737 rudder problems

[Mulligan]

As a 737-200 driver I've been trying to educate myself about the "hardover" problem for a number of years with varying degrees of success. I'd appreciate a little help from all you "sharp pencil" types.
The checklist for "uncommanded rudder" seems to be a typical process of elimination affair. First get rid of the autopilot, then the yaw damper, then switch system B flight control to STBY RUD.
I can see it up to there but I wonder why there aren't more steps. By switching system B to standby rudder you have the standby PCU on line in (I assume) an attempt to use it to position the rudder where you want it. OK but what if the main PCU was the problem in the first place and it is still trying to push the rudder the wrong way with A pressure? Will the standby PCU overpower the main PCU? They both operate at 3000psi so I don't see how. It seems that the next logical step would be to remove A pressure from the main PCU by selecting sys A flt control switch to STBY RUD or perhaps to OFF. I'm sure I'm missing something obvious here. Anyone care to point it out?

[mainwheel]

No-one can actually put their finger on the problem,otherwise it would have been fixed yonks ago.
A mate was flying a 737 in asia and something got stuck under the rudder trim wheel.He picked it up in time but nearly lost it.
I think the problem is the actuator and feedback,no proof though.
My advice if it does happen would be to kick the s..t out of the peddles.

[BOAC]

I have burnt my 200 Tech manual:), Mulligan, but the 3/4/5 says, under Hydraulics, that actuating the 'Standby Rudder'.... 'SHUTS OFF the related hydraulic system to ..... and RUDDER by CLOSING the flight control shutoff valve. I'm sure it is pretty standard Boeing stuff.

Have a look in the manual!

[Mulligan]

Sure enough. I have the manual in front of me and it says switching the B switch to STBY RUD "isolates corresponding hyd sys press from .....and rudder" just as you point out. However A sys is still "on" and is powering the main PCU, is it not? So are they working at cross purposes?
My point is that if you selected B to STBY because there might be a problem with the main PCU, why would you not also remove A from the main one as well?

[BOAC]

Sorry, Mulligan - RTFQ for me! I cannot come up with an answer to that - I'll ask the 'experts'. I (reading between the lines) think that the emphasis is on the Yaw Damper (Sys B) malfunctioning and not the PCU (otherwise think of the liability problems.............)

[Caio]

Hi ! That´s my first post in this great forum and I hope to post a lot more. Please I´m from Brazil and my English isn´t that good so be patient :_)
About the 737 rudder as far as I know by disengaging the Yaw Damper system the hydraulic power will be shutted off to the yaw damper components in the Rudder PCU,this will disable the yaw damper function and will also eliminate any Yaw Damper system anomalies. So even if you switch system B to stby. it wouldn´t make any difference if you don´t engage the Yaw Damper.Well that´s my opinion,hope I didn´t say any foolishness :_)

Best regards,Caio França.

[Chryse]

This is my first post so please be patient.

1. Sys A,B operate at 3000 psi. Stby operates at 2900 psi as far as I know.

2. With the installation of the RPR (Rudder Pressure Reducer) Sys A pressure to rudder PCU is reduced to 1400 psi above 700 ft. The RPR valve is monitored through the Sys A LOP light which would illuminate below 700 ft while descending if there was a failure of the RPR.

3. Therefore the step as given by Boeing are quite logical to me:
a. Disengage the Autopilot (Process of Elimination)
b. Switch the Yaw Damper Off (Process of Elimination and it would trip off anyway when you select Sys B FCS to Stby Rudder in the next step)
c. Select Sys B FCS to Stby Rudder. This closes the Sys B Primary Flight Control Valve, whereby the rudder PCU is now receiving pressure from Stby at 2900 psi and Sys A at 1400 psi due RPR. Therefore Sys A pressure will be overrided.

Hope this helps. But I welcome your comments or suggestions.

Bye

[Mulligan]

By Jove I think you've got it Chryse! I'd forgotten about that RPR and it's lower pressure in flt. A pot of ale with you sir!

[xpc]

with the yaw damper off shouldn't the rpr be deactivated as well?

MEL -- with yaw damper inoperative rpr deactivation required as part of maintenance procedure.