Violent Shimmy on Landing

Stingray Don

Stingray Don

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Stingray Don
Okay, I just got the 172n back from annual and went for a trip around the pattern for a test flight today. Taxi and takeoff were normal as was the remainder of the flight until landing. Upon landing, the plane began to shake so violently that I was briefly unsure of my ability to even get it off the runway. Once I turned off the runway, the shaking stopped instantly. I have never experienced even the slightest shimmy in this plane. On the rare occasion when I have experienced a shimmy in other planes, it was just a minor annoyance and was easily corrected by changing the weight on the nose with the elevator. Today, the elevator had no affect on the shaking.

I'm assuming the shop did something wrong when they worked on the plane but I don't know. During the annual, the nose wheel fairing was replaced and the control cables were adjusted. I can't think of any other work which would have affected the aircraft during taxi. I'm not flying the plane again until the shop checks it out. Anyone have any thoughts on what could have caused such a violent shimmy?
 
A cursory inspection of your own shows nothing? No low/overinflated tires? No collapsed front strut? No squawks that you can see?

Bounce the airplane up and down on the front nose strut and see if it returns to normal.

If you have a shimmy damper, is it connected? Leaking?
 
Could be the shimmy damper,on the nose wheel.
 
txflyer said:
A cursory inspection of your own shows nothing? No low/overinflated tires? No collapsed front strut? No squawks that you can see?

Bounce the airplane up and down on the front nose strut and see if it returns to normal.
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I checked the gear, tires, shimmy damper, and front strut prior to take off. After landing I took another look at everything and nothing looks amiss to me. Whatever it is, it is not visually apparent.
 
Not trying to be a jerk here, but did you touch down hard on the nose wheel on that landing?
 
Replacing the nose pant requires removing the nose wheel. It's possible that the nose wheel axle bolt is not tight enough. With the nose wheel off the ground (tighten the tail tiedown to pull the tail almost to the ground) grasp the wheel at the bottom and shake. If you hear any clunks for feel any play about the axle, the bearings are loose and the axle bolt needs tightening. This is an owner-repair operation.

The nose gear is attached to the rudder pedals through spring-loaded rods. They must be adjusted properly by an A&P. While the nose wheel is still off the ground, twist it about the vertical axis looking for looseness. There are no cables in landing gear.

Also check the shimmy damper for solid attachment. It will be some vertically play but should feel fairly solid in the horizontal (it works like a shock absorber).

If it's none of those things it is probably the torque link (the elbow scissor thingie on the back of the nose gear). It may need to be shimmed. If the mechanic disassembled this during the annual (not typical) he may have not re-shimmed it properly. Look for new grease around the three zerc fittings on the torque link - that would be evidence that it was at least serviced.
 
James331 said:
Not trying to be a jerk here, but did you touch down hard on the nose wheel on that landing?
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Actually a good question. No, I landed on the mains and it was not a hard landing. I will admit to a little side loading on touch down due to a direct crosswind. But the side loading was minor and I can't imagine that would have caused a problem. This was not my best landing but far from my worst!
 
AuntPeggy said:
Replacing the nose pant requires removing the nose wheel. It's possible that the nose wheel axle bolt is not tight enough. With the nose wheel off the ground (tighten the tail tiedown to pull the tail almost to the ground) grasp the wheel at the bottom and shake. If you hear any clunks for feel any play about the axle, the bearings are loose and the axle bolt needs tightening. This is an owner-repair operation.

The nose gear is attached to the rudder pedals through spring-loaded rods. They must be adjusted properly by an A&P. While the nose wheel is still off the ground, twist it about the vertical axis looking for looseness. There are no cables in landing gear.

Also check the shimmy damper for solid attachment. It will be some vertically play but should feel fairly solid in the horizontal (it works like a shock absorber).

If it's none of those things it is probably the torque link (the elbow scissor thingie on the back of the nose gear). It may need to be shimmed. If the mechanic disassembled this during the annual (not typical) he may have not re-shimmed it properly. Look for new grease around the three zerc fittings on the torque link - that would be evidence that it was at least serviced.
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Great feedback. I'll have to check on that when I have a chance to get back out to the airport. Thanks!
 
Stingray Don said:
Actually a good question. No, I landed on the mains and it was not a hard landing. I will admit to a little side loading on touch down due to a direct crosswind. But the side loading was minor and I can't imagine that would have caused a problem. This was not my best landing but far from my worst!
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Hmmm, I'd take it back to the shop, something changed and not for the better while it was in their custody.
 
Sounds like tire balance or a loose axle nut giving some play in the wheel bearings.
 
If it happens again, pull back on the elevator and take some weight off the nose to see if it goes away. One of the local rentals has a bad damper and I have to do this on every landing to stop the shaking. Not saying you shouldn't get it fixed -- just something to keep in mind for the next time.
 
ollopa said:
If it happens again, pull back on the elevator and take some weight off the nose to see if it goes away. One of the local rentals has a bad damper and I have to do this on every landing to stop the shaking. Not saying you shouldn't get it fixed -- just something to keep in mind for the next time.
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Also, to make it stop, come to a complete stop, then taxi slowly.
 
ollopa said:
If it happens again, pull back on the elevator and take some weight off the nose to see if it goes away. One of the local rentals has a bad damper and I have to do this on every landing to stop the shaking. Not saying you shouldn't get it fixed -- just something to keep in mind for the next time.
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Yes, that was the first thing I tried but it didn't help.
 
I've seen axle nut wasn't tight enough on the nose wheel do this.
 
How about checking for the tire being out of round.
 
Stingray Don said:
Okay, I just got the 172n back from annual and went for a trip around the pattern for a test flight today. Taxi and takeoff were normal as was the remainder of the flight until landing. Upon landing, the plane began to shake so violently that I was briefly unsure of my ability to even get it off the runway. Once I turned off the runway, the shaking stopped instantly. I have never experienced even the slightest shimmy in this plane. On the rare occasion when I have experienced a shimmy in other planes, it was just a minor annoyance and was easily corrected by changing the weight on the nose with the elevator. Today, the elevator had no affect on the shaking.

I'm assuming the shop did something wrong when they worked on the plane but I don't know. During the annual, the nose wheel fairing was replaced and the control cables were adjusted. I can't think of any other work which would have affected the aircraft during taxi. I'm not flying the plane again until the shop checks it out. Anyone have any thoughts on what could have caused such a violent shimmy?
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Nose wheel axel assembly incorrectly assembled.
 
If you do a search on POA for shimmy you'll find plenty of discussion.

As a mechanic, I can tell you that nosewheel shimmy hs the same root cause that cars used to face when I was young. Their steering wheels would shake, sometimes rather strongly, at cruise speeds, with one particular speed being pretty bad. Old guys like me will remeber this sort of hassle. The cause is dynamic imbalance, and unless it is addressed, all other fixes are treating the symptom rather than the disease. The shimmy damper is there to deal with minor dynamic imbalance, not major stuff, and even that damper can't do much when the torque link hardware is all worn out or not tightened properly. Too often I find that previous mechanics have not tightened the link bolts enough to pinch the spacer so that it can't move, so the spacer wallows out the inside of the casting ears and the bolts wallow out their holes and everything ends up slopped out.

But even then I have eliminated shimmy by dynamically balancing that nosewheel. it's not easy doing it without a decent machine but it can be done. And the usual static balance will not fix the problem. Static and dynamic balances are worlds apart, and your car's wheels get dynamic balances when you get new tires so that they run smooth like silk. No wheel hop from static imbalance or shimmy from dynamic imbalance.

We get smooth-riding wheels on our cheap cars but can't get it on $300K airplanes. It's sad. Nobody makes a machine to dynamically balance nosewheels. I once modified an old car balancer to do it. Worked like a charm. Never a shimmy.

You should know that no car has shimmy dampers. Doesn't need them.

Dynamic balancing requires that the wheel be spun to determine where the balance weights need to go. None of the so-called "aircraft" balancers do that, and they are no more than variations of 1950's gas station static bubble balancers. They can actually make things worse by getting the mechanic to stick weight in a place that balances it statically but pulls it even farther off dynamically.

If the mechanic adjusted the rudder control system in a 172 without doing it in accordance with the service manual, he might have everything out of rig. The nosewheel's torque link cam stop is the centering device in the rudder control system and those bungee rods have to be set just so, or things get spongy and can add to shimmy problems. I have found that way too often, too.

http://www.gsp9700.com/technical/4202t/6glos003.htm

Dan
 
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Tom-D said:
It would have shimmied on take off.
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Often not. When at full power, the prop blast over the stab lifts the nose enough that the nosegear centering cam is locked straight ahead. On landing, the blast isn't there and when the nose comes down it ends up in a lower attitude and disengages the cam, allowing the wheel to swivel. I learned to fly in an old 172 that never shimmied on takeoff but would rattle the eyeballs out of your head on landing.

Dan
 
Dan Thomas said:
Often not. When at full power, the prop blast over the stab lifts the nose enough that the nosegear centering cam is locked straight ahead. On landing, the blast isn't there and when the nose comes down it ends up in a lower attitude and disengages the cam, allowing the wheel to swivel. I learned to fly in an old 172 that never shimmied on takeoff but would rattle the eyeballs out of your head on landing.

Dan
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Did it have an out of round tire? my response was to that guess.

This nose wheel wasn't worn out prior to the annual.

The 172 uses a 1.125" tube as an axel. with plugs in both ends and two large bushings to bear weight on the bearing to load them and stop the bearings from slipping side to side. that is all that is normally disassembled during annual to lubricate and inspect the bearings.

It is common to not fit the tube axel into the lower fork assembly properly, and not seat the bearing properly when he landed and the centering device in the strut allowed the wheel to caster the axel tube found its place in the fork, and allowed the bearings excessive side play.
 
Any reason not to go do an aborted takeoff or some fast taxi, get it up to a little higher speed then a normal taxi, and try to get a better feel for where it's at?
 
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Bonchie said:
Any reason not to go do an aborted takeoff or some fast taxi, get it up to a little higher speed then a normal taxi, and try to get a better feel for where it's at?
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He'll just find a speed where it's bad, that's all. There will be a resonant frequency of the entire nosegear assembly that will cause the most severe shimmy when the wheel is at the RPM that coincides with the resonance. When the wheel's dynamic imbalance steers it to one side, the whole nosegear begins to flex to that side as it tries to turn the airplane that direction, but soon the wheel has rotated 180 degrees and now all that flex also pulls the wheel back toward center and causes the wheel to caster the other way, in sync with the imbalance which is now trying to steer things the other way, and the shimmy gets violent.

Dan
 
They didn't set the preload on the nosewheel bearing correctly most likely.
 
I own a Cessna and I've noticed that if I land fast and lower the nose without slowing down it will often induce a shimmy or violent oscillation that sometimes doesn't respond to elevator input (short of lifting the nose again). If I am taking off with a forward CG it will sometimes shimmy until I use elevator to lighten the load.

What I have read on this board is that it's related to the play in the nose gear linkages and that no shimmy dampener will fix that - you just have to tighten up those linkages.
 
If you get violent shimmy, it means there is excessive tolerance in your nose gear somewhere. If you didn't have shimmy issues before, and you just had an annual, it's most likely the nosewheel bearing preload being incorrectly set after re packing the bearings. Not particularly uncommon.
 
As I have noted in similar nose wheel shimmy threads here, my own solution (after repeated attempts at all of the above fixes that didn't last long) was to trust the Lord.
 
Henning said:
If you get violent shimmy, it means there is excessive tolerance in your nose gear somewhere. If you didn't have shimmy issues before, and you just had an annual, it's most likely the nosewheel bearing preload being incorrectly set after re packing the bearings. Not particularly uncommon.
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On a Cessna the nose fairing (if installed) may not fit the axle plugs, fork, or ________ and the nut tension isn't transmitted correctly to where it needs to be.

I've had a doubler on the inside faces of the pant that actually interfered with the fork and bottomed out before it touched the axle plugs (22). If the fork is slightly sprung apart it takes a lot of nut torque to get the proper bearing preload.
 
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bnt83 said:
On a Cessna the nose fairing (if installed) may not fit the axle plugs, fork, or ________ and the nut tension isn't transmitted correctly to where it needs to be.

I've had a doubler on the inside faces of the pant that actually interfered with the fork and bottomed out before it touched the axle plugs (22). If the fork is slightly sprung apart it takes a lot of nut torque to get the proper bearing preload.
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Yep, that's why I always give the wheel a spin before putting it on the ground. If a firm flick produces more than around a revolution, something isn't right.
 
Henning said:
Yep, that's why I always give the wheel a spin before putting it on the ground. If a firm flick produces more than around a revolution, something isn't right.
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While flying?
 
With steerable nose wheels (and tailwheels), when you land in a crosswind, the rudder gets pushed and the wheel is in the air, so it follows the rudder. This makes the wheel at an angle to the forward motion of the plane so when the tire touches, it gets kicked around. So some of this is inevitable and normal. If it is out of balance its worse. If they can balance the wheel/tire, have them do that. Sometimes there is no balancing the wheel and you can try just rotating the tire to another position, hoping it is better. Sometimes there are worn out parts, bearings and such. But this just happens to some extent. Learn how to deal with it to minimize it in technique. And yeah, take the weight off the wheel if you can to encourage it to settle down.
 
coloradobluesky said:
With steerable nose wheels (and tailwheels), when you land in a crosswind, the rudder gets pushed and the wheel is in the air, so it follows the rudder. This makes the wheel at an angle to the forward motion of the plane so when the tire touches, it gets kicked around. So some of this is inevitable and normal. If it is out of balance its worse. If they can balance the wheel/tire, have them do that. Sometimes there is no balancing the wheel and you can try just rotating the tire to another position, hoping it is better. Sometimes there are worn out parts, bearings and such. But this just happens to some extent. Learn how to deal with it to minimize it in technique. And yeah, take the weight off the wheel if you can to encourage it to settle down.
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You're mistaken. On the Cessna's I've flown, the nose wheel locks straight ahead when the strut extends once airborne. Moving the rudder airborne does not move the nosewheel left and right.
 
I've had that before from an under-inflated nose gear tire. Looked good, but when I put a guage on it pressure was about half what it should have been.
 
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