14 CFR Part 91: General Aviation
  Accident occurred Saturday, May 07, 2011 in Conroe, TX
  Aircraft: WOODWARD HAROLD L RAVIN 500, registration: N913RA
  Injuries: 2 Fatal.

  This is preliminary information, subject to change, and may contain
  errors.
  Any errors in this report will be corrected when the final report
  has been completed.

  On May 7, 2011, approximately 0855 central daylight time, a Woodward
  Ravin 500 experimental amateur built airplane, N913RA, impacted
  forested terrain shortly after takeoff from Lone Star Executive
  Airport (CXO), Conroe, Texas. Visual meteorological conditions
  prevailed at the time of the accident. The personal flight was being
  conducted under the provisions of 14 Code of Federal Regulations
  Part 91. The pilot and student pilot rated passenger were fatally
  injured. The flight was originating at the time of the accident and
  was en route to Temple, Texas, for the airplane to be displayed at
  an aviation event.

  According to CXO tower personnel, the pilot was cleared to depart
  runway 14 with a right turn on course. The airplane departed and as
  the pilot began his initial right turn he reported smoke in the
  cockpit. The pilot was cleared to land on any runway and giving the
  wind as being from 160 degrees at 8 knots. The airplane was observed
  to perform an approximate 240 degree right turn and crossed back
  over the departure runway before entering the left downwind leg for
  runway 14. While on the downwind leg of the traffic pattern the
  airplane was observed flying, â<80<9Cslowâ<80<9D, wings level,
  at an estimated altitude of 700 to 800 feet above the ground. While
  approximately 4,500 feet along the downwind leg the landing gear was
  extended. Shortly thereafter, the airplane started a descending left
  turn towards the closer runway 19. The airplane overshot the
  extended centerline to runway 19 and the airplaneâ<80<99s bank
  angle increased. The airplane continued in the descending left turn
  toward the runway until approximately 300 feet above the ground. At
  which point the airplane appeared to â<80<9Cstallâ<80<9Dand
  impacted into forested terrain. A post impact fire ensued. Tower
  personnel reported that during the event they did not observe any
  smoke or flames coming from the airplane until impact.

  A pilot rated eyewitness located near the crash site reported
  hearing what he described as an engine that was
  â<80<9Cscreamingâ<80<9Dand varying in RPM. When he saw the
  airplane it was flying â<80<9Cvery slowâ<80<9Dat an estimated
  altitude of 300 to 500 feet above the ground. As the airplane made a
  high bank angle turn back towards the airport the witness observed
  the nose and left wing drop before the airplane went out of sight.
  Seconds later he heard it impact trees followed by an explosion.

  The airplane came to rest in an upright position about 1,600 feet
  out on the extended center line from the approach end of runway 19.
  The measured impact heading was 200 degrees magnetic.

  ------------------------------------------------------------
  Sounds like they will never know what happened.

  I am guessing. One possible scenario is the automotive clutch Bud
  used started slipping causing the smoke and the
  [WINDOWS-1252?]ââ?¬Å?screamingââ?¬Â engine. The clutch disk could wear
  rather
  rapidly due to normal high torque fluctuations at high continuous power.

  IMHO the TBO on an automotive V8 in aircraft use should be 300 hours.
  Any piston engine can develop a crack in the critical parts leading
  to a hand grenade failure without warning. The main purpose of the
  TBO is to magnaflux the critical parts to detect cracks early before
  the engine explodes. Through years and millions upon millions of
  hours of experience and detail refinement the TBO on aircraft
  engines has increased to 2000 hours.

  The TBO on my C182 engine was 1500 hours. By adding another
  torsional damper on the crankshaft the TBO was increased to 2000
  hours. However unlike a piston engine in a car the aircraft crank
  and rods are made of the finest material possible. Only the best
  methods of manufacturing and finishing are used.

  Parts in an automotive V8 are made as cheaply as possible as the
  average required power is only about 40 HP despite a 400 HP rating.

  We are so lucky the Wankel rotary does not see the same kinds of
  reverse loading on the e-shaft and the rotor that are seen on a
  piston engine cranksaft and rods. Fatigue failures of the e-shaft
  and rotor are unheard of.

  Paul Lamar

  It sounds like the pilot might have stopped flying the airplane, but
  instead was trying to save it. My training was that in case of fire,
  you get the airplane on the ground immediately, as you really don't
  know what's burning. I can understand the smoke could have been
  incapacitating, so that might have been the issue, but if it was the
  clutch burning, it seems unlikely somehow.

  Matt-

  I agree but people that build airplanes have so much invested in
  them (more than money alone) they appear to be reluctant to crash
  land straight ahead as we were all told to do in case of engine
  failure on take off. Here is a map of the area and there does not
  appear to be many clear fields other than on the airport itself. In
  the case of rotary crashes at least two fatals were caused by the
  pilots attempting to turn back. In both cases there were suitable
  fields straight ahead. One rotary crash was caused by engine seizure
  after the loss of all oil pressure due to a split cheap oil cooler
  and the other an interruption in the fuel supply due to a poorly
  designed fuel plumbing system.

  IMHO there is another more subtle factor. One is naturally reluctant
  to do a 180 and land downwind on a controled airport or busy
  airport. We had an incident at SZP where a pilot had an engine
  failure in a Bonanza and successfully made a 180 and landed down
  wind. He slammed it in so hard on the 2500 foot runway it bent the
  spar and broke the motor mounts totaling the airplane but he was not
  hurt. He skillfully ground looped it when it looked like he was
  going to run out of runway at thew other end. I was standing 50 feet
  from the point of impact.

  I think people should practice 180's at altitude with the engine at
  idle. Just to get a feel of what the airplane will do. Note the loss
  of altitude.

  Paul Lamar

  --
  Persons close to the parties blame it on a bad oil line connection
  to the engine....not the clutch......FWIW.... Marc

  Why the "screaming" engine? Or did it sound like that all the time?

  Paul Lamar


  Re: Matt's comment about clutch burning unlikely. Yes unlikely, but
  possible.
  Hi-way haul trucks have got the brakes on fire coming down long hills. I
  worked on a Ford 1/2 ton that had stopped on the side of the road.
  Questioning
  the driver/owner he said it slowed and stopped going. The engine ran
  fine.
  The
  fellow said when he looked under the truck there was fire underneath.
  When I
  removed the clutch cover, a couple of handfuls of clutch fiber came out.
  The
  flywheel, pressure plate, trans input shaft and the back of the
  crankshaft
  were blue from the heat of the slipping clutch. The trans input and the
  crank
  just had small spots. The fire was the release fork boot burning
  from the
  radiated heat. Surfaced the flywheel and put new clutch in. I also
  had to
  replace the trans input bearing as this fellow would shift into OD at
  40MPH.
  Ford dealer had replaced the noisy brg 3 times previous. Only 45K
  miles on
  the
  truck. He came back two weeks later to see if I had the clutch core.
  Said he
  was talking to his insurance agent and they figured the fire burnt the
  clutch
  out. There we go with the only thing more dangerous than a dumby, is a
  dumby
  that thinks he is smart.

  Dale Davies

  Burning brakes happened to me when I was driving a semi as a teenager.

  Paul Lamar



  Wouldn't the clutch gripping strength be stronger than the other
  components in the drive train? I'm thinking rev up and pop-the
  clutch.. And the clutch clamping surface is quite a distance from the
  center of the shaft, lowering the slipping force (torque) required.
  In automotive use, clutches fail when they are slipped, or engaged
  suddenly. If the clutch is healthy to start with (not worn out from
  miles or abuse), it won't slip. I think your 1/2 ton truck customer
  rode (left his foot on) the clutch pedal. That's a quick way to wear
  one out - abuse.

  As a side note, I went hunting a while back with a friend.. We were
  in his pickup, he was driving. I have never ridden with anyone that
  slipped the clutch as much as this guy.. I mean, probably half the
  time we were accelerating, the clutch was slipping. I could tell he
  was doing it by watching his leg/foot.. I have no idea how long that
  clutch lasted, but it couldn't have been many miles.

  I guess the crash airplane might have had the clutch
  linkage/hydraulics mis-adjusted so that it didn't allow the clutch to
  provide its full clamping pressure.. That brings up a thought...
  Maybe airplanes with clutches (or anything that could slip) should
  have a tachometer on both the prop and the engine.. And a little
  circuit that keeps track of the ratio between them. And which sets an
  alarm if there's any discrepancy between the engine and prop RPM -
  indicating slipping - which might not be immediately detectable to the
  ear.

  I understand that there may be torsional resonant loads at certain RPM
  ranges, but I'd guess that they would threaten the prop shaft, or
  output gear.. There's lots about this that I don't know - obviously.
  Predicting the RPM at which resonant forces build doesn't seem to be
  something that can be done by guessing or TLAR.. None the less, I
  think the torsional loads would tear the center out of the clutch
  before other failures, assuming proper engagement.



  Matt-


  Bud claimed the clutch damped the torsional pulses which implied it
  had to slip.

  Paul Lamar

  Almost all automotive clutch packs have a spring loaded center section
  intended to help soften any shock to the drive train resulting from
  injudicious engagement of the clutch and/or throttle.  Perhaps this
  action was what he was referring to when he said that the torsional
  pulses were damped by the clutch.

  I doubt that the clutch would slip in response to any torsional pulse
  that could be generated by the engine.  What is far more likely is that
  the spring loaded center section would fatigue from the varying forces
  resulting from the referenced torsional pulses.

  My thought is that a damaged oil line would allow the clutch to be
  contaminated resulting in excessive slippage under full load resulting
  in functional failure.  The oil leak probably occurred before the clutch
  was engaged as the symptoms appear almost immediately after take off.

  I'm not familiar with the crank design on the Chevy engine involved but
  I do know that on some Chevy crank flanges the bolt holes provide a path
  for engine oil to leak from the crank case if one of the bolts becomes
  loosened.  Those engines require the application of some type of
  anerobic sealing compound, i.e. "Loctite", on the threads of the
  flywheel bolts.  Were the crank bolts lock wired?  Did the sealing
  compound fail?

  Mark LaPierre

  "What is far more likely is that
  the spring loaded center section would fatigue from the varying forces
  resulting from the referenced torsional pulses."

  Has happened in Ken Welters rotary powered Coot.
  Tracy uses rubber bushing which makes more sense.
  Here too is a rubber damper in Randy Lewis's belt drive PSRU.
  I don't know where it came from.

  An ounce of rubber is better than a Pound (British monetary Pound) of
  engineering :)

  Paul Lamar



  There seems to be a lot of confusion on the application of a
  "CLUTCH' for an automotive engine used in aircraft.  There is NO
  reason to have a clutch on any aircraft engine since there is no
  need to disconnect the engine from the propeller.  The function of a
  clutch is to disconnect the engine power going to the drive train
  (Propeller).  What IS needed is a mechanism which will absorb or dampen
the
  torsional pulses from the engine to the reduction gears or PSRU.  That
device
is generally incorporated into the clutch disk and can
  be found on almost all standard transmission vehicle, however it is
  independent of the clutch function.  (Automatic transmissions have a
  torque converter which accomplishes the dampening.)  A series of
springs, that
can be seen on the clutch disc picture
  above,  is designed so that the center of the disc can rotate up to
  3/8 inch which compresses the springs and absorbs the pulses.  There
  are stop pins which limit the rotation.  The spring size is
  determined by the maximum torque which can be applied to the system.
The
steel springs began replacing rubber dampers which were used
  into the mid 1930's and have been used in millions of vehicles and
  are still used today.  The springs are much more reliable and non
  flammable.Looking at the ken Welter torsional dampener, it
  appears that a little too much metal was removed in an attempt to
  save weight.

  What some people will do to have a torsional dampener, that is
  inexpensive, is to use an automotive clutch system. However, the
  clutch release mechanism should be disconnected so that the clutch
  friction pads are permanently  compressed against the flywheel.  With
this
setup there will be NO slippage in the drive train (NO
  HEAT) and the torsional dampener will still function.  If you are
  still apprehensive about slippage, you can place several bolts
  through the clutch disc into the flywheel.

  There is nothing in the clutch assembly that will burn.  A clutch
  that is slipping will generate a large amount of heat and will have
  the capacity to ignite nearby flammable parts such as rubber, oil
  lines, etc.  Asbestos was used until the 80's when the EPA limited
  its' use.  Now ceramic materials are used.

  Joel

The clutch was set up as a centrifugal. Torsional resonance is usually at
low revs so the idea is to have the engine engage at higher than critical
rpm.  Some of the gearboxes for the small engines have a centrifugal
clutches for the two strokes where there are other benefits.  The faster
the clutch turns, the harder the lock up.  If the engine was running away
it would indicating a loss connection and power to the prop.


Murry I. Rozansky


Joel - Porsche, and others, still use the rubber dampers in the clutch disc
today.  Modern Porsche's are so quiet, that customers can hear the gear noise.
Hence they seems to go back and forth between the metal springs (crisp shifting)
and rubber dampers (noise reduction).


On the rally Mazda's, we ran solid clutch disc (no springs), with a pretty solid
on/off disc action.  Horrible on the street, but on dirt you just rev'd and
dumped the clutch.  Rotary torque pulses being less than a Piston motor, but we
would still trash the gear boxes with our bridge ported motors.  At the time I
was buying "blown engine" RX2's for $100 each, hence low buck racing with the
RX2's.


As to the Raven crash... pure speculation.   I was talking with Bud, Phyllis,
and Larry after the '07 fire that consumed the Wheeler. That was a split oil
line.  Bud was cool in an emergency, and would have kept flying the plane. It
may have been a cockpit fire, but in the Raven perhaps just too much speed loss
for a 180 return.  Too much of the plane was consumed after the fire, so
unlikely we will ever know the real issue.

I did go out in the 172, with a CFI, and explained I wanted to try 'turn back
180' routines to simulate take off engine failures. We went up to 4000 AGL, and
the tighter the turn, the more vertigo set in.  We picked some reference points
to simulate our home runway (GTU) and kept at it for a while.... My best case
was 400' loss (worst was 800').  Bottom line, at 500 AGL on take off, it MIGHT
be possible.... but without constant practice, the skill gets old and the "do I
have a problem.... and few second delay...  meant I'd hit thr ground first. 500'
with the 172 was my magic number (rather than 180... best options being from 10
O'clock to 2 O'clock.  Worth while training, but if you are in an unknown plane,
and haven't practiced... keep flying into the trees will increase your chances
of surviving.


Tom


Thanks Tom. Good information.
I like the rubber clutch as the rubber actually damps by heating
up while the springs give it all back. I was wonder where Randy's
damper came from. Now I know.

Paul Lamar