1. The A300-600 is typical of all largish
twin jets inasmuch as its tail is sizeable and the rudder powerful. A
wake vortex can admittedly be very persistent but at a distance of 5
miles or more behind, its wake-speeds will still be down by about 60%.
That is to say that a 747 at 250kts will be laying a wake that at 5
miles astern has rotational velocities approximating 100 knots (and at
10nms astern about 40 kts max only). i.e. the rate of dissipation
increases as you drop further astern. As you have discerned, it is the thwartwise component (only) that is of interest in ascertaining what
the sideload effect might be upon an A300 fin.
2. When you fly across a wake slightly
transversely you are going to hit it (the core) with a significant
part of your airplane. I know that full well from station changing in
a formation aero team making smoke. If you took that hit on the tip of
your vertical fin, well you were unlucky - but that's certainly where
it can do the most harm for a number of reasons. What do I mean? Well
think about it like this:
a. Flying very obliquely across a preceding
aircraft's wake, one that is essentially upon the same track as
yourself, will give you two wake "hits". The initial one (from his
starboard wing-tip's trailing vortex) will be followed by the
inevitable next one about 2 to 3 seconds later (assuming about a 15
degree across-track heading differential). Both hits will be equally
hard and they will be essentially upon the same bit of your airplane
(here we might assume a worst-case of impact effect being upon the
uppers of the A300 fin, such that the long moment arm wrench on the
fin attachment points will be greatest).
b. Should that initial wake impact be
sufficient to wrench a serviceable (to design spec) attachment lug
point apart? Not really. So what could do it?
The timing of the aircraft flight control system's response reaction
being coincidental with the next wake impact might just do it.
3. A Data Review
- From my input to the Air Safety Week Article:
"When the Average Isn’t Good Enough (and the Median too Mean)"
- refers to the A300's DFDR
sampling and filtering - and the frustration of the NTSB
investigation, because of it.
Timings
Four Complete Reversals
Inside Seven Seconds
Travel 11º to right for
0.5 seconds.
Travel 10.5º to left for
0.3 seconds (first reversal).
Travel 10.5º to 11º to
right for about 2 seconds (second reversal).
Travel 10º left for
about 1 second (third reversal).
Finally, travel 9.5º to
right before the data became unreliable (fourth reversal).
(Note: these are the
last seven seconds during which the tailfin and rudder were well
enough attached to give reliable FDR readings. The FDR shows four
complete rudder reversals inside seven seconds, but the sum of the
intervals shown above only comes to 3.8 seconds, and the travel time
of the last rudder movement to the right is not at this time a matter
of public record, if known. The last reliable FDR reading shows the
accident aircraft in a left yaw of 8º to 10º.) Source: NTSB
A great deal was
happening in about a seven-second period in which four rudder
reversals occurred inside a period of seven seconds. The accident
aircraft was equipped with an FDR capable of capturing 167 parameters
and recording 25 hours worth of information, but investigators say for
all that flood of information, key items (i.e. data-points) are
missing.
“The issue is not the
number of parameters,” said an NTSB official. Rather, the sampling
rates, in combination with the use of filtered data, may mean the
extreme points in the Flight 587 accident sequence have been lost
courtesy of the averaging function by which the data was recorded. In
addition, while rudder pedal movement was recorded, the amount of
force applied on the pedals was not captured. The data deficiencies
have set up a situation where it may not be possible to resolve
whether actions of the machine or the man (or a combination of
man-machine interaction) caused such extreme aerodynamic loads that
the tailfin separated from the airplane.
“It took us some time to
discover that filtering [of the raw data] was going on, and how it was
being filtered,” said the NTSB official. “Given the filtering, we can
never recapture the exact motion of the controls and control
surfaces.”
“Averaging will, by
definition, tend to produce a value that’s less than the extremes,”
the NTSB official said.
In truth, there are two
aspects of the data clarity problem. The first is the rate at which
the raw data are sampled. The rudder movement on the accident
aircraft, for example, is sensed at a rate of twice per second. The
movement of the rudder pedals is captured at the same rate. In the
interval between sampling, extreme movements could have occurred in
the accident sequence. One source advised that the flight control
system (FCS) is capable of moving the rudder more than twice in the
time that the FDR records one motion, and such rapid oscillatory
motion may provide insight into the rattling noise captured on the
cockpit voice recorder (CVR).
Thus, the sensing rate
of twice per second is especially important in this case. “How good
the data are depends on how often you sample,” the NTSB official said.
The rudder is capable of moving at 39º per second, which means it
could move about 19.5º between sampling intervals – which is a lot. As
an A300 pilot explained, “Consider that the rudder limiter restricts
the movement of the rudder to just under 10º at 250 knots. That would
mean the rudder, at 250 knots, could conceivably go stop-to-stop and
never be recorded.” (complete article
http://www.iasa-intl.com/store/avionicsFDR.doc here - in Word
format). But before we go on, just remember why the DFDR and cockpit
data had to be filtered and sampling-rate slowed…… because the FCS
system itself was sampling at a chatterbox rate.
4. So para three above tells us what happened
(as far as can be ascertained) - but how might we interpret it
further? Firstly it is possible that, because of the low data-sampling
rate and only 3.8 secs accounted for (out of a clock-run of seven secs),
the rudder was actually wagging the tail at a much greater rate (i.e.
building up to a max of twice as fast). In fact a freq breakdown of
the CVR audio spectrum indicates that to have likely been the case.
What does that remind us of, what aviation phenomena? Well, flutter of
course. But flutter is a divergent undamped aerodynamic response of a
flight-control surface that quickly slips out-of-phase. Aileron
flutter for instance can be caused by vertical wing-flexing (and the
aileron bouncing perversely around its hinge-line). Why not a PIO
(pilot induced oscillation)? Wrong periodicity. In addition it is just
not instinctive for a pilot to pedal rudder pedals because he is well
used to the forceful damping effect upon turbulence-induced yaw of a
large fin. It would be wholly contrary to a pilot's instincts.
Besides, PIO's are usually limited to the elevator circuit. So if it
was flutter, what sort of flutter - and what evidence is there for it?
The answer = System-induced flutter – and the A300 has a revealing
anecdotal and recorded history of it.
5. Firstly, what else might be affected by a
wake encounter – that might contribute to a system-led wind-up (aka
destructive rudder reversals)? There are very sharp-edged pressure
discontinuities within a wake vortex and these are not damped or
averaged but fed directly into the CADCs. The CADC needs to make rapid
inputs to the chatterbox processing of the FCS so it normally works
with delta type info (linear trends - vice stabilised data). It’s
likely that the correction for the wake-induced yaw was processed and
sent and applied at about the 0.5sec elapsed moment that it took for
the effect of the first slip to maximise (nose 11 deg right). But
there were three things that may have compounded to make that first
corrective rudder movement way off-base (and subsequent rudder-throws
likewise inappropriate):
a. the feedback conflicts engendered
by unfiltered input and filtered output. (leading to incorrect CADC
interpretation of airspeeds) i.e. apply a filter to the output of a
system that works on I/O feedback and you might get away with it if
the environment remains non-dynamic. Any dynamism will beget a
degree of exponential error (which in non-extreme conditions may
still be fairly benign - such as a mild A300 tail-wag).
b. the trend-type operation of the
CADC combined with the sharp-edged pressure discontinuities.
c. the FCS calculation of a rudder
response (to restore balanced flight) that didn’t account for the
yaw-state of the airplane. Because the airplane had encountered wake
(and hadn’t yet physically yawed) the rate gyros hadn’t sensed that
the new relative airflow was only due to the thwartwise wake
component - and so didn’t allow for the fin’s considerable damping
moment. The resulting first physical yaw therefore overshot balanced
flight significantly – and the scene was set for the second wake
encounter to happen just as the FCS applied a larger 3rd
dose of righting rudder (2nd reversal).
That was likely the starboard fin attachment’s
initial breakpoint – quickly followed by separation (8 to 10 deg
nose-left side-slip and rudder deflected 9.5 deg to right). After the
first RH lug broke, relative motion (lateral rocking) of the loosened
fin was possible, so subsequent fractures were inevitable (as the
fin’s inertial overtravels at each yaw apex reinforced the effect
(upon the fracturing attachment pivot-points) of the miscomputed
rudder applications). Whether the rudder limiter was affected by the
errors in CADC output or whether the yaw damper was a factor? Hard to
say, but they are two ancillary systems that would not have benefited
from any error compaction in a CADC output affected by a cycle of
unresolved chaotic feedback.
Proof or example?
a. From the
12 Apr 02 NTSB Release:
“The Safety Board is interested in another
upset event last year involving an Airbus aircraft. On November 25,
2001, a Singapore Airlines A340-300 departed Singapore for a scheduled
flight to Dhaka, with 96 persons aboard. Shortly after takeoff, the
pilots noticed a problem with airspeed indicators. Among other things,
there were overspeed warnings and large rudder movements without pilot
input. The aircraft returned to Singapore and made a safe landing;
there were no injuries.
Inspection subsequently found problems with
the pitot and static connections to the air data computers, which may
have been introduced during recent maintenance. The Civil Aviation
Authority of Singapore is investigating the incident.
“Inspection subsequently
found…….” Well that is de rigeur isn’t it? Can’t have an investigation without an
explanation can we? It may (or may not) have been a genuine case of
”problems with pitot and static connections” – but either way it
completes the nexus between my theory, the CADC, airspeed (i.e.
pressure) error and the rudder dancing to the tune of the CADC – does
it not?
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b. From the 25
Feb 02 - NTSB Rel
Upon re-examination of the data from a 1997
event, the investigation team has determined that another American
Airlines Airbus A300-600 (N90070) likely experienced high vertical
stabilizer airloads. On May 12, 1997, American Airlines flight 903 was
near West Palm Beach, Florida, when it entered a series of pitch, yaw,
and roll maneuvers as the flight controls went through a period of
oscillations for about 34 seconds, during which the aircraft dropped
from 16,000 to 13,000 feet. The Safety Board determined that the
probable causes of this incident were the flight crew's failure to
maintain adequate airspeed during level-off, which led to an
inadvertent stall, and
their subsequent failure to use proper stall recovery techniques.
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Conclusion
Wake event = pressure discontinuities (and so
does a stall induce
sharp pressure gradients – albeit not as sharp as in a wake event).
The evidence for A300 system-induced flutter exists, it’s only the
exact process that has yet to be resolved. The dynamics of stall and
wake event airflows and pressure flux within a one-way filtered I/O is
a good start-point. My 11 Feb 02 post on
pages 2 & 3 (and later) of this Pprune thread complements this
post.
Extract: (from prior
Belgique Pprune Post)
These fins and rudders on the big
twins are quite powerful because they have to accommodate OEI any old
time. Courtesy of the system that tempers that power, the FCS is quite
capable of breaking the fin off - both due to it being a composite
structure (that won't just partially fail/fatigue crack) and because
the range of rudder-throw action is large. At 250 kts it shouldn't
exceed 9.3 deg, but if you fool it into thinking it's at 165 kts, you
can have the whole 30 degrees of throw. Obviously that's going to be
enough to break it at the higher speed. I'm not suggesting that it
went full-scale deflection - just that induced static errors caused
the ADC to continually re-schedule the FCS to permit inappropriate
deflections for the actual airspeeds - leading to an aperiodic oscillatory
flutter as each deflection proved inappropriate - yet gave rise to the
need for a prompt further deflection (and thus becoming oscillatory).
That's not overswing (per the FAA release), it's system error - where
the rudder's power is pitted against the inherent dampening of the
fin. At a certain point it will become exponential and destructive.
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