STBY RUD ON - B737 NG

[owl-attack]

Dear All

As per the FCTM the STBY RUD ON NNC the first line where it says - if the STBY RUD ON light has illuminated with no other flight deck indication or crew input - avoid large and abrupt rudder pedal movements. As per the FCTM this NNC is written for the FFM inadvertently activating and now 3 PCU's are powering the rudder, thus avoid large and abrupt rudder pedal movement

My question:
1. What will happen if large or abrupt rudder pedal movement is applied. Will the rudder break?

2. With STBY RUD ON light illuminated, and i verify and confirm that there is no loss of either hydraulic system , and thus the light has illuminated because of the FFM inadvertently activating - (because this is what the NNC is written for) Can I land on a runway with strong cross winds reported.
If you see the crosswind landing video below, I will surely have to kick in a lot of rudder after touch down.
But the STBY RUD ON NNC or FCTM does not specify anywhere that I should avoid landing with crosswinds, because large rudder inputs will be required.
https://www.youtube.com/watch?v=7P9OAng32F0


Thank you for inputs

[FlyingStone]

1. Not only rudder, the entire vertical stabilizer can fall off - see American A300 crash.

2. You have the whole rudder authority available during flare in normal operations anyway, so I suspect it would be the same. Read FCOM again and again until you at least somewhat understand the rudder system on B737...

[owl-attack]

Thank you flyingstone.

So with STBY RUD ON light illuminated because of inadvertent activation of FFM, during a crosswind landing, while i apply rudder to de-crab. the rudder can break off. Since the NNC not only says avoid abrupt input to rudder but also says avoid LARGE input also.

Am i correct?
So with STBY RUD ON I will have to divert to some other airfield than land in my destination with strong crosswinds

Am i correct?

[Lord Spandex Masher]

One of the reasons, as you stated, is there are now 3 PCUs powering the rudder. It is, therefore, possible for you to apply excessive mechanical force to the rudder which could result in structural damage. There is also no pressure reducer on the standby system as there is on the main systems and this will allow full system pressure to be available above 135 knots which will only be a problem if you're ham footed or stupid.

Or it may indicate a pressure differential between the 2 main PCUs which might suggest a jam. This is less likely and you still have enough rudder control with the standby system so it's not considered a problem.

All the NNC is trying to get you to do is be gentle with your feet. Landing in a crosswind does not require full rudder, don't bloody kick anything and nothing will break off.

[Amadis of Gaul]

Notice that the manual doesn't say abrupt AND/OR large, it says abrupt AND large, so, theoretically, if your "de-crab in a strong crosswind" is not, as others have suggested "too hamfisted" (hamfooted?), you shouldn't have a problem.

My bugger input would be that there is a time to read the manuals, and then there is a time to be an airman. It's important to know which is when.

[latetonite]

During a crosswind landing, your airload at typical landing speeds, will not shear off your rudder.

[Skyjob]

Alternatively to de-crab in the flare, don't de-crab!

Just fly it onto the runway in crab and straighten it after touchdown, as per FCTM

[BARKINGMAD]

RELAX!

Anyway, the 737 has castoring mainwheels with adequate damping, just like the NLG, so just think of it as the aircraft will sort the first 15kts of x-wind as per the Cat111A autoland case?

Don't fret, if the s'by rudder is on, go ahead and land and give the 'plane to the engineers which is it where it belongs!

[cosmo kramer]

As per the FCTM this NNC is written for the FFM inadvertently activating and now 3 PCU's are powering the rudder, thus avoid large and abrupt rudder pedal movement

First there are only 2 PCU's on a 737. A main and a standby PCU. The FCTM refers to "3 control values" (where 2 are controlling system A and B pressure inside the main PCU).

Second my FCTM say that:

"FULL rudder inputs should be avoided", in this condition. Not "Large and abrupt rudder pedal movement", which should be avoided at all times (see American Airlines A300 crash).

My question:
1. What will happen if large or abrupt rudder pedal movement is applied. Will the rudder break?

To reformulate your question: What will happen if FULL rudder pedal movement is applied?

I think you mix up aerodynamic forces with internal loads on components inside the rudder.

As mentioned, the system A and B hydraulic pressure powers 2 pistons in one PCU. The standby system powers a separate PCU. The rudder won't break, but perhaps (my best guess) the main and standby PCU may not operate perfectly synchronous, and at full deflection, put unnecessary load at the components.