Leaning the mixture in the climb
 

When flying with my mixture leaned in the PA 28 warrior 151/161s I fly, lets say at FL55 and I wish to climb to FL75 should I put the mixture back to fully rich for the climb or will that make the engine inefficient at this altitude. Likewise when decending at what point is it best to return to fully rich?

Will be interesting to hear the views of the experts (and there are always plenty of them on any subject relating to leaning!) But, for the record, this is what I was taught, and it sounds sensible to me:

During the climb, select full rich. This may not be the most efficient way of operating the engine, as you say, but at high power and low airspeed you want all the cooling you can get, and that overrides the efficiency concerns. Lean after levelling off.

During the descent, select full rich. Failure to do so will result in the engine running too lean, and this will cause damage. Lean after levelling off, if appropriate.

FFF
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I, too, would like to see expert opinion on the question of climbing at the example levels with the mixture lean or not.

Certainly, as far as descending is concerned, I have been taught to restore the mixture to fully rich, then descend.

The trouble is that this type of aircraft is probably not over-equipped with engine instrumentation.

The full rich mixture is set to provide enough fuel to retard the combustion event sufficiently to cool the engine at full-power sea level conditions.

When climbing (say 5000ft) the non-turbocharged engine cannot make full power anymore, so less rich mixture is required. However in the absence of a JPI engine analyser this is going to be difficult to judge how much to lean.

The trick is to gradually lean the mixture during the climb to maintain a constant EGT temperature. This will provide better climb on less fuel since too rich mixture will reduce horsepower and waste fuel.

Have a look at this presentation to understand the concept.

http://www.advancedpilot.com/leaning.pps


In descent, keep the mixture lean, not full rich, to avoid lead fouling of the spark plugs. It is perfectly OK to go to peak EGT in descent.

You'll probably find that in the manual (or on the back of the sun visor) it says something along the lines of 'don't lean the mixture at more than 75% power'. You'll also probably find that a normally aspirated piston engine won't develop 75% power at that height so you'll be OK to leave the mixture lean (it will richen as you climb). The problem could arise when you descend if you forget to richen the mixture before adding power on levelling off, as lower down the same mixture setting will be too lean and high power settings could cause nasties.

"RTFM

= "Read the FLIGHT (!) Manual"...yet again.

Engines vary.

UV

There is no problem with leaning in the climb. Taking off from some high altitude fields requires leaning before take-off!
You can find the altitude at which the engine is no longer able to develop 75% power in the engine handbook.

If you have a Cylinder Head Temperature gauge you set up a stable cruise and just lean until it peaks. This is the setting you want.

With an Exhaust Gas Temperature gauge the technique is almost the same except that you want to increase the mixture again to 25 degrees F rich of peak temp. for best economy or 75 dgrees F rich of peak temp. for best power.

It is easier to understand the process and thereby get dispell the myths and mysteries if you remember that all the fuel which doesn't get burnt (used economically) is used for cooling. Ergo, the point at which there is no excess fuel in the combustion chamber means there is no fuel for cooling and the highest temperature will be achieved.

Please read this column, and look very closely at the graph called 'Landmarks..'

This graph is straight from the engineering sections of the LYC-TCM-Wright-P&W handbooks.


http://www.avweb.com/news/columns/182583-1.html

Miserlou

You might lean for peak EGT but definitely NOT for peak CHT!

Hmm, thats a new one. Close the cowl flaps, bring the mixture back and see how hot you can get those cylinders :D

I would be inclined to go full rich in a climb, but I suppose this depends on altitude and temperature, yada, yada, yada. As mentioned, sometimes you lean before take-off, so if you're at an altitude where this is nescessary, then its probably best to lean a bit in the climb, assuming you need full performance. This depends on how hot the engine is getting though, if its getting hot, go a bit richer to help it cool down....

Just my view, I'm not sure there is a definitive "right or wrong" in this discussion.

EA

It is almost impossible to hurt most standard carburated normally aspirated small flat engines by overleaning, especially below the 75 percent power setting. They will start to run rough due to poor fuel distribution before you are lean enough to hurt anything. So if you lean untill roughness and enrich untill smooth (plus maybe enrich just a touch more for good luck );) you will be fine. You will also get quite a noticable increase in power especially at higher altitudes. IMO the real danger is overheating in long climbs at low airspeeds. Since most little airplanes have :mad: engine instrumentation you won't know you have cooked the engine untill the expensive repair bills start:{ This is especially true with the Continental 0-470 series. My Flying club had a 182. It required replacement of all the cylinders twice:*
to make the 2000 hour TBO. Most cylinders removed displayed signs of overheating and or shock cooling cracks.

IO540.
I've checked it up. "The Advanced Pilot's Handbook" by William K. Kershner, page 166. Apparently, you do. If you don't agree, please give some indication as to why.

Obviously you don't exceed any stated limit but common sense would dictate that you don't just accept messages on these boards for being kosher, definitive answers.

Just to lighten the serious discussion with a small anectode if i may :O

During training, my instructor had told me of a small incident when he was flying circuits at night with a student. On the downwind leg, the student was doing his pre-landing checks and had accidentally pulled the mixture knob completely out instead of the carb heat knob :eek:, upon realising he immediately put it in again without any reaction from the instructor ;)

P16

I use the constant EGT technique
 

Since I read one of PelPerch's articles, I've been using John's leaning technique.

Just after departing, I set the actual EGT as a target. As the plane climbs and the EGT goes down, I lean a bit to mantain EGT at the target.

I think there's no problem with climbing with cruise mixture, unless you are cruising at peak CHT mixture (around 50-75 ROP).
Above a certain altitude, my engine cannot be overheated at any mixture setting, I think, so I go as lean as possible. If the climb rate is not good enough, usually above 8000ft, I just increase the RPM (I fly an aspirated Bonanza).

My Flight Manual says climb fully rich to FL050 and then lean enough maintain smooth running. Personally, I would never use advice that contradicts the Flight Manual.

Hmm, I've done that during my early hours :O

Miserlou

I accept under constant conditions there is a good correlation between CHT and EGT but while EGT gives you instant response to leaning, CHT has a big time lag on it, and anyway is hugely affected by the airflow speed.

To lean by CHT, you would need to be in stabilised cruise, or a stabilised cruise climb/descent.

If one is going to spend money on a CHT gauge, one might as well spend money on an EGT gauge. Or get an EDM700 and do it properly :O

Thanks, IO.
Fuel flow meter woiuld do the trick too.
We only used to use that on the Cardinal. Temps were only checked for proper readings.

Old timers method..works great for non-turbo

To lean at cruise: switch to one mag, adjust mixture for start of roughness, enrich until engine just starts to run smoothly, switch-on both mags.

EGT runs about 50-75 deg cooler (richer) on two mags vs one
mag...ergo you will be just on rich side of peak.

Guys/Gals:

The following link to John Deakin's fabulous article answers all your questions on leaning during the climb.

http://www.avweb.com/news/columns/182176-1.html

JD's technique of noting the EGT on take-off (assuming the engine has been properly leaned, before the takeoff roll if at altitude); and then maintaining a constant EGT reading by gradually leaning every 1000 or 2000ft until the FTH limit (about 8/9000ft) works extremely well (talking non-turbocharged engines here). It also means that by the time you have reached cruising level the aircraft is essentially correctly leaned and, if the climb has been protracted, you have also made some significant fuel savings whilst climbing with full available climb power.

The same site also has other articles covering recommended leaning techniques during ground running, cruise and descent.

Definitely mandatory reading for any serious pilot so they are worth printing off, studying carefully and filing in a safe place so one can regularly refer to them for refresher training.