"Shock cooling"

[Roger]

Originally Posted by sns3guppy

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.

[sns3guppy]

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.

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.

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.

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.

[Roger]

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 ".

And as for any inference that a piston engine can not " run" at less than 12"MP, once again this misses the point. If an aircraft is "flying" and the prop is spinning, engine is "running" but not producing power . Thereby the MP can easily indicate less than 12" of merc, provided that the induction side is somehow constrained.

And / or it wasn't powerful enough.

[sns3guppy]

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.

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 ".

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.

[Roger]

"Condescending"? Now that's funny