FAO Atrude 777

[3WE]

The following is 100% true. However, as usual, I have added some sarcasm and humor which some will appreciate.

May 30, 11:00 CTD, STL Rwy 30 R (SWA 9:30 flight from DAL). Something breaks on the landing gear resulting in a severe shimmy/vibration- to the point that.

-Trim panels fall from overhead bins.
-The plane stops without exiting the runway.
-Passengers applaud.
-Pilots quickly come on PA to say, "we think we blew a tire- please sit tight"
-FA's say "They have never experienced anything like this in 18 years of flying"
-Fire/rescue equipment is dispatched.
-The aircraft is towed to the gate.
-APU-powered air conditioning fails to start. (This plus being stranded literally on the runway for an hour, sucks).

I’m not completely sure- but it sure looked like everything was wobbling including the cabin walls, and I wish there was a way to learn what the follow up was on the aircraft.

Brief statement: Thank God it was a Boeing product and not made from cheap composites.

Interesting and possibly relevant factors:

-A fairly strong, almost direct crosswind (wind socks fully horizontal).
-Strong rocks and rolls and power adjustments on final approach.
-The pilots actually reduced the flaps as they got onto final approach! (This fascinated me as I had never seen that happen- in fact, I thought we were a bit high and going around at first)
-I think we had a significant bounce on the landing; however, the touchdowns were firm, but average.
-We touched down left side first, and I think there was a little side-load; however, I have felt a lot worse in terms of sideways scoots.
-Upon arrival at the gate, the flight crew says, We have been informed that our tires are fine, but some landing gear linkage that prevents shimmying broke- "we don't know why".

Discussion points:

It was interesting was that the shimmy increased as we slowed (not a huge surprise as I have an old 1976 chevy 4 by 4 that does the same when the stabilizer/shock-strut on the tie rod is worn out.)

However, in second-guess mode, it seemed that the pilots were braking very lightly…even as the shimmy got worse- even at the end of the roll out he seemed to be totally off the brakes and let us coast to a stop (with three or four slow big shimmys). In my worthless opinion, I felt the dudes should have braked a little harder, especially since he decided it was too hazardous to go ahead and exit the runway.

Random thoughts during the incident:

Well, good, we have slowed enough that maybe things are survivable, but man, if this keeps getting worse, the plane is going to start breaking up.

Thought immediately following the roll out:

- Damn, why didn’t I have my digital camera out in video mode- then I could post it on YouTube for ADI folks to view.

[Dmmoore]

Typical nose wheel shimmy. Tell SWA to check the total accumulated wear on the steering system. The linkage that broke is the result of the shimmy, not the cause.

The nose gear on all Boeing's is very similar in design.

On the 727 and 747 we did a mid life nose gear refurbishment specifically designed prevent nose wheel shimmies.

[E-Diddy!]

Sounds like the shimmy damper in the nosewheel failed. The shimmy gets worse as you slow down because of the gyroscopic principles, and a failure of the nosewheel would probably constitute a failure in the steering system. The hard touchdown of a crosswind landing is probably what resulted in the failure, but I'll guess is that was just the final straw in a worn component.

One of the things about aviation is that if you have a problem and you take an action and that action makes the problem worse, then don't continue taking said action. The pilots probably got on the brakes, got the shimmy, then let off and let the airplane coast itself to a stop. If they rode the brakes then its very likely the entire nose gear would fail and collapse.

[screaming_emu]

It might not have broken, may just have been low.

I'm not familiar with the 737, but the CRJ 700 has what is called a shimmy damper on each main gear. I can't find a picture online, but its on the front side of the gear and looks like a coke can.

The shimmy damper does two things. First it allows there to be some play in the gear if the plane touches down a bit sideways. Second, it (as its name implies) reduces the amount of shimmy that can be felt in the cabin during takeoff and landing. If anyone has flown a plane where one doesn't work, its very unpleasant.

One of the things we have to check during preflight is to make sure that the dampers are properly charged/filled.

Basically what I'm saying is that if the 737 set up is similar to the CR7, a shimmy damper that is low (but within standards for departure) plus a somewhat tough landing could certainly make it feel like something is really broken.

[3WE]

Thanks all.

I conclude that 1970's GM steering systems, 172 nose gear, 737 nose gear and CRJ nose gear all need a good dampener.

On another board, Don mentions 747 nose gear (same fundamental design), where instrument panels have been shaken loose.

:)

[Dmmoore]

The shimmy dampener on most light aircraft is usually nothing more than a small shock absorber attached between the movable portion of the nose wheel steering system and the fixed portion of the strut.

A new Cessna 172 can be flown with the dampener removed without a shimmy resulting. However add some wear to the nose steering system and / or an out balance of the nose tire and a shimmy will result that will continue until the aircraft is almost stopped.

The Boeing nose steering system contains an accumulator in the hydraulic system. If the accumulator were flat (broken, the service pressure in this unit is checked daily) the result should be a minor shimmy event. It is wear in the steering system along with tire balance that causes severe vibrations.