Thread: "Shock cooling"
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Unread 01-08-12, 11:27 PM
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Quote:
Originally Posted by sns3guppy View Post

I was once told by a former military aviator that he had just experienced an engine failure in his skymaster. When I asked how he knew he had an engine failure, he told me that he was in cruise in the "jet" (this is how he referred t the skymaster), and stated that he saw the manifold pressure "roll back to zero," and that this is how he knew he had experienced an turbocharger failure, and therefore an engine failure. He stated that the aircraft lost 300 feet, and there was the proof.

I was somewhat bewildered by the accounting, as manifold pressure can't roll to zero, nor would a turbocharger failure result in an engine failure (except in catastrophic circumstances which become immediately apparent), nor would a turbocharger failure result in what he experienced, or thought he had experienced. I had never heard of a 300' altitude loss representing an engine failure, but he was absolutely convinced that he'd experienced an engine failure.

I was asked to investigate the matter, consult with the mechanics assigned to the airplane, and then to ferry the airplane a short distance for an inspection. Setting aside the notion that ferrying an airplane with a failed engine might not be the brightest act of the day, I sought out the mechanics associated with the airplane, and these rocket scientists informed me that the airplane had a failed turbo, because the manifold pressure had been reported to roll back to zero.

I tried explaining to them the concept of barometric pressure and that manifold pressure couldn't have gone below that with a failed engine, but they didn't seem to grasp the point. I had a fairly good idea what really happened, so I loaded up both mechanics, and taxied to a run-up area. Exactly as predicted, the airplane started and taxied fine, but had roughness on one mag, and this was easily cleared up during a power run. The ex-fighter pilot had been flying with the mixture rich all the time, fowled the plugs, and had a rough engine and power loss. The manifold pressure never rolled to zero; it was the action he thought he saw because it was what he expected based on his turbojet experience; he saw what he wanted to see. Not at all an uncommon experience.
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.

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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