots feet roughly).  You need to see if the oil drips are from these drains because that will guide your troubleshooting.  Oil will normally drip out the case “breather” tube.  That’s normal.  The sniffle valve might drip a little fuel but shouldn’t drip much.  Oil out the mechanical fuel pump drain is a sign of a failing pump diaphragm on your mechanical pump.

Unfortunately, almost all the oil leaks in the engine find their way to the nose gear.

[cruiserflyer]

Great responses, thank you! The elevator is perfectly in line with the horizontal stab on the ground. I am definitely not in favor of unnecessary maintenance and will take all these answers to the hangar on Thursday. My main goal at the moment is to locate the oil leak source. My A&P mentioned it might be the fuel pump diaphragm but we'll clean it up first, then run it and see if we can source it. On our ferry flight from Spokane to eastern Montana we only were down half a quart, hardly anything to worry about.

[PT20J]

All M20 models through the M20J have trim assist bungees (later models have a different system not relevant to this discussion). The Mooney trims primarily by adjusting the angle of incidence of the horizontal stabilizers which is a common trimming method used on many airplanes. What makes the Mooney different is that rather than adjusting the horizontal stabilizer itself (like a Piper Cub, Cessna 180 and many jets) the Mooney stabilizer is rigidly attached to the empennage and the trim, through a jackscrew arrangement, pivots the entire empennage. Since the trim has to move a considerable amount of structure, it has to have a lot of mechanical advantage to keep trim forces within reason. But, that also means that it would take a lot of turns of the trim wheel to adjust the stabilizer angle through the range necessary for all trim conditions. So, the bungees are added to add an additional force to the elevator to assist trimming so that the stabilizer doesn't have to move as much. The bungees also act as centering springs on the stick which improves longitudinal stability. 

You can see the effect of the bungees if you move the trim while sitting on the ground. With the trim set to the takeoff position, the elevator should be approximately aligned with the stabilizer. If you set full nose up trim the elevator should move trailing edge up. If you move the trim full nose down the elevator should move trailing edge down.

The TCDS (and service manual) will have rigging information for adjusting the bungees. I would check this before doing anything else.

[PT20J]

8 minutes ago, cruiserflyer said:

My A&P mentioned it might be the fuel pump diaphragm but we'll clean it up first, then run it and see if we can source it.

The fuel pump has two diaphragms for redundancy. There is a drain line between the diaphragms that should exit behind the left cowl flap. It's a good preflight item to check this line as it should be dry. It can be wet with fuel or oil depending on which diaphragm has failed. Be sure you are checking the correct hose. There should also be hoses for the intake manifold drain (sniffle valve), the crankcase breather and perhaps the vacuum pump in the same area.

[Captnmack]

On 9/21/2025 at 3:15 PM, N201MKTurbo said:

There were a few different elevator arrangements in the Mooneys. Some of them have a definite force while sitting on the ground. The arrangement you have does not. It will just center around the trimmed position. The bungees put a spring bias to the elevator depending on trim position. The bungees are responsible for the elevator not lining up with horizontal stabilizer in cruise. The bungees put the elevators in the optimal position for minimum drag, which is not lined up.

 

[PT20J]

In engineering there is an old saying that there is no such thing as a single change. I believe the short bodies trim out with the elevator aligned with the stabilizer. However lengthening the fuselage for the mid bodies threw things off and they trim in flight with the elevator slightly trailing edge down. I was told years ago by an engineer at Mooney that Lopresti looked into this when modifying the M20F to make the M20J and decided that fixing it would require significant rework to the empennage to change the angle of incidence of the horizontal stabilizer and the drag was minimal and not worth the cost to fix it.  

[cruiserflyer]

8 hours ago, PT20J said:

In engineering there is an old saying that there is no such thing as a single change. I believe the short bodies trim out with the elevator aligned with the stabilizer. However lengthening the fuselage for the mid bodies threw things off and they trim in flight with the elevator slightly trailing edge down. I was told years ago by an engineer at Mooney that Lopresti looked into this when modifying the M20F to make the M20J and decided that fixing it would require significant rework to the empennage to change the angle of incidence of the horizontal stabilizer and the drag was minimal and not worth the cost to fix it.  

Got the bungees apart on Saturday. One spring is broken, of course it's not in stock. Trying to figure out how to replace it by finding an equivalent.

Edited October 20 by cruiserflyer
Spelling

[takair]

Try @Alan Fox or @Jerry Pressley  

I may have a spare if they don’t. 

[DCarlton]

On 9/21/2025 at 3:21 PM, PT20J said:

So, the bungees are added to add an additional force to the elevator to assist trimming so that the stabilizer doesn't have to move as much. The bungees also act as centering springs on the stick which improves longitudinal stability. 

Appreciate the explanation.  Trying like heck to understand the purpose of the springs.  Having a hard time understanding how they "add additional" force.  I could see them providing a force or elevator position that varies with airspeed at any given trim position.  There's something tricky I'm missing.  Great thread though.  I've been curious about the springs; never seen them before this thread.  

[cruiserflyer]

2 hours ago, takair said:

Try @Alan Fox or @Jerry Pressley  

I may have a spare if they don’t. 

That would be great. I'll call LASAR today and update this conversation when I know. But it's safe to say they won't have it. Maybe they can give me the specs so I can procure one myself. Just very crudely I was able to compress the spring 1" with about 25lbs of force.

[PT20J]

2 hours ago, DCarlton said:

Appreciate the explanation.  Trying like heck to understand the purpose of the springs.  Having a hard time understanding how they "add additional" force.  I could see them providing a force or elevator position that varies with airspeed at any given trim position.  There's something tricky I'm missing.  Great thread though.  I've been curious about the springs; never seen them before this thread.  

The idea behind any trim system is to remove stick force and this can be done in several ways, or by a combination of methods. The most common method is a trim tab that replaces the stick force necessary to hold a particular elevator position with an aerodynamic force. Another way is to allow the incidence angle of the stabilizer to be changed in flight. A third method is to have variable force bungees in the control system so that the bungees can provide the force necessary relieving the pressure on the stick. Mooney chose a combination of variable stabilizer incidence and bungees.

[skykrawler]

https://www.ebay.com/itm/145627163740?_skw=Mooney+elevator+bungee&itmmeta=01K816TY9AQCMB5A6Y67T863QP&hash=item21e80e2c5c:g:IEYAAOSw6mVlz6iW&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1dLbFasi9WirHNRViH8d9NWsaQgHfsYpKZFB0SSac9Bx1PXxgiVoZ%2BOD1HMPO0IcIkxlKFWI1RYl0TDQLc0%2BApAD%2F1VRgnT6n81MFR3GnA%2FdCgFhKwLLtEXT%2BWb35THqoo%2FlmSXt8avX9D%2BWFxD7cH7smgYFrD78mrY81neKp%2FVvANI7rM1r2HkXqvqJEatJQBkxY97dKCvLHbQtmwH5mZhM7AQNJIF1VG3OrNbIBZ8qSB5MRJcBXj4WyOKXKICGBlotiWJQ0fajEmD96%2FIHFEaoIVXbwp7Te5Z39u2p6CKiw%3D%3D|tkp%3ABk9SR-7k66bAZg

 

[MikeOH]

1 hour ago, PT20J said:

The idea behind any trim system is to remove stick force and this can be done in several ways, or by a combination of methods. The most common method is a trim tab that replaces the stick force necessary to hold a particular elevator position with an aerodynamic force. Another way is to allow the incidence angle of the stabilizer to be changed in flight.

All of that makes sense to me.

1 hour ago, PT20J said:

A third method is to have variable force bungees in the control system so that the bungees can provide the force necessary relieving the pressure on the stick

I'm still struggling with this method.  I thought the trim wheel in the Mooney adjusted the angle of incidence of the stabilizer via a jackscrew.  But I don't understand how the spring bungees play into this.  Does the trim wheel also provide the preload to the springs?  If so, why are they even necessary as I would think you would just adjust out the force with the stabilizer AOI.

If not, where does the preload come from?  If it is fixed, then it is really not being adjusted by the pilot and is not really an 'active' trim adjustment.  More like what @DCarlton was referring to: the spring force would be dependent on the aerodynamic load, i.e. airspeed dependent.

I'm probably missing something obvious...

[takair]

There is a fixed pushrod to the bungee assembly attached to the airframe. When the stab moves the pushrod imparts an effective force on the bungees. Or another way to look at it is that the bungees don’t move while the tail does and thus imparts the appropriate spring force on the elevator. 

[PT20J]

A clue to the purpose of the bungees is that Mooney calls them "trim assist bungees."  If you rotate the trim wheel from stop to stop you will notice two things: First, because of the arrangement Rob @takair noted, the springs will move the elevator in a direction that aids the aerodynamic effect that the stabilizer is trying to achieve. The second thing to notice is that it takes a lot of turns of the trim wheel to get from stop to stop. If the stabilizer incidence change alone had to remove all the stick force, the stabilizer would have to move over a larger range and this would require even more rotations of the trim wheel unless the gearing was changed which would reduce the mechanical advantage and make the wheel harder to rotate. But by using the spring arrangement to assist the stabilizer, less stabilizer movement is needed to achieve the same stick force reduction.

 

 

'

 

[EricJ]

16 hours ago, cruiserflyer said:

Got the bungees apart on Saturday. One spring is broken, of course it's not in stock. Trying to figure out how to replace it by finding an equivalent.

See whether there is a spring fabricator in your area and what it would take to get a new one wound.    I suspect ebay or BAS may be a lot cheaper, but having a new one fabricated is always an option.

 

[DCarlton]

7 hours ago, takair said:

There is a fixed pushrod to the bungee assembly attached to the airframe. When the stab moves the pushrod imparts an effective force on the bungees. Or another way to look at it is that the bungees don’t move while the tail does and thus imparts the appropriate spring force on the elevator. 

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.    

[N201MKTurbo]

27 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.    

Take off the pilots side faring between the tail and fuselage and the big and oval access panels on the same side. It will be visible. 

[cruiserflyer]

6 hours ago, PT20J said:

A clue to the purpose of the bungees is that Mooney calls them "trim assist bungees."  If you rotate the trim wheel from stop to stop you will notice two things: First, because of the arrangement Rob @takair noted, the springs will move the elevator in a direction that aids the aerodynamic effect that the stabilizer is trying to achieve. The second thing to notice is that it takes a lot of turns of the trim wheel to get from stop to stop. If the stabilizer incidence change alone had to remove all the stick force, the stabilizer would have to move over a larger range and this would require even more rotations of the trim wheel unless the gearing was changed which would reduce the mechanical advantage and make the wheel harder to rotate. But by using the spring arrangement to assist the stabilizer, less stabilizer movement is needed to achieve the same stick force reduction.

 

 

'

 

Here are links to videos in response to a previous post where you told me to set the trim full up/down to see how the bungees were working. It led me to the disassembly that generated this post.

https://drive.google.com/file/d/15ejSrzggeifHAhGH_MJ3vJTEsrVK_tRN/view?usp=drivesdk

https://drive.google.com/file/d/1dE9TUsvxeIF79-m0d4JMrmB49EBXs8A9/view?usp=drivesdk