Questions on proper leaning.

[M_LIEBERMAN]

I have had my liscence for about 1 year. I tend to like to understand what is going on with things I use. I have asked several experience pilots, read in several books, read in several magazines, and asked a mechanic, what is the proper method to lean an engine. I have a 1976 Cessna 150 (2745J).

For general info, the plane is often flown at 2800 to stay above class D airspace and below class B airspace.

The POH states that at or below 55% power, you lean to peak and lean 50 RPM additional.
(The plane is most often flown at 2800 feet at 2400 RPM, which fits this profile.)

Most pilots do this, or lean to 50 RPM rich of peak to keep EGT down. Many pilots do not lean at all until 5000 feet. I have an EGT guage and the temp does rise a small amount when leaned as I do.

The mechanic says that leaning at all at these altitudes will ruin the engine. This directly contradicts the POH.

I am very familiar with the damage that can be caused by running TOO lean for TOO long, however, if I am running richer than the POH says is proper, I am confused by the mechanics statement that I may be ruining the engine.

I would like to hear from anyone who could spread some light on this subject for me.

Thax,
Mark Lieberman 74040,1537@compuserv.com

[HighFLyer]

On leaning, the most common thing I have heard is to only lean when your density attitude is at least 3000 to 4000 feet. And this is only necessary when cruising for long periods of time. A short trip at 3 to 4000 does not really nessicitate leaning, and the chance of accidently leaving it too lean can become a serious danger.

What you heard about running an engine too rich seems a little strange to me. As far as I know, the only danger of a too rich engine is fouled plugs, and if you are running 100LL this shouldnt be a problem. Your fuel burn might be a little over what is expected if you dont lean out your engine, but this wont hurt at thing.

My advice is to lean out your mixture any time you plan on flying above 3000 feet for an extended period. Or if the temperature is extremely high and the air is very dry, then you may benefit from leaning at lower altitudes. Hope this helps.

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The sky's not the limit, its a Playground!

[HighFLyer]

:0

[This message has been edited by HighFLyer (edited 08-24-1999).]

[This message has been edited by djschaut (edited 09-12-1999).]

[This message has been edited by djschaut (edited 09-12-1999).]

[HighFLyer]

[This message has been edited by HighFLyer (edited 08-24-1999).]

[HighFLyer]

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[This message has been edited by HighFLyer (edited 08-24-1999).]

[This message has been edited by djschaut (edited 09-12-1999).]

[djschaut]

The question of leaning is of critical interest to the owner of the aircraft. Improper leaning can cause a premature overhaul or top end.
If you run your engine too lean at high power settings, your cylinder heads heat up too much and valves can burn or chip--become unseated properly. On the other hand, if you run the motor too rich, you can get carbon deposits on the valve seats from improper combustion. On a turbocharged engine, you typically lean to 50 degrees less than the manufacturers recommended TIT, or turbine inlet temperature. This is similar to exhaust gas temperature. This usually runs around 1550 degrees. However, I recently saw a graph on an injected Continental engine that recommended leaning 25 degrees less than peak EGT for lower power settings, but enriching the mixture to achieve lower temperatures at the higher power settings, such as 75% power. At altitude, you don't need as much fuel mixed with air to achieve a similar mixture ratio as at sea level, because the air is thinner. However, especially with a turbocharged engine above 10,000 feet, or expecially at the flight levels, the engine burns hot and any slight mistake in mixture control can burn valves. Less air is flowing over the cylinders, so cooling is less efficient. The point at which a turbocharged engine reaches maximum power for the amount of air available and then begins to show increased cylinder head temperatures for higher power settings is called its 'critical altitude.' For the higher altitude turbocharged airplanes, the only way to adequately know what the proper mixture setting is would be to install a graphic engine monitor or GEM, which shows cylinder head temperatures as they change with different mixtures. Right now, Continental is working on a throttle system for its turbocharged aircraft that will automatically adjust the mixture to keep cylinder head temperatures within a specified range for specific power settings. There is no mixture control. It uses computer chip technology to precisely control fuel mixture to extend engine life.
So, you are thinking and asking questions about a very important issue regarding the life of your engine. Look at the leaning requirements for different power settings at different altitudes for your airplane. You may notice a requirement to enrich the mixture--cooling the engine--for the higher power settings at higher altitudes. The best test would be to fly it according to the POH and have your A&P inspect the valves for either wear from heat or carbon deposits from running too rich and modifying your mixture habits from those results. Your A&P should be able to scope your engine and visualize the valves. At the altitudes you are flying, the A&P you've already contacted is probably right--lean to 25 Degrees below peak EGT for all power settings. But when you take the bird up to 10,000 feet, your POH may want you to enrich the mixture for the high power setting you'll need to get it up there. You're beginning to experience the fun of flying--temperatures, flows, pressures, airspeeds, and the effect on engine life. The fact that you're thinking about it is most important.

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DJSchaut