Mixture rich for taxi?
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5000' rule of thumb
I figured that the "not below 5000' " rule was more about the constant power changes, climbs descents and manouvering in the training area and was a simple explanation given to students in preference to a proper explanation. 
As others on this thread have noted, the person giving the explanation probably wouldn't know why themselves and couldn't work it out.
I teach the guys to lean as soon as you are established in steady cruise, be it 500' or 10,000'. Every flight manual I have ever read gives fuel burn figures based on a properly leaned mixture.
Sunfish:I have never ever seen an Ops manual (read many and written a few myself) even suggest that aircraft should be planned, let alone operated, at full rich all the time.
As others on this thread have noted, the person giving the explanation probably wouldn't know why themselves and couldn't work it out.
I teach the guys to lean as soon as you are established in steady cruise, be it 500' or 10,000'. Every flight manual I have ever read gives fuel burn figures based on a properly leaned mixture.
Sunfish:I have never ever seen an Ops manual (read many and written a few myself) even suggest that aircraft should be planned, let alone operated, at full rich all the time.
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The one over-riding thing I get from this thread is that EDUCATION is what is needed here... not arbitrary 'rules' that essentially achieve nothing (no leaning on the ground, 3000', 5000' et al) and may even be DAMAGING your engine -they're certainly distorting your fuel-burn figures. Students deserve the respect of being taught the correct way to handle their engines from the ab initio stage, and if, as an instructor you don't know or are not sure then it is YOUR RESPONSIBILITY to train yourself first, then pass that knowledge on to your students.
Grandpa Aerotart
Interesting innit that people complain about the cost of hiring but never give a second thought about what they cost the aircraft owner by running around full rich in a wet hire aircraft.
Late last year I flew a mates 1956 C180 in the UK for the first time since that aircraft was a P2 registered C180 that I had last flown in 1987 in PNG.
He had sold a share to a retired BA captain who had spent a career in jets.
They operated it full rich all the time based on 'advice' received that leaning was of no value in the 0470..I suggested that was a load of ****e and proceeded to explain in detail why.
I flew it three sectors from the farm strip, where they keep it, to IOW, West Waltham and back to the Farm totalling around one hour. I leaned it on the ground as soon as it was running smoothly, used full rich for takeoff and climb, leaned in cruise/descent/landing, taxied leaned. We never got above 2000'.
We used 15 liters/hr - GBP20.00 less fuel per hour than they usually plan on. They previously worked on GBP120/hr DOCs...I suggested that GBP2000.00++/annum was probably a worthwhile saving even ignoring that the engine would give less trouble because it was being operated properly..."Point well made!" was the comment from the retired BA captain as we drove away from the farm.
In general:
ALL 'modern' piston aircraft should be leaned brutally on the ground. You will never hurt the engine doing that and for those engines that foul plugs it will make a big difference.
FULL RICH for every takeoff except takeoffs above about 5000' DA when they should be leaned for best power...as in the point being made by tinpis about PNG.
Lean appropriately in cruise no matter the cruise altitude. Appropriately means 80-100 ROP at power settings above 65% power and anywhere you like at 65% or less in aircraft not fitted with balanced injectors and an all cylinder monitor. In aircraft so fitted appropriately can mean anywhere between 50F LOP and 80F ROP depending on altitude and how fast you want to go..as long as CHTs remain below 380F.
In climb lean above 5000'...if you read the POHs they generally say 'don't lean in climb below 5000' not 'don't lean below 5000'. 5000' in normally aspirated engines equates to roughly 65% power at full throttle and leaning a little will help climb performance enormously. Just pull the mixture back 'a bit'...you want to be 100F+ ROP in climb above 5000'. The power curve is very flat in this area of the mixture spectrum so precision is not required...if you have an all cylinder monitor lean until hottest cylinder says 380F...and keep leaning 'a bit' each 1000' on climb above 5000' until cruise alt is reached.
On descent I don't touch the mixture at all in my Bonanza. I don't touch it usually until I pull it to cut off after I have parked. This because I am almost always cruising LOP. When I reduce MP around 4000' to keep speed in the green arc I am effectively enrichening the mixture because I have reduced the air while leaving the mixture control unchanged. When I bring the throttle back from wide open to around 18 in you can actually see the EGTs increase, peak and then fall a little onto the rich side of peak...this is the hottest place to run your engine and on highspeed descent the best place...even at top of the green IAS and 18in MP my cylinders rate of cooling slows and almost stops.
You do need to remember to select full rich if doing a go around. It is very good airmanship to check mixture, prop and throttle in that order whenever you are about to apply maximum power in a piston engine..whether taxi-> takeoff, cruise-> climb or approach-> go around.
Any 'Instructor' that tells you different should not be let anywhere near a student.
Late last year I flew a mates 1956 C180 in the UK for the first time since that aircraft was a P2 registered C180 that I had last flown in 1987 in PNG.
He had sold a share to a retired BA captain who had spent a career in jets.
They operated it full rich all the time based on 'advice' received that leaning was of no value in the 0470
..I suggested that was a load of ****e and proceeded to explain in detail why.I flew it three sectors from the farm strip, where they keep it, to IOW, West Waltham and back to the Farm totalling around one hour. I leaned it on the ground as soon as it was running smoothly, used full rich for takeoff and climb, leaned in cruise/descent/landing, taxied leaned. We never got above 2000'.
We used 15 liters/hr - GBP20.00 less fuel per hour than they usually plan on. They previously worked on GBP120/hr DOCs...I suggested that GBP2000.00++/annum was probably a worthwhile saving even ignoring that the engine would give less trouble because it was being operated properly..."Point well made!" was the comment from the retired BA captain as we drove away from the farm.
In general:
ALL 'modern' piston aircraft should be leaned brutally on the ground. You will never hurt the engine doing that and for those engines that foul plugs it will make a big difference.
FULL RICH for every takeoff except takeoffs above about 5000' DA when they should be leaned for best power...as in the point being made by tinpis about PNG.
Lean appropriately in cruise no matter the cruise altitude. Appropriately means 80-100 ROP at power settings above 65% power and anywhere you like at 65% or less in aircraft not fitted with balanced injectors and an all cylinder monitor. In aircraft so fitted appropriately can mean anywhere between 50F LOP and 80F ROP depending on altitude and how fast you want to go..as long as CHTs remain below 380F.
In climb lean above 5000'...if you read the POHs they generally say 'don't lean in climb below 5000' not 'don't lean below 5000'. 5000' in normally aspirated engines equates to roughly 65% power at full throttle and leaning a little will help climb performance enormously. Just pull the mixture back 'a bit'...you want to be 100F+ ROP in climb above 5000'. The power curve is very flat in this area of the mixture spectrum so precision is not required...if you have an all cylinder monitor lean until hottest cylinder says 380F...and keep leaning 'a bit' each 1000' on climb above 5000' until cruise alt is reached.
On descent I don't touch the mixture at all in my Bonanza. I don't touch it usually until I pull it to cut off after I have parked. This because I am almost always cruising LOP. When I reduce MP around 4000' to keep speed in the green arc I am effectively enrichening the mixture because I have reduced the air while leaving the mixture control unchanged. When I bring the throttle back from wide open to around 18 in you can actually see the EGTs increase, peak and then fall a little onto the rich side of peak...this is the hottest place to run your engine and on highspeed descent the best place...even at top of the green IAS and 18in MP my cylinders rate of cooling slows and almost stops.
You do need to remember to select full rich if doing a go around. It is very good airmanship to check mixture, prop and throttle in that order whenever you are about to apply maximum power in a piston engine..whether taxi-> takeoff, cruise-> climb or approach-> go around.
Any 'Instructor' that tells you different should not be let anywhere near a student.
Last edited by Chimbu chuckles; 31st Oct 2006 at 07:44.
Great advice, you guys.
I've only ever leaned in the cruise - can't imagine why anyone wouldn't do that, but the leaning on the ground bit sounds like something everyone should know about and at least consider.
The Cessna 172 Pilot's Guide I've got (by Jeremy Pratt) calls for:
Full rich for takeoff & climb except when departing an airfield > 3000 AMSL, where it calls for leaning for max power;
Leaning in the cruise to 25-50 rpm less than peak rpm on the lean side for best economy;
Use of full rich with more than 75% power set, noting that above about 5000 ft, full throttle will be less than 75% so leaning would be OK.
Doesn't talk about leaning on the ground, but it seems like a bloody good idea to avoid plug fouling.
I've only ever leaned in the cruise - can't imagine why anyone wouldn't do that, but the leaning on the ground bit sounds like something everyone should know about and at least consider.
The Cessna 172 Pilot's Guide I've got (by Jeremy Pratt) calls for:
Full rich for takeoff & climb except when departing an airfield > 3000 AMSL, where it calls for leaning for max power;
Leaning in the cruise to 25-50 rpm less than peak rpm on the lean side for best economy;
Use of full rich with more than 75% power set, noting that above about 5000 ft, full throttle will be less than 75% so leaning would be OK.
Doesn't talk about leaning on the ground, but it seems like a bloody good idea to avoid plug fouling.
Grandpa Aerotart
Yes of course...F not C
AOTW and others...if you lean on the ground, and I highly recommend you do, you should lean so brutally that the engine is not capable of much more than 1000 rpm. I find in my Bonanza that I never need much more than 850-900 rpm taxiing around and because I am not worried about fouling plugs etc I use power not brakes to keep my speed under control...that is I will quite happily taxi at 500 rpm if that's all it takes...so I save on brakes as well. If I am parked on level ground all I need do to commence taxiing is release the park brake and the aircraft moves off at 850 rpm. The way I set rpm after start is to use the throttle to idle the engine at around 700-750rpm and then lean the mixture to get peak rpm and then lean a little more...as you lean on the ground from around 700-750rpm you will see about a 30-50 rpm rise before it peaks and falls away again.
If it does not the idle mixture is set up wrong.
The reason I suggest leaning on the ground to such an extent is that if you forget to enrichen the mixture for takeoff you will be incapable of taking off...when you push the throttle forward the engine will suffer a lean mixture cutout before any damage is done. Embarrassing but not expensive. If you do a mag/prop check before takeoff you won't be able to get enough RPM for that either...so when doing the mag/prop check select mixture full rich and then leave it there for takeoff...unless operating at high DA.
In my Bo I don't do a 'classic' mag check as taught in flying school because it shows you pretty well nothing other than each mag is earth properly...first flight of the day I cycle the prop once or twice and in cruise LOP I select each mag and sit and watch the EDM 700 for 10 seconds or so at each setting. Before shutting down I check for 'dead mag/live prop' and that is it.
When I select L or R mag all the EGT bar graphs increase in lockstep...that shows that combustion has slowed considerably due to less efficient combustion caused by only 1 mag operating...as a result the temp shown on the EGT increases. IF one EGT bargraph does not increase I know which mag in which cylinder is not operating properly and in concert with smooth operation on one mag have just tested the system completely.
In the old days of big arse radials they checked Mags at 'field barometric' which equates to 29 inches MP. At that power setting and with the instrumentation available in the old prop airliners you actually learned something...you don't learn much at 1700 rpm in a Cessna.
AOTW and others...if you lean on the ground, and I highly recommend you do, you should lean so brutally that the engine is not capable of much more than 1000 rpm. I find in my Bonanza that I never need much more than 850-900 rpm taxiing around and because I am not worried about fouling plugs etc I use power not brakes to keep my speed under control...that is I will quite happily taxi at 500 rpm if that's all it takes...so I save on brakes as well. If I am parked on level ground all I need do to commence taxiing is release the park brake and the aircraft moves off at 850 rpm. The way I set rpm after start is to use the throttle to idle the engine at around 700-750rpm and then lean the mixture to get peak rpm and then lean a little more...as you lean on the ground from around 700-750rpm you will see about a 30-50 rpm rise before it peaks and falls away again.
If it does not the idle mixture is set up wrong.
The reason I suggest leaning on the ground to such an extent is that if you forget to enrichen the mixture for takeoff you will be incapable of taking off...when you push the throttle forward the engine will suffer a lean mixture cutout before any damage is done. Embarrassing but not expensive. If you do a mag/prop check before takeoff you won't be able to get enough RPM for that either...so when doing the mag/prop check select mixture full rich and then leave it there for takeoff...unless operating at high DA.
In my Bo I don't do a 'classic' mag check as taught in flying school because it shows you pretty well nothing other than each mag is earth properly...first flight of the day I cycle the prop once or twice and in cruise LOP I select each mag and sit and watch the EDM 700 for 10 seconds or so at each setting. Before shutting down I check for 'dead mag/live prop' and that is it.
When I select L or R mag all the EGT bar graphs increase in lockstep...that shows that combustion has slowed considerably due to less efficient combustion caused by only 1 mag operating...as a result the temp shown on the EGT increases. IF one EGT bargraph does not increase I know which mag in which cylinder is not operating properly and in concert with smooth operation on one mag have just tested the system completely.
In the old days of big arse radials they checked Mags at 'field barometric' which equates to 29 inches MP. At that power setting and with the instrumentation available in the old prop airliners you actually learned something...you don't learn much at 1700 rpm in a Cessna.
Last edited by Chimbu chuckles; 31st Oct 2006 at 09:50.
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Some good stuff coming out in this thread at last. The chap who mentioned the Pelican's Perch articles had it spot on, that is the definitive reference as far as I'm concerned.
I've flown in formations where I held position with judicious mixture adjustments while leaving the throttle and RPM alone. Those flights made it glaringly apparent to me that the amout of power the engine produces is heavily dependant on the amount of petrol you pour in - and more gas doesn't always = more power.
Also, there is more to setting power than throttle and mixture. If you fly a type equipped with a CSU, RPM is an integral part of engine management, and again, more RPMs isn't always equivalent to more knots.
My rule of thumb was to minimize RPM, maximize MAP, and set mixture to just above rough running/just below CHT limit. I also leaned on the ground, but only aggressively (ref Chimbu's comment), and generally only on long taxi's.
And I managed to learn that at less than 29" and without a "big arsed radial", so it would appear one can learn something in light aircraft if one pays attention.
I've flown in formations where I held position with judicious mixture adjustments while leaving the throttle and RPM alone. Those flights made it glaringly apparent to me that the amout of power the engine produces is heavily dependant on the amount of petrol you pour in - and more gas doesn't always = more power.
Also, there is more to setting power than throttle and mixture. If you fly a type equipped with a CSU, RPM is an integral part of engine management, and again, more RPMs isn't always equivalent to more knots.
My rule of thumb was to minimize RPM, maximize MAP, and set mixture to just above rough running/just below CHT limit. I also leaned on the ground, but only aggressively (ref Chimbu's comment), and generally only on long taxi's.
And I managed to learn that at less than 29" and without a "big arsed radial", so it would appear one can learn something in light aircraft if one pays attention.
Last edited by Capt W E Johns; 31st Oct 2006 at 08:45. Reason: Spelling
Grandpa Aerotart
My reference to 'big arsed radials' was only for magneto checks...typical mag checks as taught by most flying schools in C152s etc indicate a lot less about the condition of the system than you'd think. That is the reason I do my mag checks in cruise. I learned that from talking to Deakin about 5 years ago.
I agree about RPM...unless you are talking MP/RPM AND Mixture as a whole you really are not discussing the complete picture...having said that the only engine control that can adjust power from 0%-100% is the mixture control.
The throttle position and prop rpm control the air side of the fuel/air ratio and the mixture controls the fuel side with a slight overlap in the prop control because prop rpm also governs engine driven fuel pump rpm...I can lean or enrichen the mixture within a small range just with prop rpm
The mixture control is the most important engine control and the least well understood/least well taught.
I agree about RPM...unless you are talking MP/RPM AND Mixture as a whole you really are not discussing the complete picture...having said that the only engine control that can adjust power from 0%-100% is the mixture control.
The throttle position and prop rpm control the air side of the fuel/air ratio and the mixture controls the fuel side with a slight overlap in the prop control because prop rpm also governs engine driven fuel pump rpm...I can lean or enrichen the mixture within a small range just with prop rpm
The mixture control is the most important engine control and the least well understood/least well taught.
Last edited by Chimbu chuckles; 31st Oct 2006 at 09:21.
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Ahh indeed, on re-reading that is apparent! Consider my hat doffed, sir.
May I say, though, that I have caught more than a couple of dodgy mags on the pre-takeoff mag checks, returning to the line to find that they were indeed unservicable.
May I say, though, that I have caught more than a couple of dodgy mags on the pre-takeoff mag checks, returning to the line to find that they were indeed unservicable.
Grandpa Aerotart
that I have caught more than a couple of dodgy mags on the pre-takeoff mag checks
That shows me as much as I need to know before getting airborne and as much as I would learn doing the 'classic mag check' as taught to me 26 years ago.
In cruise WOTLOP selecting L or R and leaving it at each position for an extended time while closely monitoring the EDM shows me whether my system is functioning perfectly or not....it shows me how well the plugs AND magnetos are functioning under load as opposed to just functioning.
I actually lean the mixture on climb from about 1500' on up but only because I have an all cylinder monitor...I have actually climbed LOP on many occassions if only climbing below 3-5000'....but climbing LOP requires knowledge, confidence and a good all cylinder monitor.
Last edited by Chimbu chuckles; 31st Oct 2006 at 09:59.
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climbing LOP requires knowledge, confidence and a good all cylinder monitor
More rule of thumb from where I worked (*dons helmet and flak jacket for incoming criticism*) was to lean to nearly rough running, squeeze in about another 10 l/hr, and monitor CHT to ensure limits were maintained. Seemed to work (never broke one, anyway).
CC to hear is to obey. I keep asking myself "What don't I know that I should know?" Pprune often provides the answer in the least painful way. I wish I'd learned about "pilot induced oscillation" here....it would have been much cheaper
Grandpa Aerotart
No flack jacket required...I did the same for about 3500 hrs of piston single and twin flying, both normally aspirated and turbocharged, in 'the good old days'...but then we knew no better and all cylinder monitors had yet to be invented.
In PNG we were nearly always above full throttle height...taking off at MTOW with 23-25 inches being all the MP available was something you did 6-10 times a day. Knowing how to lean the mixture in the first seconds of the ground roll was essential to longevity...your's not the engine's
My early instructors treated the mixture control as a normal control to be understood (within the constraints of the technology of the day) and used the way any other control is...not as though red meant 'don't touch'. Any modern day CFIs allowing/encouraging such ignorance, and there clearly are a number judging by this thread, are guilty of negligence bordering on criminal...there is just no excuse when information of the type in Deakin's articles is freely available and has been for over 6 years.
Even if the aircraft you fly does not have the latest injector/engine monitor technology the information is still essential...so you know what NOT to do....and more importantly why.
EDIT for Sunfish Just click on the link in OZBUSDRIVERS post on page two, save it to favorites and then read every article down the right side of the page that link leads to. Some will take a LOT of thinking about to understand..when you do understand the bulk of what is in those articles you will be a better pilot than now...you will know more than most instructors. From an engine handling point of view you will have every aeroplane you are ever likely to fly covered...he also covers a hell of a lot of basic airmanship stuff. Based on reading Deakin's stuff after meeting him years ago I fitted Millenium cylinders, Gamijectors and and EDM 700 to my IO550 equiped Bonanza when it was having an engine overhaul. I can personally vouch for everything he says....it works, I see it work every time I fly my aeroplane.
In PNG we were nearly always above full throttle height...taking off at MTOW with 23-25 inches being all the MP available was something you did 6-10 times a day. Knowing how to lean the mixture in the first seconds of the ground roll was essential to longevity...your's not the engine's
My early instructors treated the mixture control as a normal control to be understood (within the constraints of the technology of the day) and used the way any other control is...not as though red meant 'don't touch'. Any modern day CFIs allowing/encouraging such ignorance, and there clearly are a number judging by this thread, are guilty of negligence bordering on criminal...there is just no excuse when information of the type in Deakin's articles is freely available and has been for over 6 years.
Even if the aircraft you fly does not have the latest injector/engine monitor technology the information is still essential...so you know what NOT to do....and more importantly why.
EDIT for Sunfish Just click on the link in OZBUSDRIVERS post on page two, save it to favorites and then read every article down the right side of the page that link leads to. Some will take a LOT of thinking about to understand..when you do understand the bulk of what is in those articles you will be a better pilot than now...you will know more than most instructors. From an engine handling point of view you will have every aeroplane you are ever likely to fly covered...he also covers a hell of a lot of basic airmanship stuff. Based on reading Deakin's stuff after meeting him years ago I fitted Millenium cylinders, Gamijectors and and EDM 700 to my IO550 equiped Bonanza when it was having an engine overhaul. I can personally vouch for everything he says....it works, I see it work every time I fly my aeroplane.
Last edited by Chimbu chuckles; 31st Oct 2006 at 18:40.
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Where I learned to fly they mixed MOGAS in with the AVGAS, attempting to lean that caused the engine to stop, so fully rich in all stages of flight and altitudes. One guy 400+ hours decided to lean the engine at 6000', the engine stopped, and he descended, it took him until 4000' before he got it restarted.
On another point, flying in California during the summer, if you didn't lean the engine for taxi the plugs fouled before we got to run ups, sometimes they would foul up if they were leaned and you were at a high altitude airport (1000'-2000' amsl) and a couple of times we couldn't burn them clean and had to taxi back to get a mechanic to actually remove and clean them.
As has been said before operate the aircraft to the POH.
On another point, flying in California during the summer, if you didn't lean the engine for taxi the plugs fouled before we got to run ups, sometimes they would foul up if they were leaned and you were at a high altitude airport (1000'-2000' amsl) and a couple of times we couldn't burn them clean and had to taxi back to get a mechanic to actually remove and clean them.
As has been said before operate the aircraft to the POH.
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Chimbu,
Excellent, excellent posts! Just have a question for you though. You say that you leave the mixture control in the leaned position from the cruise as you descend. As you descend the air gets more dense and so wouldn't that require a more rich mixture? So if you leave it in the leaned position shouldn't the engine cut out?
Excellent, excellent posts!
Just have a question for you though. You say that you leave the mixture control in the leaned position from the cruise as you descend. As you descend the air gets more dense and so wouldn't that require a more rich mixture? So if you leave it in the leaned position shouldn't the engine cut out?
Grandpa Aerotart
tinny
Creampuff is correct.
On a typical descent from say 7-9000' I initially just poke the nose down to achieve a desired ROD. Typically this will give me a TAS around 200kts in my A36 even while staying in the green arc...I like going fast
Obviously during the descent TAS drops off and IAS increases so I will first wind the prop back...all the way back, to keep IAS in the green arc or at most, in smooth air, in the lower part of the yellow arc.
This has the effect of reducing the speed of the engine driven fuel pump which, taken alone, leans the mixture somewhat, but it also reduces the speed of 'the air pump', the cylinders move more slowly at the reduced RPM so less air is sucked into the combustion chambers and that, taken alone, would enrichen the mixture.
The net effect is slightly enriched mixture but remember my starting point is LOP, Lean Of Peak EGT, so enrichening the mixture increases the EGT back towards Peak...slightly. I am putting a little more heat into the cylinders.
By the time I am around 2-3000' I will have 26 or 27 inches MP (the throttle stays wide open from takeoff until this point) and say 2000 rpm and I will be losing the battle to keep IAS down to an acceptable value.
At this point I reduce the MP in one fell swoop from 27in to 17-18in MP. There is nothing scientific about this figure...it is just above the gear warning horn. When I do this I drastically reduce the air available to the combustion chambers. Fuel flow has not changed hugely so I have drastically enrichened the mixture.
To this day I always watch the EGT columns and CHTs on the EDM 700 when I do this because I still find the effect amazing. In lockstep all the EGT columns march up to peak EGT and fall slightly over onto the rich side of peak EGT...that is they all rise LOTS and then fall slightly. The CHTs which have all been cooling gently due to the increased airflow actually cool at a slower rate. Between Peak EGT and 50F ROP is the range where you're putting maximum heat into the cylinders...it is the hottest part of the mixture spectrum.
I carry this power setting into the circuit and decelerate level aiming to be comfortably below gear extension speed abeam the landing threshold...I lower the gear and then bring the MP back to 14-15in MP and fly the approach at that power setting merely adding drag to control speed reduction rate down to a target threshold speed of about 75kts...if I judge it just right
Many reading this might say "well that is all good but I don't fly an aeroplane with that technology so it doesn't apply to my operation".
That is BS.
When you understand how an engine works and all the things that control 'mixture' you can apply it in any aeroplane...even if it is just to not do certain things which may damage an engine.
Altimeters asked whether leaving the mixture leaned all the way through descent might not cause the engine to stop...clearly I have shown it won't operating the engine the way I do with the technology I have had installed in my 36 year old Bo.
What about a bog standard Bonanza, 210, 182 etc..how do you apply this knowledge toany aeroplane?
In aeroplanes that are limited to the rich side of peak you are obviously going to be starting your descent at a mixture setting somewhere around 50F-80F ROP.
Everyone is taught to enrichen the mixture 'a little' each 1000' or so on descent...I was years ago too...but why?
My engine operates happily 50F LOP without cutting out...it does so smoothly because of the balanced injectors...the only reason any engine can't operate LOP is because the fuel flows to each cylinder are so all over the place that the engine won't run smoothly LOP. When a standard engine runs rough when leaned it is not about to stop it is just suffering from 4 or 6 different fuel/air ratios. You effectively have 4 or 6 single cylinder engines running at different horse powers bolted to the aeroplane...and that is why they rattle away.
So if the engine is not about to stop why enrichen it constantly on descent.
To be quite frank I have no feckin idea...and I am willing to bet it is not a knowledge based technique...that removes it from the field we are discussing and puts it squarly into the legend/Old Wives Tale sphere.
Lets look at it from the point of view of what we already know about what things control mixture and where that mixture needs to be to help us achieve long engine life.
We know that the mixture range between Peak EGT->50F+ ROP = Peak CHT.
So you are cruising along FD&H leaned to about 75F ROP. You commence descent and as you descend the mixture is leaning out due to increased air density.
Common dogma suggests enrichening the mixture a bit but if you do that you're just pouring cold fuel into the engine for no real reason.
If you don't your engine will be moving from 75F ROP back towards peak EGT...back towards peak CHT which is good because it will keep you cylinders nice and warm in that increased airflow. You might even end up somewhere near peak. If you then reduce MP or RPM (in a fixed pitch aeroplane) you will enrichen the mixture back towards the 50F odd ROP mixture setting because whether you reduce the airflow via a throttle controlling MP or RPM the effect is the same...LOTS less air/slightly less fuel flow = richer mixture.
Whether starting from a LOP setting or a ROP setting you still want to control mixture to help control the rate of cooling of the CHTs.
It is NOT and NEVER WAS about lean mixture cutout.
Edited to make something a little clearer. When you close the throttle you obviously effect fuel flow as well as airflow...but if the RPM/Mixture settings remain unchanged the over riding effect is much less air and a little less fuel flow so a much richer mixture entering the combustion chambers
Creampuff is correct.
On a typical descent from say 7-9000' I initially just poke the nose down to achieve a desired ROD. Typically this will give me a TAS around 200kts in my A36 even while staying in the green arc...I like going fast
Obviously during the descent TAS drops off and IAS increases so I will first wind the prop back...all the way back, to keep IAS in the green arc or at most, in smooth air, in the lower part of the yellow arc.
This has the effect of reducing the speed of the engine driven fuel pump which, taken alone, leans the mixture somewhat, but it also reduces the speed of 'the air pump', the cylinders move more slowly at the reduced RPM so less air is sucked into the combustion chambers and that, taken alone, would enrichen the mixture.
The net effect is slightly enriched mixture but remember my starting point is LOP, Lean Of Peak EGT, so enrichening the mixture increases the EGT back towards Peak...slightly. I am putting a little more heat into the cylinders.
By the time I am around 2-3000' I will have 26 or 27 inches MP (the throttle stays wide open from takeoff until this point) and say 2000 rpm and I will be losing the battle to keep IAS down to an acceptable value.
At this point I reduce the MP in one fell swoop from 27in to 17-18in MP. There is nothing scientific about this figure...it is just above the gear warning horn. When I do this I drastically reduce the air available to the combustion chambers. Fuel flow has not changed hugely so I have drastically enrichened the mixture.
To this day I always watch the EGT columns and CHTs on the EDM 700 when I do this because I still find the effect amazing. In lockstep all the EGT columns march up to peak EGT and fall slightly over onto the rich side of peak EGT...that is they all rise LOTS and then fall slightly. The CHTs which have all been cooling gently due to the increased airflow actually cool at a slower rate. Between Peak EGT and 50F ROP is the range where you're putting maximum heat into the cylinders...it is the hottest part of the mixture spectrum.
I carry this power setting into the circuit and decelerate level aiming to be comfortably below gear extension speed abeam the landing threshold...I lower the gear and then bring the MP back to 14-15in MP and fly the approach at that power setting merely adding drag to control speed reduction rate down to a target threshold speed of about 75kts...if I judge it just right
Many reading this might say "well that is all good but I don't fly an aeroplane with that technology so it doesn't apply to my operation".
That is BS.
When you understand how an engine works and all the things that control 'mixture' you can apply it in any aeroplane...even if it is just to not do certain things which may damage an engine.
Altimeters asked whether leaving the mixture leaned all the way through descent might not cause the engine to stop...clearly I have shown it won't operating the engine the way I do with the technology I have had installed in my 36 year old Bo.
What about a bog standard Bonanza, 210, 182 etc..how do you apply this knowledge toany aeroplane?
In aeroplanes that are limited to the rich side of peak you are obviously going to be starting your descent at a mixture setting somewhere around 50F-80F ROP.
Everyone is taught to enrichen the mixture 'a little' each 1000' or so on descent...I was years ago too...but why?
My engine operates happily 50F LOP without cutting out...it does so smoothly because of the balanced injectors...the only reason any engine can't operate LOP is because the fuel flows to each cylinder are so all over the place that the engine won't run smoothly LOP. When a standard engine runs rough when leaned it is not about to stop it is just suffering from 4 or 6 different fuel/air ratios. You effectively have 4 or 6 single cylinder engines running at different horse powers bolted to the aeroplane...and that is why they rattle away.
So if the engine is not about to stop why enrichen it constantly on descent.
To be quite frank I have no feckin idea...and I am willing to bet it is not a knowledge based technique...that removes it from the field we are discussing and puts it squarly into the legend/Old Wives Tale sphere.
Lets look at it from the point of view of what we already know about what things control mixture and where that mixture needs to be to help us achieve long engine life.
We know that the mixture range between Peak EGT->50F+ ROP = Peak CHT.
So you are cruising along FD&H leaned to about 75F ROP. You commence descent and as you descend the mixture is leaning out due to increased air density.
Common dogma suggests enrichening the mixture a bit but if you do that you're just pouring cold fuel into the engine for no real reason.
If you don't your engine will be moving from 75F ROP back towards peak EGT...back towards peak CHT which is good because it will keep you cylinders nice and warm in that increased airflow. You might even end up somewhere near peak. If you then reduce MP or RPM (in a fixed pitch aeroplane) you will enrichen the mixture back towards the 50F odd ROP mixture setting because whether you reduce the airflow via a throttle controlling MP or RPM the effect is the same...LOTS less air/slightly less fuel flow = richer mixture.
Whether starting from a LOP setting or a ROP setting you still want to control mixture to help control the rate of cooling of the CHTs.
It is NOT and NEVER WAS about lean mixture cutout.
Edited to make something a little clearer. When you close the throttle you obviously effect fuel flow as well as airflow...but if the RPM/Mixture settings remain unchanged the over riding effect is much less air and a little less fuel flow so a much richer mixture entering the combustion chambers
Last edited by Chimbu chuckles; 1st Nov 2006 at 13:59.
When you live....
I'm starting to believe that Chimbu Chuckles and John Deakin are the same person........
Listen carefully to what chuck says and read (no, understand - you'll have to read it many times!) the Pelican's perch articles and prepared to have to learn a whole new approach to engine management.
I'm still working on it - my leaning techniques have improved out of sight and I'm not scared of the little red knob and detonation anymore - but not quite up to the descent techniques just yet - planning a few flights with a second pilot so that I can concentrate on verifying the theory. Get your head around the fact that it's almost impossible to cook an engine at <65% power and then go and play.
At least I was taught at PPL level what leaning was all about and that it was required but without really understanding it found myself nervous about it and not doing it properly - it is this that has the potetial to harm an engine and so I suspect is the reason why schools go with the just don't touch it and the plane won't fall out of the sky theory.
Besides, the looks on the faces of some of your 'educated' passengers when you reach for the mixture soon after takeoff can be priceless!
UTR.
(A John Deakin/Chimbu Chuckles convert)
Listen carefully to what chuck says and read (no, understand - you'll have to read it many times!) the Pelican's perch articles and prepared to have to learn a whole new approach to engine management.
I'm still working on it - my leaning techniques have improved out of sight and I'm not scared of the little red knob and detonation anymore - but not quite up to the descent techniques just yet - planning a few flights with a second pilot so that I can concentrate on verifying the theory. Get your head around the fact that it's almost impossible to cook an engine at <65% power and then go and play.
At least I was taught at PPL level what leaning was all about and that it was required but without really understanding it found myself nervous about it and not doing it properly - it is this that has the potetial to harm an engine and so I suspect is the reason why schools go with the just don't touch it and the plane won't fall out of the sky theory.
Besides, the looks on the faces of some of your 'educated' passengers when you reach for the mixture soon after takeoff can be priceless!
UTR.
(A John Deakin/Chimbu Chuckles convert)
