Concerning the fatal prop separation on N320L (the red factory 320) and Jim
Frantz' comments on propeller suitability --

1)  Jim is absolutely right.  Much analysis and sometimes testing is done on
engine and propeller combinations.  Personally, I would not fly behind an
unapproved combination -- but some people call me a wimp.

2)  N320L may not have crashed due to an unacceptable propeller and engine
combination.  The following are exerpts from the NTSB report on the crash.

"The propeller was not found in the engine crater nor recovered near the
wreckage site. The crankshaft was sheared at the propeller flange. The
crankshaft was retained for further testing <snip>  A farmer who lives near
the accident site discovered the propeller located approximately 115 degrees
at 3,000 feet from the accident site while harvesting his crops. Both
propeller blades and hub assembly appeared undamaged with the crankshaft
flange still attached to the hub."

Why did this happen?  More excerpts...

"The crankshaft flange was examined by the NTSB's Metallurgical Laboratory in
Washington, D.C.. The NTSB metallurgist's factual report on the crankshaft
flange stated "...examination of the propeller mounting flange showed that
five of the six fracture areas between lightening holes contained evidence of
fatigue cracking." Further examination of the fatigue cracking in the flange
revealed no evidence of scratch marks, abusive machining damage, or other
defects that may have contributed to the initiation of the cracking. The
thickness of the propeller mounting flange was measured as 0.255 inches. A
representative of Lycoming stated that the specified thickness of the
propeller mounting flange on a P/N 74780 crankshaft is 0.26 inches to 0.28
inches. Hardness measurements on the flange averaged 30.4 HRC, below the
specified range of 32 HRC to 36 HRC. <snip> The propeller mounting flanges on
the crankshaft from IO-320-B1A engines installed in Piper PA-30 airplanes are
the subject of Airworthiness Directive (AD) 65-03-03 (referencing Lycoming
Service Bulletin 300B). This AD requires visual or magnetic particle
inspection of the propeller mounting flange before the next flight following
certain aerobatic type maneuvers not approved for normal category aircraft.
The compliance of this AD note is not required for experimental aircraft. The
Pilot's Operating Handbook for the Lancair 320 allows for aerobatic
maneuvers. "

Why did the airplane crash after the crankshaft sheared?  The report goes on
to say that losing the mass of the prop pushed the CG well aft of the aft
limit.

I'm not pointing fingers at Lancair here, although it seems like the NTSB was
trying to suggest there was a defective part which might have been detected
had the AD been complied with.  The point is that it's not clear that the
320L crash was caused by a prop failure due to an unapproved prop/engine
combination.  However, this sort of thing CAN happen with an unapproved
engine/prop combination, which is the real lesson here.  A prop failure may
result in worse than just gliding to the nearest corn field, so if you think
your risk with an unapproved combination is limited to an unscheduled
off-airport landing, think again.

Another lesson this teaches us, although off-the-topic, is a caution to pay
attention to an AD even if compliance is not mandated by law.  You may choose
to not comply, but make sure it's for a better reason than "I don't have to."
 (And no, I'm not suggesting that Lancair did that.)

- Rob Wolf