Lean It

Set the throttle to run-up rpm--or a little higher--then lean the mixture as in cruise flight. Leave the mag switch on Both and your feet on the brakes.

This brand-new spark plug shows the gap between the electrode and the plug. Carbon, oil, or lead deposits can foul the gap, preventing a normal spark.

Your preflight magneto check can ground you even on the prettiest of flying days. But there may be a simple fix that, if it's effective, will let you fly after all.

In the spring of 1993, I had planned a flight from the Washington, D.C., area to Hanscom Field, west of Boston in Bedford, Massachusetts, to attend a work-related conference. The weather forecast called for widespread instrument meteorological conditions (IMC), although ceilings were not expected to go below 500 feet. My reservations were made, my expenses had been approved, my work schedule was cleared up for several days, and everyone knew where I was going to be. I had completed an instrument rating the previous summer, I was current, and the airplane was ostensibly just as well prepared.

The airplane was packed and preflighted, I'd been issued a clearance for an instrument flight rules (IFR) flight, and all I had to do was complete the runup. I would be remiss if I didn't add that I was, um, really looking forward to making this flight, and attending the conference. Yes, I had a serious case of get-there-itis.

Back to that run-up area, anticipating my departure for Massachusetts, I wheeled the airplane around to face the wind, assuring that the prodigious blast from the Cessna 182's mighty Continental O-470 was directed harmlessly away from other aircraft. Applying the brakes, I advanced the throttle to 1,700 rpm. I switched the ignition from the Both position two clicks over to the right magneto, and observed an rpm drop of somewhere between 75 and 100 rpm, and then switched back to Both. Then it was over to the left magneto, at which point the drop was...several hundred rpm. I was, to say the least, crestfallen; my heart sank, along with the rpm. After all, the before-takeoff checklist for this airplane stated: "Rpm drop should not exceed 150 rpm on either magneto or 50 rpm differential between magnetos."

I remember shrugging, thinking about all those plans, and wouldn't they be surprised to see me back at work later that morning? (The 12-hour drive was an option, but not one I cared to exercise.) So I taxied back to the tie-down spot, un-did all my packing, buttoned up the airplane, wrote a note in the squawk pages of the airplane's logbook, and also made a note to call the flying club's maintenance officer the moment I got back to my office.

As I was driving back into the parking area from the "airplane" side of the airport I passed our resident designated examiner, Bob Gawler, and told him what had just happened with the 182. I can to this day remember his immediate question, just as he asked it in his touch of a Canadian accent: "Did'ja lean it out?"

Well, no, I hadn't, as a matter of fact. I certainly had not known about leaning a rough-running engine to improve engine performance to airworthy status. Rest assured that I've since learned. Here's the run-down on improving your chances of a successful run-up.

If the power drop exceeds allowable limits, it isn't necessarily a bad magneto. The problem could simply be that the plugs are fouled, either by oil or by deposits of lead. (If you're lucky enough to have temperature readouts for each engine cylinder, you will even be able to identify the culprit; the cylinder with the fouled plug will be much cooler than the others.)

It is important to mention at this point that proper engine maintenance (including regular plug cleaning, gapping, and rotating), as well as correct engine operating technique (such as proper leaning procedures, as well as the smooth application of takeoff power) will go a long way toward preventing fouled plugs. But if you someday experience an unacceptable rpm drop during your mag check, here are some points that you might wish to note for future reference. This is admittedly just one small dragon; but it's a dragon with a soft spot that you might someday be able to handily dispatch with this additional arrow in your quiver.

First, don't reduce power to idle; leave the engine rpm set to the run-up level; better still, set it higher (say, 2,200 rpm or even 2,400 rpm). Even though you've set the parking brake, keep your feet pressed firmly on the brake pedals.

Turn the ignition switch back to Both, so the engine is running on both magnetos. Returning the mag switch to the Both position is important, because if you don't, you might not be able to generate enough heat to burn off the oil or carbon deposits and clear a fouled plug. (Since each cylinder has two spark plugs--the reason your run-up includes separately checking those powered by the left magneto and those powered by the right mag--you want to use one to help the other.)

Then slowly and carefully lean the mixture, just as you would in cruise flight. If you have an exhaust gas temperature gauge, lean the mixture until the EGT peaks. (If you don't have an EGT gauge, simply lean the mixture until the engine begins to run rough, and then enrich it just enough for the engine to return to running smoothly.) Doing so may burn out the excess oil or carbon deposits. You need some patience and a bit of discipline here to run the engine hard and lean, and for as long as a full two minutes, unless the cylinder head temperatures or other engine indications exceed tolerances. (Lead deposits probably will not go away with the application of power, however.)

Then return the mixture to the normal takeoff position (usually full rich), retard the throttle if necessary, and try the magneto check again. If the engine is still running poorly, you will fly again another day--just not today.

Jeff Pardo is an aviation writer in Maryland with a commercial pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,300 hours since 1989. Pardo has also flown for the Civil Air Patrol.

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