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Annual - Landing Gear U-Bolt

NorCalPilot

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Oct 31, 2021
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Northern California, Central Valley - 1968 172I
Just completed annual and among a few other items, our A&P discovered this broken u-bolt. There is some question as to if this was ever properly checked during past annuals. It is our understanding that these u-bolt's could be something that some A&P's might overlook or miss? We are fairly new to this so I thought I would share. I only found the following after the Annual & replacement.


U-Bolt History and Maintenance

By Dave McFarlane

U-Bolts take immense vibration and cyclic stress loading from landing and taxiing on imperfect runways. The long Cessna landing gear legs have tremendous twisting leverage on the U-Bolts. Every imperfection you feel while taxiing or operating on the runway is magnified to large stress forces on the U-Bolts that clamp the gear to the outboard fuselage support forgings. The Cessna U-Bolts are made from standard 4130 Steel with inclusions and other imperfections in the metal. The standard sharp threads are cut during Cessna manufacture. The cut threads allow stress concentration of the large stress forces in the sharp “V” of the thread where the U-Bolts are often found broken. When the aircraft is operated with broken U-Bolts, the weight of the airplane and the inboard attachment bolt keeps the landing gear in place, but the twisting gear stress results in hammering vibration on the gear forgings.

If you listen for it, you can hear a broken U-Bolt in the cabin during taxi or you can check for a broken U-Bolt by kicking the tire while holding your finger between the gear leg and the fuselage structure. You will feel movement if the U-Bolt is broken. This hammering will eventually start cracks in the aluminum gear forging and the forging must be replaced - a very expensive repair. It is also recommended to replace the inboard gear support bolt if the aircraft has been operated with a broken U-Bolt. The inboard bolt is a special high strength magnafluxed bolt that is critical to landing gear security. Because of the U-Bolt failure problem, Cessna issued Service Bulletin SE78-68 requiring periodic inspection of the U-Bolts and replacement of the U-Bolts every 1000 hours to prevent failure and to prevent continued operation with failed U-Bolts. Proper torque process and value are also very important to prevent U-Bolt failure.

McFarlane U-Bolts are manufactured from higher strength 4340 Crankshaft Steel that has been re-melted under vacuum to remove imperfections in the steel. The threads are roll formed, which strengthens the threads by perfecting the grain structure of the metal that drastically improves fatigue resistance. The “J” style rolled threads are formed with a radius in the bottom of the thread “V” that spreads out the stress loading. McFarlane furnishes new nuts that are designed to work effectively with the “J” thread design. The McFarlane U-Bolts are then carefully heat treated and tempered for perfection in strength and fatigue resistance. Our FAA approved Instructions for Continuous Airworthiness (ICA) conservatively recommend 1200 hours between U-Bolt replacements.

Ubolt.jpeg
 
Thanks for posting this. This is new information to me. :)
 
This is a known issue with the spring steel gear. Look for NOS on Ebay and get two if you have the flat gear legs. Sooner or later you'll need them. Cessna has a service bullietn requiring changing these at 1000 hours.

You can't interchange the nuts betwen Cessna and Mcfarlane bolts.
 
Inspection of the Landing Gear area is a required of an Inspection.

Flat spring gear components ( Gear Box) are also subject to failure .

Why does OP say it was not properly checked?

Bringing up replacement often results in “ if it’s not broken don’t fix it”.

While bolts are expensive ignoring them until broken can result in

damage to the Gear Box.

Now that is serious Ka-ching!

fyi. Every cable passes through the Gear Box. So ALL controls must be re-rigged

during this procedure. And it’s not the hard part!
 
Just completed annual and among a few other items, our A&P discovered this broken u-bolt. There is some question as to if this was ever properly checked during past annuals. It is our understanding that these u-bolt's could be something that some A&P's might overlook or miss? We are fairly new to this so I thought I would share. I only found the following after the Annual & replacement.


U-Bolt History and Maintenance

By Dave McFarlane

U-Bolts take immense vibration and cyclic stress loading from landing and taxiing on imperfect runways. The long Cessna landing gear legs have tremendous twisting leverage on the U-Bolts. Every imperfection you feel while taxiing or operating on the runway is magnified to large stress forces on the U-Bolts that clamp the gear to the outboard fuselage support forgings. The Cessna U-Bolts are made from standard 4130 Steel with inclusions and other imperfections in the metal. The standard sharp threads are cut during Cessna manufacture. The cut threads allow stress concentration of the large stress forces in the sharp “V” of the thread where the U-Bolts are often found broken. When the aircraft is operated with broken U-Bolts, the weight of the airplane and the inboard attachment bolt keeps the landing gear in place, but the twisting gear stress results in hammering vibration on the gear forgings.

If you listen for it, you can hear a broken U-Bolt in the cabin during taxi or you can check for a broken U-Bolt by kicking the tire while holding your finger between the gear leg and the fuselage structure. You will feel movement if the U-Bolt is broken. This hammering will eventually start cracks in the aluminum gear forging and the forging must be replaced - a very expensive repair. It is also recommended to replace the inboard gear support bolt if the aircraft has been operated with a broken U-Bolt. The inboard bolt is a special high strength magnafluxed bolt that is critical to landing gear security. Because of the U-Bolt failure problem, Cessna issued Service Bulletin SE78-68 requiring periodic inspection of the U-Bolts and replacement of the U-Bolts every 1000 hours to prevent failure and to prevent continued operation with failed U-Bolts. Proper torque process and value are also very important to prevent U-Bolt failure.

McFarlane U-Bolts are manufactured from higher strength 4340 Crankshaft Steel that has been re-melted under vacuum to remove imperfections in the steel. The threads are roll formed, which strengthens the threads by perfecting the grain structure of the metal that drastically improves fatigue resistance. The “J” style rolled threads are formed with a radius in the bottom of the thread “V” that spreads out the stress loading. McFarlane furnishes new nuts that are designed to work effectively with the “J” thread design. The McFarlane U-Bolts are then carefully heat treated and tempered for perfection in strength and fatigue resistance. Our FAA approved Instructions for Continuous Airworthiness (ICA) conservatively recommend 1200 hours between U-Bolt replacements.

View attachment 12096
They just break. Mine was fine, but 8-9 months later, while cleaning up the interior, found a floor cap loose. As I was placing it back, noticed a shiny reflection down the hole....broken bolt. Thanks for the info on the service bulletin. I went back and checked my log books, those have never been replaced (or there was no record of replacement) since 1961. I didn't expect to find any record of compliance, since SB are not mandatory like an AD. But still, good to know and something else to watch for

And yes, those castings suck!!! I found short hairline cracks aft of the slot for the spring gear. Was hoping it was dirt/grease, no such luck. The outboard castings run about 5k new, but you can find used/tested with 8130 tickets on the used market for less than half the price. I replaced mine so was able to save on labor, but it still takes about 30-40 hours.
 
I did not have the bolt in hand and it was misplaced when I posted this thread. I probably should have waited to post and not drawn out this thread. I do not know anything about corrosion other than it happens. The plane is hangered and has been pretty much a hanger queen for the last two years while it was undergoing maintenance & upgrades. My assumption based on the photos below is that this bolt was missed at one or more prior annuals. I have been wrong before and most certainly will be again before the day is out... ;-)

IMG_5417 Large.jpeg


IMG_5418 Large.jpeg

IMG_5419 Large.jpeg
 
Due to jacking difficulties most high wing aircraft seldom have the landing gear

in an unsupported condition during an inspection. Hence the first indication

might be the broken bolt/ nut lying in the “ well” . There are usually pop off

covers of a about 1 1/2 inch diameter for access on many.

You do not have to be an IA to check this.

Folks doing Owner Assist would be prudent to conduct their own inspection

when opening up. ALL aircraft have issues. This is just one of them.
 
Doesn’t look like a fresh break to me.
Thank you for that confirmation magman, which is why I started this thread. Also, thank you for the advice in regard to Owner Assisted Annual. We just remove inspection plates, seats, flooring etc., but when we are able, we like to be involved as much as we can. Always something to learn. Thanks again.
 
There are times Cessna folk consider other aircraft.

Many times a J-3 , Champ or similar that has “ low maintenance “ is the target.

Low is not zero.

If these aircraft did not get the needed service they can get very pricey.

Fabric Pipers should have the attach bolts removed and greased .

Piper says at 100 hr intervals. Rarely done.

Taylorcraft has drilled ports in the landing gear to lube the bolts.

Nearly always they are painted over and ignored.

Champs/Chiefs have the attach points wear that necessitates welding

a doubler in place. Try that with fabric on.

Just a few drops of oil may save thousands of $$ in repairs.

This is authorized as Preventative Maintenance.

Do it or assure it gets done.

”Pay me now etc ,etc”
 
They just break. Mine was fine, but 8-9 months later, while cleaning up the interior, found a floor cap loose. As I was placing it back, noticed a shiny reflection down the hole....broken bolt. Thanks for the info on the service bulletin. I went back and checked my log books, those have never been replaced (or there was no record of replacement) since 1961. I didn't expect to find any record of compliance, since SB are not mandatory like an AD. But still, good to know and something else to watch for

And yes, those castings suck!!! I found short hairline cracks aft of the slot for the spring gear. Was hoping it was dirt/grease, no such luck. The outboard castings run about 5k new, but you can find used/tested with 8130 tickets on the used market for less than half the price. I replaced mine so was able to save on labor, but it still takes about 30-40 hours.

The initial notification was a SB but they are listed as an airworthiness requirement now in the Cessna manuals - making them a mandatory item.
 
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