Re: 727 rudder

From:         Rob Tremblay <dfens@netaccess.on.ca>
Organization: NetAccess Systems Inc.
Date:         16 Feb 96 04:33:27 
References:   1 2
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(Ken O'Rourke) wrote:
> I believe that having multiple surfaces allows you to have independent
> actuators on each so that in the event of a failure you maintain
> sufficient control authority to land.  Takeoff with potential engine
> outs and crosswind landings require the greater  rudder authority and
> hopefully a pilot could find a suitable airport to bring it in.
>
> Also, perhaps it is desirable to lock out one of the surfaces in
> cruise to minimize the exposure to actuator failures resulting in a
> full authority hardover.

In a way you are right, Ken. The 727 has 3 hydraulic systems- a "A"
system, a "B" system and a "standby" system. The upper rudder is
powered by the "B" system- if the "A" system fails, the upper rudder
continues to function (hopefully). The lower rudder is primarily
powered by the "A" system, but can also be powered by the standby
system if the "A" system fails. There are also two independant full
time yaw dampners on the airplane- one on each rudder. If one fails,
the other will continue to work (hopefully). If both primary hydraulic
systems fail, the standby system will continue to power the rudder.
If all hydraulics are lost, the x-wind limitaion is zero knots. The
yaw dampners will also not function with the standby system alone,
if I remember correctly. If the yaw dampners both fail at high
altitude, the potential pilot/passenger can kiss his/her ass goodbye-
the 727 is very unstable at almost any altitude above FL280 with no
yaw dampners. The most interesting screwup with the rudder design
is the fact that it was originally not powerful enough. Each rudder
was consequently split in half- the most rearward tab is actually an
anti-servo tab. This means that if the forward tab moves left, the
rear tab moves further left. I can assure you that the rudders are
now very powerful.
The rudder has no capabilties in the event of a total hydraualic
failure- ie. "manual reversion". Only the controls with trim tabs
(elevator/ailerons) can be moved with no hydraulics. I don't know
if this has actually ever happened to a 727- I've never heard of it.
The 727 has tons of redunancy- many independant systems etc.- very
safe. One FAA 727 actually hit a small aircraft and lost most of one
of the elevators- since the other elevator is completly seperate (as
you will see on some unpowered a/c- ie. one elevator up, one down) the
airplane was able to land without too much problem. The light aircraft
did not fair as well.
Later...