Clearly you were there, and I wasn't, weren't you?
This particular smart-ass fresh-from-the-jet expert didn't see zero and he didn't see bottom of the gauge. He didn't see anything but what his imagination told him was there, and my assessment of his accounting and the event, my subsequent investigation, and examination and operation of the aircraft proved it to be the case, which is why I presented it as I did. I noticed, incidentally, that you weren't there.
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Piston engines 101-a : When a pilot says his aircraft manifold pressure goes to "zero" on a gauge that only goes down to 10, he is really saying that his MP went to what amounts to .33 bar's. Well within the probability of compression braking caused by intake obstruction. Zero is a euphemism for "bottom of the gauge ".
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Investigation 101: witnesses often make the worst witnesses. Often five people who see the same event see five different events. Ten see ten, and so on. I can guarantee that this witness, the pilot involved, didn't see the manifold pressure indicating at the bottom of the gauge. He couldn't stop referring to the piston powered airplane as "the jet," and was quite condescending in his accounting. He told me that "I was at XXXX feet in the jet when I saw the manifold pressure roll to zero." When asked how he knew he had an engine failure, he told me "The jet lost 300'." I asked what else indicated to him that he'd had an engine failure, and he told me that was it: the airplane lost 300' and wouldn't recover the lost altitude. He returned to land.
Icing was not a consideration. Induction obstruction was not an issue. Engine failure was not an issue. He had a partial power loss, and in fact landed under power.
You seem very hung up on the issue of manifold pressure, but the issue at the time, particularly for the mechanics, was the insistence that a turbo failure would cause manifold pressure to drop to zero, when in fact that is entirely inaccurate. A complete power loss with a closed throttle plate does not equate to zero, or even bottom of the gauge. This is particularly true at the speeds the airplane was operating, and the resultant windmilling force on the propeller and RPM. High speed was not an issue, nor was high RPM, and manifold pressure in a windmilling piston engine is a function of windmilling RPM and throttle position.