Primarily for general aviation discussion, but other aviation topics are also welcome.
#1323627
Hi there,

First post so please dont shoot me down for what is a stupid question!

So my shiny new PPL license arrived (finally) this week and tomorrow i am going on my first flight as a PPL holder! :D

Whilst i have been waiting for my license i have been filling my time thinking about what i am now going to do with it and how i can slowly improve my flying technique.

Something that didnt come up much in training was leaning the engine. When i did ask my instructor he was very unsure of the proper process and kinda bumbled through it - and then said but i fly fully rich all the time and as such he wasnt 100% sure!

So i then looked at the POH which gives a clear explanation (which incidentally is different to what the instructor showed me!). The POH says that leaning should be done below 75% power. So here is the silly question.....

What is 75% power??? I was taught to fly a lot of things by engine RPM..... so would 75% power be 75% of maximum revs? If so 75% seems to be a lot less than the RPM i was also told for cruise (i was taught 2300 - 2400 for cruise). Or is 75% power something else that i am missing.

See told you it was a stupid question!!

You will be please to know i dont intend playing with this on my first PPL flight - i am going to keep it simple..... but this has been bugging for a while so thought i would sign up to flyer and ask the question!

Oh and one last totally unrelated point - thank you to the people on here who have also done the blogs (and Leia Airborne in particular). I read through the them before and during in training. Before hand a lot of it bamboozled me but it was nice reading about the exciting times that were coming up! During training I could totally relate to what was written…. Including the days after a bad flight where you have messed up, you feel really down about the flight and then you remember you now have to hand over a small fortune for the privilege of feeling so down about it!!!! It was nice to know I was not the only one with those feelings!!
#1323644
Hi, congrats on getting your licence!

Funnily enough, I've been wondering the exact same thing, too. We didn't cover leaning at all during my training. So I'm glad you asked :)

I had my first flight as a PPL holder on Wednesday. Such a great feeling. Enjoy yours tomorrow!
By Jon Mercer
FLYER Club Member  FLYER Club Member
#1323645
Leaning as I was taught it:

Never below 1,500 ft. Bring the mixture back very slowly until there is a very slight but perceptible drop in revs. Move the mixture forward a clear inch.

You'll find that at higher altitudes you can lean off more than at low altitudes - which ought to serve as a warning when descending - beware a lean cut. On a long cruise descent, re-lean by giving full rich then leaning out again, providing the revs are maintained at a reasonable level, above 2000 RPM.

Always remember that you need a full rich mixture below, say, 2000 ft for safety. And in a climb - it helps cool the engine due to evaporation of the excess fuel in the cylinders.

As ever, have a plan about what to do if the engine does stop! Mixture rich, change tanks, etc, etc, just as you have been taught.

And congratulations to you both. I got my ticket as recently as July.
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1323653
1) Why oh why oh why don't flying instructors learn this? I think there are instructors out there with no real world experience.

2) Power settings. Well, this will depend on your engine/prop combination, but generally there should be a graph or table in the POH showing what RPM corresponds to what power. It'll vary by altitude as the air gets thinner when you climb and the prop characteristics will change. Generally speaking though, for general flights at low level around the UK, you can look up a particular RPM at (say) 2500ft which corresponds to 75% power and then just use this as a rough guide. Lycoming say that you can lean as much or as little as you like below 75% power, it won't hurt the engine. I would give a caveat of "at normal cruise speeds".

3) "Do not lean below xxx" - nonsense. Lean at any altitude. Not doing so is like driving with the choke on, it'll soot up your engine and waste fuel.

4) Technique for the cruise. Above 75% power you need to be very careful in leaning and there's a specific technique for this by setting EGTs, monitoring CHTs, etc. Below 65% power, knock yourself out. Lean back until the engine starts to run rough and then just push it in a little bit. Doesn't have to be much.

Lycoming say that you can lean as much as you like below 75%, but Continental say 65%, so it'll depend on what engine you have...from 65% to 75% you might want to give it the "inch" that Jon mentions above but in my experience it isn't really necessary in this country at normal cruise speeds. If you have a CHT gauge, just keep an eye on it.

5) Technique for climbing. On full throttle/full power, this is where you "shouldn't lean below a certain altitude". As you'll be on high power and lower airspeed, you'll need to keep the cylinders cool, which means putting in an excess of fuel. In my aeroplane I start leaning almost straight away, but to a value of fuel flow/pressure described in the POH (fuel injected engine). If I'm flying a carb aeroplane, I'll perhaps start leaning at 2000ft. Once you get to 5000-6000ft, you can lean more aggressively even on full throttle as the engine power will be down because of altitude. It's best to cruise climb to keep the cylinders cool though rather than continuing to climb at (say) Vy. In this country, with a common cruise being 2000ft or so, I'd just keep it full rich while climbing to that altitude, then set up and trim for the cruise, then lean.

6) Technique for descending. If descending from 2000ft or so, just stick it full rich for the last bit. If on a cruise descent from higher altitudes, you just want to increase the mixture bit by bit as you descend. You'll get the feel for it. If reducing power for a long descent, you don't really need to enrichen it until you need more power again, then as Jon says, best to go to full rich and then lean again from scratch. If you're then just joining the circuit, stick it on full rich until you land.

This all assumes roughly sea level airports like we have in this country! If you're flying from Big Bear or such like in California, then come back and ask us! :D
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By Keef
#1323667
I'm disappointed that there are still instructors around who don't know about and understand leaning. How can you "have a feel" for the aeroplane and its engine if you don't know about mixture and fuelling? I'd have hoped that was in the written exams too. (Chorus of "in my day...")

As Paul says. Don't lean when in the climb on full power unless you are high enough to need to do that to obtain climb power (read up on "hot and high" for more).

Other than that, lean at ANY height if in the cruise. I'm a "flying by numbers" type - know the RPM and/or MP setting for 75%, 65% power. Likewise know the settings for 500fpm up or 500fpm down, for approach speed, and so on.

If you have the gauges, learn how to use them. If you don't, then set cruise power (75% or whatever, refer the POH) and then lean till it starts to run rough, then put mixture back till it's smooth, and add a bit. The CHT or oil temperature or EGT (or all of the above) will soon tell you if you got it right.
NOT leaning risks sooting up the plugs, as well as wasting a lot of fuel. It gets embarrassing when you pull back the power to land, and the engine stops.
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By anglianav8r
#1323702
My mixture is always 50:1 :lol:
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By Rob P
#1323713
Not a stupid question.

Even outside the protection of the stude forum, here questioning is encouraged, flaming is frowned upon.

Rob P
By OhNoCB
#1323726
If the airplane you are flying has good engine instrumentation (EGT and CHT, preferably for each cylinder) then you can really lean whenever you want, regardless of altitude or % power. Its not the leaning itself that can damage the engine but more the increased temperatures as a result of doing it (especially incorrectly).

If you don't have great engine instruments then you can play it safe by doing it below 65/75% power (depending on engine). You can still do it at any altitude but the lower you are, the lower the rpm you will need to stay below 65/75%. There should be a table in the POH to show the rpm/altitude to % power.
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By Rob L
#1323727
itchybumba wrote:...See told you it was a stupid question!!...
<heavy snip by Rob L>

There is no such thing as a stupid question; only questions to which you don't yet know the answer, so well done for sticking your head above the parapet and asking :D . I fly a machine without a mixture control, so I'd be interested to learn the answer too.

Congratulations on getting your PPL :thumright: (a lot who start don't get that far).
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1323730
No, lots of people aren't familiar so not a stupid question at all. If you really want us to take the pee, then I suggest we should focus more on your chosen username! :D

Going back to the power thing, percent power is definitely not related to percent RPM on a 1:1 basis. It could be that 2000rpm is something like 45% power whereas 2200rpm is 65% power. I don't know the actual relationship, it would take an aerodynamics expert to explain what the prop was doing and how much air it was moving how much power it was absorbing, etc. I isn't one of them!
By riverrock
FLYER Club Member  FLYER Club Member
#1323735
I was taught to lean right at the beginning of my PPL and it was expected of me anytime when cruising. It really isn't scary and I think it is atrocious that your instructor wasn't able to demonstrate. It's true that it makes more difference in some aircraft compared to others, some do it automatically and some semi-automatic (by height) but if it has a mixture control you should know how to use it.

Bulldog that I fly has full rich, Best power and economy tables to follow at different parts of the flight and likes bring leaned during taxi to keep the plugs in order. You can pretty much half fuel consumption with proper leaning.
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By Rich T
#1323737
If you want to understand a LOT of detail about leaning, Mike Busch's video on the subject is excellent. Mostly the routine is as others have described, but I always prefer to understand the principles rather than learn by rote.