LOP Descent

[teethdoc]

I've been trying to read everything I can about LOP vs. ROP.  When you are running LOP, how do you handle your descent?  At what point do you run your mixture back up?

[1964-M20E]

I generally leave the mixture and prop where they are during the decent and monitor FF, CHT and EGT.  If I level out at some alt before entering the airport zone or established on the approach I’ll assess the fuel flow, MP, mixture, EGT and CHT.  I generally do not push the mixture in unless I’m going around for some reason.    During my initial decent, generally 500 to 600FPM, from cruising altitude to pattern or approach altitude I’ll manage MP to no more than 25” unless I need to descend faster then I’ll throttle back as necessary.

[John Pleisse]

Leave everything at cruise LOP setting until abeam the numbers. However, when you get a vector after being told to level off, then get a traffic alert, watch your airspeed bleed, advance the throttle while looking for the traffic, then hear a loud POP? That would also be a great time to advance the mixture.

[Super Dave]

I keep it LOP all the way in. As fuel flow creeps up in the descent, I adjust mixture to keep fuel flow at the same value. This results in a constant HP as the engine goes farther and farther LOP as MP increases in the descent. At some point, throttle is reduced, and I leave the mixture alone till tweaking it for the taxi to the hanger.

I've also experimented with maintaining a constant EGT in the descent, which works fine, but this increases HP as you descend and it may be possible to move into the "red box" as you descend. Maintaining a constant fuel flow keeps a constant HP and you actually move farther away from the red box as you go farther LOP.

I have a throttle quadrant, so I remind myself that in the event of a go-around I'll need to push a handful of levers forward. If I had vernier controls I might consider going full rich at the final GUMP check.

My IO360 with stock injecters stays smooth using this method. But if yours starts to run rough as it goes farther LOP, you could enrichen just enough to keep things smooth.

[Earl]

If you take the advancedpilot course they will tell you they don't touch anything other than MP until you are on roll out after landing.  If you have to go around you push everything forward.  But I agree with Super Dave that with a Mooney it might make sense to push the mixture forward as part of the GUMP check.  That's what I do but I don't fly LOP because my GAMI spread is a little too much to run LOP.  At my next annual I am going to have GAMI injectors installed.

[teethdoc]

I keep it LOP all the way in. As fuel flow creeps up in the descent, I adjust mixture to keep fuel flow at the same value. This results in a constant HP as the engine goes farther and farther LOP as MP increases in the descent. At some point, throttle is reduced, and I leave the mixture alone till tweaking it for the taxi to the hanger.

I've also experimented with maintaining a constant EGT in the descent, which works fine, but this increases HP as you descend and it may be possible to move into the "red box" as you descend. Maintaining a constant fuel flow keeps a constant HP and you actually move farther away from the red box as you go farther LOP.

I have a throttle quadrant, so I remind myself that in the event of a go-around I'll need to push a handful of levers forward. If I had vernier controls I might consider going full rich at the final GUMP check.

My IO360 with stock injecters stays smooth using this method. But if yours starts to run rough as it goes farther LOP, you could enrichen just enough to keep things smooth.

So say you are 25deg LOP and burning 10gph.  You constantly bump up mixture as you descend to keep it at 10gph?

[Bob_Belville]

I'm thinking leaner is better.

 

I took up my M20E Monday for 30 minutes. I flew around some checking everything since we just finished an annual. I didn't get above 3000' because of some broken clouds. I pulled back the throttle and prop but I didn't touch the mixture. Normally I would but I was interested in other things.

 

 I was the only plane in the area when I headed back to the airport. I was high when I made the decision to land straight in so I pulled back the throttle got the gear out and even the speed brakes. On short final I pushed the prop in and turned on the boost pump. SOP. I landed started rolling out and when I was slow enough to push in the throttle to taxi the engine quit, prop stopped. I coasted as far as I could but could reach the turn off. The engine started right up and after I taxied in front of the maint. hanger I could not make it quit by pulling throttle back to idle which was about 650RPM. We're theorizing the engine just loaded up, flooded. So... SOP for landing checklist of mixture full rich, boost pump on in anticipation of possible go around needs to be taken in context. I seem to recall 40 years and 3000 hours ago in trainers that clearing the engine with a momentary burst of throttle was part on the landing routine.

[yvesg]

Bob, what is SOP?

Yves

[Super Dave]

So say you are 25deg LOP and burning 10gph. You constantly bump up mixture as you descend to keep it at 10gph?

I don't know your model or engine. But for my IO360A1A, I actually need to tweak the mixture back a little in the descent to keep FF constant. If your FF naturally remains constant or decreases in the descent, I would leave it alone, unless I needed to enrichen for smoothness or more power.

[Bob_Belville]

Bob, what is SOP?

Yves

standard operating procedure

[BorealOne]

My practice is to keep MP to 20' on descent, and to gradually enrich. I find the EGTs get too hot if you don't touch the mix. Mind you, I'm usually coming down from the teens - I'll go from about 14 gph to 16-18 gph once I'm levelling off for an approach, and lower once things get slowed down.

[captainglen]

  Your engine, primarily the exhaust valves do not care about lean of peak or rich of peak, all they care about it temperature. Some older EGT indicators had a bulls eye at 1,400 degrees which at one time was considered ideal; current logic says 50 or 75 degrees off peak (I was taught 100). In carburated engines it is customary to run rich of peak because a slight vacuum leak could cause a cylinder or two to run a bit leaner than the richest cylinder. Typically this kind of imbalance will cause the engine to run rough when leaned to lean of peak. It is also fairly common for injected engines to run rough lean of peak which is why most mechanics do not reccomend it unless the engine is equipped with a matched set of injectors. A good graphic engine monitor that gives EGT for each cylinder can give you a lot more confidence when leaning your engine.

  As to when to enrich the mixture it is always best to go by the Pilot Opperating Handbook which assumes that leaning is an ongoing process except when at full power or just before landing in preparation for potential go around. At takeoff power a full rich in fact excessively rich mixture is used because the excess fuel is used as part of the engine cooling process. As for decent it is most important due to the high speed involved that all descents be made power on to avoid shock cooling of the front two cylinders. I will add a comment on shock cooling in that as a mechanic I find it far more common to find the back two cylinders declining in compression faster than the front two than the other way around. This could be because most pilots prefer power on descents or could be that shock cooling is really a bugaboo.

[Jeff_S]

I will not touch the mixture if I get a normal cruise descent into the traffic pattern and normal landing. As you descend, the mixture is only getting leaner and leaner so there's no harm you can do to the engine as long as you are making the power you need to achieve your goal. However, if I get put into an excessively long pattern or get vectored at a lower altitude where I need to maintain speed and altitude for any length of time, I will rebalance the mixture then to suit the needs of the moment.

[mike_elliott]

  Your engine, primarily the exhaust valves do not care about lean of peak or rich of peak, all they care about it temperature. Some older EGT indicators had a bulls eye at 1,400 degrees which at one time was considered ideal; current logic says 50 or 75 degrees off peak (I was taught 100). In carburated engines it is customary to run rich of peak because a slight vacuum leak could cause a cylinder or two to run a bit leaner than the richest cylinder. Typically this kind of imbalance will cause the engine to run rough when leaned to lean of peak. It is also fairly common for injected engines to run rough lean of peak which is why most mechanics do not reccomend it unless the engine is equipped with a matched set of injectors. A good graphic engine monitor that gives EGT for each cylinder can give you a lot more confidence when leaning your engine.

  As to when to enrich the mixture it is always best to go by the Pilot Opperating Handbook which assumes that leaning is an ongoing process except when at full power or just before landing in preparation for potential go around. At takeoff power a full rich in fact excessively rich mixture is used because the excess fuel is used as part of the engine cooling process. As for decent it is most important due to the high speed involved that all descents be made power on to avoid shock cooling of the front two cylinders. I will add a comment on shock cooling in that as a mechanic I find it far more common to find the back two cylinders declining in compression faster than the front two than the other way around. This could be because most pilots prefer power on descents or could be that shock cooling is really a bugaboo.

Hi Captain Glen,

Could it be the back to cylinders declining in compression could be caused by a lack of cooling relative to the front to vs. shock cooling? If you subscribe to shock cooling, wouldn't it follow that you must also subscribe to shock heating and could be really damaging the cylinders by pouring the coals to it with CHT's below 325 or so on take off? How would flying into a rainstorm/coldfront affect shock cooling? It would seem this could really cool down the cylinders in a hurry, shock cooling them. Just curious of your take on shock cooling.

As a LOP'er, I just leave the mixture alone during decent, until setting up for a go around on final, where I might have to shock heat that sucker and go around , so I enrich to about 1325 EGT on my plane which is close to max power setting of around 14.1 air to fuel.

[aaronk25]

If the landing requires a lot of workload or if I have a plane full of non-flyers I'll slide the mixture in a bit to a point where I know the engine won't miss but never ever ever ever full rich unless go around and I don't touch the mixture until down wind.

If I think my cruise setting is so lean that it might be to lean for the decent just richen it enough so you know it won't miss.

Lop decent is great to help clean or keep the combustion chamber clean at end of flight!

[Shadrach]

Your engine, primarily the exhaust valves do not care about lean of peak or rich of peak, all they care about it temperature. Some older EGT indicators had a bulls eye at 1,400 degrees which at one time was considered ideal; current logic says 50 or 75 degrees off peak (I was taught 100). In carburated engines it is customary to run rich of peak because a slight vacuum leak could cause a cylinder or two to run a bit leaner than the richest cylinder. Typically this kind of imbalance will cause the engine to run rough when leaned to lean of peak. It is also fairly common for injected engines to run rough lean of peak which is why most mechanics do not reccomend it unless the engine is equipped with a matched set of injectors. A good graphic engine monitor that gives EGT for each cylinder can give you a lot more confidence when leaning your engine.

As to when to enrich the mixture it is always best to go by the Pilot Opperating Handbook which assumes that leaning is an ongoing process except when at full power or just before landing in preparation for potential go around. At takeoff power a full rich in fact excessively rich mixture is used because the excess fuel is used as part of the engine cooling process. As for decent it is most important due to the high speed involved that all descents be made power on to avoid shock cooling of the front two cylinders. I will add a comment on shock cooling in that as a mechanic I find it far more common to find the back two cylinders declining in compression faster than the front two than the other way around. This could be because most pilots prefer power on descents or could be that shock cooling is really a bugaboo.

Shock cooling is a bugaboo. In my experience the hottest cylinders tend to go first and those are typically the rear cylinders (which tend to get less airflow, and therefor are less susceptible to "shock cooling"). A vacuum leak will cause any cylinder to run leaner of where it is set... No matter if the engine is injected or carbed; the leak is aft of whatever is metering fuel and therefore the cylinder with the leak will run leaner. Engine monitors are great. I am however of the opinion that without one, it's just as risky (or more so in some cases) to run ROP as LOP. Also when running ROP, a clogged injector or vacuum leak has the potential to fry a cylinder. When LOP, it just get cooler, makes less power and/or runs rough. Keeping all cylinders at peak or beyond is the safest operating regime IMNSHO...

[yvesg]

Could shock cooling only be an issue when the OAT is low like in the minus region? I flew the Mooney at -30 C (ground temp) several years ago and it was not pretty. I had more issues with the dials in the cabin than the engine though.

Yves

[1964-M20E]

-30C on the ground    I wouldn’t be anywhere but my house buried under blankets.  However, I do live on the gulf coast where a cold winter day is 35 to 40 degrees F and rarely do we see below freezing.

[Shadrach]

Could shock cooling only be an issue when the OAT is low like in the minus region? I flew the Mooney at -30 C (ground temp) several years ago and it was not pretty. I had more issues with the dials in the cabin than the engine though.

Yves

I doubt it.  At -30C the engine will likely never get that hot...  Folks in Alaska take off with those kind of surface temps regularly.   I'm with John...stay home.

[danb35]

When you are running LOP, how do you handle your descent?  At what point do you run your mixture back up?

I don't unless/until I have to go around.  Otherwise, I just keep my cruise mixture setting until shutdown, +/- giving a little more gas if the engine stumbles (which, if it happens at all, is usually during taxi).  Don't touch the prop either.  Normal descent for me is to just trim nose down to maintain ~ 500 FPM descent, and pull back on the throttle about 2000' above pattern altitude.

[jetdriven]

Remember the RSA-5 injection unit leans the mixture out when you retard the throttle.  Part of this is when the throttle is at the forward stop, it allows additional fuel via a different circuit.  Bringing the throttle off full leans it quite a bit.  So I usually leave the throttle full forward for the whole descent and dont touch the mixture.  Getting below 2000', I just limit FF to 10.0 GPH, and if any cylinders are >360, a little less to cool it down some.

 

2 miles from the airport, at pattern altitude and 160 knots, MP to 20", which usually requires some slight richening to keep it smooth. Abeam the numbers, I just set the mixture about 10-20% from full rich for a go-around.

[Shadrach]

I've been trying to read everything I can about LOP vs. ROP. When you are running LOP, how do you handle your descent? At what point do you run your mixture back up?

The answer is, it depends... If I'm landing at St Simon Island in GA in the early spring, I would surely treat the mixture differently than I would going into Colorado Springs in July. The question is where do I want the mixture based on the destination airports DA...

[Bob - S50]

We have and M20J.  I criuse about 10-30 LOP at WOT and 2400 RPM.  When its time to head down I roll the RPM back to 2200 RPM and start down at 500 fpm.  As I descend the MP will rise.  When it gets to 20" I keep pulling it back to keep it from going over 20".  That keeps my power low enough that I can descend at 500 fpm without exceeding the green arc, yet high enough to generate some heat.  As I descend I will also periodically try to enrichen the mixture slowly while I watch the EGT.  If the EGT goes down I quit.  If the EGT goes up, I continue until it peaks.  That's to help keep the engine warm.

 

When I level off near pattern altitude I'll leave it set at 2200/20".  That will give me about 135k true.  As I approach the pattern I pull it back to 17" which will slow me to 105k.  Abeam the numbers its gear down.  That slows me to 90k.  Approaching the base turn I go full flaps and when I've slowed to 80 I start the turn.  Since I usually have to pull the power back even further to keep the airspeed and glidepath where I want them, once I've done that I finally push the mixture and prop full forward in case I need to go around.  At the very low power setting, it usually makes no difference in engine power at that point.  I slow to 75k on final.

 

Hope that gives you one more option.

 

Bob

[fantom]

We have and M20J. 

 

By your tail number, I figure your J is a '77, Bob.

 

You would help yourself and us out if you post the model year on the left, along with your location.

 

And welcome to MooneySpace!!!

[Bob - S50]

Thanks for the suggestion.  I've updated my profile a bit to include a picture.  It is a '78 based at S50 (Auburn, WA)