The Oracle @ Delphi #1 1/31/06 - On the Wings of Mercury (Induction Leak)
I was very fortunate to be able to participate in the pre-purchase inspection of a 1966 Cessna Cardinal (C177) this last weekend (28Jan2006). For those of you who have never had the opportunity to fly a C177, it's similar to a Cessna Skyhawk (Better know as a C172). The main differences are that the C177 engine has 180 horsepower (compared to the C172's 150-160 horsepower), and the propeller is a constant speed (variable pitch) propeller.
With the exception of a moderate amount of light surface corrosion (on most internal surfaces) and a small amount for moderate corrosion on the top of the spar where the skin is attached, the airframe was in fairly good condition. Most general aviation aircraft of this vintage were considered disposable, therefore most were not internally treated with any form of corrosion protection such as Zinc Chromide.
During the portion of the engine inspection performed with the engine running, the Mechanic With Inspection Authorization (I.A.) noticed when the throttle was returned to idle after running up the engine, the manifold pressure would initially show a near vacuum then creep up to about 10 Inches of Mercury (in.Mg.). For those of you who are less familiar with a constant speed propeller ,remember, the manifold pressure is used as an indication of how much power the engine is producing. The more air available to the engine intake (at the manifold); the more power the engine will produce. This will lead to a greater in.Mg displayed on the gauges, and less vacuum! A great web resource for more detail on this type of problem is available in the article "Pelican's Perch #15: Manifold Pressure Sucks!" on http://www.avweb.com. The successful diagnosis of the problem showed that somewhere after the throttle butterfly valve, outside air was being sucked into the engine through a leak. This lead to less vacuum, and the manifold pressure gauge was showing more in.Mg. The engine was not producing the power indicated by the manifold pressure because this air leaking into the manifold did not go through the carburetor, effectively leaning out the fuel mixture. Due to the leak, the in.Mg level no longer correlated properly to the power output of the engine.
Normally, the story would end there, however, someone with a bit of foresight decided at some point in the past to install an engine monitor with probes for each individual cylinder head temperature and exhaust gas temperature. This non-standard piece of equipment allowed a more in-depth look into the actual state of the engine. After discovering this induction leak, the engine was brought to idle, and then leaned out. As soon as the extra fuel (that the engine was using for cooling) was removed, a disproportionate rise in temperature of the number 1 cylinder was noted. The conclusion is that the leak must be specific to the number 1 cylinder because the number 1 cylinder was running leaner than the other three which also results in it running hotter. At a lower power setting such as idle the engine is not producing sufficient heat for any of this to be a problem, however, if this airplane was running at a high power setting (say around 75% HP) and properly leaned out (Say 50 Degrees Rich of Peak) the number 1 cylinder could have exceeded it's critical temperature, resulting in an in-flight engine failure.
Being able to observe first-hand exactly how each control change affected each cylinder individually (as opposed to an average or a single cylinder as is more common in engine monitors, if there is one) emphasized the importance of this type of engine monitor. This is the only way a pilot can really know what the engine is doing on a routine basis during flight. The two major players in the market are J.P. Instruments (http://www.jpinstruments.com/) which markets the EDM series and Insight Avionics (http://www.insightavionics.com/) which markets the GEM Series.
A careful inspection, as well as the installation of a GEM engine monitor with individual probes for the cylinders has provided the prospective buyer with a wealth of information on which to make the final decision.