No Back Spring Update

[exM20K]

On 2/1/2023 at 10:16 PM, StevenL757 said:

Exactly right…the quality control issue is accurate; whereas, the defective springs will simply not install correctly inside the actuator; hence, not rotate as they should per the install instructions.  The defect is obvious when you try to fit the spring.

Thanks, this is good to know.  Mine was replaced with an off -the-shelf from a major  MSC last Spring.  The whole actuator was rebuilt as it was grinding.

-dan

[PT20J]

6 hours ago, EricJ said:

FAR 23.729 specifies the legal requirements for gear extension.   If there is no manual extension method (I don't know if the crank or pull handle counts); 

c) Emergency operation. For a landplane having retractable landing gear that cannot be extended manually, there must be means to extend the landing gear in the event of either—

(1) Any reasonably probable failure in the normal landing gear operation system; or

(2) Any reasonably probable failure in a power source that would prevent the operation of the normal landing gear operation system.

I don't know if "reasonably probably failure" is defined anywhere, or it's left up to the manufacturer, but it's clear that single point failure items just need to be "reasonably high reliability" or at least above the threshold of "reasonably probable failure".

 

CAR 3 was less specific

[T. Peterson]

On 2/4/2023 at 8:42 PM, 1980Mooney said:

Reality check - how big is this market? TINY and shrinking.  Incredibly unlikely that any new units will ever be built again.  No one is going to spend 2 cents re-engineering the Mooney gear extension system - Not Eaton and certainly not Mooney.

If this becomes too big of a safety issue the more likely outcome will be an AD to lock the gear down permanantly......  

Well, that’s encouraging!

[Bigdaddie]

So, the question is replace or do not replace?  My spring has 2,000hrs on it.  Is it worthwhile to replace it or just continue with a known good part?  I hate doing preventative maintenance on an item that may create a problem.  Any time you open up a system, be it cleaning fuel injectors or landing gear, you create a potential problem by doing maintenance on an item that worked perfectly.  Any thoughts?

[Schllc]

On 2/4/2023 at 3:19 PM, A64Pilot said:

 

If there are of have been cracks at the bend of the tang and or broken tangs, it to me could possibly indicate that the tangs were formed after the springs were heat treated, which is normal, as I don’t think it’s normal to heat treat small springs after being formed, but bending heat treated steel in a small radius can fatigue it

I don’t remember manufacturing springs during my engineering days,  but we did do a lot of fea and design for parts and assemblies in high stress environments and the very purpose of heat treating in all of our parts was specifically to reduce stresses from the metal and prepare it for its purpose.
I would not say it’s normal to bend any metal after heat treating because it generally defeats the purpose. 
if these spring tangs were bent after heat treatment that would be considered bad practice, at least in my experience, and is almost certainly the reason for the failure. 

[A64Pilot]

You guys get me to thinking that maybe it’s time to sell, and buy something maintainable, while it’s still sellable.

So far as “simply” converting to hydraulic etc That’s not happening, we have no down locks etc that would be required, it may sound simple but I can assure you that it would get complicated quickly

It could be done, with enough money anything could, but I don’t see it happening, due to the lack of money

PMA’d parts would seem to be our only hope.

I think I need to call Top Gun and ask for my $1,000 back, it seems due to no fault of theirs, but new NBS springs or I guess any other parts for the actuator are unavailable, and seems there is no plan to make them available.

 

[A64Pilot]

2 hours ago, Schllc said:

I don’t remember manufacturing springs during my engineering days,  but we did do a lot of fea and design for parts and assemblies in high stress environments and the very purpose of heat treating in all of our parts was specifically to reduce stresses from the metal and prepare it for its purpose.
I would not say it’s normal to bend any metal after heat treating because it generally defeats the purpose. 
if these spring tangs were bent after heat treatment that would be considered bad practice, at least in my experience, and is almost certainly the reason for the failure. 

The heat treat process itself seems to cause changes in springs in particular, by changes I mean bends unbend slightly etc, because heat treating steel of course involves quite high temperatures.

However things like valve springs are of course heat treated I’m sure after manufacture, but even simple spring steel landing gear legs we had problems with them varying the curve of the bend during heat treat so we came up with shims to adjust if you will the gear legs on assy. I’m not a fan of spring steel gear.

But the NBS has to be a simple fix, just I think there isn’t enough money for anyone to make one that meets drawing specs and works.

A lot of the springs we had manufactured like elevator down springs that looked a lot like spring door springs the supplier would cut to length and bend the loop down, this I’m certain caused a stress riser but it seemed to work.

I wasn’t saying that bending after heat treat is a good way, but one that I think is often done. If you contacted a spring supplier and gave them the drawings I wouldn’t be surprised if they took an existing spring, cut to size and bent the ends to make tabs.

[PT20J]

2 hours ago, Bigdaddie said:

So, the question is replace or do not replace?  My spring has 2,000hrs on it.  Is it worthwhile to replace it or just continue with a known good part?  I hate doing preventative maintenance on an item that may create a problem.  Any time you open up a system, be it cleaning fuel injectors or landing gear, you create a potential problem by doing maintenance on an item that worked perfectly.  Any thoughts?

Based on my research, I personally would not replace the NBS.

1. The 1000 hour recommendation seems to be based on a couple of failures of Plessey actuators and carried over to Eaton. This has never been a serious enough problem to warrant an AD.

2. The failure of an Eaton actuator (and recall of a particular range of serial numbers by Eaton) appears to be due to a particular lot of springs years ago. Those have likely been replaced or have failed by now if they are going to fail.

3. Most of the very few documented failures have been Plessey actuators, the most recent only a few hundred hours after spring replacement. If I had a Plessey actuator, I would consider spending the $10K or whatever the going rate is to replace it with an Eaton if I could find one.

4. Hours isn't really important -- it's cycles. But cycles are not tracked in single engine piston airplanes. According to Don Maxwell, the spring is rated for something like 20,000 cycles. So if you cycled the gear once an hour on average, that would be 20X the recommended replacement interval.

5. A new spring would have some risk because they are not in current production. People are going to have to dig out a 50 year old drawing and make a part that they likely have never made before. Maybe they are simple to manufacture and maybe there is some technique to it. Processes were often not as precisely documented in the age before everything was done on CAD. Who knows? There was some post to the effect that Mooney rejected a batch that didn't fit correctly. I have no first hand knowledge of that.

Skip

 

[A64Pilot]

I don’t think they would mind me posting this and I don’t know any other way so they are photos.

‘I’ve paid for and have had on order a NBS for about a year now.

‘What I want is the actuator disassembled, cleaned, inspected, relubed and reassembled, that way I think I would feel better.

First Email below was Jan, second June

While I agree cycles are an issue, but so is calendar time. My actuator has 42 yr old grease in it, and grease that’s decades old often changes, it sometimes dries out and sometimes separates into liquid and a thicker solid, but most of it eventually degrades and should be replaced every so often. That’s why we repack wheel bearings.

Having said that most automotive wheel bearings are now sealed, and apparently do fine. I don’t know if grease is better now or were we just repacking for all those decades when we didn’t need to? But I have had to replace many a bearing on farm equipment because the grease had dried out and just wasn’t grease anymore, so who knows?

 I think I need to pull the trigger on having my actuator serviced and did email Top Gun asking about that, but never received a reply.

Are there serviceable actuators out there is the “pre flown” world?

 

[Fly Boomer]

1 hour ago, PT20J said:

Based on my research, I personally would not replace the NBS.

Mine was replaced once.  It was a while back.  I think once per lifetime should be enough.

[201Mooniac]

I've replaced mine 3 times in 28 years but they were never gard to get back then and Tom Rouch at Top Gun would do a full disassembly and service.  I'm 6 years and about 600 hours into the current one and given the situation as it stands, I likely won't replace it again.

[Pinecone]

What happened to the guy that was going to make them under PMA?

Last I remember seeing, he wanted a broken one to determine the exact alloy used.

[1980Mooney]

On 8/18/2023 at 1:33 PM, Bigdaddie said:

So, the question is replace or do not replace?  My spring has 2,000hrs on it.  Is it worthwhile to replace it or just continue with a known good part?  I hate doing preventative maintenance on an item that may create a problem.  Any time you open up a system, be it cleaning fuel injectors or landing gear, you create a potential problem by doing maintenance on an item that worked perfectly.  Any thoughts?

As @PT20J says - I would not replace the NBS.

But as @A64Pilot says - I would have the actuator disassembled, cleaned, inspected, relubed and reassembled.

Back in 2003 when SB-282 first came out, I watched while my former A&P (now deceased - an old timer who started working at Cessna in the 50's) replaced my NBS, cleaned, relubed and reassembled my Eaton actuator on my 1980 J.   It wasn't rocket science but it had to be done right - the parts are unforgiving.   It has been 20 years since but I have no plans to replace the NBS again.  If anything. I will  have the actuator disassembled, cleaned, inspected, relubed and reassembled again.

Also - if you or former owners have been practicing with the Emergency Landing Gear Extension system a lot - Per Mark Rouch of Top Gun Aviation MSC:

"Another problem with the newer actuators is the wear on the emergency extension cable. It is Teflon coated and the Teflon can tear and peel loose and that loose Teflon can actually jam the actuator. It actually happened during a practice emergency landing gear extension.

The other wear item is the emergency clutch, which is made out of soft brass. If the emergency cable is not rigged correctly, this may allow partial connection of the emergency clutch to the actuator drive gear, which causes the brass on the emergency clutch to wear off, and the clutch will not engage."

SBM20-282-revA.PDF (mooney.com)

Edited August 19 by 1980Mooney

[Danb]

23 minutes ago, 1980Mooney said:

As @PT20J says - I would not replace the NBS.

But as @A64Pilot says - I would have the actuator disassembled, cleaned, inspected, relubed and reassembled.

Back in 2003 when SB-282 first came out, I watched while my former A&P (now deceased - an old timer who started working at Cessna in the 50's) replaced my NBS, cleaned, relubed and reassembled my Eaton actuator on my 1980 J.   It wasn't rocket science but it had to be done right - the parts are unforgiving.   It has been 20 years since but I have no plans to replace the NBS again.  If anything. I will  have the actuator disassembled, cleaned, inspected, relubed and reassembled again.

Also - if you or former owners have been practicing with the Emergency Landing Gear Extension system a lot - Per Mark Rouch of Top Gun Aviation MSC:

"Another problem with the newer actuators is the wear on the emergency extension cable. It is Teflon coated and the Teflon can tear and peel loose and that loose Teflon can actually jam the actuator. It actually happened during a practice emergency landing gear extension.

The other wear item is the emergency clutch, which is made out of soft brass. If the emergency cable is not rigged correctly, this may allow partial connection of the emergency clutch to the actuator drive gear, which causes the brass on the emergency clutch to wear off, and the clutch will not engage."

SBM20-282-revA.PDF (mooney.com)

Here’s your example of the torn cable in a newer Mooney 2005 Bravo, I was lucky and found the problem during the NBS replacement 

[blaine beaven]

So it looks like my NBS was replaced in 2002, about 950 hours ago. That was the first time it was replaced, and the aircraft had 2000 hours at that time. 
 

Since these NBS seem to be impossible to obtain, I am considering having the actuator opened up, inspected, cleaned, and lubricated, but not replace the NBS. 
 

Do many of you do this at the 1000hr mark? Or do you just leave it be, and not open it up at all? I’m somewhat hesitant to have a system that is working looked at “just because” and introduce errors in reassembly. 
 

Thanks! 

[PT20J]

I'm firmly in the camp of if it ain't broke, don't mess with it. 

I've looked into this a lot. Eaton never had a recommendation about maintenance on these as far as I can tell until they had a failure of a no-back spring that was probably due to a bad manufacturing lot of springs. That was many years ago. The 1000 hours seems to have come from Mooney and was due to the failure of two springs in Plessey actuators (different design) at a little under 1000 hours each. 

The 1000 hours is really meaningless. It's cycles that are important. Don Maxwell told me that the Eaton actuator is rated for 20,000 cycles. If you cycled the gear once per hour of flight time, that would be 20,000 hours.

So, figuring that any bad springs have worked their way out of the installed base by now, I see no value in messing with it. I think about it this way: The only reason to mess with it is fear of failure. After I had it serviced, I would have to worry about a maintenance induced failure. Same difference -- I'd still have something to worry about and there is no objective means to tell which is better. By the way, when I asked Don about it he said not to mess with it unless it was making an unusual noise.

Skip

[blaine beaven]

Thanks Skip, I appreciate your opinion on this. 

[amillet]

I am picking up aircraft from annual tomorrow. One of the squawks I listed was the gear horn and gear warning light would chirp occasionally while taxiing. Advanced Aircraft Services fixed it. In the process I got a new no back clutch spring that I didn’t intend to change out. The one removed had about 1400 hours on it.

 

The invoice describes the fix as follows :

Discrepancy
Landing gear motor momentarily actuates un-commanded when master switch is turned on.
Corrective Action
Complied with Mooney SB M20-282-A per Mooney Service Manual and Eaton instructions by installation of new No Back Spring. Landing gear system was functionally tested for proper retraction and extension. Manual extension check was performed and down lock tensions were verified and adjusted as needed.

[1980Mooney]

34 minutes ago, amillet said:

I am picking up aircraft from annual tomorrow. One of the squawks I listed was the gear horn and gear warning light would chirp occasionally while taxiing. Advanced Aircraft Services fixed it. In the process I got a new no back clutch spring that I didn’t intend to change out. The one removed had about 1400 hours on it.

 

The invoice describes the fix as follows :

Discrepancy
Landing gear motor momentarily actuates un-commanded when master switch is turned on.
Corrective Action
Complied with Mooney SB M20-282-A per Mooney Service Manual and Eaton instructions by installation of new No Back Spring. Landing gear system was functionally tested for proper retraction and extension. Manual extension check was performed and down lock tensions were verified and adjusted as needed.

Interesting - Well they are an MSC and no doubt you got what you are going to pay for - "Complying with SB M20-282-A and a new No-Back Spring."

But I would like to understand how the "No Back Spring" has anything to do with the "Landing gear motor momentarily actuates un-commanded when master switch is turned on."  The No-back Spring keeps the landing gear up when they are fully retracted.  For the "Landing gear motor momentarily actuates un-commanded when master switch is turned on." there would have to be failure(s) in the gear down limit microswitches. - i.e. the actuator motor momentarily thinks that the landing gear are not fully down and locked.

Perhaps they did more work on the limit switches which was not shown in detail on the invoice but shows up in your Aircraft Maintenance Log.

As Skip @PT20J explained

"it is clear from the descriptions in M20-282-A that the spring serves primarily as a brake to keep the gear in the up position. This is necessary because the actuator uses a ball screw (recirculating ball bearings between the nut and threaded jackscrew), likely to reduce friction under load, and ball screws can back drive the actuator. The Mooney landing gear has no up locks, so the weight of the retracted gear could back drive the actuator unless it had some sort of locking mechanism. This is not a problem in the down position because the gear is locked down by an over-center mechanism in the linkages. Thus, only a one-way brake is needed.

The spring is a form of what is known as a wrap spring. It fits over the input hub driven by the motor and the output hub that drives the screw. This whole assembly fits inside a cylindrical housing with the spring having a slight interference fit in the bore. The spring can be thought of as "brake shoes" and the housing a "brake drum" in analogy with old style automotive drum brakes. Any attempt to back drive the actuator unwinds the spring slightly and cinches it in the housing stopping rotation of the ball screw."

 

[amillet]

I too questioned why it made any difference with the gear down. This is Greg’s explanation:

The gear actuator is the biggest issue we seem to have.  The reason the gear horn is sounding intermittently is because the gear actuator is turning backwards releasing the gear tension.  The reason for this is the No Back Spring is slipping.  When the tension drops off, the gear actuator kicks on to gear back up against the stop.  Not the best thing for the motor or the actuator.

[amillet]

Advanced had a no back clutch bring in stock, that was ordered for another aircraft. Greg said, for some reason it didn’t work in that aircraft and neither he nor Mooney could explain why. It worked fine in our actuator.

[201Mooniac]

2 hours ago, amillet said:

I too questioned why it made any difference with the gear down. This is Greg’s explanation:

The gear actuator is the biggest issue we seem to have.  The reason the gear horn is sounding intermittently is because the gear actuator is turning backwards releasing the gear tension.  The reason for this is the No Back Spring is slipping.  When the tension drops off, the gear actuator kicks on to gear back up against the stop.  Not the best thing for the motor or the actuator.

I'm not disputing what he says but I thought when the gear was down it was over center and wouldn't move off the stops until driven off.  I'd like to understand the mechanism here as if the gear can come off the down stops by the actuator turning backwards, why doesn't the gear ever collapse when parked with no power to the actuator?  Maybe the preload wasn't set high enough?  Any help in understanding would be appreciated.

[Pinecone]

It seems you might not understand how an over center link works.

When you over center a linkage, you go past the point of maximum extension.  So you are at the point that tension on the linkage cannot open the link, it makes it lock tighter.

http://www.mjvail.com/destaco/images/intropage3a.gif

 

[kortopates]

It seems you might not understand how an over center link works.
When you over center a linkage, you go past the point of maximum extension.  So you are at the point that tension on the linkage cannot open the link, it makes it lock tighter.
http://www.mjvail.com/destaco/images/intropage3a.gif
 

Not only that, but all the additional force to hold it comes from the gear actuator compressing springs on the gear to create that pre-load force. Let the gear actuator slip back a tad and that pre-load force goes, relieving the compression on the springs. From the description above of the issue, we know it’s slipping back enough to at least open the gear down limit switch. Probably not by much.


Sent from my iPhone using Tapatalk

[MikeOH]

6 hours ago, 201Mooniac said:

I'm not disputing what he says but I thought when the gear was down it was over center and wouldn't move off the stops until driven off.  I'd like to understand the mechanism here as if the gear can come off the down stops by the actuator turning backwards, why doesn't the gear ever collapse when parked with no power to the actuator?  Maybe the preload wasn't set high enough?  Any help in understanding would be appreciated.

If I'm following this discussion correctly (and I may not be!), I think that once on the ground the weight of the aircraft tries to drive the link open, but since it's over center, it just locks more. The motor is no longer needed once weight is on the wheels.

The part I'm not following is if the no-back spring is causing this, i.e. it has failed, then how come the gear works at all?  I thought a no-back spring failure resulted in the gear refusing to even extend, not just allowing the gear to come off of over center?  What am I missing?