Mike’s Maintenance Corner: Rich or Lean of Peak?

Photo Courtesy: AOPA

Mike Shekari/Staff Reporter

Flight training is a central aspect of what makes Embry-Riddle a unique environment to obtain a college education. However, the amount of knowledge that students have about the aircraft flying overhead varies greatly. This new series aims to answer questions commonly asked by students about piston-powered general aviation aircraft like the ones in the university’s fleet from an A&P mechanic’s perspective.

This week’s article focuses on a question that always draws controversy among pilots. Should an aircraft reciprocating engine be operated rich or lean of peak exhaust gas temperature (EGT)? The short answer from a maintenance standpoint is to operate rich of peak EGT by approximately 100 degrees Fahrenheit.

For those who are not familiar with terms used to describe the operation of aircraft engine fuel systems, running a system richer means operating with more fuel and leaner means operating with less fuel. EGT is the temperature of post-combustion gases going through the exhaust manifold after it has exited the cylinders.

The EGT reading on the instrument panel helps pilots determine how rich or lean their engine mixture is running, and the peak EGT is where the engine is combusting the mixture most efficiently. However, running the engine below peak EGT will help ensure engine longevity. Both enriching or leaning the mixture from peak will lower the EGT, which  is where  the controversy begins.

Supporters of lean of peak operations often cite benefits such as fuel savings, lesser environmental impact, and reduced sludge deposits inside engines at overhaul. However, aggressively leaned engines are also more prone to having higher cylinder head temperatures (CHT), causing unnecessary stress on cylinder valves. Not to mention the reduced fuel flow also causes the cylinders to have less lubrication and increased wear. Additionally, the lack of fuel creates a reduction in performance, demonstrated by slower cruising speeds.

Running an engine approximately 100 degrees rich of peak delivers higher engine performance, maintains lower CHTs, and reduces overall cylinder wear. However, it is important to note that this guidance applies mostly to carbureted and older injected aviation engines, which comprises most of the general aviation fleet.

In the future, the answer to this question might change as manufacturers work to create engines better suited for lean of peak operations and engine monitoring instruments to help pilots observe the health of their aircraft’s engines. The question might even become a non-issue with the proliferation of electronic engine controls in the general aviation industry.