Walter Sobchak

Hello,
I read it over and over again and most probably one time too much because now I am puzzled.
With the FFM detecting a force fight and automatically turning on the Standby Hydraulic Pump to power the Standby Rudder is Hydraulic System A and Hydraulic System B shut off to the Main PCU? It would do so in manual operation of the related FLT CONTROL switch to STBY RUD but how would it know which system is faulty during a force fight and in automatic operation?
Any suggestions or answers appreciated.
Brgds.

Flight Detent

Hi WS,

To the best of my knowledge, in the 737NG, when the FFM triggers the introduction of standby hydraulics to control the rudder, both system A and B are shut off.
Just like you set both flight control power switches yourself!

Cheers, FD

atuk

To the best of my poor knowledge, if Standby rudder on light illuminates when descending below 700ft you could suspect it's RPR failure to transition to normal press causing automatic switching to sby rudder n illumination of the light.
At other time the most probable cause is either manual switching of flt control switch to sby rudder or...FFM trigger. There's no way of knowing which system (A or B) that cause the trigger... This is on the -400 that is fitted with RSEP...cheers

CaptainSandL

WS,

You have two questions, first when the standby rudder PCU is activated automatically by the FFM is hyd A or B shut off to the main PCU? Answer No, when the FFM detects a pressure diff on the main rudder PCU it simply activates the standby rudder PCU and illuminates the STBY RUD ON light.

Second, how would it know which system is faulty during a force fight? Answer, it doesn’t need to because both hydraulics remain on at the main rudder PCU.

There is no facility to switch off hydraulics just at the main rudder PCU. If you switch the flight controls switch to either OFF or STBY RUD it will switch off hydraulics to the ailerons and elevator as well as the main rudder PCU. The only difference between these two positions is that at STBY RUD you will also have standby hydraulic power to the standby rudder PCU.

I guess what you are asking is what if there is a rudder hardover, will hydraulic power be removed from the main PCU if this is the source of the hardover? Answer, NO unless you switch the flight controls switch to either OFF or STBY RUD. This is the last step in the Uncommanded Rudder/Yaw or Roll procedure. It is only necessary to do this on the B system because you have an RPR reducing the pressure on the A side. In fact on the NG you also have two load limiters reducing pressure on both A and B sides so the potential problem is reduced further.

BTW I wrote a 13 page article on this subject here and it still confuses me!

S&L

Walter Sobchak

Thanks a lot for the answers and the link. I always thought the FFM system was developed for the hardover case which is not the case then.
Brgds.

CaptainSandL

The FFM was developed as a result of the suspected rudder hardover incidents but it does not detect a rudder hardover. The maintenance manual says it “finds opposite pressure on the A and B sides of the tandem actuator”, yet on the schematic it is shown as a “Delta P sensor” which suggests it reacts to any difference (rather than opposite force) in hydraulic pressure, much like the elevator feel diff pressure system. Either way, once triggered it starts the standby rudder PCU which should resolve the problem.