Hi guys,

I have just completed one of the practice CyberExams for CSYA and came across this question:

An engine is running with the mixture fully rich. Progressive leaning to peak EGT will produce:

A. Best economy at peak EGT
B. Best power first, followed by best economy then peak EGT
C. Best economy first, followed by best power then peak EGT
D. Best power at peak EGT

My answer is: A.

My understanding was that best economy occured at peak EGT (or leaner than). However I seemed to have gotten this question wrong and just wanted to make sure my understanding of mixture control wasn't completely flawed!

Anyone care to explain the reasoning behind it?

Cheers,
Carambar

Carambar

Yes many modern engines do run at best economy at peak or even lean of peak. If this is the case it will be described in the aircraft's handling notes. However it is also true that most manufacturer's handling notes give economy setting as finding peak on the EGT then richen to about one division rich of peak. This technique is what the question assumed. However I must admit that it is increasingly common for economy to be at peak. If I got that question in a CASA exam I would select answer B.

It should be remembered that best economy is not just as lean as possible, it also depends upon the resulting airspeed. If you were too lean, you would have a lower TAS and therefore it is possible you could burn more fuel on a given trip even at the lower fuel flow.

Assume we are talking about Lycomings. Am I right in thinking rich of peak is better from the standpoint of getting the best life from the engine also ? Runs cooler and smoother ? Or is it only going to damage the motor if you run it stupidly too lean?

Hi Bob, I noticed its been 8 years since your comment, is this still true for CASA exam? if leaning from full rich, we get best power first, followed by best economy then peak EGT?

While its always true that best economy will always be lean of best power, it is not always rich of peak. Some engines have best economy at peak and some have best economy a little lean of peak. Trust you Pilot's Operating Handbook, or engine manufacturers manual.

What about for CASA exam? I remember U wrote somewhere, Is the order peak EGT, Best economy then best power as mixture richened ?