No back spring in landing gear actuators

[Yooper Rocketman]

2 hours ago, M20S Driver said:

Mooney  recommends to replace the no back spring on the new planes.  This is an extra cost that works against Mooney for selling new planes.  If this was not a safety issue.  I am sure that they would have removed it from the list of recommendations.

 

Owning a franchise with a large OEM (Daimler Trucks N.A.) I find a real contrast (or shall I say "Irony") between what a manufacturer thinks needs to be repaired/replaced when THEY have to pay for it verses what recommendation they will give for the same procedure when the end user will bear the cost.  There have been a ton of recommendations given by aviation related manufacturers that have been contrary to their future business model.  The liability department always trumps sales and marketing.

[M20S Driver]

On 3/20/2016 at 2:26 PM, Yooper Rocketman said:

Owning a franchise with a large OEM (Daimler Trucks N.A.) I find a real contrast (or shall I say "Irony") between what a manufacturer thinks needs to be repaired/replaced when THEY have to pay for it verses what recommendation they will give for the same procedure when the end user will bear the cost.  There have been a ton of recommendations given by aviation related manufacturers that have been contrary to their future business model.  The liability department always trumps sales and marketing.

with all due respect, I do not agree.

If this was a starter, alternator, speed brake, or any other component that did not have a disastrous failure mode, I would agree.   Given the cost, I am happy to replace it even if it improves the chance of not having a failure by a few percent.  

After looking at the spring that was removed, I am even more convinced that it does not has indefinite life.   Maybe 1000 hours replacement is overkill but I would not wait too long to do it.  

[cnoe]

Still, I must respectfully suggest that a "failure to extend" does not qualify as a disastrous failure. I've read dozens and dozens of posts arguing that a GU landing is no big deal which should have little (if any) effect on the value of a plane. Neither can I recall hearing of any injuries in such incidents.

I'm not telling anyone not to replace their no-back-spring but the potential consequences are not nearly as dire as skipping the 500 hour mag inspection or neglecting to sump the tanks when appropriate.

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[donkaye]

1 hour ago, cnoe said:

 

Still, I must respectfully suggest that a "failure to extend" does not qualify as a disastrous failure. I've read dozens and dozens of posts arguing that a GU landing is no big deal which should have little (if any) effect on the value of a plane. Neither can I recall hearing of any injuries in such incidents.

I'm sorry, but you've got to be kidding.  You would risk the safety of your family and/or yourself, the potential of a fire when scraping the belly of your plane, a $45,000 repair charge, the lack of use of your airplane, a devaluation of at least 10% due to accident history, a black mark on your certificate, a bruised ego, all because you didn't want to preemptively replace the back spring at a total cost of $1,000?  That just seems like very poor aeronautical decision making from my point of view.

I had a much worse situation cost wise.  I had to replace the whole gear actuator, since it was a Plessey, and no more springs were available because Plessey went out of business.

 My spring gets replaced every 1000 hours--preemptively.  "Failure to extend" DOES qualify as a disastrous failure to me.

[cnoe]

I'm not trying to be difficult, nor am I one to forgo necessary maintenance at the expense of myself or my family. But from all evidence I can gather the level of risk related to this issue appears minimal. I may not have all the information but the previously referenced SB calls for an inspection and replacement of all these springs even though it appears that the problem only exists with Eaton actuators. And then even Eaton states in the SB that "it is believed that the no-back clutch malfunction was an isolated incident".

My plane does not utilize an Eaton actuator and I can find no evidence that a failure has ever occurred in this model. The problem cited in the SB is a crack or fatigue failure in the spring and unlike aluminum, steel components are not predisposed to fatigue failures unless caused by a manufacturing or innate material defect. The problem as I understand it is also unrelated to "wear".

For the record my plane's no-back-spring was replaced prior to my purchasing it and still falls within the prescribed replacement period. What I am saying is that I am giving strong consideration to not initiating another replacement in the future. I cannot give advice to others on what they should or shouldn't do; I can only speak for myself.

I wish that more information about this subject were available so that owners could make an informed decision rather than relying on guidance from what appears to be the standard CYA legal remedy.

We could all spend many AMUs improving our safety of flight, whether it be in training or equipment, but at some point one must decide which expenses are reasonable and have an actual positive effect on safety.

[donkaye]

The problem is that if there was a 1% failure rate and you were the 1% then for you the failure rate is 100%.  This is one of the few single point of failure items where, if the spring fails, the gear will neither come up or go down.  In my opinion this is one area where an AMU should be spent.

[cnoe]

Fair enough Don.

BTW, I also use Camguard and run LOP so take my opinion with a grain of salt.

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[donkaye]

Just now, cnoe said:

BTW, I also use Camguard and run LOP so take my opinion with a grain of salt.

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I think running LOP and  using Camguard is just fine.  My engine just won't run LOP well even with Gamis.

[Danb]

Mine won't either Don, seems like there against us.

[carusoam]

 

When did steel become resistant to fatigue failure?

Imagine for a moment...

1) Bending a steel rod into a 90° bend.

2) the surface of the inner bend radius of the rod is trying to compress.

3) the surface of the outside radius is under stress of being stretched.

4) under ideal physics the rod has no radius or surfaces.

5) in real life the surface is constantly being attacked.  

6) stretching the material is called strain.  Strain can lead to changes in the material, like surfaces cracks or work hardening.

7) metals have a certain amount of memory that allows them to stretch and return to there initial state. 

8) going beyond the yield limit.  The material will be changed.

9) The material's resilience keeps it from being yielded

10) Spring materials have been chosen for their springiness first, yield strength after that...

11) a good spring design allows for very little actual strain over the length of the spring's actual stretch.

12) the reason springs fail near their ends is the amount of stretch that is being applied in twisting (the analog of bending the bar)

13) small amounts of strain evidence seems to be gathering and appearing on the surface of the springs.

14) spring design is important.

15) spring manufacturing plays a role.

16) spring materials play a role.

17) The environment the spring is in May effect it.  Especially if the environs induce rust.

18) Spring mounting technique plays a role.  Over Stretching or damaging the spring is bad.

19) The history of at least one bad batch of springs lives on longer than the springs do...

20) A reminder to always make a good quality product all of the time.  The cost of one bad batch will be tough on your business.  One bad design will cause you to lose customers at a higher rate than you can gain customers.  

Reminds me of Champion's law of spark plug quality...

The list goes on...

Best regards,

-a-

Curvy intake tubes + balanced FIs, make LOP possible through a wide range of rpm. 

[co2bruce]

Don, I agree with you 100% on this issue. I've taken my lumps before on this site for believing we should do everything possible to make each flight as safe for ourselves, our passengers, and people on the ground as possible. If that means additional expense and maintenance so be it. That's just my opinion. I would be willing to bet there have been injuries, and deaths involved in gear up landings.

[Marauder]

Don, I agree with you 100% on this issue. I've taken my lumps before on this site for believing we should do everything possible to make each flight as safe for ourselves, our passengers, and people on the ground as possible. If that means additional expense and maintenance so be it. That's just my opinion. I would be willing to bet there have been injuries, and deaths involved in gear up landings.

I'm actually a proponent of preventative maintenance as well as redundancy. I just don't like getting stuck somewhere because my tired thingamajig decided to go TU. That out of my mouth, I'm still flying around with my 12 year old Concorde battery. Does that make an oxymoron or me just being a moron?

I saw the thread on the spark plug leads and I think it was Clarence who said they could last forever. Mine are tired looking and have 1,500 hours on them and my mechanic said the same thing as Clarence.

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[ArtVandelay]

Does anybody know how much the back spring has to flex? If it's not much it could last forever, our wings flex all the time, we don't replace them. With out knowing exactly how the spring, actuator works, it's hard to make an inform opinion on how much stress the spring is under.

[N601RX]

If there are 1000 of these in service a few will eventually break.  If all 1000 are replaced a few will be replaced incorrectly or have a manufacturing defect. 

[donkaye]

Well, the first question I would ask if I were buying a Mooney is what type of gear actuator it had.  If it was a Plessey, I would discount the plane by $18,000, the cost of a new actuator.  Anyone buying it without the discount does so at their own peril.  I would consider non disclosure of such an issue significant enough that I would never deal with a broker brokering it again.   As for a Seller, I would consider him a crook.  Putting my money where my mouth is, I went with a rebuilt actuator recently to eliminate the issue in my plane.  Also, remind me not to fly with or try to teach anyone who pushes back on this type of issue.  Sorry, but I personally question such judgment.

[DonMuncy]

The decision to take any risk in life, whether it be piloting a plane, stepping outside your front door or preemptively changing a no-back spring should be intelligently based upon several factors. (1) What is the consequence of the risk going badly. Ie. Do you die, take a monetary loss (and how big is that loss), or have to accept a few minutes of inconvenience. (2) What is the percentage of likelihood of the bad result occurring. Ie. is a meteor going to fall on you if you step out the front door; how often do people die piloting a plane; how many no-back spring failures occur per year/per plane, etc. (3) What is the side effect of taking or not taking the risk. ie. How will you support yourself if you don't leave your house; how often do malfunctions occur due to the no-back spring being tinkered with; how will your enjoyment of life be affected if you don't fly at all. (4) What is the cost. If it costs $3 to eliminate or mitigate a risk, it is a lot different than spending $10,000 to repack a parachute every x years.

Naturally there are a lot of variations on all of these. There are also a multitudes of unknowns in all of these. So we (or at least we should) make the best estimates of the factors, and decide how much risk we are willing to accept. Some people won't fly in small planes because "they are too dangerous", and to an extent they are right; we know there are risks in piloting. But everyone who posts here accept some risk because they like flying. And I know that people who are afraid of GA flying take risks of other sorts in whatever they do.

We all pick our levels of risk and live with the results. Hopefully intelligently.

[cnoe]

7 hours ago, carusoam said:

 

When did steel become resistant to fatigue failure?

 

-a-

Again, not to be combative...

From the "Fatigue (material)" wiki - 

ASTM defines fatigue life, N_f, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs.^[2] For some materials, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength.

Reference: https://en.wikipedia.org/wiki/Fatigue_(material)

A good spring, properly designed, manufactured, and employed should last essentially forever. Just sayin'.

Cnoe

[carusoam]

CNOE,

no combat intended.

 

Metals in general all follow the same stress/strain relationship with yield being their limit.  Steel, aluminum, titanium, spring steel...

With enough stress, they can be stretched.  When they are stretched beyond their limit they yield.  The Wiki page doesn't do the justice it deserves.

 

This comes from one of the more fun engineering classes simply called material science.

Steel and titanium are strong compared to their weight, but still are not impervious to these limitations.

 

From what I am learning here...  

1) The nbc spring is capable of yielding small amounts over the cycles.  More cycles, more yield damage.  

2) The SB gives guidance of how to inspect and determine the need.

3) Plessey seems to be the odd man out.

4) With MS you get a real feeling for the effect it has had on other pilots.  JimR's is my favorite.

5) For other dissapointing metal vs. stress/yield examples see Jim Peace's photos from today or look up flap mounts that have been ruined by extension Beyond speed limits.

Our Mooneys are an endless source of metals experience.  Dissimilar metals in contact with each other, oxidation, wear, the ill effects of a prop's ground strike, bird strikes, and inter granular oxidation issues on spars.

Some of our Mooneys have seen a lot of cycles over 50 years.

Best regards,

-a-

[yvesg]

On March 20, 2016 at 1:50 PM, Cyril Gibb said:

I found the same thing.  My previous post admitted that CarolAnn Garrat (sp?) had a failed infant mortality "new replacement" no-back spring.  One incident.

Although there's been posts that talk loosely about a bad batch of springs failing causing gear ups, I haven't read about another. 

We can come to end of job here with some facts.  Does anyone here know, beyond just rumours, of another no-back spring failure.  Clarence ? Don ? Anyone?

There was this one in Gatineau summer 2014.

 

Yves

[donkaye]

1 hour ago, donkaye said:

 Also, remind me not to fly with or try to teach anyone who pushes back on this type of issue.  Sorry, but I personally question such judgment.

The issue I was referring to was the backspring replacement.

[donkaye]

45 minutes ago, carusoam said:

CNOE,

This comes from one of the more fun engineering classes called material science.

Fun engineering classes?  As electrical engineering students we were forced to take that class in either our sophomore or junior year.  Not much fun for me.  

[Cyril Gibb]

13 minutes ago, yvesg said:

There was this one in Gatineau summer 2014.

 

Yves

O.K., I stand corrected, assuming we know this gear up was as a result of the spring failure.  I just looked up the incident report and it's clear that attempts were made to drop the gear manually also without success.

Yves, was it a spring failure? 

[carusoam]

Cyril,

That is the insidious failure that the nbc spring has.  It disables the only two methods of lowering the gear while flying...

Best regards,

-a-

[carusoam]

Don, I used 'more fun'....  As compared to Physical Chemistry (PChem is less fun).

Best regards,

-a-

[cnoe]

CNOE,

no combat intended.

 

Metals in general all follow the same stress/strain relationship with yield being their limit.  Steel, aluminum, titanium, spring steel...

With enough stress, they can be stretched.  When they are stretched beyond their limit they yield.  The Wiki page doesn't do the justice it deserves.

 

This comes from one of the more fun engineering classes simply called material science.

Steel and titanium are strong compared to their weight, but still are not impervious to these limitations.

 

From what I am learning here...  

1) The nbc spring is capable of yielding small amounts over the cycles.  More cycles, more yield damage.  

2) The SB gives guidance of how to inspect and determine the need.

3) Plessey seems to be the odd man out.

4) With MS you get a real feeling for the effect it has had on other pilots.  JimR's is my favorite.

5) For other dissapointing metal vs. stress/yield examples see Jim Peace's photos from today or look up flap mounts that have been ruined by extension Beyond speed limits.

Our Mooneys are an endless source of metals experience.  Dissimilar metals in contact with each other, oxidation, wear, the ill effects of a prop's ground strike, bird strikes, and inter granular oxidation issues on spars.

Some of our Mooneys have seen a lot of cycles over 50 years.

Best regards,

-a-

This is an excellent discussion and I respect everyone's position.

My opinion is that if the NBS is repeatedly (or ever) reaching it's yield point then it was improperly designed and/or manufactured. Indeed if you push something beyond its yield strength it will eventually fail. Our wings experience an exponentially higher number of cycles than the NBS yet they don't catastrophically fail, even being made of a (theoretically) inferior material. FWIW I'm no expert on the subject but I have endured a course of study in metallurgy at the collegiate level.

That being said a properly designed (i.e. doesn't reach its yield point) ~$1,000 spring shouldn't fail, and there's little available evidence that this is an actual problem with our planes. So far there have been 3 cases mentioned; the original one reported to Eaton, the infant mortality case, and now the Canadian GU in this thread. There may be others and it would be great to have this information along with the incident specifics such as manufacturer, time in service, etc. so that we could all make an informed decision whether to replace our own or not. Just because a corporate lawyer says it is doesn't make it so.

If indeed we discover that all Mooney gear actuators were improperly designed then I too will jump on the spring replacement bandwagon.

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