Bill wrote>

Been trying to puzzle this one out and so far rudder flutter seemed

likely, but

with three physically separate actuators, that seems harder to do.  Going

on

your knowledge of the Boeings with separate actuators what will the
actuators do if they lose hydraulic pressure (severed line, drain out

fluid)?

Free the rudder to flutter or do they lock up mechanically some way?

If they free up the rudder (basically like a hydraulic cylinder drained of
fluid) the rudder could still flutter, but would have had to had all three
systems severed.

Bill
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor

I doubt that Bill. It is very unlikely that an maintenance was done on those
actuators (PCU's) recently, especially since there has been no media word of
that. The actuators on airliners are incredibly reliable.

On the 777 each actuator assembly includes a pressure reducer valve to trim
hyd pressure back to 1850psi in normal (3 actuator, flight) operations to
reduce stress. If one actuator fails, or during take off and landing, the
pressure reducer solenoids are de-energized to allow full 3000psi ops.

There are three modes of operation.
Normal - All PCU's powered.
Bypass - One PCU fails, it is put in bypass mode and back driven by the two
operating PCU's
Damped - Two or three PCU's fails, they are put in Damped mode, connecting
both sides of each actuator through a restrictor. A remaining PCU can
operate the rudder at greatly reduced speed.

If flutter occured, it sounds more likely that a weakened attach fitting
bulkhead/rib was the culprit rather than a maintenance error (thankfully for
the mechanics).

 
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