posted 3rd October
I was asked
by a few people to elucidate whatever theory might have been
behind the vague comment above (that was in response to
FDXMech’s info). My reply may be of interest.
Think in terms of two computers driving a singular servo-valve
actuator (only one of the three A300 rudder actuators is
connected to the yaw-dampers and that actuator can drive the
rudder at 39deg/sec). One yaw damper drives the actuator
mechanically and the other soft-drives it by virtue of something
akin to a shock-absorber strut or oleo (that is concentric to
that same singular primary actuator valve) (i.e. soft =with a
40% flex), Now develop a theory whereby the "soft-drive"
suddenly becomes a hard connection because its travel seizes
under the angular displacement of a rudder at near to full throw
(or, due to stiction, is acting very slowly). The other system's
yaw-damper computer sees a bad result (and takes over - as it's
designed to do). The soft-drive connection suddenly frees itself
and its computer wants to argue about who has precedence. The
rudder actuator returns full-stroke and the soft-drive again
seizes. etc etc
Get the idea? In an ideal world the "failed" yaw damp system
would trip and hand over. My guess is that it doesn't do that in
conditions of partial failure when the actuator goes well
outside NORMAL travel (such as in a wake event). i.e. it has
probably been allowed some latitude (by design) because
yaw-damping can be a rather dynamic event. Now also think about
an actuator that spends 99.99% of its existence near motionless
over a very limited travel zone. It's going to have a particular
wear characteristic, yes? But suddenly there's a wake event and
the yaw-damper sends it into the Twilight Zone of Travel for
that particular actuator. It's now well outside its "comfort
zone" of wear and starts twitching, sticking or seizing (or even
suffers an internal hyd leak) - perhaps due to encountering
corrosion in the untraveled zone.
This might also explain the anecdotal (and incidental) incidence
of mild to severe tail-wagging on the A300.
It was a similar mechanism (with the rudder power control unit)
that led to the 737 rudder hard-overs. However, before reading
further, you should read the 747 SP yaw-damper event narrative
at this URL first.
The dismissive comment in that incident report is very telling
when you think of AA587. It says:
"System redundancy had operated as
required to limit the effect of the upper yaw damper anomaly"
(Well, that's very true for the 747’s split rudder system! The
A300 however doesn't have a split rudder AND its yaw dampers
operate through a singular dual-action valve). It ain't got no
real redundancy at all. And just keep remembering that
AA587 had a yaw-damper fail its
checkout on pre-start at JFK - and required a maint reset. Do
you think it might have been trying to tell them something?
Commercial pressures notwithstanding, a failure of a system in
my opinion requires more than giving it a quick kick in the guts
reset and telling it to work (or else). A pessimist might think:
"Jeez, that's failed its checkout and that's a warning that
everything might not be mechanically right. We've now convinced
it to reset but......well what might’ve caused it to fail
What else might crank up a yaw damper disagreement? Well most
(all?) of these URL links below addresses situations where air
data computers, fluctuating airspeed and uncommanded yawing all
played a part. In AA587 it was flying
in amongst the pressure-spike sharp discontinuities of a wake
encounter. At West Palm Beach it was the pressure variations and
static port asymmetry that you’d encounter in an inadvertent
stall - when the copilot then (tellingly) tried to recover using
97 West Palm Beach.” As the plane's nose pitched up 12 degrees
and the bank angle exceeded 50 degrees, the first officer
applied full left rudder to correct the roll, NTSB
B. 25 Nov 2001 - Singapore Airlines A340-300 incident [departing
Singapore for Dhaka] (problems with airspeed indicators,
overspeed warnings and large rudder movements without pilot
input). Post-flight inspection revealed problems with the pitot-static
connections to the Air Data Computers (CADC).
C. 17 May 99 - American Airlines A300 experienced uncommanded
yaw (crew being unable to control the rudder with their pedals).
Problem was attributed to the autopilot.
D. 26 Jul 00 - Kansas City (FEDEX A310-203). N409FE returned
shortly after take-off due to overspeed warning actuation
followed by rudder system 1 and 2 fault lights illuminating.
Maint replaced #2 Air Data Computer and a/c checked out
E. 05 Nov 99 - Miami (American A300B4-605R) Operator reported
erratic rudder movements during a manual approach, caused by a
"double fault" with the autopilot yaw actuator. After
replacement of the yaw actuator the system functioned normally.
F. An American A300-600 crew departing Lima, Peru, reported
"fish tailing" soon after the plane took off, an NTSB report
said. The aircraft, N7055A returned to Lima and made an
G. 26 Jul 2000 Fedex A310 (N409FE) – overspeed warning followed
by rudder system 1 and 2 fault, returned to Kansas City.
Replacement of air data computer #2 fixed fault. CE20001AC071
H. The FEDEX A300 that broke a rudder actuator (the yaw-damp
connected one) in the hangar whilst undergoing maint checks.
So IMHO, the tail didn't fall off - it got blown off. That's
where the "kid's swing" analogy comes in. A little rudder
repetitively at just the wrong time can build the restorative
dampening up to quite a yawing gyration. I used to do it to
great extremes at the Jet FTS on my graduation parade aero
shows. The final pass was inverted yawing of up to 30 degrees
nose left and right. You might have seen it done at air
displays. That simply requires in-phase rudder-kicks to build
Once the yawing was underway, it only needed the rudder to be
significantly deflected whilst the fin was highly loaded and its
ultimate load could be exceeded.
I'd be surprised if something similar to this theory doesn't
rate a mention by the NTSB at their public docket show later