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Perhaps you should have considered that the aircraft most likely experienced partial or complete induciton icing (or some other transient induction/butterfly obstruction) which created Compression Braking due to the spinning prop. This would have drawn down the MP to a negligible reading, i.e. zero +/- as well as caused a decrease in thrust and the subsequent loss of altitude.
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I did consider that possibility. Given the lack of visible moisture, time of year, desert location, and impossibility of induction ice in the injected airplane under those conditions, I discounted that possibility. However were the induction to be partially blocked, manifold pressure still would not roll to zero. Piston engines 101:
Manifold pressure is a function of a)ambient air pressure, or current real barometric pressure, or b) boosted pressure by turbocharging, supercharging, or both, or c) suction by the engine which has drawn manifold pressure down to a lesser value.
While it's not done with most horizontally opposed recip engines these days, part of the checking on a piston engine should be idle manifold pressure checks. Most are, or should be, familiar with idle mixture checks, and idle RPM checks, idle manifold pressure is part of the equation. Most engines cannot drop below about 12" of manifold pressure except for a complete occlusion. In such cases, the engine can't continue to run, and still won't show zero inches of manifold pressure. In fact, in cases I've seen in which the induction manifold failed, or in cases of icing occlusions, I've never seen anywhere remotely close to zero manifold pressure. I've seen manifolds collapse and induction doors and alternate air doors come loose and plug the manifold, and still it doesn't drop that low.
What the pilot saw wasn't what he actually viewed; he saw what he expected to see, which was a manifold pressure rolling back to zero. In truth, whereas a turbojet aircraft (or the advanced tactical fighter he had been flying previous to this airplane) don't have manifold pressure gauges, but instead deal with other parameters such as EPR, various spool RPM's, and turbine interstage temperatures or exhaust gas temperatures. He saw what he thought he expected to see, not what really happened.
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The fact that someone misinterpreted the event as an turbo failure would be consisent with a non-mechanic, but to assume that the MP can't roll back to virtually nothing is I beleive an errant review of the possible events.
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The problem is that the two individuals who interpreted the event as a turbocharger failure were both mechanics, assigned to the airplane. The individual who was flying interpreted the event as an engine failure. None of the three individuals were correct. The event was a partial power loss due to improper use of the mixture, and subsequent spark plug fouling.
The "possible events" as I described them are the actual events, as I not only interviewed each person involved, but examined the aircraft, ran it up, and flew it. I knew exactly what had happened.
The tendency for pilots with advanced aircraft experience to underestimate light aircraft is a dangerous and all too common one. Some months prior to the event above, an individual had a double engine failure. He lost power on one engine, switched tanks, and shortly thereafter lost power on the other. No checklists were used, before, during, or after the flight. When I queried the matter, I found that the pilot was insistent that the mechanics has "misrouted" his fuel. I took that to indicate that he thought the mechanics had somehow routed his fuel lines improperly. I checked with the mechanics; they'd done nothing to his fuel system, other than move the fuel tank selctor valves during maintenance.
Checking with the pilot, and his superior I learned that neither used the checklist for preflight, before takeoff, takeoff, after takeoff, cruise, descent and approach, landing, after landing, or parking. No checklist use. After all, they said, it's a light airplane. They were experienced in far more complex and demanding aircraft; these airplanes were like toys to them.
What actually happened was that the fuel selector positions weren't checked prior to takeoff. One engine was run dry, and shortly thereafter, for reasons unknown, the individual switched to the dry tank and killed both engines. He landed without power. He sharply blamed the mechanics for moving the fuel selectors, but placed no blame on himself for failing to use the checklist or verify his fuel routing before takeoff.
The light airplane can kill you, but just barely. So they say.