Ed,
The ACS (Airworthiness Concern Sheet) from the Sebring crash was
discussed in great detail on Tech back in September.
Observers heard the engine winding up as if in a dive. NTSB evidence
confirms that the top spar cap failed under negative G force, and that
the seat with occupants was ejected from the airframe. (read the last
paragraph through now before proceeding further here...and yes, Bob
Sanders wore a parachute while testing Ercoupes!) A high speed low
pass with a sudden pull-up would result in positive spar loading. A
"pull-up" is, however, consistent with accounts from observers on the
ground, none of whom mention seeing the seat and occupants thrown from
the cabin. The eyewitness accounts and the physical evidence do not
seem to match.
Speeds in excess of 130 mph require a dive. Considerable forward yoke
pressure is required and must be maintained. Let us speculate that the
sudden loss of some 400 lbs. of occupants and forward yoke pressure
could cause a structurally failed and unoccupied airframe to appear to
"zoom", then turning and physically disintegrating. The very sight of
such a sudden "zoom" would likely hold every watcher's absolute
attention.
Just as the "art" of the magician relies, in no small part, on
sufficient distraction of the audience that they do not "see" all that
goes on before them, the departure of the occupants and seat as the
flight path transitioned from dive to zoom could have gone unobserved.
We don't have a diagram of the topography, the flight path in three
dimensions or where each reporting observer stood. Ejection of the
occupants would have been unexpected, and no observer mentioned seeing
it . The human mind tends to dismiss that which our eyes "see" if
those images are radically unfamiliar.
Per my (certified) letter to the FAA's Roger Caldwell of 9/23/09, pages
5 & 6 (and email to Tech of that date):
On November 8, 1948, Bob Sanders of Sanders Aviation issued "Ercoupe
TakeOffs" #G3 entitled "Discouraging Aerobatics". This describes
instances of "wingtip failures" and other structural failures reported
in "recent months" and that "...wewere not surprised or alarmed since
evidence presented strongly indicates misuse of the Ercoupes involved."
He explains that the 144 mph red line "indicates themaximum speed for
which the airplane is designed", pointing that all airplanes havea
similar restriction and that speeds above this "will endanger the
airplane and occupants inasmuch as excessive speed may induce wing
flutter or other unpredictable difficulties which will, in some cases,
cause immediate collapse of the structure". We have all seen reports
of difficulties of other popular make airplanes caused by pilots
exceeding design limitations." His last paragraph urges dealers to
cooperate "in bringing to the attention of all Ercoupe owners and
pilots the seriousness of exceeding the air speed red line and the
inadvisability of doing aerobatics in the airplane since they may
easily exceed the speed or accelerations which can be safely borne by
the structure."
On December 1, 1948, W. L. Greene, then Chief Engineer of ERCO, wrote
the CAA Aircraft Components Branch, Airframe & Appliance Engineering
Division(1-301) following "several airplane accidents involving wing
failure in flight". He states that "We are of the opinion that the
most probable cause...is aileron flutter caused by a loose aileron
control system. The wreckage of these airplanes...all
indicated...failure of the main wing spar was caused by a negative lift
load on the wing tip. This load could be applied in normal flight on a
smooth day by a substantial twisting of the wing to provide a negative
angle of attack of the tip. It...appears that a violent distortion of
the wing took place from some cause. The only obvious cause appears to
be wing tip or aileron flutter." In test flights investigating wing
flutter, and with 9/16" looseness in the control system "we obtained a
violent wing flutter, or aileron flutter, which caused the right
aileron tobe torn from the hinge attachments and leave the airplane at
a speed of about 190 miles an hour...".
On April 20, 1950, Norman A. Hubbard, D.E.R. 1-21 for ERCO wrote the
CAA,Attn.: Chief, Aircraft Division (1-565) relative to flight tests
for C.A.A. approval of the installation of the Trim-o-matic in Models E
and G only. He mentions "The recent accident during flight test of the
Trim-o-matic installation...occurred at about 180 mph calibrated air
speed." The next week, on April 27, 1950, Bob Sanders of Sanders
Aviation issued "Ercoupe Take-Offs" #P-7 entitled "TEST FLIGHT--1950
Ercoupe". This described the decision "to dive the airplane to the
higher speed.required to approve the...installation on earlier
airplanes, which were not limited against aerobatic maneuvers. In the
process of this dive, which went to 185 m.p.h., something happened to
the airplane causing it to nose down sharply, throwing me out of the
airplane and causing substantial damage to the airplane, which resulted
in its disintegration. ...initial and unofficial opinion is that the
nose ribs of the outer panels deflected, initiating the failure. The
rapidity with which I was thrown out, without any advance warning of
failure...impels me to again...discourage aerobatics which may
encourage unskilled pilots to get into speed ranges and attitudes which
are dangerous to them and their airplanes."
Cordially,
William R. Bayne
.____|-(o)-|____.
(Copyright 2010)
--
On Mar 6, 2010, at 21:22, Ed Burkhead wrote:
But what CAUSED negative g-load sufficiently strong to break the spar
in a
negative g manner and eject the seat/pilot/passenger out of the plane?
I cannot imagine a pilot with the experience and ratings listed doing a
barrel roll or pushing negative g's close to the ground, over
civilization
(the golf course).
I could, maybe, understand a high speed low pass with a sudden pull-up.
I've seen this done perhaps a hundred times at various Coupe flyins.
May I
say CLEARLY that I think this is a BAD IDEA.
Any plane with slop in its control system can get surprise control
surface
flutter when it encounters an airspeed and load combination that
allows a
resonant vibration to get going. I had that happen to me and am very,
very
glad that I reacted correctly and almost instantly (turned the yoke to
load
the aileron control system and pulled [with moderation] up to slow
down).
I'm not any kind of genius pilot. In retrospect, I think I must have
been
thinking about flutter and been alert for it as I did my power-glide
to see
what 144 mph felt like - only that would explain how I diagnosed the
vibration and did the right thing so quickly.
You'd better believe I got my control system tightened up to
specifications
after that and never again tried to power-glide up toward 144 mph!!!!
The NTSB probable cause report mentions nothing about flutter being
involved
in the accident chain other than repeating the quote from the ATP
pilot who
said he observed both ailerons fluttering. I wish they'd addressed
this -
but, I suspect they just had nothing solid to go on.
To me, vibration sufficient to shake the rear windows out of the plane,
knock the inspection hole covers off the plane and shake PAINT CHIPS
off the
plane might, just possibly, be important.
I'd urge everyone to do the checks necessary to see if your control
systems
are within specs.
And, please, don't show off by doing a dive down to a high speed pass
with
or without a sudden pull-up at the end.
JMHO
Ed Burkhead
-----Original Message-----
From: [email protected]
[mailto:[email protected]]
On Behalf Of William R. Bayne
Sent: Saturday, March 06, 2010 8:14 PM
To: ety Ercoupe
Subject: Re: [ercoupe-tech] Re: NTSB probable cause...
Hi Craig,
You make an excellent point. They may well have MEANT that
"cross-sectional strength" was reduced by 10% using "design code"
practices.
But what they SAID was that the "cross-sectional AREA of the top spar
cap" was reduced by 10%! On that basis that I deemed the COMMENT
"utter nonsense". Professionals should express clear and accurate
findings in any accident report.
The NTSB seems to conclude that aerodynamic stresses substantially
exceeding applicable design criteria preceded the observed
catastrophic
structural failure of subject wing and spar assembly such that even
had
the "extra" hole not been drilled the structure would have likely
failed in much the same way with unchanged result. If the possible
reduction in "cross-sectional strength" from this one non-factory hole
was dwarfed by the structural design load safety factor, no one would
bother to "do the calculations".
Regards,
WRB
--
On Mar 6, 2010, at 14:39, craig wrote:
The 10% figure probably came from design codes. Any hole no matter
how small and the design codes say that you must at a MINIMUM deduct
10% of the structural strength of the metal. In any case if you drill
more holes you would have to do the calculations to determine how
much
tensile strength remains.ie, drill enough holes and you could end up
with NO tensile strength left.
