Proper Leaning At Cruise Aids Safe Flight—And Saves $ Dollars

This Article Now Incorporates Material From "A Special On Fuel Management" Which Has Been Eliminated

Various Lycoming Flyer articles have emphasized proper leaning at the manufacturer’s recommended cruise power. Before delving into the savings to be obtained by leaning, it may be appropriate to again review those factors that affect leaning at cruise.

First we must know that cruise power for Lycoming normally aspirated engines is generally considered to be 55% to 75% of the maximum power for which the engine is rated. At these power settings, the engine may be leaned at any altitude. There has been confusion about the reference to not leaning below 5000-feet density altitude. Remember that this reference only applies to those power settings above the cruise range — those normally used for takeoff and climb. Once cruise power has been set, leaning to best economy should be standard procedure as damage to the engine will not occur from leaning at cruise power settings.

In this article we will expand our discussion of leaning and explain (1) how it saves dollars, and (2) how it aids safe flight. In a practical approach to our subject, let’s look closely at the chart printed below:

Leaning the normally aspirated, direct drive Lycoming engine at cruise vs. full rich at 4,000 feet density altitude, 75% power.
Engine Airplane

Model Model

300 HP Piper Cherokee 300

Full Best Economy Hours Hours

Rich (Peak EGT) Rich Lean

19+ gals 15.6 gals. 4.2 hrs. 5.1 hrs.

Engine Airplane

Model Model

250 HP Piper Aztec

Full Best Economy Hours Hours

Rich (Peak EGT) Rich Lean

16.2 gals. 13.6 gals. 4.3 hrs. 5.1 hrs.

Engine Airplane

Model Model

180 HP Cessna Cardinal

Full Best Economy Hours Hours

Rich (Peak EGT) Rich Lean

11.9 gals. 9.7 gals. 4.1 hrs. 5.1 hrs.

To put the cost of operating at a full rich mixture setting during cruise flight into perspective, let us assume that the cost of aviation gasoline is $2.10 per gallon. In each case it is only necessary to multiply the difference in gallons burned at "Full Rich" vs. "Best Economy" times the fuel price. The number obtained will be the amount saved each hour of flight by operating at best economy during cruise. Using the examples above, these are the savings for each of those aircraft and engines:

Aircraft Fuel burn difference Fuel cost savings

Cherokee 300 3.4 gallons X $2.10 = $ 7.14 per hr.

Aztec 5.2 gallons X $2.10 = $10.92 per hr.

Cardinal 2.2 gallons X $2.10 = $ 4.62 per hr.

While we are on a discussion of unnecessary cost of operation, another important factor is the damage often done to engine accessories by operating at full rich at cruise power. Engines operating at full rich in cruise tend to be rough, resulting in shaking engine accessories and engine mounts, thereby considerably reducing their life and often resulting in expensive early replacement. A properly leaned engine at cruise power is a smooth engine - and will save money.

In earlier issues of the Flyer, we have been telling all concerned about the benefits to the spark plug of proper leaning at cruise power. That information can be repeated in this discussion because it helps to illustrate our point on saving dollars. Proper leaning at cruise helps prevent spark plug fouling. The maintenance cost to remove and clean spark plugs can be reduced by good leaning techniques. Frequent cleaning of spark plugs reduces their life and requires early replacement. Furthermore, badly fouled spark plugs could also become a safety-of-flight problem.

For a very interesting safety-of-flight item, let’s look at the chart again. Notice the difference in hours of flight at full rich vs. lean at cruise. In the illustration of the 180 HP engine there is one full additional hour of flight when properly leaned. The other engines provide nearly an additional hour of flight time when leaned to best economy during cruise. This is a very real safety of flight fact.

These are some of the more important facts that illustrate how proper leaning at cruise power aids safe flight - and saves dollars.