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).

Let me relay a story from a retired 767 captain. "One day while being pushed back with a full passanger load, a warning light warned that there was an error in the flight control system. We were pulled back to dock. After a few minutes of fiddling with the equipment below decks, the mechanic anounced that there was a glitch and that all was well. Again we bushed back and eventually were aligned for departure. A thumping sound started that was quite pronounced. A steward called on the phone and anounced that there was one hell of a racket going on in the tail. The rudder pedals were cycling to the ends of their travel very rapidly. As we started to taxie back in the problem stopped. We deplaned and the passengers left on another aircraft.
The problem aircraft left a number of hours later with another crew. The mechanic told me that the problem was a bad engine yaw sensor".

I add to this a reminder that one of the first 300s landed in a forrest after a slow flyby with news people on board. I think three were killed. The pilot said he applied full power but the computer had already determined that he wanted to land, so land it did. He was charged with manslaughter. Better to charge the pilot than to admit the computer program running the aircraft is junk. Also, I saw a vidio of an a-320 trying to roll over to the right while on final. It did this on three accaisions and was recovered by the crew without injury. Add to that the uncommanded hard over rudder problems of the 373. Boeing says there is nothing wrong with it, then sends out new actuators to everyone to replace all of the perfectly OK actuators now in service. The crews practice hard over rudder recovery in a simulator, because of course, it could never happen. Then some of the new actuators have part of a valve sleeve crack and cause further control problems, but there is no problem is there?
So you don't need to find a mechanical reason for the rudder to flutter, if the computer decided the cycling the rudder at the highest possible rate was suddenly required.

I hate computers. The computer people say that it is because I don't understand them. I say put the hydraulic boosted cables back in, and let the pilot fly the plane. Have the computer work on the coffee water and spraying deoderizer in the toilets. Something a computer can screw up without killing everyone, and setting all of aviation back 50 years. I bet the Airbus test pilot hates computers to.

Lynn E. Hanover

 
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