Oscillations - NOT TECHNICAL

[Colonel]

Probably a lot of people will be turned off by the title of this, and not read
it, and that's kind of a pity, because oscillations are very important to pilots.
(I just altered the title in the hope that people don't think there will be
equations, which would be bad)

The guy in the hanger next to me, Andy Phillips, died when his rudder fluttered
on his RV-7A.  That's what an oscillation can do to you.



Oscillation in pitch during the landing of any nosewheel aircraft, can wipe your
nosewheel right off, and seriously damage the aircraft.





Oscillation in yaw during a tailwheel landing can groundloop and wreck the airplane.



Oscillation - shimmy - of just the nosewheel or tailwheel itself, can be
violent enough to damage the landing gear, or even the aircraft. 



Very similarly, head shake on a motorcycle can be extremely scary, and
really hurts your thumbs:


Your job as a pilot, is to be aware of oscillations.  Avoid them, and dampen
them out when they occur.  How?

If you care, systems have natural frequencies.  Drinking from a beer bottle,
blow across the top and as the level of the fluid drops, and the size of the
cavity changes, you will hear the frequency change.  Note that a larger
cavity results in a lower frequency.

You can sometimes alter the system so that the natural resonant frequency
is high enough, that you don't encounter it.  That's what you're doing when
you put an extra tie-wrap on a wiring bundle.

The worst frequency is when you encounter a divergent oscillation, and
the amplitude rapidly increases.  That's when really bad shit happens.

Anyways, I have tried really hard not to mention differential equations in
this post, and to keep it practical.  In the not too distant future, you will
encounter a nasty oscillation, and I hope you can deal with it, because
that's your job, even if no one ever told you.



Life is a prick of a teacher.  First comes the test, then comes the lesson,
if you choose to learn from it.

[Eric Janson]

At an Airline I used to fly for operating very old A320s - a few of the aircraft had a vibration issue in flight.

It was clearly lateral and it was severe enough on one aircraft that it was spilling coffee from cups in the aft galley.

We were given forms to fill in - didn't help.

So in cruise flight I turned off the autopilot and tried a few gentle turns to see if that helped. Possible spoiler float but that wasn't it.

Next thing I tried was centering the rudder trim - vibration stopped instantly. When the autopilot was turned back on the rudder would trim to one side and the vibration would start again.

This was written up and despite the source being identified all that was happening was more forms had to be filled out.

In the meantime on one aircraft I had to hand fly it in cruise with the rudder trim at zero to keep the vibration at a minimal level.

Safety department also got involved - and finally an investigation of the rudder system was performed.

They found the bushings on the rudder actuator were worn allowing some play in the rudder. They found the same in a second aircraft.

The whole thing took several months to resolve which is unacceptable.

[Colonel]

Diagnosis is extremely difficult and performed quite badly in perhaps
every human technical pursuit.

All you can hope for, is a large population, and not to be at the leading
edge, so that if you encounter a problem, someone else has already
seen it and fixed it, so you can merely pattern match instead of diagnose.

It is my opinion that as we continue to dumb things down, our collective
ability to diagnose will worsen.

[vanNostrum]

" Life is a prick of a teacher.  First comes the test, then comes the lesson,''
Truer words could not have been spoken