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To lean or NOT to lean?

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To lean or NOT to lean?

Old 28th Feb 2006, 16:56
  #1 (permalink)  
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To lean or NOT to lean?

I know that there was a thread recently regarding high density altitude takeoff's and the implications of not leaning correctly, but my question is; should I lean below 3000ft during the cruise?

I fly the PA28 181 Archer. The POH does not recommend leaning below 3000ft, but I'm begining to get the impression that there are mixed feelings about this ?????
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Old 28th Feb 2006, 17:09
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Hee Hee - please remember to recycle the can after all the worms have got out...

Do a search on this subject - my memory tells me there are a few good threads to increase your knowledge. Talk it over with your friendly local FI too...
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Old 28th Feb 2006, 17:19
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Yes you should....not doing so is how people run out of fuel tooling around close to home.
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Old 28th Feb 2006, 17:57
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Sigh Here we go again. My Archer II POH and Lycoming allows (if not actually requires) leaning anytime below 65% power. This varies with the altitude, power setting and the temperature relative to ISA. There's a table in the POH that explains all the combinations that add up to 65%
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Old 28th Feb 2006, 18:45
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The 181 Archer II POH actually recommends leaning above 5000'.

Leaning should always be done, to ensure you get close to the forecast fuel burn rates and speed.

All tables are based upon the engine being leaned correctly.
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Old 28th Feb 2006, 19:44
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Without wishing to start yet another massive thread, it's important to understand a little bit about engine efficiency as applicable to the spamcan market.

For the last few years I have been flying an aircraft whose engine is operated precisely lean of peak (LOP) and I have had an accurate flowmeter - usually better than 1% when correctly calibrated etc. So I knew I had 2 hrs and 35 mins before the juice ran out; I also knew I would have 2:10 at the planned destination (corrected dynamically for current ground speed) and this would be right within a few mins. I have not flown spamcans at all - until recently when doing the IR and this reminded me of the dodgy fuel practices that prevail in the flight training business where just about nobody (including the instructors) understands what they are doing.

Normally, spamcans are flown well rich of peak (ROP). This wastes fuel massively (up to 30% for the same speed obtained otherwise) but has two advantages:

a) the combustion is cooler, which is good for the heavy handed flying that takes place in that business

b) every cylinder is getting more fuel than it needs which circumvents the poor fuel/air distribution that is common in carburetted engines (if you try to get this just right, the engine vibrates too much because some pots are making more power than others)

The trouble with flying massively ROP is that nobody actually knows the flow rate. It isn't in the POH. In there (PA28-161) you get typically two lots of figures: "best power" and "best economy".

BP is about 100F ROP. This isn't quite the full rich position most spamcans fly at, but nobody knows where it is on the lever position.

BE is peak EGT, close to the best efficiency, but most carb engines can't make peak EGT without vibrating (reason above)

So, the traditional way, taught here and everywhere, is that you lean until

a) the engine gets a bit rough, or

b) until the power drops off

whichever occurs first, and then you go a bit rich and check the carb heat still works

With carb engines, a) usually occurs first but nobody knows the actual flow rate because .... well.... the engine will get rough when the leanest cylinder goes LOP and this depends on how badly matched the fuel and air delivery is ... b) is a good indicator of the peak-EGT point but most spamcans can't get to it for reason given above.

So, most people have no idea of the flow rate, not better than +/- 20% of the POH figures.

So, the smarter people fly with massive reserves; say 3 hrs max endurance on a plane which could actually do 5:30 if optimal. Those who are a bit too smart and try to calculate things properly (as they have been taught) are likely to end up in a field - because nobody told them the actual flow rate is anything up to 20% out. Or they end up in somebody's loft and the CAA takes them to court, but the CAA loses because it is "revealed" (shock horror, absolutely nobody had any idea) that this pilot was only following what is standard practice in the training business.

Instructors often get to know a particular plane really well (obviously) and they know just how far they can push it, and they often go very close to the edge, but this is no good to others.

So, to briefly answer the original question, leaning is OK anytime en route or in descent. Not in a max-perf climb when all should be fully forward.

A prohibition on leaning below say 3000ft is standard school/club practice (for reasons given above) but it doesn't relate to engine reality.
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Old 28th Feb 2006, 20:33
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The simple answer - Lean the engine any time that it is operating at less than 75% power.

Use the POH to determine what your chosen power setting is in % power and base the decision on that.

The POH recomends leaning in the climb when above 5000ft because leaning is necessary to produce maximum power above 5000ft and also even when leaned at that level the engine is not capable of producing more than 75% power at full throttle.

So simply - in the climb lean when above 5000ft, in the cruise lean at any level unless you are using 75% or more power.


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Old 28th Feb 2006, 21:18
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As IO has mentioned, actually controlling an engine with the standard gash set of monitoring instruments is not particularily easy or accurate.

The fuel/air distribution is generally very poor in your standard Lycoming or Continental so even if you are using an EGT gauge for leaning purposes, you will only be checking one cylinder, the same with the CHT.

Proper engine monitoring and blue-printed engines are worth their weight in gold if you do long distances and have a machine worth spending the money on. For your average club hack. It isn't really worth it (unfortunately...)
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Old 1st Mar 2006, 01:12
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The simple answer - Lean the engine any time that it is operating at less than 75% power
The answer is easy - lean the engine anytime it is burning too rich......simple.
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Old 1st Mar 2006, 02:18
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I lean depending on the a/c I'm flying.

If I have a fuel flow gauge or EGT (that actually works) I will lean as per POH/instructers training (providing >3000ft in single, whatever specified in a twin)

I am much more dubious if the a/c doesn't have either, I am not a fan of leaning till the engine splutters then riching the mixture slightly, I do not trust the reliability of this method. I nearly had an engine cut once using this method. Some clubs teach you to do this purely to keep their costs down.

Certain types have fuel vaporisation once an engine cuts if the engine is warm, that tends to prevents an easy restart.

I wouldn't lean any single below 3000ft either way, to give myself a good choice of landing options if the engine cuts.
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Old 1st Mar 2006, 02:25
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If you trouble to take out the book and the Cessna Power Computer -- and lean properly, you do come quite close to the book figures.

I did land once with just one hour as planned in the tank and the line boy remarked that he had never put that much gas in a C-172 before.

This was a plane I had flown frequently and had verified a standard burn rate.

At a 3500' airport in the summer, you lean for best power to takeoff. Remember that you are getting considerably less than full power.
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Old 1st Mar 2006, 09:22
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Why do it if it's not fun?
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The simple answer - read the POH!

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Old 1st Mar 2006, 09:37
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It's not quite so simple because what do you do if the POH authorises peak-EGT ("best economy") but the engine can't make peak EGT without excessive vibration?

Then you can fly either at the "best power" setting (which cannot itself be determined accurately), or at some unknown flow rate which is what you get if you lean "a little bit".

I don't think there is a solution, without decent instrumentation. You just have to be very conservative on fuel planning. A Shadin flowmeter costs about £3k. An EDM700 is something similar; costs only US$1200 to buy but the rest is UK disti markup and installation.
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Old 1st Mar 2006, 10:22
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I am flying a PA28RT-201 and we fly it on the EGT to obtain best economy. Havent had any problems with the engine shaking itself apart and we seem to be getting consitent fuel flow figures by running it at this. Yes we do lean below 5000' as well, its not alt dependant!

I could of course go back to Piper and tell them there is something wrong with the engine as the vibration is missing

If your fuel planning is so tight that you are out of fuel before you reach your destination then suggest you are better driving or let someone who can flight/fuel plan properly do it for you!!!

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Old 1st Mar 2006, 10:43
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Ok, my answer of reading the POH was a little flippant, and IO540 is correct to say it's not quite so simple.

However, it's a good place to start. TotalBeginner has read his POH, and told us that it does not recommend leaning below 3000'. The answer to his question, therefore, is to not lean below 3000'.

Whilst there are lots of debatable areas when it comes to leaning (rich or lean of peak, leaning for take-off, etc, etc, etc), and the instrumentation you've got available to you might well be a big factor in answering these questions, I would have thought that there can be little doubt that if the POH says don't lean, you don't lean???

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Old 1st Mar 2006, 18:44
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Not really, because there isn't any engineering reason for not leaning below 3000ft.

If it's in the POH, it's because somebody has made it up.

Is there really a Lyco or Conti engine manual (I mean an engine manual, not an aircraft manual) which prohibits leaning below 3000ft? I've never seen such a thing.

Sure it isn't a club/school operating rule, perhaps inserted into the POH to make it look more official? Much more likely.
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Old 1st Mar 2006, 21:27
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Originally Posted by englishal
The answer is easy - lean the engine anytime it is burning too rich......simple.
Most certainly not.

When the engine is operating at high power (75% and above), it is designed to have a slightly rich mixture. This excess of fuel is used for cooling purposes.

Do not lean at 75% power or above.

Here is the exact information from the Engine manufacturer (Lycoming);



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Old 1st Mar 2006, 22:06
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Current automotive technology enables race engines to be tuned in a very sophisticated manner. Mapping an engine on a rolling road is a fascinating process. It is startlingly apparent how a map from one engine does not suite an apparently identical second engine. Determining the correct air fuel mixture at a range of pot settings has a stunning impact on the power output of the engine, in how smoothly it runs and in the amount of “pollution” it produces.

Why is this relevant?

I was encouraged at some point in my flying to lean the engine using the EGT as the main tool for achieving the “correct” lean. In theory this seemed entirely reasonable before having a better understanding of engine mapping. Whilst I appreciate automotive engines are in many ways different animals they do bring home the extreme danger of operating an engine at peak fuel mixture efficiency unless you can be absolutely certain you can monitor and control all the factors that might just mean the engine has become over lean, even for a few minutes. For example, in a “modern” automotive engine the air pressure is constantly monitored, as are the emission gases, the air temperature and the head temperature never mind that during the mapping process the EMU will be programmed dynamically for that specific engine at 50 rpm intervals and the EMU will in many respects manage each cylinder individually! I can get 200 bhp from a 1,600 cc engine at 8,500 rpm with sound reliability.

In comparison whilst many of the tolerances on the engines we use are far greater equally the engine monitoring is primitive.

The most obvious result of over leaning an engine is a rapid rise in head temperature.

How often have you "leaned the engine correctly" for a given power setting and altitude and "forgotten" to adjust the mixture when you have changed power setting or altitude until some moments after or maybe even reaslsied you had left it just as is until you realised on short final. Poor airmanship, I guess, but you might only have to do it once to cause significant damage.

How often do engines require a top end overhaul before TBO? - nearly always. It is the top end that so often starts “playing up” before anything else. Of course one of the greatest causes of damage to the top end is over heating - I am not suggesting that is the only cause or the only reason for top end overhauls.

So in short I have come to the conclusion that most of the tools at our disposal are pretty useless at reliably telling us in all circumstances whether we are “cooking” the top end. An EGT, the usual next best thing (short of nothing but the mark 1 ear) is a pretty course tool, as it only gives an indication of what is happening to one cylinder and then some little while after it has happened. (I appreciate some monitor more than one cylinder but it is certainly not the norm). Of course a multi gauge CGT goes a long way to improve things but it stills relies on the human computer rather than an EMU to react to changes.

So in short running an engine rich of peak is less efficient, may cause in some engines a little more plug fouling, and is not environmentally friendly but it IS economically sound. Ah, I hear you cry, what about the extra fuel you are wasting. Well you are not because the extra fuel is providing a safety net, a cooling buffer if you will so it may cost a bit more every hour but its petty cash compared with the cost of an early top end overhaul!

In my view, lean a bit, and keep the bank manager happy, lean aggressively and keep your engineer happy and if the POH says differently stuff the POH.

Last edited by Fuji Abound; 1st Mar 2006 at 22:19.
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Old 2nd Mar 2006, 06:20
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Originally Posted by FlyingForFun
TotalBeginner has read his POH, and told us that it does not recommend leaning below 3000'.
My bet is that it doesn't say that at all, or that it's been written by somebody who has misunderstood the engine manufacturer's recommendations. The Lycoming advice has always been that you must lean above a certain altitude to prevent the engine running grossly over-rich. Unfortunately, this advice has evolved through Chinese whispers into "you mustn't lean below x thousand feet", which is total garbage.
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Old 2nd Mar 2006, 21:07
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For those interested in engine management options not offered by the POH, or even an opinion about the inaccuracies in some POH's, the articles by John Deakin in AVweb might be interesting reading. They can be found at:

In particular: #75 and #76 “Those Dreadful POH’s” and #63 through #66 “Where should I run my engine” (discussing in detail engine management during Takeoff, Climb, Cruise, and Descent.)
You can decide what or who is the better to follow, the POH or John, but all his articles are highly informative.
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