F

Frank Smidler

Well Known Member
Last Friday I was heading home to Indiana from Iowa at 11,500 ft with a good tailwind pushing me along at over 200 kts ground speed. I couldn't believe it when the vnav on my Garmin 296 notified me that it was time to start my decent to KLAF when I was still 70 nm out. I have it set for 500 ft/min decent. Gotta love these planes.

As I started my decent I began to think about the best way to handle the high altitude leaned out mixture during a long decent. I have a 0-360 A1A carburated engine and I usually push the mixture full rich as I start the decent. Is this the best way to manage the mixture during decent or are there other proven procedures that I should be using?

Thanks
 
I don't enrich in descent at all.. I usually pull the power back every so often to keep manifold pressure from building up too much.. I don't have exact numbers that I follow every single time.. I kinda go by ear/feel instead.. You can go with pull 1" of MP every 1-2k ft.. either way, no need to enrich mixture downhill.

Also, as a general guideline, your mixture should never be full rich at 11.5k... that's way too rich...
 
mixture and MP info

Hi guys,
I may be one of many who was never properly 'trained' in the use of mixture, and what the heck the MP really MEANS to operation of our engines.
Can anyone suggest a good source of practical info on this? There seems to be as much ( or more) opinion than fact out there.

thx
 
I enrich further periodically as I descend. If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.

My airport is near 6860 feet MSL so I rarely see full rich.
 
Ron Lee said:
I enrich further periodically as I descend. If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.

My airport is near 6860 feet MSL so I rarely see full rich.
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At our 4200-4600' airports, we seldom see full rich either. You can even tell the difference when taxiing with fuel rich. Normal procedure is to start the engine at full rich, then immediately pull the knob out a bit. Usually 1/2 to 5/8".

I never go full rich at landing, with these altitudes either. It has a tendency to foul plugs, which is easily noticeable on the mag check.

L.Adamson --- RV6A
 
I do not even start full rich. I know about where the mixture should be and start there. Then lean out more to taxi.

Verify correct mixture during runup.
 
What settings are you using?

Ron Lee said:
If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.
Click to expand...

Which "setting" should you know? Are you talking about EGT? CHT? Fuel Flow? MP?

I am getting a better understanding of leaning as I climb, but descent still has me somewhat confused. I guess my saving grace on descent is that I have already reduced power (2300 rpm and 18" MP gives me a relaxed descent), and I go over-rich as I descend.

What I'm thinking (haven't tried this yet) is that I should maintain altitude and pull power/prop to 2300/18" and let EGT stabilize - maybe 30 seconds. Then begin descent and maintain EGT at the stabilized temp by adjusting mixture. At low power, that technique should keep my engine leaned for the current altitude and avoid fouling the plugs.

What do you think?
 
Descent

Lycoming says it is unnecessary to richen mixture during descent at less than 75% power if engine is running smoothly. We have followed this for years and several Lycomings for more than 2000 hrs each before uneventful o'haul. So there you have it - both authoritative and anecdotal!
 
Scott, at my field elevation (>6800 feet), the mixture is pulled out a certain distance at optimal mixture setting as determined during the runup. By this I mean max RPM while varying the mixture setting at around 1700 RPM. At sea level, you may not do that.

As I climb, I lean out more as determined by EGT. So the difference in how much the mixture is leaned out (knob pulled aft) at altitude and where it is on the ground is gradually reduced as I descend. If I land closer to sea level, the mixture knob gets closer to completely in as I approach pattern altitude.

I am not an expert but I would not enter the traffic pattern near sea level with the mixture knob set for 12,500' or higher. My guess is that a go-around full power situation could result in the engine quitting if it does not during the descent. By gradually pushing the mixture in during descent, instead of doing it as part of a pre-landing checklist, I avoid that possible outcome.
 
Frank Smidler said:
Last Friday I was heading home to Indiana from Iowa at 11,500 ft with a good tailwind pushing me along at over 200 kts ground speed. I couldn't believe it when the vnav on my Garmin 296 notified me that it was time to start my decent to KLAF when I was still 70 nm out. I have it set for 500 ft/min decent. Gotta love these planes.

As I started my decent I began to think about the best way to handle the high altitude leaned out mixture during a long decent. I have a 0-360 A1A carburated engine and I usually push the mixture full rich as I start the decent. Is this the best way to manage the mixture during decent or are there other proven procedures that I should be using?

Thanks
Click to expand...

While I do not have an RV (at least not yet, I am still working on the wings), what I do is when ascending, is when the plane gets to 5000' MSL, I start leaning. Then at 6000' MSL, lean a bit more, at 7000'MSL, a bit more, and so on in 1000' increments. I find that doing so makes it easier for me to keep track of the leaning.

By the same token, when descending, I also enrich in 1000' increments, and by the time the plane is at 5000' MSL, it is at Full Rich.

I hope this helps!
 
Crossbow said:
By the same token, when descending, I also enrich in 1000' increments, and by the time the plane is at 5000' MSL, it is at Full Rich.
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On the other hand, if you were taking off at my 4607' msl airport, you would be at less than full available power.......because it wasn't leaned. And I'll never be full rich when I return to this airport.

Mountain flying books, and even some POH's these days, suggest to think about leaning above 3000' msl, and especially when density altitude is higher.

But, many pilots who fly from sea level airports, do much of what your doing.

I had a heck of a time convincing a pilot from the UK to lean before takeoff at my airport's altitude. But once they do, they can really tell the difference.

L.Adamson --- RV6A
 
mixture

I also use the 3000' msl altitude as a good place to start leaning. Above 3000' there becomes a noticible roughnesss and power reduction when the mixture is at full rich. I like to takeoff and make the initial climb set at full rich (down here at 1285' msl) because I think the evaporative cooling of the extra rich fuel mixture helps keep CHTs cool during takeoff and climb. On a descent from altitude, as long as the engine is at a mid or lower power setting, I don't enrichen the mixture until I'm on downwind for landing.

CDE
 
Don't have an RV, and my Cessna 180 has a Continental O-470, but I think this might be generally applicable. My home field is at 5,050 feet.

1. On take-off, I lean (even at sea level) for best power at 1,800 rpm at run-up.

2. I lean more as I climb.

3. At cruise, lean using either the six-cylinder EGT gauge or by the sound of the engine. The engine sound is more reliable, and at high altitude and low power, this engine will easily go lean of peak. I generally prefer to run at close to best power, which is easily felt. The EGT gauge almost always will suggest a pretty rich setting and it lags, too.

4. Gradually go more rich during descent. One day I forgot to. I'd been cruising at 9,500 and while descending to a field at about 1,000 feet, on short final, the engine quit. I knew immediately what had happened (that low-altitude oxygen helped) but I had the runway made so I elected to land. I completed the landing, stopped the airplane and restarted the engine with a better mixture setting for that elevation. Repeat - gradually enrichen the mixture during descent.
 
My experience and technique has been very much the same as the other high field elevation airport guys (5050' here at Stead).

I spent a little time after I brought the airplane here trying to figure out what Larrry and Bill have wired down. For takeoff, I was looking for a mixture setting that gave me max RPM on takeoff, yet did not drive the temps too high during initial climb. After leaning to peak RPM during runup, it turns out I need to enrichen it slightly to keep from getting hot during climbs. At our 5K field, that turned out to be Larry's 1/2" to 5/8" back from full rich. I often reduce RPM to 2500 after initial climb, and have been also looking for a fuel flow at that reference RPM that works well for temps, as its easy to make a quick mixture adjustment based on the FF, then get the eyes back outside. I've found that, with my IO-540, that turns out to be about 18gph at 5K', and a bit over 20gph at sea level. Temps stay cool, and the motor is happy (not too rich, not too lean...the porridge is just about right ;)). The numbers will vary based on the engine type, but its a good tool for ball park estimates as you refine your technique.

I'm also in the camp of enrichening slowly as I descend, and I target that same 1/2" to 5/8" back for home field, full rich for SL, and in between for, well, in between field elevations. Nothing real scientific, just don't want to be too lean as I descend, nor do I want to be pumping avgas overboard in a plume of over-rich exhaust. Just rich enough to keep the motor cool and happy.

Just one possible technique, for sure.

Cheers,
Bob
 
Long descent

When my Garmin gives a start descent advisory, I decrease power to establish the descent rate. Generally I have to enrichen the mixture(?) slightly to keep the engine running. Your engine's best power usually comes at an A-F ratio of 12.5:1 which will give an EGT about 100F lower than peak EGT. You can use this or an even leaner mixture for lower fuel-flow when operating at less than 75% power. At higher power settings a richer mixture is used to lower CHTs and possible detonation. When flying along at lower altitude with the throttle pulled back to lower rpm and MAP, you can lean your mixture to get better mpg. BTW, I generally use a higher descent of -700 fpm to -1000 fpm in descent beacause it keeps me higher over intervening terrain to keep me out of the lower-altitude turbulence longer.
 
As a rule it is bad form to add more than 1 cooling mechanism to these engines at once.

When you descend you've added cooling already by increasing airflow, so don't add more by enriching or reducing power. My technique is to touch nothing upon initial descent. As you get lower you can pull the power in stages to keep from getting too lean. Managing the power is better than managing the mixture directly during descent.

This is the simplest way of managing your descent from both a power and mixture perspective and will keep your engine warmest at top of descent where the air is cooler and you want to go fast anyway.
 
Read Deakin !!!!!!

By the wide range of replies here, I can tell that lots of people that probably should be familiar with what Deakin says haven't read the Pelican Perch series
that is on the AVWEB and other places.

READ IT

This guy knows his stuff, he's studied and he's run engines in test cells as well as extensive flight testing and demonstration.

Short answer - don't richen on descent. The only reason I richen in the pattern is in case of a go-around. If I knew 100% sure that I wasn't going around, I would leave the mixture at the cruise setting all the way to the tie-down.
 
Wonder if he has ever bounced a taildragger...

...and needed a quick instant burst of power. Probably still available even leaned. I will have to try it, in a controlled environment. I dont go full rich but I do enrichen in the pattern. My engine will also stumble if I do not enrichen some when decending from high altitude.
 
On a decent I don't worry one bit, why, no load on the engine, you can leave the settings alone until... your in the pattern, than I richen the mix mainly for the possibility of a go around. I don't want to forget the mix in a crucial time. So I richen a bit for such a task, along with turning on the electric pump.Generally I'll poke around the local area, MSL is 2000 give or take a couple hun and fly around 3000 to 3500MSL with the map at no more than 23 and rpm set at 2400, cs prop, and lean to 50lop, you heard it, lop. I'm below 23 map and the engine doesn't care or know my altitude. Runs great and if I want to drop down I pull the throttle and do nothng else and do a drop and do anything I want, I don't touch the mix. It's flying at it's best. If I deside to run into the airport at full speed I richen to 50ROP and let her rip. Works for me.
 
i fly the same...........

setup as you. in level or descending flight below %75 power lean until rpms start to drop then back in a bit. its that simple with fixed pitch prop. thats the basics in a nut shell. any questions mate? well start leaning. during taxi you will be surprised how far out the mixture will be. :D
 
Since I run lean of peak in cruise, I would be concerned about "leaning some" in descent and putting myself right at peak EGT. Instead I only start adding a bit of richness as the engine starts to run rough, never bringing it up to max EGT (staying LOP). My EFIS helps with that, displaying LOP when it calculates that the combination of MP and FF = LOP.

Once I am under 3000' AGL (e.g. approaching pattern altitude) I will lean to ROP, and select full rich only on entering the pattern at low altitude fields on cooler days. On hot days in the SouthEast, the density altitude is almost always high enough to warrant a bit of leaning.
 
JonJay said:
snip in> ... if he bounced a taildragger landing...and needed a quick instant burst of power. Probably still available even leaned. I will have to try it, in a controlled environment. I dont go full rich but I do enrichen in the pattern. My engine will also stumble if I do not enrichen some when decending from high altitude.
Click to expand...

Deakin is indeed a good read...well worth it. Good call, and I reviewed it again it tonight. Great stuff!

On takeoff, I look for rated FF (if at SL) as he recommends, and watch the temps per his recs. That was where I culled the idea of using a rule of thumb on setting fuel flow in the climb (from a high altitude airport like home), and then watching for his gouge temps. The pull to 2500ish is for noise (as he discusses) when I feel the need for it, but I have no problem keeping it at max RPM (especially in a SARL race!) Just revisiting this, as I don't want to pass bad gouge...Deakin has some great info that is easily incorporated into the bag of techniques.

Jon, enrichening in the descent to keep from stumbling, or to stop stumbling, seems to be in concert with Deakin's article. That's why I posted that I slowly enrichen it...just wasn't so eloquent. I'm probably a bit ahead of where he would richen, but its certainly not at top of descent, and is in very small increments (quarter tuns, etc.).

After reviewing his articles again, I may give his techniue of waiting a bit longer to enrichen, to see where it starts to run a little rough before twisting the red knob, and also play with reducing RPM before coming off of WOT as he describes (I'm ususally at cruise RPM and WOT and let it run untill I have prep to slow for the pattern...maybe back off the MP down lower if I'm headed for a low elevation airport). For those of you that never or rarely touch the mixture in the descent, do you find that the engine runs smoothly all the way from, say mid to low teens until the pattern? Interested, as I may be being overly cautious, and always willing to learn.

And Jon, as one who may have bounced my taildragger...once or twice ;), I was thinking the same thing, and I do like to be at or near an approximate go around mixture for landing, just for that reason (or other go-around inducers). I know Deakin preaches Red, Blue, Black for go-arounds, and I practice that, but the bounce is a place where a little expedience pays off. Presetting the mixture as you enter the pattern and do the checklist makes it a little more Murphy-proof too (just one technique and opinion).

Great discussion and thanks for the jog to read Deakin again!

Cheers,
Bob
 
cropdusterengineer said:
On a descent from altitude, as long as the engine is at a mid or lower power setting, I don't enrichen the mixture until I'm on downwind for landing.
CDE
Click to expand...

Ditto. If my carbureted O-320 isn't being tasked to make any significant power during a descent, I don't want to waste fuel or risk fouling my spark plugs. Just being throttled back for the descent seems to offer more than plenty enough cooling for the cylinders, and the low compression Lyc with 100LL fuel is not going to have any detonation risk under such condition either.
 
Just remember the old days when you had a manual transmission. If you took off in 2nd or 3rd and floor it, chances are you will ping. If you had regular gas and the timing was to far advanced this would for sure be the case. I would think the main thing for causing concern in our lycs are going to be the timing and how much load we are putting on the engine. Load on the lyc would be the prop and than if we are climbing and a third... it's aerodynamics of the airplane. Our RV's are so slippery that we put way less stress on the engine over lets say a cessna. So to over load our airplanes are much less of a factor. Sure the mix will come in, but I think we can get away with more with our slippy airplanes than most. Just something to think about. Also if we are using 100LL we are already on the game because our engines can use much less octane than that, but we are using the 100octane, so we are already ahead of the game.
 
What Works for Me....

I've read most of Deakin, and pretty much agree with him - although I do always remember that he is generally talking about big motors and Continentals....

I am fortunate to have an EFIS that displays real-time "Percent Horsepower", so I can manage the engine the way Lycoming recommends, without having to mentally figure out where I am on the power curve from RPM and MAP. Knowing where you are on the curve is HUGE!

I am generally cruising the RV high enough that I have the throttle full open, RPM at a comfortable 2350, and the mixture leaned to where the engine is just about stumbling (generally Lean of Peak). This is always going to be less than 75% above 8,000' or so.

When I begin a descent, I blip the trim a couple of times to get the nose headed down, and don't touch the power levers - why give up sped when I don't have to? I manage airspeed in the descent to stay just below the airframe limit (200 knots TRUE for the RV-8), so long as the air is smooth. As I come down out of altitude, the %HP starts to climb. When it reaches 75%, I start throttling back to maintain 75% (which is where I have it set almost all the time at low altitudes). I generally find that as the power comes back, i have to enrichen the mixture just a nudge now and then to keep the engine running smooth. So long as I am below 75%, I keep it leaned as far back as I can until my pre-landing checklist.

That's pretty much it - simple management made really easy with %HP and TAS being computed and displayed. I have all the fancy EGT plots and graphs on my GRT, and watch them to keep the spread as low as possible. Engine seems real happy as we reach 1100 hours since new....

Paul
 
Paul - As I understand it, when you are LOP, then %HP would be directly proportional to fuel flow and not read off a Lycoming HP chart using MP and RPM. Is it possible then to get %HP from the GRT display?

regards

erich
 
erich weaver said:
Paul - As I understand it, when you are LOP, then %HP would be directly proportional to fuel flow and not read off a Lycoming HP chart using MP and RPM. Is it possible then to get %HP from the GRT display?

regards

erich
Click to expand...

Yes, the GRT EFIS displays % HP directly from the measurements it has available, plus a look-up table that is customized for the engine youhave installed.

Paul
 
Hmmm. Not sure I made my point - or perhaps Im just not understanding.

My point from previous post was that the GRT EFIS would have to know whether you were operating LOP or ROP, and then depending on that answer, decide how to calculate %HP; its not done the same way LOP as ROP. Is it really capable of this? If so, news to me.

erich
 
No, it's not that smart :) But in this particular case, it ends up being on the high side, so you're really safe as actual %HP will be lower than the readout.



erich weaver said:
Hmmm. Not sure I made my point - or perhaps Im just not understanding.

My point from previous post was that the GRT EFIS would have to know whether you were operating LOP or ROP, and then depending on that answer, decide how to calculate %HP; its not done the same way LOP as ROP. Is it really capable of this? If so, news to me.

erich
Click to expand...
 
Radomir's answer is right on - close enough for this type of job! Too many folks expect high precision in the measurements we use, but that's just not the case - consumer-grade transducer's and the like give us real accuracy within a couple percent. Fortunately, Lyc's are pretty forgiving - and frankly, the instruments they used to establish the "rules" probably weren't all that god 60 years ago (by today's standards).

Paul
 
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