Leaning the mixture.

[FuturePilotJB]

How do you lean the mixture properly? I mean I know how to lean it in a straight level flight for best performance, so this question is more about climb/descent. I always read that during climb and descent you use rich mixture. But what if I wanted to climb to 10,000 feet. Can I start leaning it off during the climb (after 3,000 feet)? And what about a descent from 10,000 feet; rich mixture? (ofc with carb heat) before starting the descent?

Thanks for your expert advice.

[A and C]

As the aircraft climbs you will need to lean the mixture but not be too aggressive looking to keep the EGT well rich of peak. At above 8000ft a normaly aspirated engine can't make more that 75% power so no damage can be done by the leaning alone but the lower airspeed in the climb and the the reduced ability of the thinner air to cool the engine are good reasons to keep the mixture rich of peak to ensure that the CHT is maintained below 450F.

Initaly in the descent I would not bother to lean off but would start to go towards rich at about 4000 ft.

As long as 75% power is not exceeded lean mixture is not a problem so low power settings are not going to to do any damage.

[piperboy84]

I asked the same question a while back on this form, with the feedback from that coupled with reading the engine manufacturers operation manual and the "Lycoming Flyer" engine tips I came up with this for my 180HP fixed pitch Lycoming. Full power/full rich for TO unless density altitude, when climbing thru approximately 700 AGL and after retracting the flaps I power back to just under 2350rpm at 500fpm and lean it out which brings the RPM back up to 2350 for a economy cruise climb, after climbing thru 2000ft I nudge in the power 25RPM for each 1000 foot of climb which gets me to 2450 at 6500 ft, I then re-lean and if climbing higher power up to 2550rpm and keep it like that on the remaining climb and level cruise.

Seems to give me a a good compromise between performance and economy

Edit, my engine is rated for 180hp at 2700rpm (red line) with the type of fixed prop i have it rarely gets above 2600rpm in normal operations

[BackPacker]

How to lean the mixture properly depends on a number of factors. Air density, quality of the induction system, carbureted or injected, turbocharged, -normalized or normally aspirated, amount of instrumentation you have, fixed pitch prop or constant speed, ...

The engine manufacturer has taken all these into account, and the airframe manufacturer has lifted the bits from the engine manual that apply to that particular configuration, and put it in the POH. So your first and best source of advice should be the POH.

From your post it looks like you are flying a normally aspirated, carbureted engine, and most likely with a fixed pitch prop and limited instrumentation. In other words: A basic trainer such as the PA28 or C172. In those situations, the advice in the POH will basically come down to this:

1. When using full power below 3000 or 5000 feet (the number varies slightly), use full rich.
2. When using full power above said number, lean the mixture for maximum RPM.
3. When flying with any power setting lower than 75% (cruise and descent), regardless of altitude, lean until the engine runs rough, then enrich to restore smooth running.

Enrich the mixture before making any power change, lean as per the above when you are again in a stable configuration - climb, cruise or descent. So even during a cruise descent you can lean until rough running. And in preparation of a possible go-around, enrich the mixture somewhere as part of your pre-landing action.

If your engine configuration gets better (injected vs. carbureted for instance) and if your instrumentation gets better (EGT, per-cylinder EGT, CGT, per-cylinder CGT, ...) you will find the POH prescribes more refined methods. But the crude method above will already be very close to optimal.

Oh, and also note that a properly leaned engine will run with hotter EGTs than a full rich engine. That makes carb heat - which draws air from a muff around the exhaust - more effective.

[Big Pistons Forever]

I agree with BP's excellent advice. He did not address descents however so here is my 02 cents

When cruising at the higher altitudes (eg above 5000 feet) Never go to full rich before you start your descent. The rush of extra fuel will cause the cylinder head temps to rapidly drop which is a recipe for cracked cylinders.

If you are doing an A to B trip and need to descend as you near your destination, first plan a 500 foot min descent. The easiest way to figure this out is start down when your time to destination is twice the vertical distance (eg if you have 5000 feet to lose, start down 10 mins from the destination). This is dead easy if you have a GPS which is IMO a must have item for any smart pilot.

When it is time to go down simply pitch the nose down and trim for a 500 ft/min descent. The airspeed will increase which will help make up for what you lost on the climb. Leave the mixture where it is or if you want slightly enrichen it. As the airplane descends the RPM will increase so you will have to reduce the throttle periodically to maintain your cruise RPM.

[Agaricus bisporus]

if you have a GPS which is IMO a must have item for any smart pilot.

Fookinell! So "smart pilots" can't read maps then?

"Must have" - What is the world coming to?:ugh:

[therealdooga]

A GPS device is an extremely useful and quite affordable tool, especially when coupled with a moving map application.
Anyone who makes use of extremely useful tools that are readily available is definitively smart.

For me, the moving map reduces uncertainty, so it reduces stress - which is definitively a good thing when flying. The one thing to remember is to always stay aware of where on the map one is all the time, in case something goes wrong...

[piperboy84]

Anyone who does not have even a basic vfr GPS that can be bought for the price of a few tankfuls of avgas is crazy, the features they offer make for increased safety, efficiency and economy in every aspect of flying.

[A and C]

Any one who cant navigate without a GPS is a danger to themselfs and the rest of aviation, these people are only a flat battery away from serious trouble.

[Jonzarno]

Is that a reason not to carry and use one?

[Johnm]

Anyone who does not use GPS as the primary means of navigation is nuts. Anyone who *relies* on only *one* method of navigation in the air, be it GPS, radio beacons or maps and watch, is also nuts :ugh:

As mentioned above GPS has relevance to leaning because it can help with approach planning and make it really easy to work out when to start enrichening.

[flyme273]

agree with Back Packer and Big Pistons Forever

Amazing how many pilots will refuse to lean when flying below below 5,000 ft.

Teaching on this subject is very poor. Welcome the FADEC.

flyme

[flyinkiwi]

Amazing how many pilots will refuse to lean when flying below below 5,000 ft.

Teaching on this subject is very poor. Welcome the FADEC.

In my PPL Air Technology night class when we were discussing air/fuel mixtures I asked the instructor how come we have to lean manually in our training aircraft when my car's engine does it automatically 20 times a second? He didn't answer, he just smiled at me.

[Maoraigh1]

Any one who cant navigate without a GPS is a danger to themselfs and the rest of aviation, these people are only a flat battery away from serious trouble
I use a Garmin Etrex Voyager occasionally to help tell ATC where I am in relation to somewhere they know.
Otherwise it has to be "Unable Comply".
It's also useful for visits to inconspicuous and unfamiliar airfields.

[therealdooga]

In the airplane you don't change operating points of the engine nearly as often as in the car.
Once you're in cruise, you usually stay there for a while. No question, the results of manual motor management won't be even close to what a modern engine ECU does in a car, but still the mode of operation is so different that in airplanes you still get reasonably useful results if you manage manually.

[FlyingKiwi_73]

I've always been shown to use the RPM as guide to leaning and listening to the engine, i.e. pull the mixture out until you hear a tone change in the engine or a drop on the Tacho then push in slightly??. a CHT gauge is also a good guide too.

Anything wrong with this method guys? serious question for the boffins?

As for GPS i flew the 172S G1000 on the weekend and took both maps had them unfolded to the area and on my lap, PAX asked why i had paper maps I told her "if that goes black (pointing to screen) then at least i'm ready and i know where i am". plus i find the airspace heights easier to read from the sectionals than the garmin....

[flyme273]

Flying Kiwi

Further to BackPacker above, I quote from Lycoming literature, for float-type carburettor (for best economy) lean to roughness, then enrich to smooth running. Lycoming do not foresee an advantage in fitting an EGT over this method. The problem is fuel distribution over all cylinders. If you have an EGT then lean for peak EGT. However, if this results in rough operation of the engine, then still enrich for smooth engine operation = no advantage in fitting EGT.

Do not try “Lean-of-Peak” with these simple engines. Charles Lindbergh had big radial engines.

For a more sophisticated engine set up (injectors, individual EGT’s, GAMI’s) “Lean-of-Peak” is probably permitted and in deed the recommended cruise setting. One would normally refer to the Flight Manual, however in my experience many Flight Manuals do not adequately cover this subject.

While on the subject also investigate if “over-square” engine settings are appropriate. Also a legacy from radial engine operations.

I once knew a pilot who insisted full rich was better for the engine as it operated cooler (no evidence to support this) and would not be told otherwise. Utter rubbish, waste of fuel and risk of plug fouling. This pilot made a long over-water flight to an island and managed to run out of fuel 10 miles short resulting in a water ditching and loss of aircraft. I’ve long held the opinion that if the pilot had aggressively leaned, the outcome would have been successful.

flyme

[BackPacker]

I've always been shown to use the RPM as guide to leaning and listening to the engine, i.e. pull the mixture out until you hear a tone change in the engine or a drop on the Tacho then push in slightly??. a CHT gauge is also a good guide too.

You're doing just fine. That tone change is actually the onset of rough running, where one of the cylinders is starting to run too lean, and not contributing the same amount of power as the other cylinders anymore.

I regularly fly a R2160 and the induction system is such that the onset of rough running actually happens at peak RPM. The POH doesn't even have instructions for economy cruise, just for best power leaning - which is simply considered identical to best economy.

We tried it but were never able to achieve a smoothly running engine when running leaner than peak RPM.

As far as the CGTs are concerned, yes, they are a good guide, but you've got to take into account that they lag behind tremendously. So if you want to use CGTs for leaning, make a slight adjustment, give it a minute to stabilize, and then make another slight adjustment. I have seen people using precisely this method in the climbout. The engine was doing 100% power (0' AMSL), with full throttle and max RPM (this was a C/S prop). They were carefully monitoring the cylinder CHTs (per-cylinder CHT indicator) and were leaning until the hottest cylinder ran at - from memory - 450F. But it took them a few minutes to reach that setting. They also had the advantage of having a fuel injected engine. It's one example of getting a more sophisticated engine and engine monitoring, allowing you to do more sophisticated leaning.

Note that with that method you're not reaching a stochiometric mixture. You're still using an overrich mixture to help with engine cooling - and since the engine is producing more than 75% power you need that help. But you're running less rich than full rich, so you are already saving some fuel.