Differential Fuel Burn question...

scottygofast · #1 12-16-10, 10:27 PM

Hello all..

Just wondering if anyone else has had a similar problem with their 337. On a 5 hour flight, I had a 15 gallon difference in fuel burn between the front and rear. The only fuel burn/quantity info I have is standard cessna gagues and the front egt is inop. Ive been leaning the mix according to the POH, any both levers are in the same position.. any ideas what might be going on? I did notice a 10 min difference in fuel burn time when exhausting the 18 gal aux tanks, but dont know how to explain any further? Thanks!

Roger · #2 12-17-10, 07:40 AM

How can you lean to POH standards with an inop EGT ?

Gord Tessier · #3 12-17-10, 08:06 AM

The best way to answer that one would be to invest in an engine monitor. The position of the levers, though even. does not mean the settings are the same. An engine monitor will tell you right away if you have a cold cylinder and a host of other issues that you can't tell otherwise. I had a rear engine failure and was able to detect it almost immediately because I could see the cylinders going cold. It may have saved my life as I was bout to go over water.

WebMaster · #4 12-17-10, 09:42 AM

Fix The Egt!!!

WebMaster · #5 12-17-10, 09:59 AM

The good news is you weren't too lean. Obviously running way rich of peak.

Ernie Martin · #6 12-17-10, 10:41 AM

I agree that you do have to get the EGT gauge fixed to properly lean per the POH, but if you are coming close to leaning both engines per the POH* , my main concern is that the 3 gallon per hour difference may be a problem far worse than unbalanced leaning. You might have a tank leak or, even worse, a leak elsewhere in the fuel system. I would have this looked at ASAP.

Ernie

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* Coming close is doable even without an EGT gauge by an experienced pilot, familiar with both engines, using the standard fuel flow gauges, relying on the roughness that accompanies peak EGT, and then enriching from peak EGT by the same flow rate needed to enrich the other engine after its peak EGT.

hharney · #7 12-17-10, 11:13 AM

Remember, the fuel flow indicator is based on pressure not flow. Without the EGT to verify the setting, there may be a pressure issue.

rhurt · #8 12-19-10, 03:39 PM

Scotty,

Did your fuel flow indicate the same value for both engines? I agree with Gord that the mixture lever position is not the best indicatoin of fuel flow.

The way I run the engines I would have burned just shy of 100 gallons in 5 hours and with 128 gallon tanks that only leaves about 15 gallons per engine. Yikes!

Randy

Walter Atkinson · #9 02-08-11, 01:53 PM

Quote:

Originally Posted by larry bowdish

The good news is you weren't too lean. Obviously running way rich of peak.

What is "too lean?"

The only mixture that is "too lean" is one that will not burn--that's WAY LOP--and called Idle Cutoff.

If one is "not rich enough" on the rich side of peak EGT, the CHTs and ICPs can be dangerously high. I'm thinking that's what you meant. Right? Not too lean, but "not rich enough."

As for the engine monitor... it's clearly the most important engine management tool you can have. It will pay for itself many times over in reduced maintenance costs and can save your tail, too. They tend to pay for themselves "all at once!"

Gord Tessier · #10 02-14-11, 07:27 PM

Hi Walter I just had work done to my fuel system and when it came back they has buggered the setting for the fuel flow. The answer is simple, have your mechanic adjust the fuel flow-rate on the plane with the engine running. he or she will know what to do.

Walter Atkinson · #11 02-14-11, 09:07 PM

Yes, and the FF should be adequate on that engine to result in EGT readings of about 1300dF during a sea level, full power takeoff. That will require that the FF be adjusted a little above the red line FF. Consider the red line to be a MINIMUM, not a MAXIMUM value.

That adjustment will result in cooler CHTs and lower ICPs which are healthier for the engine.

hharney · #12 02-23-11, 10:30 PM

Gord

Here is what I have used to set my fuel flows. When I performed this test it changed my OEM indicator reading from the high green arc to the red line on take off. I only adjusted the front engine and then test flew the aircraft.

During the test we had difficulty reaching the 2800 rpm value that the chart suggests. There is a factor to use if you cannot reach this value. Problem was (and I am going by memory here) is that we could only achieve 2500 rpm on the ramp. The equation to use for lower rpm tests did not go that low. We had to graph the line out further to find the intersection and set the pressure at that point. When I perform a take off the rpm achieve the full 2800 rpm and my fuel flow (based on pressure) goes just beyond the red line on the front engine.

I have just installed a JPI engine analyzer with fuel flow and before I adjust the rear I will fly the aircraft to see the difference in fuel flow with the original settings. Should everything go right tomorrow I should be able to test the JPI for the first flight tomorrow.

See attached procedure

hharney · #13 02-26-11, 08:58 PM

The test on the new JPI EDM 760 was flawless. I flew again today to record some numbers for the warranty card. Amazingly the fuel flows were very consistent with my factory fuel flow gauge. I have flown in other aircraft including Skymasters were the numbers were miles apart. Mine seem to be fairly close. Have others out there had similar results? The amount of information that this unit provides was rather intimidating while reading the handbook but once I started using it I could see that it was very simple to use. I haven't mastered the fuel quantity and usage yet but that will come.

Hope to get in a good cross country next week and have a chance to really see it work and to be able to dial it in at cruise. Should be a trip to Dayton to scout the hotel accommodations for SOAPA meeting. I'll post my results.

Walter Atkinson · #14 02-28-11, 07:10 PM

Congratulations on your new monitor.

Engine monitors are so valuable that I consider them a "Go--No-go" item for an IMC flight and I would prefer not to fly VMC without them.

The challenge is to learn how to interpret the data that they display. Gaining that understanding will save you a lot of money on maintenance and could, one day, save your life.

(some of the info in the manual concerning the interpretation of the data is factually incorrect--like what happens to the EGT and CHT during pre-ignition. Use the manual to learn how to operate the instrument. It is quite good for that. Be cautious of the materials related to interpreting the data.)