O540 backfire on low power approach?

DesertNomad

DesertNomad

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DesertNomad
I flew with a friend today which was his first time using a noise canceling headset and he has not flown much in a long time... but has about 2500 hours from the 70s.

During approach into an airport at 4200' today, on the base leg, he thought it sounded like the engine was backfiring during power reduction. It sounded totally normal to me, so it could be an artifact of the headsets but who knows?

The mag check on the ground is very good - drop of about 125 on both sides and very smooth. The single drive dual mag was serviced 10 hours ago. He says it is common to hear some backfire on bigger engines (235hp, 300hp etc), but his experience is from the 70s flying without headsets.

I had it properly leaned for the altitude we were landing at and it all seemed normal to me, but do you folks have any comments on this?

I did notice my idle was a bit slow this morning (it was 35 degrees) and idle was about 500rpm if I pulled it all the way back.
 
You had the prop driving the engine with the throttle closed down and mixture rich, this causes the cylinders to overload on fuel and the engine to produce after fires, if you are real rich an pushing the engine hard with the prop, you can even load the muffler up with raw gas, and on cracking the throttle open a bit add the oxygen necessary to explode the muffler. That's always cool.

The solution is to lean hard for descent and pull the prop all the way back to limit the ability of the prop to drive the engine. Also planning your descent further out helps as well.
 
How quickly and how far did he pull the power

I've seen a few of the big bores do that when someone ham fists the throttle back for landing.
 
Mixture was about halfway in I guess and this was in the base leg - MP around 11 or 12" prop mostly forward (full forward on short final). Would you really have the prop pulled way back on base?

I started pulling power back about 14 miles out - an inch every 45 to 90 seconds. 500fpm all the way down to pattern altitude. I flew downwind at about 15" and base around 11-12" which was held until very short final where it goes to about 10", then idle over the numbers.
 
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DesertNomad said:
Mixture was about halfway in I guess and this was in the base leg - MP around 11 or 12" prop mostly forward (full forward on short final). Would you really have the prop pulled way back on base?
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I don't push the props forward until I have reduced the power enough that the props come down off the governor, that way the RPM/power and noise doesn't surge. Where that happens in the pattern depends on the speed and style of the approach, anywhere from abeam the numbers to short final.
 
For cruise my prop was at 2200 and it stayed there until short final.
 
DesertNomad said:
For cruise my prop was at 2200 and it stayed there until short final.
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Next time you shut down from idle, watch your tach, you should see about a 75 rise in RPM. If you don't, your carb needs to be inspected/adjusted. If that's ok, have the timing checked including the points & internal timing.

If that all looks good, it's just an artifact of big Pistons and carburation.
 
Cut power by leaning and see if it stops the backfire. IF it does, the engine is just backfiring because its rich and being throttled down. Not much you can do about it except lean it. My carbed 0360 does it (some). Doesnt if I lean the heck out of it. Doesnt if I cut power REALLY really sloooowwly...
 
My engines always pop when I pull the power. In my 540 biplanes I would enter the pattern at 25/25 setting usually, haul ass on downwind (165 mph plus) pull the power to idle (pops like crazy) Start my base to final turn when even with the numbers and land. I have never started reducing power 14 miles out???? That must be some boring flying :)
 
Yeah, I am boring.

My friend who heard it did describe it as a few pops... It certainly was not a "bang". When I took off from Reno, I leaned for best power during the run up and made a note of that position. After flying for an hour 4000' AGL with it leaned back beyond the t/o mixture, I started the descent. At about 500 to 1000 above TPA, I returned the mixture to the t/o setting we had leaned to back in Reno as I was landing at basically the same elevation.

It is possible that I just pulled back power too quickly at some point, but I try to do everything slowly. When would you typically enrichen the mixture for landing? I tend to do it a bit above TPA and then move the prop forward after turning final.
 
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Lycoing specified a really rich mixture for the O-540 when at full rich. It makes the engine stumble like that, especially if the carb heat is on and the throttle is reduced. The POH will tell you to lean it when above 5000' DA for smoother operation, even for takeoff.

I once talked to the guys at Precision Airmotive, the guys that used to make the HA-6 carb on those things. He told me that Lycoming had specified that rich mix and that the carb wasn't defective. We had an R182 (182RG) that did all that popping and stumbling in the circuit unless it was leaned some. In a long climb it would begin to smoke as altitude was gained. Way too much fuel.
 
My O540 doesn't pop like that as far as I've ever noticed. It's turbonormalized but at low power that shouldn't really matter. I do not richen it very much for landing, though, so that might be part of it.
 
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