The NTSB released its preliminary report on the Pine Mountain Lake crash.  As usual, the preliminary report contains no conclusions concerning the cause of the crash. For that, we’ll have to wait up to 4 years.  The preliminary report does, however, hint that the NTSB’s investigation will focus on whether the pilot pressed on into weather beyond what the regulations allowed.

The full text of the report is here.  Some excerpts:

Instrument night meteorological conditions prevailed at the accident site, and no flight plan had been filed.

Instrument weather conditions are those that require a pilot to fly by reference to his instruments rather than by looking out the window. To fly in instrument conditions, a pilot must be instrument-rated, his plane must be properly equipped, and he must have a clearance from air traffic control.  He is not necessarily required to file a flight plan.  For example, instead of filing a flight plan, the

pilot may have departed San Carlos in good weather and then obtained a "pop-up" clearance from air traffic control before entering instrument conditions at Pine Mountain Lake.  Nothing unusual or unsafe about that. 

A pilot, who stated that he flies to the airport most weekends, reported attempting to Cessna 510land a Cessna 510 while on an instrument flight plan, about 1 hour prior to the accident. He reported that throughout the instrument approach he was unable to identify the runway environment. He performed a missed approach, and diverted to Modesto where he landed uneventfully. He stated that he has flown into the airport utilizing the instrument approach regularly over the last few years, and this was the first time he had to divert to an alternate airport.

As discussed in this post, crash, a pilot on an instrument approach to runway 27 must "go missed" if he descends in the clouds to the minimum allowable altitude  — in this case 770 feet above the ground —  and still can’t see the runway.  Instead of going missed as required, some pilots will descend "just a little further" believing that, in just a few more seconds, they will break out of the clouds and the runway will appear before them.  Descending below the minimum altitude set forth in the instrument approach procedure is a violation of FAA regulations and a leading cause of instrument approach-related accidents. The NTSB seems to suggest that the pilot of the accident aircraft, N4175A, must have ventured below minimums to get beneath the clouds because the Cessna jet had to go missed.  However, the fact that the Cessna was forced to execute a missed approach at the airport one hour before the accident means little. Weather can change in an hour. 

The remaining two propeller blades were attached at the hub. All of the blades exhibited leading edge gouges, and varying degrees of tip twist. 

Gouges and blade twist is an indication that, the time of impact, the engine was developing power. Engine trouble can likely be ruled out.

A third witness, located 1/2 mile northeast of the approach end of runway 27, heard a low flying airplane, which he presumed was flying directly over his house, with engines running "full bore."

What was the pilot doing 1/2 mile northeast from the runway? (See image below.) As discussed in this post, the pilot should have been lined up for a straight-in approach.  And during the approach procedure, the pilot should have been throttled back for descent.  A pilot typically applies full throttle only when going missed.

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Location of NTSB Full Throttle Witness