Landing Gear collapsed during taxi

[jasona900]

After a fuel stop, I taxied my '64 C (manual gear) onto the active after my run-up.   I was going maybe 3-4mph, and all of a sudden, the nose gear collapsed, followed by both mains about a second later.  After the initial shock of what had just happened, I quickly hit the master switch off, shut off the fuel, and exited the aircraft.  The J-bar never moved out of the block.  Fortunately, there was an A&P on the field, along with a group of other very nice folks who helped me put the plane on jacks, and then onto a flatbed trailer.  3 hours after the collapse, we towed my Mooney into a hangar.  We just recently got it back on jacks and off the trailer, and the inspection of what happened started.

Backstory:

My Mooney (most of you probably recognize it as N6XM, a fantastic example of a C model) underwent an Annual at a MSC in October.  Since the annual, I have put roughly 10 hours, and only a handful of landings on it.   This MSC (I'll keep their name out of it) treated me very well, and conducted a very detailed annual.  Because 6XM was so well maintained before, they really didn't find much wrong with it.  Among other things, they shimmed the nose gear steering assembly, and adjusted the preloads on all three landing gear.   Before they adjusted any preloads, they said one of the mains required sufficient torque to break the overcenter joint, but the other main and nose were low.  So they adjusted the retraction trusses accordingly and brought the other main to within spec, and the nose to about 125 in/lbs

After the collapse, I called the MSC who offered to fly out and preform an inspection to determine what went wrong. 

One of the retraction trusses broke when the gear collapsed.  I will include pictures later, but it broke in two places.  Looking inside the truss that broke, there doesn't appear to be very much corrosion.  Once side is clean and shiny, the other has just a few specs of brown in it.  The walls of that tube may appear to be slightly thinner but it is hard to tell.  The other truss bent during the collapse. The weld that connects the vertical portion of the J-bar to the block on the bottom tore like a piece of paper.  An additional weld separated (not completely) on point that the J-bar attaches to the retraction truss that did not break.  Unfortunately, this makes it impossible to do a preload check on the nose gear.  The overcenter joint doesn't appear to have any play in it.

So we don't know exactly what happened.  The best that we could come up with is that the truss walls were thin due to aging.  When they adjusted the truss, (even if it was to spec), that an enough stress on that tube to cause it to fail.  Once that tube went, the overcenter joint could have broke center, and the nose gear went down.  That pulled on the other retraction truss, which pulled on the J-bar causing it to tear, and allow the mains to come down.

I have read that the aluminum block the top of J-bar seats in can wear, causing it to lock in place, but not as far forward as it is meant to be.  This can limit the travel of the overcenter joint, and cause the gear to collapse.  I need to inspect the block, and will post the results.  For those of you who haven't read it, this is a MUST READ if you have a J-bar in your Mooney:

http://www.knr-inc.com/shoptalk-articles/25-shoptalk/41-200305-manual-gear-mooneys

So now I am waiting on a quote to see how much this will cost.  Fortunately, the insurance company is willing to work with me.  I know I can't find another Mooney this nice (at least not right now) for what I got 6XM.    Pretty upset, as I have put a decent amount of money into 6XM since I've had it...but such is life.  At least it happened with just me on board, I didn't get hurt, and it was only a 25 min drive from home.  But if anyone has any other ideas on what happened, (or advice on a way forward) I'm all ears!

 

[Bob_Belville]

 

Coincidentally(?) Don Maxwell just posted in the MAPA discussion group and encouraged spreading the info:

"It seems as these Johnson bar gear ups go away for years, then the planes change hands, the old mechanic die and here we go again. In the last 10 days I have had calls on three J-bar gear ups. All three just out of annual. One was the failure of the J-bar itself. Pic 2198 is of the jbar still latched and all three gear went down. Pic 2196 is the bellcrank welded to the bottom of the J-bar.  New J-bars have a strap welded around the bottom for added strength. Lasar has been doing this mod to J-bars they repair for many years. It looks as though there may have been corrosion in that area. Have your mechanic pay close attention to the weld area at the base of the J-Bar.  The one thing I see and hear over the years are new mechanics fascinated with the J-bar system and want to adjust one. It is not that difficult but should be shown by someone with experience.  If a J-bar, installed at the factory, is greased regularly I doubt it would be out of adjustment 40 years later. If a J-bar system is out of rig it’s because someone screwed with it. Be very careful about mechanics changing preloads if they are not experienced. Feel free to pass this on.

Don

 

Jason, I hope you already know that if you've put quite a bit of money in avionics, paint, interior, mods,... whatever and have not raised the hull coverage to reflect the increased value you might be at risk of having the insurance adjuster decide their best cost solution is to pay you the hull coverage and take the plane and all your goodies for salvage. Hope that doesn't apply in your case. A typical gear up (& required engine tear down) will run $30,000 +/-. 

[N201MKTurbo]

Did you notice any difference in the effort to put the gear up or down after the adjustment?

it is important that the springs in the retraction tubes are not compleatly compressed. If they are the forces put on the landing gear parts is huge.

[jasona900]

I didn't notice much (if any) difference during retraction.  And looking at the springs, they were not completely compressed.  I'll take a picture of them and post it next time I'm at the airport, but I'm not sure they had anything to do with it.

[Piloto]

Breakage at welds are not uncommon. It all depends on the welder. Weld/metal fatigue and corrosion accelerates the failures. In many instances adding a gusset at the joint point relieves the welding stress.

https://www.bing.com/images/search?view=detailV2&ccid=%2f%2buytFtB&id=0FE8F19B37FEEA839175789EF45D5E80BB4DBA13&q=Gusset+Support&simid=608023428844029950&selectedIndex=0&ajaxhist=0

 

José

 

Edited December 9, 2016 by Piloto

[cctsurf]

In the article mentioned above (http://www.knr-inc.com/shoptalk-articles/25-shoptalk/41-200305-manual-gear-mooneys), the author mentioned "Tom Rouch's" article about moving an airplane after gear up without adding more damage.  Does anyone know where I can get my hands on this article? I have no desire to land gear-up, but I would like to read it.

[jasona900]

Here are some pictures of the truss that broke, and the welds on the bottom of the J-bar.  (May need to zoom to see them).  I think it's safe to say the J-bar welds cracked during the collapse and were not cracked before.  But I have also heard this is a common spot for cracked welds and is worth spending an hour pulling the belly panel off for a look.  

[mooniac15u]

17 minutes ago, cctsurf said:

In the article mentioned above (http://www.knr-inc.com/shoptalk-articles/25-shoptalk/41-200305-manual-gear-mooneys), the author mentioned "Tom Rouch's" article about moving an airplane after gear up without adding more damage.  Does anyone know where I can get my hands on this article? I have no desire to land gear-up, but I would like to read it.

I had a gear extension failure once and had to do a belly landing.  The airport manager called a towing and recovery service that normally worked with overturned semis.  They wanted to wrap some straps around the fuselage and lift it with a crane.  I was afraid that would do significant damage so I declined.  Instead they were able to put inflatable airbags under the wings and lift the plane enough to get jacks under it.  We were then able to get the gear extended with some manual intervention and ultimately towed it back to the hangar.

[jasona900]

2 hours ago, Bob_Belville said:

 

 

Jason, I hope you already know that if you've put quite a bit of money in avionics, paint, interior, mods,... whatever and have not raised the hull coverage to reflect the increased value you might be at risk of having the insurance adjuster decide their best cost solution is to pay you the hull coverage and take the plane and all your goodies for salvage. Hope that doesn't apply in your case. A typical gear up (& required engine tear down) will run $30,000 +/-. 

The money was spent on the annual, which did include a powerflow exhaust.  But that is a good point you brought up.  I had insured the airplane for the amount I paid.  My next airplane (if I don't fix this one) will be insured for more than the final sales price.

[mike20papa]

Very sorry to hear about your gear failure. That all three gear collapsed is suspicious.  My experience is that the simple Mooney gear is converted into a complexity convenient to the overly inquisitive mind of an uninitiated mechanic.  It is really just a simple over center linkage.  The only complication is that three links are connected into one activating lever (the Johnson Bar) and a crude balance/adjustment between the three has to be made.  The throw required to be adequate to position the linkage into an over center position is given a numerical/subjective number by Mooney following the A model.  In fact, my A model's original maintenance manual suggests simple use of hand pressure to determine suitable gear "preload".  Maxwell is probably correct.  once correctly set, there's NO reason to adjust the gear as Mooney suggests (i.e. loosen them all and go figure it out..a new) If someone took your nose gear apart sufficiently to install the shim .. all bets are off.  I would guess most J bar Mooneys have the preloads set too F'n high and without consideration to the consequences.  Too high and you start breaking stuff.  The preload is not what holds the gear down, the over center linkage does that.  Preload is like insurance.  The J bar should feel as it goes into the final 2 inches of latch down - the story.  The preload is only intended to ensure the over center link is thrown.  The same mechanism is what latches the door.  Begin there, understand it, and then move on to the gear.

[takair]

The broken truss shown is half the picture.  There is the twin link on the other side.  If they are not adjusted symmetrically, one is always under load/stress and can cause what you see.  They are hard to adjust, at least the ones with the cam.  It is a matter of feel, since you can meet the torque spec with just one doing the work.  Once the one side breaks, the entire load path is thrown off.  Unfortunately, it would be difficult to figure out if that was the case.  

I think the key to Don Maxwell's comment is "properly lubricated". I'm not convinced that a once a year lube for an outdoor machine (not implying that is the case here) would be adequate to get zero wear for 50 plus years.  When the overcenter links wear, the worse needs to be adjusted.  Lube often and I suspect many of the issues go away....but it's a pain.  

 

[DaV8or]

5 hours ago, jasona900 said:

Here are some pictures of the truss that broke, and the welds on the bottom of the J-bar.  (May need to zoom to see them).  I think it's safe to say the J-bar welds cracked during the collapse and were not cracked before.  But I have also heard this is a common spot for cracked welds and is worth spending an hour pulling the belly panel off for a look.  

 

Some of those failures are part of the cascade effect. As the plane starts to fall, the weight and momentum puts loads of stress on the components on down the line many of which will break until there is equilibrium. I don't know where the failure began, but a study of the way the systems works and the broken parts will tell the tale.

[jasona900]

33 minutes ago, DaV8or said:

 

Some of those failures are part of the cascade effect. As the plane starts to fall, the weight and momentum puts loads of stress on the components on down the line many of which will break until there is equilibrium. I don't know where the failure began, but a study of the way the systems works and the broken parts will tell the tale.

I completely agree. Hard to tell if the overcenter joint went first, or if it was the truss first.  Regardless, it's safe to assume the J-bar welds were intact before the collapse.

[Cyril Gibb]

10 hours ago, jasona900 said:

So they adjusted the retraction trusses accordingly and brought the other main to within spec, and the nose to about 125 ft/lbs. 

The M20C service manual says nose gear preload torque should be 100-130 Inch lbs.  Also, some of those joints are drier than I'd expect a month after being lubricated during an annual.

[Yetti]

so the only thing I know about gear is removing my electric actuator and sending it off.  And I read that section of the S&M several times, which has nothing about removing the actuator. To remove the actuator I had to crank gear off being over center.   Since I don't like things being on jacks a simple push on the mains had them lock back in place.  And it is now setting gently on the mains.  

So Let's say the Johnson bar broke.   Unless you hit a bump, I don't thing the gear could collapse something still has to move it from overcenter locked position.  When down the johnson bar is not holding the gear down, the tension on the springs and overcenter holds the gear down. The johnson bar is just a lever to undo the overcenter and then pull the gear the rest of the way up. 

So now what caused the front gear to collapse?  There are two rods to the front wheel I would guess to provide symmetry and not unbalance things.  If those rods were not balanced and tensioned the same I could see the symmetry  being off and a rod breaking, then the whole system is toast.

Not an engineer, but I have spent about 20 hours in the last week under the mooney with the belly open.

Edited December 10, 2016 by Yetti

[Yetti]

Oh and generally the proper way to grease something with zerks is to pump until you see new grease coming out the sides.  You may check for that.  For the hemi joints (what is shown in the picture), should be hit with some teflon spray.

When I spray, I try to wipe off some of the excess so there will not be a build up.   You should also see some new grease on the zerks.  Also using a rag to wipe off the zerks will keep some of the crud off them.   It's the little things that don't take much time but makes for a nicer job.

[jasona900]

21 minutes ago, Yetti said:

so the only thing I know about gear is removing my electric actuator and sending it off.   To do that I had to crank gear off being over center.   Since I don't like things being on jacks a simple push on the mains had them lock back in place.  And it is now setting gently on the mains.  

So Let's say the Johnson bar broke.   Unless you hit a bump, I don't thing the gear could collapse something still has to move it from overcenter locked position.  When down the johnson bar is not holding the gear down, the tension on the springs and overcenter holds the gear down. The johnson bar is just a lever to undo the overcenter and then pull the gear the rest of the way up. 

So now what caused the front gear to collapse?  There are two rods to the front wheel I would guess to provide symmetry and not unbalance things.  If those rods were not balanced and tensioned the same I could see the symmetry  being off and a rod breaking, then the whole system is toast.

Not an engineer, but I have spent about 20 hours in the last week under the mooney with the belly open.

I will likely send the rod off for an ultrasound so that I can determine the wall thickness.  What you described is close to what I think happened.  Just wondering if the torque put on the truss was the right amount and it failed due to old age.. or if the tube was fine, and asymmetrical stress caused the failure.

[Cyril Gibb]

1 hour ago, jasona900 said:

I will likely send the rod off for an ultrasound so that I can determine the wall thickness.  What you described is close to what I think happened.  Just wondering if the torque put on the truss was the right amount and it failed due to old age.. or if the tube was fine, and asymmetrical stress caused the failure.

As my previous post said, if the nose really was torqued at 125 lb/ft, it was 12 times the service manual torque.  If the mains were also torqued with foot pounds rather than inch pounds, you have your answer.  Or was that a typo?

[jasona900]

13 minutes ago, Cyril Gibb said:

As my previous post said, if the nose really was torqued at 125 lb/ft, it was 12 times the service manual torque.  If the mains were also torqued with foot pounds rather than inch pounds, you have your answer.  Or was that a typo?

It was a typo.  

[Cyril Gibb]

2 minutes ago, jasona900 said:

It was a typo.  

O.K. ... just checking

[jasona900]

Just fixed the original post.  Thanks!

[Guest]

4 hours ago, Yetti said:

Oh and generally the proper way to grease something with zerks is to pump until you see new grease coming out the sides.  You may check for that.  For the hemi joints (what is shown in the picture), should be hit with some teflon spray.

When I spray, I try to wipe off some of the excess so there will not be a build up.   You should also see some new grease on the zerks.  Also using a rag to wipe off the zerks will keep some of the crud off them.   It's the little things that don't take much time but makes for a nicer job.

While greasing the gear it doesn't hurt to partially retract the it to allow grease to flow more evenly into bearings and bushings.

Clarence

[Bob_Belville]

27 minutes ago, M20Doc said:

While greasing the gear it doesn't hurt to partially retract the it to allow grease to flow more evenly into bearings and bushings.

Clarence

AeroShell Grease 22.

[jetdriven]

Switch to 33 and you can grease the entire airframe except the jack screws. 

[ArtVandelay]

10 minutes ago, jetdriven said:

Switch to 33 and you can grease the entire airframe except the jack screws. 

Ok, I gotta ask, what does the jack screw use?