Fear of Leaning

9 Answers

1. 
   

   
   

   Kent Shook on Nov 19, 2010

   The “lean until roughness, then push back in a bit” is the most basic leaning technique, and is what is usually taught in primary training, especially since most trainers don’t have the instrumentation for a more accurate leaning procedure.

   One at a time:

   1) If you lean until “roughness” the engine won’t actually quit, it’ll just be running rough. If the engine does quit, it should start right back up when you put the mixture knob back in. But, if it quits – You pulled the knob too fast and too far! If you lean slowly, the engine will start running noticeably rough LONG before it quits.

   2) If you go high enough, lean, and descend far enough, yes it is possible that the engine will quit. However, I don’t think this will happen descending from 3000 feet to 25 MSL – And if it does, it would happen at high power settings, so it would be more likely to surprise you on a go-around rather than cause any problems before a normal landing. Did your instructor teach you about the “GUMPS” pre-landing check? (Gas – correct tank selected and fuel pump on if needed, Undercarriage – landing gear down, Mixture – full rich, Prop (if constant speed) full forward, Seat belts – on and Switches – as required.) Doing a GUMPS check before landing (or better yet, several checks) should help you remember to go full rich.

   3) Yes, if you push the knob back in, the engine should start right back up.

   It would probably help you to go up with a CFI and learn the proper techniques. Also, while I know leaning may feel scary to you, remember that if you *don’t* lean you are burning a lot more fuel, which could bite you on a long cross-country flight. It can also foul up your spark plugs.

   However, here is something you can try on your own – And is a good thing to do on every flight anyway: Aggressive leaning on the ground. After engine start, check your oil pressure and then immediately lean the engine – You’ll see the RPM’s go up and then come back down. Leave it set there for taxi. (That’s the part you want to do every flight – It’ll keep your spark plugs from getting fouled, and you won’t be able to add takeoff power without enriching so you won’t be able to take off if you forget to go full rich again.)

   Now, for the learning part of the exercise – Try to increase the throttle. You’ll notice that the engine coughs and sputters until you bring the throttle back (this is what would happen if you went very high, say 9000 feet, descended without enriching the mixture, and then had to go around). Move the mixture to full rich, and increase to runup power. Now, slowly bring the mixture back until the engine runs rough, and then increase it until the engine runs smoothly. That is exactly what you’ll be doing later in the flight in cruise. If you can do it on the ground without the engine quitting, you can do it in the air with that same technique without worrying about the engine quitting.

   The key is, just don’t pull the red knob too fast – And don’t be afraid of it.

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2. 
   

   
   

   Gary Moore on Nov 19, 2010

   also – if you feel you did NOT get enough instruction on a particular topic – there is a very simple way to solve that 🙂

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3. 
   

   
   

   Brian on Nov 20, 2010

   Relax, engines are much nicer than we tend to think they are. Don’t trust me? Rent an aircraft, fire it up, make sure it is nice and warmed up (10 minutes in flight will satisfy). Now take the aircraft over the field and pull the mixture. A few seconds later, after the engine is off, push it back in. To your amazement you can do this all day till you are blue in the face and that engine will start back up every single time.
    
   Without fuel and with the propeller windmilling the engine is completely operable. The pistons are moving, oil is keeping you lubricated, valves are firing as needed (they just don’t have fuel to shoot out), and your spark is firing reliably because the magnetos are going because the engine is going. Essentially you have a system that is ready to go and willing to give you power as soon as you supply it with something that is explosive (typically this is avgas 🙂 ).
    
   So long story short, who cares if it quites. Just give it fuel again and you’re ready to go.
    
   As for waiting till 3,000 feet to lean, I ask you why. Really an engine operates much better when it is leaned near optimal performance. Look at FADEC engines, used in cars for many years now and now in aircraft. Computers keep you leaned out during all operations except takeoff where only a small amount of extra fuel is used. This is not for safety as you might suspect, but for cooling since excess, unburned fuel, acts as a coolant during the high powered climb. My procedure is to lean anytime I’m above pattern altitude, monitoring engine temps if I am in a climb.
    
   Finally, I completely agree with Gary. While it may sound as though he is just looking for more money there is a great deal of human physiology studies that agree with his recommendation. A bit of common sense can be helpful here as well. If you are operating a machine that can easily kill you and you don’t feel you understand how it works…need I really add more?
    
   The grim truth: More instruction costs you more money. On the other hand, it can save your friends and family from a funeral ahead of its time.

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4. 
   

   
   

   Jim Foley on Nov 22, 2010

   Kent and Brian did an excellent job covering all the important stuff, but here my two cents:
   1- Don’t be too afraid of leaning at cruise and forgetting it (but I wouldn’t make a habbit our of it).  I had my 172 (fuel injected) leaned for +50 degrees rich of peak during cruise at 10,500 msl, and decended to 463 msl for landing, and forgot the mixture.  Did’t realise it untilI was doing my shutdown checklist.  
   2-  One thing not mentioned, would be to lean for maximum rpm for taxi and climbout.  Only one of my flight schools has thought this, so it’s not bad if you don’t, but you will get better climb performance and fuel flow if you do.  I wouldn’t worry too much about getting too hot while it’s leaned.  A cessna representative once told me it is impossible to over lean and over heat a properly maintained engine. (I still keep and eye on temps, but no need to focus on it, just worry about flying the plane)  Also, flying a glass cockpit taks alot of the worry away, since any abnormal indication will cause many annoying sounds and/or flashing lights.

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5. 
   

   
   

   Kent Shook on Nov 23, 2010

   A few things:

   1) WRT intentionally shutting off an engine and restarting it in the air… Well, there’s a reason combat troops don’t practice bleeding, ya know? I wouldn’t recommend this. In theory it’ll fire right up again, but if you leave it off for too long, maybe it won’t. I’ve seen an engine-out demo on a 414 (granted, a much larger engine than most of us are flying, and fuel injected too) where the pilot had one helluva time getting it started again. If it’s your only one… Well, you get the picture. If you do think this is necessary, be sure you do it over a nice big runway. Better yet, do it with an instructor – And see how many instructors you have to present the idea to before you get one that’ll take you up. (That’s a hint.)

   2) It is possible to get an engine too hot, even if it’s well-maintained. If you have the proper instrumentation, watch to make sure none of your CHT’s get above 400º. If you don’t have the proper instrumentation, it’s best to err a little on the rich side. Fuel’s expensive, new engines are much more expensive. That said, if you fly at higher density altitudes, you will NEED to lean prior to takeoff so that you can develop full engine power – Your climb rate will already be bad up there, and you need every bit of performance you can get.

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6. 
   

   
   

   Brian on Nov 28, 2010

   Regarding shutting down the engine in flight, if you ever poke your head through the student pilot forums website you likely know Bob Gardner (Yes, the aviation author) posts there quite frequently. If you search through the posts you’ll find him giving this same advice some months back for a similar question.
    
   I have personally done it 100’s of times. In fact, not 2 days after that first post I went up and did this demo. Anyways, if the engine is going to die from you doing this then there is a very good chance it won’t make it till you land anyway. Something is wrong with it. 
    
   I do agree this should be done with a competent instructor, preferably one who has done this. Not for the sake of safety, it will start again. Instead for the sake of not unknowingly beating up the engine and not frying the radio and other electrical equipment which the instructor would hopefully know to turn off for such a demo.
    
   —-BREAK—-
    
   On the topic of leaning and engine temperatures, careful being on the rich side. In fact, 50 rich of peak (ROP) on the EGT is the point of highest temperature for the CHTs. Take a look:
    
   http://www.gami.com/img/articles/bttf/pt1fig2.jpg
    
   The middle two lines are the CHT for the two different power settings shown. The far left vertical line represents 50 degrees ROP. If you wish for low CHT’s you either want 100+ ROP or 25 or more LOP.
    
   I honestly never bother with any of this though. I lean till she sputters then I make her run smooth and off we go. The only time I lean differently is when I have CHTs available for each cylinder. The EGT gauge is a waste of space in my opinion.

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7. 
   

   
   

   Matthew Waugh on Dec 02, 2010

   This is all good theory stuff – but I assume you can read anyway.
   Gary has the right answer – if you don’t feel like you got enough instruction, get some more. Your instructor can’t mind read, you have to tell them what you’d like instruction on and they’ll be happy to provide it. You haven’t got enough instruction in leaning because you haven’t asked for it (or if you have you’ve got the wrong instructor – get another one).

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8. 
   

   
   

   John D. Collins on Jan 24, 2011

   I won’t repeat the advice of others, but hopefully will provide a few insights into the subject.  Most trainers have carbureted engines and they don’t do a good job of distributing fuel equally to all the cylinders.  When you have a rich mixture, there is more fuel than needed for combustion and some of the excess fuel provides for cooling.  If you were to plot the power curve from a point of just enough fuel to match the available oxygen to a fairly rich mixture, the curve would be fairly flat, with peak power (+1 to 2 percent) occurring in the vicinity of 100 to 150 degrees rich of peak EGT if you have an EGT. This also means that power is primarily dependant on how much air (throttle) you are providing to the engine. An imbalance in fuel distribution still results in a smooth running engine because each cylinder generates the same amount of power under these circumstances.  As you lean the engine, you get to a point where the fuel and air are evenly matched for combustion (this is called a stoichiometric mixture and often referred to as peak EGT if your airplane has an EGT). 
    
   As you lean further, you are operating on what is called the lean side of peak EGT, where there is more air being supplied to the engine than required for combustion. The result is that instead of the amount of air being supplied to the engine determining power, the amount of fuel supplied determines your power, less fuel, less power. If you plot this regime of the power verses fuel flow curve, it is a fairly linear relationship between power and fuel flow, with lower power following fuel flow reduction.  In the carbureted engine with the uneven fuel distribution this means that each cylinder will develop the amount of power based on the fuel it gets. The natural result with uneven amounts of power being developed by the various cylinders is engine roughness.  In fact, the engine roughness will often appear when you are close to peak EGT because the variation in fuel distribution between cylinders will place some cylinders lean of peak before others and result in less power being developed by those cylinders.  By leaning to roughness and then enriching to smoothness, you get a fairly good economy mixture setting. Enriching to the point that RPM peaks should give you best power and if you have an EGT this will correspond to a point of 100 to 150 degrees rich of peak.
    
   If you have an engine with fuel injection, the fuel distribution can be much better balanced and then operating lean of peak with smooth operation is feasible, but that is a separate discussion.
    
   So when should you lean at cruise. It depends on who is paying for the fuel.  If you are paying, then lean at any altitude.
    

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9. 
   

   
   

   David Brown on Feb 09, 2013

   My post in another thread is equally at home here.

   Hope this helps.

   First of all, I would strongly suggest education from http://www.advancedpilot.com as quite simply this is the best education available anywhere in the world. Period. I am sure John would agree. Be it a simple O-320, a Turbocharged IO540 in a Chieftan, a big Radial or a Briggs and Stratten mowevr engine, the theory is the same. Simple as that.
   Best power and best economy are very poor use of the word “Best”. Peak power is achieved around 75-80 dF ROP. The best BSFC is achieved at lower powers around 10-20dF LOP or around 60-80dF LOP at say 80+% power. And yes nothing is wrong with LOP at 80-90 or more % power. How do all these modern diesels run in your fancy cars?
   The interesting thing is, as EGT climbs to peak EGT, the CHT, Valves and guides are getting cooler once past about 40dF ROP, so it is true that leaning towards peak is getting cooler as EGT is higher.
   The APS course takes 2.5 days, and to be brutally honest, it can’t be taught on an internet forum. To fully understand answers to a question requires a bunch of other knowledge and experience which not even us folk involved in APS can do on a forum.
   By the way, I am not financially involved in APS in the USA, just Australia, but either way, its not about money, we do this for the good will of fellow pilots.
   All your questions and more are taught in the course, but leaning is only about 15% of the course, the rest is where the real value is. Understanding the whole engine process, the engine monitor, and how to diagnose problems. The ones that cost money or might kill you.
   Best return on investment you will make in aviation. Or should I say legal investment at least.

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