B737 Fight Control Modules

[Whippersnapper]

Those of us flying 737s all kknow about the FCM AD. Essentially a faulty FCM (located downstream of the control PCUs) causes a hydraulic lock in the affected Flight Contol System.

Should this happen, control is retained by manual force with assistance from the correctly functioning side. This does result in higher than normal control loads and will result in AP disconnect, and usually Yaw damper disconnection too. OK, it's still flyable by hand, and Boeing simply recommend an immediate Flap 15 landing.

What if the other FCM has the same fault? Boeing insist this can't happen (I hate it when enginners insist a failuer is impossible) because no aircraft is allowed to operate with 2 FCMs from that suspect batch. Well, they thought they only had 1 suspect batch of fuel pumps on their hands, didn't they. Alternatively, what happens if the good side pops a seal and looses all its fluid? No chance for manual reversion with 1 control system hydraulically locked now, is there?

[fruitloop]

Quote :
Those of us flying 737s all kknow about the FCM AD. Essentially a faulty FCM (located downstream of the control PCUs) causes a hydraulic lock in the affected Flight Contol System.

Excuse my ignorance but arn't FCM's (flight control modules)located UPSTREAM of the PCU's(power control units)and downstream of the HCM's(hydraulic control modules)
Also isn't the failsafe control (loss of electrical signal)supposed to shut off supply (pressure)fluid and the internal valving(duel check valves)to allow a run-around system(example electic flap operation would be impossible with a locked hydraulic system.)

I could be wrong !!:D

[comxtek]

The FCMs ARE located upstream of all flight control (ail, rud, elev) and spoiler PCUs.
the loss of one hydraulic system would still allow hydraulic asisted flight, albeit with the loss of a couple of flight spoilers. The manual reversions doesn't kick in until there is a loss of BOTH hydraulic systems. At that point, all you're flying are the balance tabs/panels.
But remember, that's why u striped shirts get paid the big bucks. handle it!!!:D

Regarding the failsafe shutoff, you would not want the loss of a signal to shutoff fluid. The valve is always open, unless a switched off on the flight controls module.

And regarding electric flaps ops, hyds has to be stopped when running electric flaps. Arming the sys will stop hydraulcis to the flaps - which woul oppose flap movement by hydraulic lock.

[steamchicken]

and there was I, thinking a "fight control module" was a new security measure....

[fruitloop]

Thanks Comxtek.

Here is a simple test for you to do.(if allowed) Start either hydraulic system (300+)then go to the w/well and disconnect the 2 plugs from that hydraulic module (simulating loss of signal)have a mate then turn on the oppersite system and operate any controls.(you won't be able to listen with all the noise but feel the inative module for fluid movement).
Cheers and again thanks.

[comxtek]

I don't need to do the test.
I can tell u in confidence that the valve is either powered open or closed. If I remove the plug from it while it is powered open, it will stay open. Power is needed for it to be powered closed.