No back spring in landing gear actuators

[donkaye]

If you go to my website:  www.donkaye.com and click on "Infamous Back Clutch Spring" I have some detailed pictures of the spring.  This one had just come out of my airplane.  It looks a little worn at the ends. This one had been in the plane for 1036 hours.  The thing that bothers me about this spring is those little end pieces.  If they were to break, it's all over relative to getting the gear to come down.  Think of the fact that those little end pieces control the fate of your plane.  So again, I recommend the 1,000 hour replacement.

That little what I would call $.25 spring cost $1,500.  They were available at the time.

[carusoam]

1) The Mooney pilot and lawyer gave a great detailed opinion above.

2) a properly designed (i.e. doesn't reach its yield point) ~$1,000 spring shouldn't fail. (Can't disagree with this)

3) the most egregious Mooney spring failure belongs to the Porsche powered M20L.  Valve springs that killed the Porsche engine.

4) valve springs have a tendency of losing their springiness in the high temperature stress environment...

Materials lab and stretching various materials using the Instron strain test is a blast.  More fun when you use polymer samples.

How is that for a memory?

Best regards,

-a-

[carusoam]

http://www.donkaye.com/donkaye.com/Infamous_Back_Clutch_Spring.html

Anyone have a picture of a broken or damaged one for comparison?

Best regards,

-a-

[yvesg]

7 hours ago, Cyril Gibb said:

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? 

I just sent an email to the mechanic who fixed the plane after the gear up. I will let you know his answer when I get it.

Yves

[DC_Mooniac]

Recognizing people have strong views about whether this is actually necessary, does anyone know who might have the SB kit in stock?  Mooney apparently does not have any currently in stock.

[Awqward]

FWIW Metal fatigue is a function of stress level and number of cycles (Google S-N curve). Fatigue failure requires more cycles at lower stress and vice versa... Fatigue failure can (and usually) occurs within the elastic range of stress....ie not yielded...

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[cnoe]

Thanks for the clarification and reference Awqward; that's a good article. The design summary states:

There are four principal approaches to life assurance for mechanical parts that display increasing degrees of sophistication:

1. Design to keep stress below threshold of fatigue limit (infinite lifetime concept);

2. Fail-safe, graceful degradation, and fault-tolerant design: Instruct the user to replace parts when they fail. Design in such a way that there is no single point of failure, and so that when any one part completely fails, it does not lead to catastrophic failure of the entire system.

3. Safe-life design: Design (conservatively) for a fixed life after which the user is instructed to replace the part with a new one (a so-called lifed part, finite lifetime concept, or "safe-life" design practice); planned obsolescence and disposable product are variants that design for a fixed life after which the user is instructed to replace the entire device;

4. Damage tolerant design: Instruct the user to inspect the part periodically for cracks and to replace the part once a crack exceeds a critical length. This approach usually uses the technologies of nondestructive testing and requires an accurate prediction of the rate of crack-growth between inspections. The designer sets some aircraft maintenance checks schedule frequent enough that parts are replaced while the crack is still in the "slow growth" phase. This is often referred to as damage tolerant design or "retirement-for-cause".

We simply need more information regarding actual occurrences of spring failure and then we can individually make a decision whether to replace the item or not based on our own risk tolerance.

Great discussion guys and gals!

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[yvesg]

4 hours ago, yvesg said:

I just sent an email to the mechanic who fixed the plane after the gear up. I will let you know his answer when I get it.

Yves

Got the response from the mechanic and his answer is "No", nothing more. Next time I speak to the aircraft owner I will ask him if he has more details.

Yves

[Awqward]

For Eaton actuators at least, the other component subject to failure is the brass manual extension clutch. This may have been the issue with the Gatineau GU....it is not a great design and the clutch can get chewed out if the red handle cable is not pulled taut enough thus allowing the clutch to partially engage in the drive spindle....

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[carusoam]

Anthony,

Is that the case of having the manual gear extension system (accidently) engaged while operating the electric gear?

There is a 'critical' part that can get ruined when the manual and electric systems clash.  The check list covers making sure that the back-up system is not accidently engaged by errant passengers...

Best regards,

-a-

[Yetti]

Is this not the whole reason for certified airplanes and PMA so you can trace the "bad" batch of springs.  That would keep us all from having to guess.  If they are a wear (does not sound like it reading the SB) issues then it should be a PM type item added to the Maint manual/100 hour/annual inspection.   As regard to springs in general these debates can go on a long time with respect to gun magazines (should you keep them loaded or not).   I have an original 1911 (not A) that the magazine spring is just fine.  When I got it the magazine had been loaded for probably 40 years.

[Awqward]

31 minutes ago, carusoam said:

Anthony,

Is that the case of having the manual gear extension system (accidently) engaged while operating the electric gear?

There is a 'critical' part that can get ruined when the manual and electric systems clash.  The check list covers making sure that the back-up system is not accidently engaged by errant passengers...

Best regards,

-a-

Yes...although if the lever is fully raised, and the clutch fully engaged, it should in theory stall the electric motor and cause the breaker to pop...many people put some additional cover over the handle to prevent accidental deployment...the photo is the errant part from my J....discovered at annual...on inspection, the cable that runs from the red handle to the actuator was not tight enough and had allowed the clutch to only partially engage...thus instead of jamming the actuator it just got chewed out... I called DMax and he diagnosed it instantly... (as an aside I called Steve Rue on a Monday morning and had the new part in my hand by Wed afternoon...deep in rural Scotland!! amazing service...although nowadays I believe you can't go directly to the factory any more...).. Anyway they sell quite a few of these clutches....meaning they fail regularly....I don't know how many failed no-back springs have caused GU landings...probably the brass clutch has been responsible for just as may if not more and we never hear about it on here...you would only know when you had an electrical failure or some other event requiring manual extension...or at annual...

Edited March 23, 2016 by Awqward

[Awqward]

17 minutes ago, Yetti said:

Is this not the whole reason for certified airplanes and PMA so you can trace the "bad" batch of springs.  That would keep us all from having to guess.  If they are a wear (does not sound like it reading the SB) issues then it should be a PM type item added to the Maint manual/100 hour/annual inspection.   As regard to springs in general these debates can go on a long time with respect to gun magazines (should you keep them loaded or not).   I have an original 1911 (not A) that the magazine spring is just fine.  When I got it the magazine had been loaded for probably 40 years.

Inspecting the no-back spring is not a trivial job....removing and dismantling the actuator and returning it to service unnecessarily would probably lead to more maintenance induced failures than saved GU landings!

[jetdriven]

24 minutes ago, Yetti said:

Is this not the whole reason for certified airplanes and PMA so you can trace the "bad" batch of springs.  That would keep us all from having to guess.  If they are a wear (does not sound like it reading the SB) issues then it should be a PM type item added to the Maint manual/100 hour/annual inspection.   As regard to springs in general these debates can go on a long time with respect to gun magazines (should you keep them loaded or not).   I have an original 1911 (not A) that the magazine spring is just fine.  When I got it the magazine had been loaded for probably 40 years.

This is essentially a clock spring.  But clocks run for 60 years and their springs don't break. So something else, either the design of the part was inferior, or the quality control of the spring is substandard.  

But just like Lycoming camshaft a and Continental airmelt cranks, a mad rush to change our thousands of proven parts because of 2 or 3 failures in a fleet of thousands.  Then only to find out the new "improved" parts are actually worse than the old stuff. 

[Yetti]

The inspection of "looking for a linear crack"  would say bad batch.

There seems to be the same issue with the oil pump gear.   Supposedly their were three different versions of three different material in a short period around the late 80s

[yvesg]

4 hours ago, Awqward said:

 

For Eaton actuators at least, the other component subject to failure is the brass manual extension clutch. This may have been the issue with the Gatineau GU....it is not a great design and the clutch can get chewed out if the red handle cable is not pulled taut enough thus allowing the clutch to partially engage in the drive spindle....

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Is this part between the motor and the transmission? I was at the airport a few hours ago and the mechanic came to see me and he said it was the part between the motor and the transmission... not a spring issue.

 

Yves

[Yooper Rocketman]

This has been a good discussion.  Even those not jumping on the wagon to change their springs right now clearly have this issue in the back of their mind.  I do and will be looking at it this annual.  Much has been said about changing them in the interest of safety, even if no one has been able to come up with one documented failure of "original" springs.  Again, everyone needs to assess the risk, reward, and cost based on their comfort level.  But, as a wrench, I've found there are lots of areas on a mechanical machine that can present us with a failure.  We see new AD's and SB's all the time, on issues that had not previously been a problem.  Someone either had a bad day or got lucky every time a new advisory is issued (or shall we say just before it was issued due to their situation).  I'm not claiming the sky is going to fall, just stating a reality.

With that, how many of you have ever looked at the size of the bolts holding your engine mount to the firewall?  Steve, my friend, Mooney owner and A&P, marveled several years ago on how four, on his E model, six on my TSIO520, 1/4" bolts secure the engine mount to the firewall, yet we use six or more 1/2" bolts to hold the prop on the crank?  Just for reference, the bolt size difference is to the magnitude of 5 times more strength on the crank studs.  My Lancair, with the same engine weight, uses 7/16" bolts on the mount.  We determined after that observation neither of us would tolerate any MAJOR engine vibrations without, at the least, dropping the engine to idle, and in the worse case scenario, engine shut down.  If you think you will have a bad day landing with the gear in the wells, I can only imagine my last thoughts as the engine departs the front of my aircraft and I tail slide to the ground.

Maybe those engine mount bolts should be changed on hours too?

Tom

[Awqward]

5 hours ago, yvesg said:

Is this part between the motor and the transmission? I was at the airport a few hours ago and the mechanic came to see me and he said it was the part between the motor and the transmission... not a spring issue.

 

Yves

No, this part is on the opposite side of the gearbox to the electric motor....it allows the cable pull system to engage on the gearbox drive spindle....

[M20S Driver]

On 3/24/2016 at 5:57 PM, Yooper Rocketman said:

With that, how many of you have ever looked at the size of the bolts holding your engine mount to the firewall?  Steve, my friend, Mooney owner and A&P, marveled several years ago on how four, on his E model, six on my TSIO520, 1/4" bolts secure the engine mount to the firewall, yet we use six or more 1/2" bolts to hold the prop on the crank?  Just for reference, the bolt size difference is to the magnitude of 5 times more strength on the crank studs.  My Lancair, with the same engine weight, uses 7/16" bolts on the mount.  We determined after that observation neither of us would tolerate any MAJOR engine vibrations without, at the least, dropping the engine to idle, and in the worse case scenario, engine shut down.  If you think you will have a bad day landing with the gear in the wells, I can only imagine my last thoughts as the engine departs the front of my aircraft and I tail slide to the ground.

Maybe those engine mount bolts should be changed on hours too?

Tom

Good point Tom.  Our planes were originally built for 2400-2500 rpm vibration and axial load from 244-280 HP.  My plane now runs at 2700 rpm and 310 HP.  Perhaps it is not a bad idea to keep an eye on those bolts...

[RobertE]

This is a slight deviation, not a hijacking of the thread, but can someone tell me exactly how this spring functions?  We're debating the stresses it endures but does it, in fact, bear much or any load in normal activity?  If so, is it a significant strain or modest pressure?

[M20S Driver]

On 4/5/2016 at 7:03 PM, RobertE said:

This is a slight deviation, not a hijacking of the thread, but can someone tell me exactly how this spring functions?  We're debating the stresses it endures but does it, in fact, bear much or any load in normal activity?  If so, is it a significant strain or modest pressure?

The spring transfers the motor torque for extension and retraction.  It is also used when the manual extension is used.  if it fails in the gear up or partial extension position, there is no way to extend the gear.  As far as the cyclic load levels are concerned, I have no idea.  

I just changed mine after 1100 hours and will do it again at around 2100 hours.  Don Kaye has the pictures and more info on his website  www.donkaye.com.

[nels]

Good thread but my opinion after reading is this: if I had a plane with 4000 hrs on the landing gear box and backup spring was still in good shape, I would be totally nervous flying if I indeed elected to have that spring changed out for a new uncertain component. A properly designed spring should last way beyond the life of the airframe. I believe there was one or maybe several bad springs that got out the door and these may fail within the first hundred, five hundred cycles or more. If it's a proven, time/cycle example I would hesitate to change it. Sorta like I would be more comfortable with an overhauled engine with 400 hrs on it rather than one with 10 hrs.

[ArtVandelay]

22 minutes ago, nels said:

 Sorta like I would be more comfortable with an overhauled engine with 400 hrs on it rather than one with 10 hrs.

What if the engine had 4000 hrs vs 10?

I'd like to know how much stress,flex the spring goes under, then I could make an educated decision.

[jetdriven]

Bluehighwayflyer's 201 has like 8k on the back clutch spring, still going fine. 

[ArtVandelay]

My emergency landing mechanism locked up, so when i sent it in they replaced it as part of the repair.