Hey everyone, I have another question, this one about fuel mixture.

1. Takeoff: The checklist says the mixture should be adjusted according to the plate next to the fuel flow indicator. (Image attached).

Then, in the take-off section of the POH, it says, "To obtain maximum engine power, the mixture must be adjusted during the initial take-off run to the fuel flow corresponding to the airfield altitude.
(Refer to the 'Take-off and Climb Adjustments for Maximum Performance' plate, located next to the fuel flow indicator.) The power increase is significant above 3,000 feet, and this procedure should always be used for altitudes above 5,000 feet above sea level." Finally, where the EGT table is located, it shows the following (image attached).
* So, as I understand it, if I'm at an airport at 8,400 ft, I should adjust the fuel flow indicator to approximately 68 lbs/hr?
* When I try to adjust using the information in the EGT table, those references don't reach the EGT indicator. I have to adjust between 150° and 200° from the reference peak and increase the mixture, but the maximum it reaches is 100°. This also happens during the climb; it doesn't reach 125°, only 100°.

2. Climb: The checklist says lean mixtures up to a fuel flow of 78 lb/hr.
The POH for climb says, "Cruise climbs should be made at approximately 78 lb/hr up to 5500 ft and at 6 lb/hr more than the normal lean fuel flow shown on the power computer at higher altitudes and with reduced power."

* Since we are climbing above 8400 ft, what should I adjust? If 78 lb/hr should be maintained up to 5500 ft, and considering this, with a takeoff at 68 lb/hr, am I correct in the first question?

* Do I add those 6 lb/hr mentioned to the 68 lb/hr for takeoff or to the 78 lb/hr for the normal climb?

 

You recalled all that great detail.... but didn't tell us which aircraft you're referring to?

 

51 minutes ago, JYW said:

but didn't tell us which aircraft you're referring to?

B737 ?

🤣

Edited November 16, 2025Nov 16 by turbomax

50 minutes ago, JYW said:

You recalled all that great detail.... but didn't tell us which aircraft you're referring to?

 

Carenado C337 Skymaster

22 minutes ago, turbomax said:

B737 ?

Carenado C337 Skymaster

4 hours ago, wilivarob said:

So, as I understand it, if I'm at an airport at 8,400 ft, I should adjust the fuel flow indicator to approximately 68 lbs/hr?

Yes.

As for the rest, Carenado is not renowned for their systems modelling. They do a descent job, but I generally would not expect that you must follow the real POH to keep this plane in the air, or that fuel flow is modelled accurately. I remember that, in the last Carenados that I flew, I just pushed the throttle to max and it worked fine for the entire flight.

If you want to fly by the book, I would recommend A2A, Blacksquare (any model from each developer), or the Cessna 310R from Milviz/Blackbird. These models usually have a gauge that tells you the optimal setting for mixture.

Peter

For others following along: This is a continuation of a previous discussion on leaning in the Skymaster.

4 hours ago, wilivarob said:

So, as I understand it, if I'm at an airport at 8,400 ft, I should adjust the fuel flow indicator to approximately 68 lbs/hr?

No -- that number is from the "twin engine climb" row for 8,000 ft. For takeoff, you should use the first row ("takeoff or single engine climb"), which gives you a fuel flow of 78 lbs/hr at 8,000 ft.

4 hours ago, wilivarob said:

The POH for climb says, "Cruise climbs should be made at approximately 78 lb/hr up to 5500 ft and at 6 lb/hr more than the normal lean fuel flow shown on the power computer at higher altitudes and with reduced power."

For additional context: What throttle setting (manifold pressure) and prop rpm does the manual say you should use for the cruise climb?

4 hours ago, wilivarob said:

Do I add those 6 lb/hr mentioned to the 68 lb/hr for takeoff or to the 78 lb/hr for the normal climb?

Neither of these. (As an aside, you've got the numbers backwards -- it's 78 lb/hr for takeoff at 8,000 feet and 68 lb/hr for climb.)

What this is telling you to do is to lean according to the "normal lean" setting given in the "normal cruise" row of the table, which is 50 degrees Fahrenheit rich of peak. You should then note whatever fuel flow you're getting at that point, add 6 lb/hr, and enrichen the mixture to achieve the number you calculated.

As to why the manual is asking you to do this, I can only speculate that they want to enrichen the mixture to keep the engine running cooler because the airspeed will be lower in a cruise climb than in cruise, and so cooling won't be as effective.

As far as I understood after some googling, it's pretty simple - mixture full rich until cruise, then lean slowly until you see a bump in RPM, and follow EGT to max it out, of course by following POH and keeping the temperatures below maximum allowed.

2 hours ago, martinboehme said:

For others following along: This is a continuation of a previous discussion on leaning in the Skymaster.

No -- that number is from the "twin engine climb" row for 8,000 ft. For takeoff, you should use the first row ("takeoff or single engine climb"), which gives you a fuel flow of 78 lbs/hr at 8,000 ft.

For additional context: What throttle setting (manifold pressure) and prop rpm does the manual say you should use for the cruise climb?

Neither of these. (As an aside, you've got the numbers backwards -- it's 78 lb/hr for takeoff at 8,000 feet and 68 lb/hr for climb.)

What this is telling you to do is to lean according to the "normal lean" setting given in the "normal cruise" row of the table, which is 50 degrees Fahrenheit rich of peak. You should then note whatever fuel flow you're getting at that point, add 6 lb/hr, and enrichen the mixture to achieve the number you calculated.

As to why the manual is asking you to do this, I can only speculate that they want to enrichen the mixture to keep the engine running cooler because the airspeed will be lower in a cruise climb than in cruise, and so cooling won't be as effective.

OK, so:

1. For takeoff, I always use the data for “takeoff or single engine climb.”

2. For climb, the second line.

3. For cruise, let's say at 12,500 ft, the configuration would be 2,600 RPM, 16 MP, 168 TAS MPH, 95 TOTAL LBS/HR....

When I reach 12,500 ft, I start to lean the mixture until I reach the peak, then I enrich the mixture by 2 lines (50°F), which will give me a value X on the fuel flow indicator, to which I add 6 lbs/hr.

Now, the question I have always had is whether that value would be 95 lbs/hr from the performance chart values or whether that value has nothing to do with fuel flow.

This article may help by giving you a better understanding of this subject.

https://aerotoolbox.com/how-to-set-mixture/

 

Basically mixture is air to fuel ratio. For take off you goal is to achieve max power. For sea level take off mixture is always full rich. If airport is at high altitude you need to air to fuel ratio to squeeze  maximum possible power output.

In twins you always care if you can make take off if one engine quit during take off run. There are two possibilities you either able to stop on remaining runway (accelerated to stop distance), or continue climb on one engine. If you continue climb on one engine your highest priority is obstacle clearance. 

As you climb you have to maintain constant power power hence you need adjust mixture "as you go".

Finally, for a cruise you need adjust mixture depending on your priorities:  endurance, range , or speed. Which means you not always need to squeeze more power, hence you have 75%, 65%, 55% BHP (brake horse power)

By the way all power setting tables that you can find in POH or manual are based on properly "lean" setting - meaning you need adjust mixture to get it right.

9 hours ago, wilivarob said:

1. For takeoff, I always use the data for “takeoff or single engine climb.”

Correct.

9 hours ago, wilivarob said:

2. For climb, the second line.

Correct.

9 hours ago, wilivarob said:

3. For cruise, let's say at 12,500 ft, the configuration would be 2,600 RPM, 16 MP, 168 TAS MPH, 95 TOTAL LBS/HR....

When I reach 12,500 ft, I start to lean the mixture until I reach the peak, then I enrich the mixture by 2 lines (50°F), which will give me a value X on the fuel flow indicator, to which I add 6 lbs/hr.

You would only add the 6 lbs/hr if you wanted to do a cruise climb. If you want to level off at 12,500 feet (which I assume you do, given that you wrote "for cruise" above), you would simply lean as you described by reference to EGT.

The one caveat here is that I'm not sure what the manual means by "extended range mixture". In the excerpts you quoted earlier, it calls 50 ROP a "normal lean", so "extended range mixture" may be something different. Does this refer to peak EGT maybe? (I think you said in an earlier thread that this is from a POH published in 1971, so I'm pretty sure they weren't thinking about lean-of-peak operations.)

9 hours ago, wilivarob said:

Now, the question I have always had is whether that value would be 95 lbs/hr from the performance chart values or whether that value has nothing to do with fuel flow.

The value on the performance chart does have something to do with fuel flow, but it's the total fuel flow for both engines (which is what's relevant for flight planning). The fuel flow that you see on the engine instruments would be half that. In your example, with a total fuel flow of 95 lbs/hr, you'd expect the fuel flow gauges to show a fuel flow of 47.5 lbs/hr per engine.

Maybe controversial opinion here but mixture leaning really isn't an exact science. There are tables etc in the POH which give you guideline figures but even amongst the same aircraft type in the real world all engines are different, aging and other external factors can mean that no single setting will give the same outcome on two even 'identical' airframes. We've got a Warrior (160hp) in our fleet which seems to cruise about 10kts faster than all the others at the same RPM setting, and an Archer (180hp) whose performance only just about matches our slowest Warrior. Nobody can figure out why on both counts.

Lean it off until the engine faulters, then push it back in a bit. Or if you're blessed with an EGT gauge that works, lean it off until it peaks, and then push it back in a bit. Takeoff and climb should be at full rich, unless you're operating from an airfield at very high altitude and power output is compromised.

Edited November 17, 2025Nov 17 by Tom Wright

6 hours ago, martinboehme said:

You would only add the 6 lbs/hr if you wanted to do a cruise climb. If you want to level off at 12,500 feet (which I assume you do, given that you wrote "for cruise" above), you would simply lean as you described by reference to EGT.

So, if I'm cruising at 12,500 ft with a fuel flow of 95 psi and for whatever reason I want to climb to 13,500 ft, do I add 6 psi to the 95 psi?

And I assume that since I'm climbing, I should lean out the fuel, not enrich it.

6 hours ago, martinboehme said:

The one caveat here is that I'm not sure what the manual means by "extended range mixture". In the excerpts you quoted earlier, it calls 50 ROP a "normal lean", so "extended range mixture" may be something different. Does this refer to peak EGT maybe? (I think you said in an earlier thread that this is from a POH published in 1971, so I'm pretty sure they weren't thinking about lean-of-peak operations.)

That table is the one that comes with the plane by default; I'll leave you the one from the 1971 POH.

6 hours ago, martinboehme said:

The value on the performance chart does have something to do with fuel flow, but it's the total fuel flow for both engines (which is what's relevant for flight planning). The fuel flow that you see on the engine instruments would be half that. In your example, with a total fuel flow of 95 lbs/hr, you'd expect the fuel flow gauges to show a fuel flow of 47.5 lbs/hr per engine.

I have 2 indicators, one that is the analog one and another that has the GPS and this one is digital and gives me the exact total.

23 minutes ago, Tom Wright said:

Lean it off until the engine faulters, then push it back in a bit. Or if you're blessed with an EGT gauge that works, lean it off until it peaks, and then push it back in a bit. Takeoff and climb should be at full rich, unless you're operating from an airfield at very high altitude and power output is compromised.

Yes, I have the CHT temperature indicator and when I make adjustments on the EGT, I observe the temperature and it is within the range that the POH says it should be.