737 NG Ailerron PCU input Pogos;aileron cable failure ; 'What ifs'

[bwheelz05]

Good afternoon,

Have looked in the FIM, AMM 1 and II for information regarding the tell-tale signs of when an input pogo (namely for the ailerons, but elevator/rudder too) has been compressed/stretched during a rate jam or lets say, a failure of one PCU..

This was a discussion at work as we received some new -800s from a customer. The conversation was amongst the mechanics as to whether or not anyone has seen an input pogo that has had its internal spring overcome... or how would one determine if an input pogo was compressed/stretched during one of the above examples.. no body seems to have experienced the situation.

I am sure a pilot write up would be immediate upon landing with a complaint about the poor roll tendency of the aircraft (poor handling) ... something indicating the need for further inspection of that PCU system (ailerons for example) But what (if any) visual clues would could one expect to see if the pogo(s) were stressed? I know there would be no separate indication in the flight deck of a 'failed PCU' (unless Hydraulic failure of a single PCU occurred, then a HYD or LOW pressure light would illuminate if lets say, a system failed) ... maybe the feel/centering unit would not allow the control wheel (in the case of the ailerons) to come fully neutral as an 'over extended' pogo would push back on the input shaft,canting the feel/centering unit ???

Surely somebody here will have the question or has experienced a input pogo that has undergone this issue.

Also, (same conversation, different topic) if a aileron wing cable were to break... am I correct in assuming that the aileron balance panels and tab would keep the aileron at a somewhat faired position relative to the wing.

So: If the LEFT wing cable were to break, this would allow the RIGHT aileron to continue operating normally... perhaps the aileron body quadrant might shear at the rivets maybe... BUT the good side would still operate. So at high speed/cruise speed; the bad aileron would not be operable but somewhat faired by the balance panels/tab ????

Ideas?

[rmm]

I'm not aware of any telltale sign on the pogo as such, but I think if were heavily stressed you might pick it up during the PCU change when you try and rig the pogo's. You might find the answer at CMM level? Not to say it doesn't happen but in the 20+ years I've spent working on the 737 I've never seen a failed pogo.

or how would one determine if an input pogo was compressed/stretched during one of the above examples

Would you really need to? Technically it's doing what it's designed to do.

Although the AMM (part 1) doesn't specifically mention a cable break it does state that forces acting on the balance panel will hold the aileron at the neutral position when there are no inputs.

Hope this helps.

[bwheelz05]

Hi and thanks for the reply!

My understanding is the input pogo(s) internal spring doesn't compress or extend until a pogo jam occurs. The bad/jammed pogo is back driven by the rest of the PCU assembly and it is then, that the spring, 'does its job'. So the pogos are usually quite rigid until a jam occurs.... is that correct?

So if that's the case, there would have to be something occurring at the control column that the pilots would feel as wrong or 'not right
.. thus they would write up the issue and maintenance would diagnose it. The FIM is unclear as to what to look for along the lines of a bad input pogo. Not even a mentioning of a visual clue or other indicator.

I agree that issue would be found during a PCU change or rigging...

I looked in the Component Maintenance Manual (CMM)... not much there for "How to find a bad input pogo. or . Indications of a stressed or jammed input pogo" .... hmmm

As far as a aileron cable breakage scenario... I figured the balance panels in conjunction with the aileron tab would try to keep the aileron in a somewhat neutral position.

If you haven't come across a pogo issue in 20+ years then I the chances of it happening must be pretty slim. ... maybe chalk it up to airplane magic! haha

[rmm]

So the pogos are usually quite rigid until a jam occurs.... is that correct?

Yes, although I've never actually tried to extend/compress one during a PCU change, so I can't give you an idea of the force involved.

I think if it were such an issue (the scenario you raise) that Boeing would have placed a telltale mechanism on the pogo's such as shear rivets. What was interesting is that the IPC states that certain part number pogo's are subject to corrosion from de-icing fluids. So maybe the opposite is more of an issue, i.e the pogo's not breaking out. We don't use alot of de-icing fluid in Oz so I doubt I'll ever see this issue either.

[bwheelz05]

great point! appreciate the info!

Makes sense. For now I guess if the AMM I and II ... CMM and other tech pubs dont mention it... the likely hood of an issue occurring is quite low.

[Yeelep]

A rate jam that causes a pogo to compress/extend will cause an extra 20lbs of control wheel force. The only tell-tale sign would be witnessing the failed PCU operating on the ground and seeing the pogo extend/compress.

But what (if any) visual clues would could one expect to see if the pogo(s) were stressed?

None, because the pogo was never stressed, it performed as designed and will return to its normal position with no damage when disconnected from the bad PCU.

My understanding is the input pogo(s) internal spring doesn't compress or extend until a pogo jam occurs. The bad/jammed pogo is back driven by the rest of the PCU assembly and it is then, that the spring, 'does its job'. So the pogos are usually quite rigid until a jam occurs.... is that correct?

Yes, except it isn't a pogo jam, it's a rate jam.

The only time I've had to change a pogo was due to external damage caused by rubbing against the metal safety loop attached to the PCU. That was on a single input PCU.