RAM has maintained a long standing position on mixture management at cruise power, which has always been to operate the engine(s) at rich-of-peak (ROP) as opposed to operating at either Peak, or Lean-of-peak (LOP). A rule of thumb for TCM 520 Series engines has been: at 75% power set mixtures 100° rich-of-peak; at 65% power set mixtures 75° rich-of-peak; at 55% power set mixtures 50° rich-of-peak for all cruise operations. Such mixtures will increase the life of the exhaust system, turbocharger, engine valves and rings, reference Cessna Model 340A POH, Section 4, Normal Procedures (amplified procedures), page 4-21, issued 1, Nov. 1977, Revision 1 - 1, April 1978.

This RAM recommendation includes all engines overhauled by RAM. Additionally, RAM OHE engines operating under a RAM Supplemental Type Certificate (STC) are FAA approved to operate only on the rich side of peak (ROP) in accordance with FAA Approved engine calibration, 150 hour engine endurance, durability, and detonation testing conducted by RAM under 14 CFR Part 33, Sections 33.45, 33.47, 33.49, and 33.19.

RAM is aware that some aircraft engines are designed and have FAA approved procedures to operate at lean-of-peak settings, and RAM knows that there are advocates for LOP operations whenever balanced fuel nozzles are installed. RAM supports the installation of balanced or matched fuel flow nozzles, but does not, in fact cannot, condone or approve their use to operate an engine at an unapproved mixture setting. RAM’s position is that LOP is not suitable for all airplanes, engines, or pilots. A major engine manufacturer has made a point that it only takes a brief episode of mis-management to bring about deep internal damage, and they point out that pilots don’t always realize when it happens, since their engine continues to operate. They suggested that lean-of-peak can be akin to operating on the edge, and that doing so requires pilots to be very precise, have good instrumentation, and watch their engine mixture and throttle full time. RAM agrees.
RAM reminds pilots that mis-management at LOP settings can cause both dynamic stresses and thermal stresses that hammer main and rod bearings, burn pistons, burn valves, and cause cumulative exhaust system damage. RAM encourages pilots wishing RAM would recommend other than (ROP) rich-of-peak to research the numerous White Papers and Engine Manufacturer’s Publications addressing the advocacy issue of LOP vs. ROP. RAM reminds it’s customers and friends in General Aviation that mixture management of a turbocharged Cessna 414A at FL230 is more demanding than the mixture management required of a normally aspirated Cessna 210 at 9,500 feet. Again, RAM cannot recommend LOP operation for any aircraft piston engine not authorized to do so by its manufacturer or an appropriate FAA approved flight manual supplement (AFMS).