spark plug fouling and alternate static source questions

[pilotod]

My 172R manual says that engine roughness could be spark plug fouling and to test the mags. The manual says to correct it by leaning or enriching after a few minutes but it didn't say if I should keep it on the mag that produced the roughness. Should it stay on that mag when leaning or enriching or put back to both before leaning, etc?

The alternate static source airspeed calibration chart is confusing to me. If it says 50 normal KIAS and 51 alternate KIAS does that mean that 51 with alternate is the same speed as normal 50? I've never tried the alternate static source.

 

[denverpilot]

My 172R manual says that engine roughness could be spark plug fouling and to test the mags. The manual says to correct it by leaning or enriching after a few minutes but it didn't say if I should keep it on the mag that produced the roughness. Should it stay on that mag when leaning or enriching or put back to both before leaning, etc?

Mags on Both. You need the "fire" in the cylinder to hope to clear the fouled plug with leaning. There's no fire when you're set to the one that's fouled. You might get lucky, you often will be taxiing back and having a mechanic pull and clean the fouled plug.

(You can also have my experience where I thought I had a fouled plug and really had a plug wire break off. That was a lot of vibration and leaning that led to nothing but taxiing in and hunting for a mechanic.)

The alternate static source airspeed calibration chart is confusing to me. If it says 50 normal KIAS and 51 alternate KIAS does that mean that 51 with alternate is the same speed as normal 50? I've never tried the alternate static source.

Yes. The airplane will be flying the same real (Calibrated) airspeed for the two different indicated speeds. Why not go try it? See if you can see the difference.

It's relatively difficult to see a one knot difference on the ASI without staring at it. If you're perfectly trimmed and stable, maybe you could see that. The chart is telling you something more subtle: That the ASI reads slightly higher when you're utilizing the Alternate Static source.

Thus, you're actually moving just a little slower than you think you are. Look for areas of the chart where they diverge more than 1 knot. At those speeds, is say a 5 knot difference important?

 

[denverpilot]

P.S. I'm assuming that's an on-the-ground section of the manual. Engine roughness in flight, use the mag that gives the least roughness. Ask Clark about that one.

 

[Clark1961]

P.S. I'm assuming that's an on-the-ground section of the manual. Engine roughness in flight, use the mag that gives the least roughness. Ask Clark about that one.

I vote for using the mag that keeps the engine running. Actually, I kept it on both in the vain hope the failed mag would magically repair itself. It never did and I had to pay Saturday mechanic rates...

 

[denverpilot]

Good point. A dead engine is reeeeeealy smooth.

 

[alfadog]

Following my top overhaul, I had a bit of trouble with one plug fouling if the engine was not leaned aggressively on the ground. It would not clean up on run-up so I asked the tower for a high-speed taxi pass on the active. That did it Had to do this on a couple of flights but the rings are seated now and no worries.