Do you prefer a video explanation? See this superb summary from Martin Pauly; it's so clear and so well done that it's added here 15 years after the original article was published. It's 25 minutes, but absolutely worth the time.

Keeping it simple... Sometimes we make aviation techniques harder than they need to be. So, if you don’t have the JPI data-trapping engine monitor....or even if you have only the standard factory nozzles... this may help:

The following correspondence came from our internet forum of Beech Baron and Bonanza owners in response to a question from David, a Baron C55 owner (IO-520CB's 285 hp engines with GAMIjectors and a factory single-needle EGT gauge). His GAMIjectors were installed but not further adjusted as he has no engine instrumentation with which to have done so. He was worried about how to try running on the lean side of best power without a full engine monitor.

I said: “Hello David, If you are above 8000 feet MSL there is NOTHING you can do with the mixture levers that will hurt the engine. The altitude alone will protect the engines as you can not possibly develop more than 65% power. Remember the information that came with your GAMIjectors? It says something like:

"CHT must be above 450F AND POWER must be above 65% for detonation to occur. No aeronautical powerplant engineer differs with that statement."

“So - even with the factory single needle EGT instrumentation - at altitude, get set up for cruise, then pull the mixtures back till the plane slows just a bit...you can easily feel it...you HAVE to be LOP at that point and you don't need any gauges to do that. A friend of mine did it successfully on his first ride in my JPI-equipped Baron with his eyes literally closed. He came within 1/2 GPH of being perfect on his first try, then nailed it the second time. It's easy. Good luck” -Fred

From David: “Thank you for the FIRST real world tip on what to do.” -David

Later, David was discussing whether his injectors were properly balanced, (he could not be certain because he did not have a full engine monitor). He asked:“...well, I don't know that my injectors are properly balanced, except that the engines run smooth when lean of peak.” - David

“Old Bob” Siegfried (retired UAL and a Lifetime Achievement Award winner from the American Bonanza Society) commented: “Good Evening David, If you can lean it enough to get at least a ten knot drop in airspeed before they get rough, the fuel/air distribution is plenty good enough to run lean of peak EGT. That is how we checked in the days before we had all the fancy instrumentation!" "Old Bob” flew heavy piston airliners for a living, and has also attended the Advanced Pilot Seminars in Ada.

“If it does run smooth that far lean of peak EGT, all you need to do is lean it enough to ascertain a knot or two drop in airspeed and it will be just fine. Fact is, you don't even have to be able to hold a stable airspeed. If the air is too rough for that, just lean one engine at a time. Lean one engine until you just note that it has lost a little thrust. (The airplane will then be slightly out of trim). Then lean the other engine until the aircraft is back in the same trim it was before you started the leaning process. In the old days, we called that power leaning.

“No need to look at the EGT gauge at all.

“Remember, it is a two part process. Leaning for a ten knot drop in airspeed should be done as an occasional test to determine whether or not the mixtures are still well balanced ... to discover any problems such as a plugged injector. It is not necessary to do the ten knot test every time you fly. Once you know that [all is well], there is no reason to run that lean unless you just prefer to.

In routine flight operations, if you lean until you can feel a little power loss, you will be on the lean side of best power. As long as you are above 8000 feet, you can even run at best power (80F ROP) all day long with no trouble at all. If you do have good CHT gauges, it would be best to lean it enough such that the heads stay below 380 F.

“Parenthetically, 6500 MSL is probably OK with a conforming engine. But if the timing is off or the ambient temperatures high, 6500 MSL may be marginal. At 8000 MSL there is no possibility of problems at all. So, 6500 MSL is the absolute minimum altitude that this old crude method should be attempted, but at 8000 MSL or higher, you can do almost anything you want!

“The primary advantage of all the fancy engine monitor instrumentation is for your education and to help in trouble shooting the engine if things are not going well. If everything is working as it should, leaning by feel works fine.” -Happy Skies, Old Bob

From Tom, in a Beech Debonair, who asks: “Would the 10 kts idea be the same for a Debbie also, or is a smaller airspeed reduction appropriate?” -Tom

From Old Bob: “Good Evening Tom. Same deal. Remember, the more engine instrumentation, the better. Without a full monitor, the ten knot reduction is just a way to check things. It was all we had fifty years ago, but if the engine gets rough, we have no way of telling why. With a full monitor installed, we can quantify things much closer and we can tell where to look for the troubles.” -Happy Skies, Old Bob

So, where did "Old Bob" Siegfried learn all this? He said “My comments are directed primarily to procedures used with General Aviation airplanes that are not as well equipped as the later piston airliners. I first started running my personal aircraft on the lean side of best power after I was exposed to such operation by my early pre-airline training.”

This power leaning technique in a twin was first shown to “Old Bob” by his boss when he was flying Bamboo Bombers before he went to work for UAL. After hiring on at the airline, he learned that it was also used by the older, prewar, UAL DC-3 captains, but was not taught by the airline company or generally used by the WWII trained pilots. These power leaning techniques were primarily used on engines that were not very well instrumented and then only by pilots who had a bit of experience and training well beyond that supplied by the military or UAL.

Any questions? just give me a call, or click on the e-mail link below