Full rich at Altitude?

[clh]

I have a mechanic that insists that the POH states to run full rich when cruising at high altitude.  I cannot find it anywhere.  I asked him if he was thinking of turbocharged aircraft, but he said no, the lycoming IO360.  Has anyone heard of this?  I cannot find it anywhere in my POH.   Other than dumping fuel and fouling plugs, I cannot think of a reason to run full rich above ~4500 ft.

 

[carusoam]

I think you would need clarification on that.  Full rich below 3,000 has been taught to me while primary training.  Your POH will indicate power settings at altitude by following a specific leaning procedure.  My M20R POH mentions 50dF rich of peak. 

[rbharvey]

He is wrong. Turbos cannot be run LOP above certain altitudes (like 23k for Acclaim) but normally aspirated motors will cough and die at full rich at high altitudes, no question about it

[DaV8or]

He's crazy wrong. Your POH will give you power settings for different altitudes with corresponding fuel burns. Usually there is a "best power" setting and an "economy" setting. Somewhere in the beginning of the performnce chart pages they will state what they mean by these setting and they are somewhere around 100 ROP and Peak I think. Anyhow both settings are achieved with leaning to some degree. If your mechanic is telling you contrary to the POH, I would consider finding another A & P. Maybe what he means is he is telling you to run ROP at altitude, but not full rich. This is the prevailing advice of the A & Ps that I have met.

[jlunseth]

I am curious about the advice to not run LOP in a turbo above 23k?  Why would that be?  I can understand it if you are above critical altitude and the MP is falling, maybe that is what is going on.  I still have throttle left at FL230 in my M20K 231.  It seems to me that you should be able to run anywhere you would at sea level.

On the OP's question, I agree, way too rich to run full rich at altitude in a normally aspirated. 

[danb35]

If he's saying the POH says this, ask him to show you where.  Of course, it isn't in there, but you already knew that.

[N9937c]

Most A&P's will not believe that a pilot and aircraft owner will question what spews from the mouth of a FAA CERTIFIED MECHANIC. After all It takes 2 years of school to know EVERTHING about ALL aircraft.

I just ran up a turbo arrow that had the fuel system overhauled by a reputable shop and installed and adjusted by an A&P IA with CERTIFIED GUAGES. Take off power 40" and 2575 rpm was set at 15.1 GAL per HR. You talk about LEAN OF PEAK!!! After "TWEAKING" fuel flow now is 23 gph as per manual. AMAZING!!!!!

larry

[FlyDave]

If I didn't lean my engine at the latest above 5,000' It would start running so rough I'd be looking for a place to land.

Full rich at altitude? I think (hope) he mis-spoke.

[FlyDave]

Also, don't forget we're talking about density altitude. On a hot summer day I start leaning around 2,000'.

[KLRDMD]

Quote: rbharvey

He is wrong. Turbos cannot be run LOP above certain altitudes (like 23k for Acclaim) but normally aspirated motors will cough and die at full rich at high altitudes, no question about it

[jax88]

Perhaps he/she is confused by item 2 of the "Cruise & Range Data Conditions" (as it appears in my POH) which states that "The data is taken from flight tests at full-rich mixture settings above 75 percent rated power and best-power mixture settings for cruise at 75 percent rated power and less."  In addition, the heading of each of the altitude Cruise & Range charts has a statement that reads "Use FULL RICH mixture above 75 percent power."

[Hank]

I don't even fly full rich TO altitude, much less AT altitude.

Since I have a carb, I usually back off the throttle ~1/2" for better mixing, then lean away per my "Owner's Manual" to 25º rich for best economy or 100º rich for best power. I'm usually in the 50-75º rich neighborhood depending on how closely I squint at the little lines on my factory EGT.

When I climb high, I pull back gently on the red lever in the climb, too, adhering to Mooney's insistence on 200º rich and CHT below 400º.

[Ned Gravel]

OK.  I finally did it!!

Took off from du Page near Chicago for my trip home today (near Toronto, Ontario) and when I was finally released to cruise altitude of 7000' I decided to try LOP for real.  I had previously tried it but the engine always went really rough on me.  This time it worked and I learned a few things.

First, I set up LOP at or below 70% when it is much easier to slowly go through peak EGT and determine the numbers myself (I know that the monitors all do that, but the process was not making me feel comfortable).  At 7000' and pulling 2500/22" is shown as 72% BHP in my owner' manual (POH) so any reduction in any of these two parameters will keep it below 70%.  At or below 70%, I am told that I do not have to worry about the Red Box where internal combustion pressures will hurt the engine.

Here are my numbers with Peak EGT determined to be 1480 degF.

7000' 50 degrees LOP as determined by Cyl 4

          Cyl 1     Cyl 2     Cyl 3     Cyl 4

EGT    1467     1472     1440     1433

CHT      313      291       325       289

 

7000' 100 degrees LOP as determined by Cyl 4

          Cyl 1     Cyl 2     Cyl 3     Cyl 4

EGT    1417     1437     1394     1383

CHT      305      285       316       268

The engine sounded only slightly less solid than it does at or beyond 70 ROP.   But it did not sound rough (Finally!!!!).  I lost about 2-3 knots indicated for 50 LOP and another 1 or 2 for 100 LOP.  I should have made careful measurement when I filled it up to leave, but I did not know I was going to try this.  I left the tanks an inch below the caps when I filled up to leave and attendent filled them up all the way after I landed.  I should have payed more attention, but the result was 33 gallons for a three hour flight.  So, slightly more than 10 gph.

One observation was the extreme sensitivity of the reference cyllinder's EGT mixture knob inputs when lean of peak.  Quarter turn and I was down 30 degF on number 4. Another is that once the system is set, it will wander as much as 15 degF up and down without making any changes other than direction (no altitude changes).  Mind you the QNH varied by as much as .4 inches during this flight.  I was travelling from a lower pressure area to a higher one.

Way forward?  Do more of this.  Get up.  Get settled and when it is safe (70% or below with all other parameters operating nominally) rotate the mixture knob through peak.  I have also been told that peak at altitude is "best power" mixture and I am OK with this so long as it is not in the Red Box.

Feel good about this.  Been wanting to do it for some time.  Just need to see a start point that did not make me worry and when I recognised the condition as less than 70% BHP, it all fell into place.

I also think I may end up swapping some injectors to even out the temperatures a little more.  (Next annual).

This is me saving gas money now.........

 

[Ned Gravel]

Double post

[KLRDMD]

Quote: edgargravel

Here are my numbers with Peak EGT determined to be 1480 degF.

7000' 50 degrees LOP as determined by Cyl 4

          Cyl 1     Cyl 2     Cyl 3     Cyl 4

EGT    1467     1472     1440     1433

CHT      313      291       325       289

 

7000' 100 degrees LOP as determined by Cyl 4

          Cyl 1     Cyl 2     Cyl 3     Cyl 4

EGT    1417     1437     1394     1383

CHT      305      285       316       268

[Ned Gravel]

Ken:

Thanks for the clarification.  What I thought was 100 degF LOP was near the edge of the engine beginning to act rough.  Now I know that the current point for that to happen in my engine is closer to 70 degF LOP. With some slight changes in setup and matching of injectors, that will improve.

I do not have fuel flow, but it is something I want to add to my JPI.

 

[KLRDMD]

Quote: edgargravel

What I thought was 100 degF LOP was near the edge of the engine beginning to act rough.  Now I know that the current point for that to happen in my engine is closer to 70 degF LOP. With some slight changes in setup and matching of injectors, that will improve.

[Capt_CrashN_Burn]

Quote: rbharvey

He is wrong. Turbos cannot be run LOP above certain altitudes (like 23k for Acclaim) but normally aspirated motors will cough and die at full rich at high altitudes, no question about it

[carusoam]

"Getting to 70º LOP with stock injectors isn't bad at all."

How LOP can you go?  

much below 50deg LOP, I start losing too much speed, and vibration starts to show up. (IO550G)

-a-

[danb35]

Quote: rbharvey

Turbos cannot be run LOP above certain altitudes (like 23k for Acclaim)

[rbharvey]

Quote: danb35

Add me to the list of people who would like some clarification on this point--I suspect it's incorrect, but I'd like to hear some explanation of why you believe it to be the case.

[rbharvey]

Quote: danb35

Add me to the list of people who would like some clarification on this point--I suspect it's incorrect, but I'd like to hear some explanation of why you believe it to be the case.

[clh]

I finally took another look at the original POH.  In the "CRUISE PROCEDURES"  section under "LEVEL-OFF PROCEDURES" it states

"4. Lean the mixture for smooth operation (refernce Lycoming's Operators Manual.)  The Exhaust Gas Temperature Indicator is to be used only as an aid in setting the mixture during cruise at powers up to 75% power, but not at take-off, climb, or descent.  For lean-out procedures at 75% power or lower, lean to peak temperature, then lean until temperature drops 25* F. minimum."

If I read this right, full rich for take-off, climb or descent.  When over 75% power, lean to smooth operation.   But under 75% run at least 25*F lean of peak!

So It looks like I need to be running LOP........

[danb35]

Which year and model is this?  I didn't know of Mooney advising LOP operations on early models.  On mine, the POH guidance is to not lean at all at or above 75% power, that best power was 100 ROP, and best economy was peak EGT.

Note, unless it's in the "limitations" section, anything in the POH is merely a recommendation--you're free to operate contrary to that guidance if you wish.

[clh]

1964 M20E.  It is the original manual. I am the third owner and have talked with the two previous ones.