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#1
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Quote:
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. |
#2
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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. Quote:
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. |
#3
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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 ".
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. Last edited by Roger : 01-10-12 at 10:44 PM. |
#4
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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. Quote:
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. |
#5
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"Condescending"? Now that's funny
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