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(Source: b737.org.uk)

Anyone knows why "STBY RUD" is named "STBY RUD" when the selection of the switch position activates the standby hydraulic system as well as the standby rudder system (by opening rudder shutoff valve)? Is it because the standby rudder is more important than the standby hydraulic system as a standby system?

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According to the Boeing 737 Flight Crew Operation Manual, the "Standby Hydraulic System" only powers the following:

  • thrust reversers
  • rudder
  • leading edge flaps and slats (extend only)
  • standby yaw damper

It does not power, for example, brakes, ailerons, spoilers and the landing gear.

I speculate that the word is chosen to remind pilots that the system powers the rudder but not other primary flight controls.

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Standby Rudder is a separate indicator light that was installed as part of the Rudder Enhancement Program, which was put in place after findings of the NTSB regarding the cause of accidents that occurred in the 90s.

Pre rudder system enhancements only 2 lights. After Enhancement program addition of STBY RUD ON light Indicating Standby rudder shutoff valve open. All aircraft should have been modified by 2008.

Standby Rudder pressurises the Standby hydraulic system which will power the separate Standby rudder PCU. Above and below pictures are from this site; picture below shows the upgraded system, with one PCU powered by hydraulic system A, one PCU by system B, and a separate Standby PCU powered by the Standby hydraulic system.

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From this site:

The 737NG also has a standby rudder PCU that moves the rudder during manual reversion operation. The wheel-to-rudder interconnect system (WTRIS) will coordinate (assist) turns by using the standby rudder PCU to apply rudder as necessary based upon the Captains control wheel roll inputs. From experience, I can verify that this makes the NG much easier to handle in manual reversion than previous generations.

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    $\begingroup$ Sorta. "Stby Rudder" was always a position on the Flight Control switch; the RSEP added a new way that the standby hydraulic pump could come on & pressurize the standby rudder PCU -- if the system detected the A PCU and the B PCU opposing each other. In that case, the Stby PCU becomes active (using the Stby hydraulic system) to "break the tie" in favor of whatever the flight controls were commanding. To tell the pilots that this was happening, the RSEP added the "Stby Rudder" light. If it's on & you didn't put a Fit Control switch to Stay Rudder, then the cause is the other PCU's fighting. $\endgroup$
    – Ralph J
    Jan 28 '18 at 16:22
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There is only one rudder on the aircraft, so speaking of "the standby rudder" would be incorrect -- to describe it that way suggests the presence of a "main" rudder and a second "standby" rudder, and that's not what's on the 737. What you do have is the standby hydraulic system, which is used to provide backup hydraulic power to a few components (listed in @Kevin's answer) when one of the main hydraulic systems (A or B) becomes inoperative.

So the better way to think of "Stby Rudder" would be "Standby Hydraulic System powering the rudder" -- and on the switch, "in lieu of Hydraulic System A" or "... B". Those switches are used either when a hydraulic system has failed, or to test the standby hydraulic system. The three positions of each switch are "__ ON" -- so the respective (A or B) hydraulic system is powering all flight controls, "OFF" -- so the respective hydraulic system is not powering any flight controls, and "STBY RUDDER" so that in addition to the "Off" functionality, the standby hydraulic system is activated & powering the rudder. (Why just the rudder? The ailerons & elevator can be adequately controlled in Manual Reversion -- zero hydraulics -- mode, but the rudder cannot. It has to have hydraulic power to provide the deflection required in case of an engine failure.)

The standby pump comes on automatically under various conditions, essentially all related to loss of a main hydraulic system. So if a main system were to fail on short final, let's say, you'll have the Standby pump running to power the Thrust Reverser on the side where the failed system would normally power it -- no need for an immediate reaction to put the flight controls to Standby Rudder during a critical phase of flight. In that scenario, your good hydraulic system continues to power all flight controls just fine.

As @Koyovis describes, the RSEP modification added some additional safety features and the "Stby Rudder" indicator light. That light is new with the RSEP mod; the switch position had always been labeled like this. Essentially, the RSEP mod added sensors to detect if the A-system and B-system rudder PCU's were opposing each other. This is driven by the theory that a rudder PCU can "run away" and cause uncommanded actuation of the rudder to a hard-over position. Were this to occur, the RSEP mod enables the Standby Rudder PCU (i.e. the PCU for the rudder that is powered by the Standby Hydraulic System) to become active & "break the tie" so that whichever PCU is responding to flight control inputs "wins" and overpowers the runaway PCU. The Stby Rudder On light will come on if the Standby Rudder PCU is powered, and if the flight crew didn't command this by putting either Flight Control switch to "STBY RUDDER" then they know it is happening because of a "force fight" between the A and the B PCU's and that the Standby PCU is working to keep things under control. I've not heard of that happening during actual operations; maybe Boeing has.

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  • $\begingroup$ Your description is that there is a third rudder PCU. You say that in addition to the "Off" functionality, the standby hydraulic system is activated & powering the rudder. But it is powering the rudder through a seperate standby-only PCU? That would explain the switch marking. The top position turns on, not only the stby hydraulics, but also the stby PCU. Thus the markings of both STB for the hyd system and RUD for the extra PCU. Am I understanding this correctly? $\endgroup$
    – TomMcW
    Jan 28 '18 at 20:15
  • $\begingroup$ @TomMcW Yes, there is (has always been) a third PCU. Essentially, an A system PCU, a B system PCU, and a standby system PCU. Until the RSEP, at most 2 of these could be simultaneously powered. The standby PCU is near the bottom of the right graphic Koyovis posted. $\endgroup$
    – Ralph J
    Jan 28 '18 at 21:51

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