leaning a PA-38
 

I'm currently flying a PA-38 and had a chat to an instructor about leaning the engine in the cruise. His advice was that once in S&L flight I should gradually lean the engine until RPM peaks then continue until about 50rpm lean of peak.

I tried doing this for the first time today when settled at 3,000ft. Unfortunately as I leaned the mix the rpm didn't change at all until the mixture lever was about 1/3 of the way back. Then the engine suddenly started running rough - richening the mix a little sorted this out.

To be on the safe side I flew the rest of the flight with a full rich mixture. I obviously don't want to end up with a knackered engine but woudl like to lean effectively since I often do 2 hour legs at 3-5,000 ft.

Suggestins greatfully received!

Lean the engine until it starts to die..then richen up again to about half way between the two..ie from when the engine starts to die to fully rich..

remember leaning is better for the engine than running fully rich all the time,saves avgas as well..

if in doubt always air on the rich side...prolonged over leaning can damage the engine..holed pistons etc

all this is if your aircraft doesn't have a EGT probe..PM if you need any more help..:cool:

If your aircraft has minimum instrumentation (basically RPM only, plus your own ears), leaning until the engine runs rough and then enriching until smooth running is re-established, is virtually the only way of leaning for best economy. As you've seen, 50 rpm lean of peak may already lead to a rough-running engine.

For best power, lean to max RPM. But do NOT lean the engine if it's producing more than (about) 70% power. (The exact figure will be in the POH.) This generally means that you should not lean for best power below 3000-5000 feet (again, depends) otherwise you cook the engine: It needs the cooling effect of the evaporating excess fuel. (Leaning for best economy can be done at any altitude, as the engine will be producing less than 70% power.)

Now if you have more instruments, like CGT, EGT, per-cylinder engine monitoring, Gamijectors, lean-assist G1000s, you name it, there's a lot more you can do. Likewise, the leaning techniques for a variable pitch/constant speed propellor are different but these types of aircraft tend to have more sophisticated instruments than a single RPM in any case.

I have similar problems trying to lean the engine on my PA-38. Very, very careful adjustment of the mixture control will result in a leaned engine and an improvement in fuel consumption but, as you've discovered, 1/8 inch too much on the mixture control and the engine damn near stops. The rpm peak is rather small and Danny Boy's method seems sensible to me.

I flew Beccles - Wickenby and back yesterday, mostly at around 5700 feet and managed to lean it enough to bring the fuel consumption down from 24 lts/hr to about 21 lts/hr. It does take a bit of practice to get it right though!

The general procedure for leaning a non-instrumented engine to peak EGT is indeed to lean until there is a sudden RPM drop and then enrich to just before the drop.

This should yield an MPG improvement of 20-30% over flying full-rich, and with the normal Lyco engines is quite safe provided one does it below 75% of max rated power, and only in cruise i.e. with sufficient cooling airflow.

If this method doesn't work, there may be something wrong with the mixture linkage mechanism. Or one could have a bad spark plug or some other issue with specific cylinder(s) which gets masked by running excessively rich. For an engine to run smoothly at peak EGT, there must not be anything wrong with it.

To be fair, a PA38 is going to be old... I started my PPL in them and all I flew were not only totally clapped out but also "maintained" (by a company then holding a CAA AOC for charter) to the absolute minimum standard possible. The control linkages and the elevator trim in particular were well trashed, there was up to 1" of water on the floor after a night's rain, and the rain would fill the fuel tanks with so much water that it once took several drain beakers to drain it out, and that was just one wing tank.

A knackered 1970s C152 was a revelation after the "PA38 experience" :)

There are many techniques to leaning, and once mastered will result in significant fuel savings. On my CPL cross country, followed by 3 colleagues on the same route - I cruised at FL060-070 using the Piper POH method, and managed the entire route without refueling. They all needed fuel at stop two.

The 'operators' method at the time was to use the EGT guage, as mentioned above - lean until peak EGT is reached, then rich the mix again to achieve a reduction in EGT of 1 division on the guage (i dont remember what the divisions are!)

The POH for the aircraft, and the Piper method of leaning (a PA28) for best economy was to push the throttle to max (firewall) and set the RPM using the mixture. This sounds a litttle crazy, but the proof was there! The economy was great.

Dig into your aircraft POH, it will include the details you are looking for

Sometimes on the PA38 the RPM rise is very small. Practice is the key. :ok:

If you can't detect a rise in RPM lean until you get a slight drop then enrichen it just enough for it to run smooth again.

Smithy

I would like to add that if you are flying a cross country then if you do not lean in line with the POH the fuel rate may be much higher than the POH figures, and it may substanially affect your fuel calcs (this may be an obvious comment to many on PPrune but perhaps not a low hour pilot)

I was reminded this by a very experienced CAAFU friend who had been asked to be an expert witness at an inquest into the death of a pilot whose areoplane had run out of fuel much earlier than expected and the only explanation that they could find was that the aircraft wasnt leaned in line with the POH which increased the fuel burn - he didnt make his destination and died in the forced landing attempt

It did make me think....

Flying any distance without an accurate fuel flow figure always struck me as highly dodgy, but it is normal in GA to fill the tanks to a certain level, and depart, assuming a certain fuel flow rate which has never actually been checked...

"The POH for the aircraft, and the Piper method of leaning (a PA28) for best economy was to push the throttle to max (firewall) and set the RPM using the mixture. This sounds a litttle crazy, but the proof was there! The economy was great."
Were there any other requirements/details for this method?
Can anyone explain the theory for an ignorant PPL such as me?
(I've used the EGT and "rough and back off" methods.)

Without a multi probe egt, all you can do is slowly reduce mixture until you either hear or see a drop in RPM, then either leave it there or enrich a tad for smooth operation.
The FF figures in the POH are based on correct leaning technique so you have to follow them. the biggest problem I see with leaning methods, is pilots tend to rush the process and get a false peak when using a single egt guage, then not re-leaning after a change of altitude, to lean properly, you have to do it slowly.

Not necessarily dodgy, just a little more limiting than necessary; take a very conservative fuel burn figure, and keep good reserves. I also tend to dip before and after, so I have an idea what it burns flown the way I fly.

Search for LOP (lean of peak) and you'll get to fill your boots. Especially look on avweb (pelican's perch) for some good articles. I'm a little suprised (sceptical?) that it'd be in the POH. Every PA28 I've flown had conventional leaning in the POH, so it's not necessarily the 'Piper' way - it may have been an amendment for that particular a/c?

In short it's (theoretically) possible to throttle the engine by reducing fuel as well as reducing air. Problem for 99% of a/c is that the fuel flows are not well enough matched, and one cyl starts to lean cut before the others are fully lean, which is where the 'rough and back off' method comes from. With 'GAMIjectors' and all the other associated setup, the flows for each cylinder are balanced and you can. Would be very suprised to find an 'out of the box' PA28 that would run LOP.

I think barrow makes a very good point about doing it slowly. First time I leaned I cut the engine completely, and stopped messing with it for a long while. With time I learned a more patient approach :E

I am just very pleased that an instructor has taken the time to teach a student to lean the mixture, most of them seem to think that the mixture is an engine ON / OFF switch.

If you look at the Lycoming web site, then you can download the 'Lycoming Flyer' -- one section has all about leaning and also how to maintain your engine.

Here are the recommendations of Lycoming:
http://www.lycoming.textron.com/supp...fs/SI1094D.pdf

Although I don't fully agree with them as in I lean in the climb already but I use a climb speed which is about 30 knots above Vy and with the G1000 I have EGT and CHT for every cylinder accurate to the degree.

These "old school" engines use extra fuel for cooling. even though they are officially air cooled engines they are actually cooled by three things:

• Air- exterior, cylinder head and cylinder barrel
• Oil- interior
• Excess fuel-combustion chamber and cylinder head temperature

Climbing 30 kts above Vy removes the extra fuel requirement which allows you to lean in the climb as soon as you leave the pattern.
Unfortunately most of the spam cans out there only climb at one speed and have no equipment installed for EGT and CHT.

There is also the excellent constant-EGT climb technique, which allows a continuous climb all the way to one's operating ceiling, without touching anything but the mixture. It does need an EGT instrument though.

I have to transition to 120kt very soon after takeoff, to keep all CHTs below 400F.

In my experience with PA38, leaning for climb usually isn't neccessary, since O-235 shows pretty good performance even at high altitudes - if I remember correctly, some 300ft/min climb with full throttle at 8000 feet on close-to-ISA day...

Before attempting leaning in climb you should consult Engine Performance chart in PA38 POH, since you should not lean if power setting could exceed 75% of engine's rated power...

As being previously said, PA38 doesn't allow for precise leaning LOP since the engine basically "dies" just a little leaner of peak. In my experience, the best way to lean it (considering fuel economy and engine health) is to find the peak (EGT/RPM) and then advance the mixture lever 0.5-1 cm forward.

Sadly there are instructors who teach students to lean as soon as they reach cruising altitude, nevertheless if it's only 2000ft. I bet the mechanics are more than pleased when they hear this...

Given the topic is PA38, the Vy+30 would be 100 KIAS, which is 5-15 KIAS above average cruising speed of an PA38 :ooh:

Unless the DA is >5000 feet leaning in the climb at full power is not a very good idea, regardless of airspeed. :hmm:

All too easy to cook the cylinder head & induce pinking. Says so in the POH of most types, which up until now I thought was the word & the law with respect to operating an aircraft...

Smithy

Flyingstone
 

Please tell me why the engine should not be leaned at any altitude with a power setting below 75% power?

Risk vs. gain: how much economy would you gain by leaning at 2000ft - half a gallon per hour? Compared to the increased possibility of engine damage if the power setting is slightly above 75% I believe the safest way is to fly on full rich on cruise density altitudes lower than 3000-3500 ft.

You definitely burn some more Avgas that way, but at least you know for a fact the engine isn't over lean...

Perhaps you misunderstood me, I didn't say leaning is bad, but you shouldn't lean automatically when you reach cruising flight level, you should take into effect other factors, such as: length of cruise (for example, you probably don't lean in downwind when doing traffic patterns?), ability to maintain altitude (I don't think it is very wise to fly very lean [peak or close to it] when you're unable to maintain +100/-50ft due to turbulence), ... Some instructors teach to fly full rich all the time, some teach to always lean when in cruise (regardless of circumstances) - the right and the safest way to do it is probably somewhere in between...