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Flutter Survivor:
“I
survived a
Piper Seneca in flight break-up”
Reprinted from Aviation Safety, April 1982
I
found your article on Seneca tail flutter quite Interesting and it
brought back some sobering memories of December 27, 1976, as I am one of
the pilots who luckily survived a Piper Seneca in flight break-up, I would just like to add some comments that may help aircraft
owners realize the potentially serious problems with trim tab free play.
The
flutter testing flights
for N33589 was
not taken as very dangerous because of our previous success with the
Seneca. In fact, I had just completed a series of flutter flights on a
Seneca II equipped with Robertson spoilers and tip tanks. The
accelerometer traces were good, but the camera that recorded the speeds
and altitude malfunctioned. The FAA wouldn't accept our data without
speed verifications, and by the time the bad film was developed the
aircraft was reconfigured for delivery. The owner was impatient and
wanted his aircraft, The FAA allowed us to deliver this aircraft with
some restrictions, and flutter testing was to be completed on N33589.
The
intended test
was
to climb to 25,000 feet, camera and recorders on and to pulse each
control at several predetermined speeds and at Vd (10 percent past red
line). The first flights on December 24th were aborted because low
temperature (-45 F) at 25,000 feet froze the elevator jack screw and
only low-speed data could be collected. The heavy grease was removed
from the jackscrew and a light silicone spray was used.
Flight
tests on December 27 appeared to be going well.
I climbed to 25,000 feet on an IFR
clearance above a 4,000 to 6,000 foot broken cloud cover. The test
equipment turned on and trim function was good. Intermediate speeds
looked good and at Vd (250 IAS) at approximately 20,000 feet, I
completed the following control pulses: aileron (spoiler equipped) left
and right, rudder left and right, elevator down, and then I reached to
start the up-elevator pulse. The aircraft was in trim and my left hand
was holding the wheel lightly. The air was smooth and there had been no
vibration.
It
was like an explosion, the elevator
failure resulted in an extremely violent nose down tumbling. Both wings
tore off, once outboard of the engine nacelle and again at the fuselage.
The fuselage was ruptured, part of the tail section and rear door were
gone, the fiberglass nose section was gone and the windshields were
blown out. Negative Gs had me pinned against the cabin ceiling and the
violent shaking had torn off my helmet. I had to protect my head with my
hands to keep from being knocked out; the survival instinct had taken
over. I knew I had to get
out of there to stay alive, but I couldn't move and couldn't do anything
but try to protect myself against the beating I was taking, As abruptly
as it started, the violence stopped. It was smooth and calm, with a
sense of slow rolling or tumbling. The cold air was blowing through the
hulk of the fuselage (-40F). Seat belt off and through the cabin door, I
opened my chute immediately, which could have been fatal.
As my chute opened, an engine with a section of wing came by from
above. I can still clearly
see that engine in my mind. The prop was feathered and the engine was
running. The total sequence of failure and getting out only took
seconds, but it felt like minutes.
I
was awed by the destructive disintegration
of
that aircraft and my survival. I was now floating down at 20,000 ft or
so. There were small light
pieces of aluminum, insulation and fabric floating down all around me.
In any direction I looked, it was like confetti.
This
was December and I was over a cloud layer
somewhere over the Cascade Mountains of Washington. I wasn't dressed for
this situation and again my thoughts turned to survival. Luck was on my
side that day. Wear some
torn clothes, a parachute under your left arm, put up your thumb and you
get a ride every time.
This
was the first aircraft lost
by Robertson in flutter testing. The FAA took a harder stand for more
testing and complete evaluation by Robertson for flutter clearance.
Prior to this loss, Robertson inspected and conformed only its
modification and assumed that an airworthy aircraft would have all
controls balanced and free play within limits. N33589 did not have the
elevator tab free play measured before that test, but I firmly believe
it was past its limits. On the walk-around inspection.
I commented to the company inspector about the apparent amount of
free play. We both agreed it was a common thing on Seneca’s.
Hindsight is 20/20, and I know where I should have stopped that
flight test.
Some
time later, Robertson elected to continue the certification of the tip
tanks on the Seneca II. The FAA insisted that we place the trim tabs at
the maximum limit. Robertson was not evaluating the tail, but was
looking for differences in wing response with the tip tanks. Robertson
lost the argument, and we instrumented the tail and went to maximum free
play on the tab. But wait: when
the inspector checked the original tab free play, he found it
considerably past the limit and we had to build new bushings to tighten
up the free play to the maximum allowed.
The flight-testing was conducted as cautiously as possible with
several flights of increasing speed and data reviewed between flights. I
had declined to do this test flight and a consultant pilot was hired.
On
one flight near Vd a problem was encountered that nearly caused the loss
of this aircraft. The cabin door popped open at 225 KIAS, The pilot
thought it was coming apart. The
accelerometer couldn't he measured because it went off scale. The door
problem couldn't be solved satisfactorily and the pilot insisted that
the front door he taped shut with aluminum tape. (His only exit now
would be through the rear cabin.) The
test was successfully completed and proved the aircraft is
flutter-free when within limits. The consulting pilot still
believes the door problem could have caused the tail failure of N33589.
It's interesting that Piper has redesigned the door latch on later
models.
I
do believe the Seneca Is one of the best light twins on the market. Most
pilots and maintenance facilities do not understand the consequence of
improperly balanced controls and excessive tab free play. Robertson
Corp. has corrected many of these problems during R/Stol modifications,
and the owners are usually unhappy at the added cost. An airplane with
controls out of limits cannot be returned to service and should be
grounded until it's corrected.
Pilots
and mechanics may think of flutter as a continuing vibration. Divergent
flutter should be thought of as an explosion. If the conditions are
correct, it's as dangerous as a lighted stick of dynamite.
My
recommendation for the owner of any aircraft is to
assure that your aircraft is maintained properly. All control surfaces
can be checked on a simple
preflight. Free play limits are design limits with no margin of safety
if they are exceeded. Replace bushings or install high tolerance
bolts to stay within these limits. An AD is not issued unless the airworthiness is
in question and this means an unsafe condition exists. These unsafe
conditions are discovered ordinarily by you, the pilot. Pay attention
and know your aircraft.
Sherman E,
Hall
(Former
Robertson STOL Test Pilot & Chief Engineer)
Owner
Advanced Aero
Safety, Inc.
Camano
Island, Washington
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