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Climbing and Descending

I wanted to get the opinion of others on climbing to altitude, particuarly in the use of the mixture control. Most flights I do are generally under 5,000ft, and I lean as appropriate. Twice I have taken my PA28 upto 12,000 ft (just about, lol).

The procedure I was shown for climbing to such altitude was to climb to about 5,000' mixture fully rich, level off and lean as required, and then start climbing again up to whatever altitude you want on the lean setting. Upon reaching the desired altitude, you check the mixture once more and lean further if need be. At this point I am on full power, and you leave it there. Obviously it isnt mandatory to stop at 5,000' to lean and start the clim again, but is this procedure OK and is it OK say at 5,000' just to lean while in the climb and continue the climb, or does anyone have better suggestions?

On descending, I have left the mixture leaned, descended by closing the throttle a little to achieve about 1,000ft per minute descent, with carb heat on. Reaching say about 4,000' I have moved the mixture level forward so it is more rich, and if decending to land, or under 2,000' then I move the mixture to fully rich and set the carb heat to cold when levelling off. I like to make sure I am doing the things right and not damaging the engine so any advice is always helpful.

I should add that I dont have any non-standard engine monitoring equipment, and the EGT currently doesnt work, so any leaning is done purely by looking for a RPM drop.

PiperArcher

The main question is whether you have modern engine monitoring equipment or not. If you don't like in your case then you have to play safe.

You risk two things by leaning: too hot CHTs and detonation.

If you do have an engine monitor (highly recommended), then there is no need to go full rich all the way up to say 5,000ft. Lyco says that you can lean below 75% power. My EDM 830 shows me the power output in % and also CHT and EGT for each cylinder so I can see when I can start leaning and how much. I also have the cowl flaps to play with. I'm turbocharged so I have to be more careful because I can do > 75% pretty much all the way up to FL200. That's why I switch from 100% takeoff to cruise climb ca 400 AGL (depending on terrain).

All I've got here is the PA28-161 Cadet POH and it's a bit funny and incomplete about leaning.

Normally with a normally aspirated, carbureted Lycosaurus and a fixed pitch prop, you'd run full rich until about 5000', and lean for best RPM above that, to achieve max power. And in the cruise, you lean until rough running, and then enrich a bit more. (There is no need for a level-off segment at 5000', you can just keep on climbing at Vy or Vy+10 while doing your leaning for best RPM.)

But the instructions in the POH are not quite like that. In the "climb" section there is no mention of leaning whatsoever. In the "cruise" section there are two descriptions. One for best power, one for best economy.

Best power is what you expect. With the mixture full rich, set the RPM with the throttle to about 35 RPM below the desired setting. Lean till peak RPM, then adjust the throttle to get the desired RPM setting.

Best economy has instructions that I have never tried. The instructions are to go full throttle, full rich and then immediately lean until the desired cruise RPM is established. (The table of % engine power vs. altitude/RPM is on the LHS visor, not in the POH... And you are running above 75% for a short period, so the POH gives you a maximum of 15 seconds to execute the procedure.) RPM will peak first and then drop off, and you set the final RPM with the mixture, while leaving the throttle fully opened. So essentially these instructions seem to suggest that the engine can run smoothly well lean-of-peak. Is that indeed true?

What I've been taught by my instructors was to lean the PA28 for economy just like any other aircraft with the same O-320 or O-360 engine: Set RPM with throttle, lean to rough running, enrich to reestablish smooth running, fine adjust the throttle.

The AFE flight guide, by the way, uses the normal leaning procedures too. Best power = best RPM, and best economy is 25-50 RPM lean of best RPM.

In a carbureted Lycoming engine, without an EGT for guidance, FT to 5000 feet, then lean for maximum RPM, This should be best power. If you have an EGT, this is probably 100 to 125 ROP. Above FL 70 you should be at or near 75% power at FT and leaned for best power. At this altitude or higher, to lean for best economy, lean until roughness (this is usually at peak EGT or slightly LOP if you have an EGT) and enrichen until smooth operation. If you have an EGT, note your EGT at takeoff power, and maintain the same EGT thru out the climb. In the FT position, the engine is substantially enriched during the last portion of the throttle movement.

KUZA, United States

If it's your own aircraft and you pay for the fuel, then I would strongly recommend to fit an engine monitor. It allows you to save fuel and monitor the health of your engine.

Most instructors are only used to wet charter where fuel economy is not the main focus. I was given very very poor advice during PPL and IR training and it was only after I started reading about this subject that I started to understand the the whole subject.

Also keep in mind that the POHs tend to be very old, back from days when there were no engine monitors and fuel was cheap so fuel economy was mainly about maximizing the range of the aircraft. My 1979 POH contains quite some nonsense.

On an Archer with no engine monitoring kit all you can do is climb on Full throttle full rich to around 5000 to 6000 ft after that you'll be hard pressed to get more than 75% power and can start leaning for max rpm as if you we're facing density altitude issue at a hot high airfield

The performance charts in the POH will help you work out where 75% cuts in agains temp and altitudes

Johnm
Gloucs, United Kingdom

I absolutely agree with those who'd like to see more engine information being available to you. More specifically CHT, EGT and fuel flow.

Assuming you've got all of that, or at-least the first two, you can do quite a lot, but I'd not follow the proceudures you've been given.

I don't subscribe to the commonly taught view that you should always be fully rich at lower altitudes. In the cruise. Leaning in the climb can give better power, and clearly leaning in the cruise obviously can give lower fuel consumption. The risk as been said is of detonation or over-temp.

So in the climb, Once above, say, 1000ft I'd fly at your selected climb or cruise-climb speed, then lean for best rate of climb. You do this by slowly bringing the mixture back and monitoring RoC -it's unlikely to be at the same condition as peak EGT, more likely a bit richer. Keep slowly leaning in stages and RoC should keep increasing then start to drop off - when you hit the last, richen again back to peak RoC. Monitor EGT but particularly CHT as you climb and if you get close to limits richen to avoid: it's most likely to be the CHT limit you hit in the climb. Signs of rough running whilst within EGT and CHT limits? - again, richen to avoid.

In a PA28 this'll give you 50+% better RoC, particularly at higher density altitudes than you'll get fully rich. BUT, don't do it unless you can monitor CHT and EGT - if you can't, then stick to climbing fully rich unless you need a short term burst of high RoC for some safety reason.

I can't see any magical reason to level off at 5,000ft and lean. But, periodically, say every 3,000ft or so, if it's a long climb, richen a little and back off again to make sure you're still getting best RoC. More importanty keep monitoring CHT and EGT.

At ToC, richen slightly (or fully if you want, it's only for a half-minute or so and won't make any significant difference to overall fuel burn) and follow the usual procedures for levelling off at your correct cruise speed, then lean for cruise. Depending upon engine and aircraft that's likely to be either just rich of rough running or, usually a little better, a hair-breadth lean of the EGT peak point. (With the proviso that if the POH says something different about the method for leaning for cruise, then it may be better to follow that.)

If you do have fuel flow, firstly don't trust it. Fuel flow gauges on aeroplanes are seldom very accurate. But, it should be accurate insofar as telling you if a change you've made gives more or less fuel flow. If you want accurate cruise data, use the POH cruise conditions - with a fixed pitch prop on a PA28-161 and normally aspirated engine but good instrumentation, I'd set RPM for the desired cruise power (generally ~60% if going a long way or trying to conserve fuel, ~80% if in a hurry), lean using the normal EGT / rich of rough method, then during the cruise (and whilst still monitoring CHT and EGT limits) fine tune to get lowest fuel flow in level flight.

G

Boffin at large
Various, southern UK.

There is no substitute for having a multicylinder engine monitor.

With it, you can do it all properly.

Without it, you are at best guessing and burning loads of extra fuel.

The standard advice to not lean below say 5000ft is OK (and that's what I do if climbing only to a few k, because it involves no work) but has no basis in physics or engineering. The optimal way is the constant-EGT method, which is to catch the EGT just after takeoff, and then lean continuously throughout the climb so as to maintain that figure. The mixture doesn't have to be done exactly; a tweak say every 1000ft is fine.

Then, when level in cruise, a simple lean for peak EGT does the job.

The descent doesn't normally require any mixture adjustment, until the usual full-rich on short final. However, if descending from a reasonable altitude, say 8000ft or more, enrichment is necessary, because peak-EGT at say 2000ft is not the same mixture setting as peak-EGT at say 8000ft. If you descended from say 15000ft (peak EGT) to the surface, the engine would probably cease combustion (and would feel very rough) before you got there.

The most important thing is maintaining the CHTs below some reasonable figure, like 400F max. This needs good airflow, which is one one should not just climb at Vx or Vy as trained. One should climb like that until clear of obstacles, and then transition to a higher speed, to cool the engine. In the TB20, I climb at 120kt and that only just manages to keep the engine below about 400F (ISA conditions).

Administrator
Shoreham EGKA, United Kingdom

Piper Archer

At the risk of being rude to my fellow posters I have to say I think great care should be taken when extrapolating advice given by pilots flying high performance singles, especially those with turbo charged engines, to your situation.

I think what you are doing is perfectly fine and any investment in an engine monitor for a simple carburated 4 cylinder Lycoming is a waste of money. If the engine is running smoothly it is impossible for it to be dangerously lean. A single probe CHT is helpfully however to prevent overheating and if you really want belt and suspenders get a Multi probe CHT with a probe on each engine.

The way your engine gets hurt is typically by overheating. Long climbs at low airspeeds on hot days are particularly problematic. Heat output is proportional to engine power which is why most POH say to run full rich below 5000 feet. However they also say you can lean at or below 75% power so there is nothing to stop you from figuring out from POH data, at what altitude full throttle will give you 75% power and start leaning as you pass that altitude. However most days that point will be close to 5000 feet and so the amount of fuel saved over just leaning at 5000 feet every time will IMO be insignificant.

I see no reason to level off at 5000 feet, Just slowly lean to max RPM as you pass 5000

As was already mentioned proper climb speed is important. Too slow and you risk overheating. I teach climb at an airspeed that will give you a climb of 500 feet min reducing to no lower the Vy + 10 kts.

The engine should be leaned in cruise flight at any altitude

For descent leave the mixture leaned and simply reduce the throttle as you descend to maintain the same RPM as the engine RPM ell increase as you descend. The worst thing you can do is go to full rich at the start of a long descent.

Finally the best insurance against engine overheating is good engine baffles and flexible seals. I regularly see private aircraft with baffles that are just trashed. In one case this resulted in 4 heat damaged cylinders and a $8000 bill. If the owner had invested $1000 in baffle Maintenence, the cylinders would probably still be fine.

Wine, Women, and Airplanes = Happy
Canada

I don't much point in telling PiperArcher how to lean with engine instruments when he told us that he didn't have any.

I don't see much wrong with what you're doing in the climb. My only comment is that you're throwing away a lot of fuel while on the climb to 5000ft but it probably doesn't take so long so not too much of an issue.

I used to be in your scenario previously and what I would do is similar except I'd stop climb at 2000ft, set 75% power or less and then lean properly and then continue the climb. The climb would take longer but at least I'm not throwing away fuel.

At 75% power Lycombing say that you can't damage the engine so no problem being agressive with the leaning.

The stopping at 2000ft is because you need a stable level speed to find the right rpm to give 75% power with a fixed pitch prop.

In the desent I think you should be reducing the leaning as you desend. Otherwise you might have no power when you needed it because the mixture is too lean for the new air pressure.

EIWT Weston
12 Posts
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