Summmer Leaning on Approach

[Boilermonkey]

A new M20C operator/owner....   This is my first summer flying the M20C.  Twice this month in 90deg OAT on approach the engine ran rough when I pulled the power back.  Carb heat was ON during the decent, so it wasn't carb ice, I'm thinking that the mixture is too rich given the density altitude ~5000ft.  What do you think?   Should I lean it back just a bit to ensure that I don't flood or foul the engine?   Any other ideas, should the carb be recalibrated?

 

[Marauder]

A new M20C operator/owner....   This is my first summer flying the M20C.  Twice this month in 90deg OAT on approach the engine ran rough when I pulled the power back.  Carb heat was ON during the decent, so it wasn't carb ice, I'm thinking that the mixture is too rich given the density altitude ~5000ft.  What do you think?   Should I lean it back just a bit to ensure that I don't flood or foul the engine?   Any other ideas, should the carb be recalibrated?
 

Are you sure you're not running too lean? Have you seen what happens when you turn off the carb heat? I'm sure a C owner will be along shortly that may have seen this before.


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[Boilermonkey]

I should have said, in the original post.  The roughness starts when I go full rich on downwind in the pattern.

[Hank]

I routinely fly my C in 90º OATs without any problem. For descents, I do not reduce power; as I come down, the air thickens up, causing MP and EGT to rise. So I periodically reduce throttle back to whatever my cruise MP setting was, and nudge the mixture forward to regain the cruise EGT. This could happen several times if I descend from 9-10K to nearly sea level, maybe once if I'm only 2500 agl.

My goal is to level off a couple of miles from my destination, and I reduce throttle as required to slow down to pattern speed by the time I get there. It's nice to be below flap speed [125 mph] by the time I turn downwind so I can lower them to Takeoff, readjust everything including trim and drop gear to begin my descent abeam the numbers.

Never had problems running rough like this. Keeping the power on regains lost time in the slow groundspeed climb, and removes the need for carb heat during descent.

My high DA landings are somewhat rare, but we do get 3000+ DA here [326 msl]. Only went out west once, stopping at KRAP, KCOD and KGCC without any problems, using the same procedure. I was worried when KRAP's AWOS said something about high DA landings and consulting your POH. At 5000 msl, just don't go full rich in the pattern. Or is something else amiss?

[steingar]

I've flown in conditions similar to what the OP is describing and never seen anything like that.  Indeed I've never seen that in an airplane.  Sorry, this smells.  Something's wrong.  Moreover, since this is a significant safety concern (engine operation in the landing pattern) I'd get it addressed sooner rather than later.  Wish I could be useful tell you want the problem is, but airplane mechanic I ain't.

Edited June 15, 2017 by steingar

[bonal]

hello Boiler,  unless your in real icing conditions its very unlikely you need to apply carb heat.  At that high of DA I would not be going full rich. I dont go rich in fact when I pull power I am already leaned to avoid plug fouling.  But be ready to go full rich if you go around.  If you dont apply carb heat does it still run rough

[takair]

We recently had another thread on idle mixture.  When at approx 1000rpm, on the ground, smoothly pull mixture to idle cut-off.  Ideally the RPM rise is between 0 and 50 RPM.  If more than 50, your idle mixture is already rich and should be adjusted.  This should give you a good starting point for troubleshooting.

[DXB]

Agree 5000 ft DA + carb heat + low power = roughness from over-rich, assuming no issue at low DA.  

One might record the expected peak EGT when taking off full rich at 0 DA - a data logging engine monitor really helped me with this.  Then figure out what mixture gives you that EGT at full power on the ground at 5000 DA (i.e. lean to target EGT), and use that mixture setting for GUMPS when landing at >= 5000 DA. Maybe one could make a sharpie mark on the mixture control to be consistent?  That would be a more desirable mixture setting anyway at high DA for more power if one is goes around rather than lands.

BTW I always use carb heat from start of descent until short final, when I turn off at the final GUMPS to optimize power for a possible a go around.  To me it's just not worth the remote but real chance of carb ice when reducing power.  There's little cost to doing it, and I ain't gonna collect atmospheric data and pull out a chart each time to figure out if conditions are perfect for carb ice.  

[DXB]

3 minutes ago, takair said:

We recently had another thread on idle mixture.  When at approx 1000rpm, on the ground, smoothly pull mixture to idle cut-off.  Ideally the RPM rise is between 0 and 50 RPM.  If more than 50, your idle mixture is already rich and should be adjusted.  This should give you a good starting point for troubleshooting.

But I assume what the OP is describing is not roughness near idle but at a more typical pattern MP of around 15" ?   Does idle mixture make much of a contribution at those power settings?  I'm guessing most peoples planes run rough at idle with carb heat on - mine sure does and it's correctly adjusted based on your method.

[takair]

4 hours ago, DXB said:

But I assume what the OP is describing is not roughness near idle but at a more typical pattern MP of around 15" ?   Does idle mixture make much of a contribution at those power settings?  I'm guessing most peoples planes run rough at idle with carb heat on - mine sure does and it's correctly adjusted based on your method.

I suspect it it has an influence.  It is a simple check so can't hurt.  I guess it comes down to our interpretation of rough.  It can be quite subjective.  You could be quite right, that it is normal to some degree with the heat on....but his roughness might be greater than your roughness.  Wonder if we can come up with a good measure of roughness for these discussions.

[gsxrpilot]

I only put 400 hours on my C. But in that time I never had carb ice in any conditions. The only time I pulled the carb heat on other than during the runup, was on the rare occasion I used a smidge of it to get better fuel distribution for LOP. 

I'd never use carb heat. Secondly, I'd never go full rich in the pattern. I'd generally leave the mixture alone all the way to the ground. Of course, if I need to go around, the black and red knobs will go full forward together.

[Hank]

22 minutes ago, takair said:

I suspect it it has an influence.  It is a simple check so can't hurt.  I guess it comes down to our interpretation of rough.  It can be quite subjective.  You could be quite right, that it is normal to some degree with the heat on....but his roughness might be greater than your roughness.  Wonder if we can come up with a good measure of roughness for these discussions.

Mine will sometimes burble when I go to idle on short final, like an old truck. I think know of it as the sound of power ready to be applied . . . .

[steingar]

Haven't used carb heat much since I flew a Cessna 150.  I certainly would if there were visible moisture anywhere.

[Htwjr]

When I first got my C it would run rough if I applied carb heat full rich with idle power.  Now I leave it leaned for landing and only go full rich on a go around.  I generally use power on descent and rarely use carb heat. 

[gsxrpilot]

Haven't used carb heat much since I flew a Cessna 150.  I certainly would if there were visible moisture anywhere.

I've spent hours in all kinds of visible moisture and prime carb ice conditions. I've never once had any hint of carb ice. Don Maxwell explained to me when I first bought the C how the combination of the carb location on the Lycoming engine and the tight cowl of the Mooney virtually eliminates any chance of carb ice. And the years I owned the C certainly confirmed it.

[Hank]

Checking Carb Heat is on my Engine Start checklist, right after verifying oil pressure after start. I'm full rich then, nary a problem.

I rarely use carb heat otherwise . . . Not much in 10 years.

[steingar]

2 hours ago, gsxrpilot said:

I've spent hours in all kinds of visible moisture and prime carb ice conditions. I've never once had any hint of carb ice. Don Maxwell explained to me when I first bought the C how the combination of the carb location on the Lycoming engine and the tight cowl of the Mooney virtually eliminates any chance of carb ice. And the years I owned the C certainly confirmed it.

It is in the POH, but I still don't use it much.  Did do some IMC training not long ago, and did turn it on then.

[midlifeflyer]

On 6/15/2017 at 11:23 AM, Boilermonkey said:

A new M20C operator/owner....   This is my first summer flying the M20C.  Twice this month in 90deg OAT on approach the engine ran rough when I pulled the power back.  Carb heat was ON during the decent, so it wasn't carb ice, I'm thinking that the mixture is too rich given the density altitude ~5000ft.  What do you think?   Should I lean it back just a bit to ensure that I don't flood or foul the engine?   Any other ideas, should the carb be recalibrated?

 

Leaving the always enjoyable  "carb heat yes or no" argument for others...

If the density altitude is about 5,000', if you are going to full rich for landing, it's probably too rich. And, as other pointed out carb heat has the effect of enriching the mixture, so you are potentially making a bad D-Alt situation worse. If you choose to use carb heat, you should be re-leaning after you apply it.

I don't know whether that was the cause of your engine roughness, but I would expect some degree of roughness in the situation you describe.

 

 

[jlunseth]

I have a turbo, and operations are quite different from your carb NA aircraft, but I have the same problem.  The 231 (my aircraft) is typically set up to run very rich to deal with turbo temperature issues.  If I leave the mixture full in during approach to landing, I will get "burbling," which is roughness from the overly rich engine.  There is nothing unsafe about it, and you can ride the plane down that way if you wish.  But there are two other better ways.  One is just to lean back, especiallly on final when you are descending. You don't need power, and the descent will keep the engine running well even heavily leaned.  This, of course, means that in the event of a go around you need to get the mixture in first.  The second way allows you to set up for a go around, and I sometimes do it when conditions dictate.  On final, I pull the power way back.  Back far enough that during the descent I need to bring the nose up to maintain speed, and ultimately must put in power, again, to maintain my desired speed.  What this does is put the aircraft behind the curve, and behind the curve it is necessary to use more power than ahead of the curve.  This in turn allows me to run the engine full rich, becaue power is up, and sets the aircraft up for a go around.  

I rarely do it the second way, because it is no great trick to push the mixture knob in for a go around, but the second method does work.

[Boilermonkey]

OP here...I flew today and left the mixture leaned (with carb heat on) throughout the approach and landing.  The result was a smooth running engine, so the assumptions about it being too rich at low power, carb heat on, full mixture was correct.   I'll try this a few more times in these conditions.  

[3914N]

G owner here, so same engine as you. My engine also runs noticibly rough with full rich mixture and carb heat on. My personal procedure is to approach with the mixture pulled out 1". It makes the engine run much better, and wouldn't damage my engine if I needed to go to full power quickly. 

[C-FRJI]

Same here. Carb heat makes the engine run rough...lean a bit. No prob.

[Mooney1]

Always shake my head at the people that use carb heat in the summer.  Or in conditions that do not require it. WHY???!!!

 

Edited May 13, 2018 by Mooney1

[neilpilot]

27 minutes ago, Mooney1 said:

Always shake my head at the people that use carb heat in the summer.  Or in conditions that do not require it. WHY???!!!

 

If you use carb heat on all large power reductions in all weather, it becomes a routine that you are less likely to forget when the weather requires.  I've read several times about a carb icing incident in weather that wasn't expected, that surprised the pilot?  I don't need that kind of surprise.

Do you fly using a landing checklist?  Do pilots who fly behind a O360 use a checklist that says, "apply carb heat only in the following weather:  xxx temp and yyy humidity range"? 

[triple8s]

41 minutes ago, Mooney1 said:

Always shake my head at the people that use carb heat in the summer.  Or in conditions that do not require it. WHY???!!!

 

I was a C flier for several years, my previous CFI always said what a good plane it was but he always warned about what an ice maker that plane could be, so after his constant advice how I need an engine monitor I bought a UGB 16 and got it installed. One of the features is it can monitor carb temp, WELL, what an eye opener! I had always heard how carb ice could form at temps up to 70df but found it hard to believe, until the cab temp was displayed right in front of my eyes. If I still flew with a carb engine I would definitely have a carb temp monitor. If you think about it an ac unit works similar to a carb. They both evaporate a liquid turning it into a gas. This causes cooling, at any outside air temp it causes cooling. How much cooling? Get a carb temp probe and you will see.